Chapter 22 - Water and Waste Water Treatment Plant Hydraulics

22.1 INTRODUCTION

Designers of water treatment plants and wastewater treatment plants are faced with theneed to design treatment processes which must meet the following general hydraulicrequirements:

• Water treatment plants. Provide the head required to allow the water to flow throughthe treatment processes and to be delivered to the transmission/distribution system inthe flow rates and at the pressures required for delivery to the users.

• WastWW ewater treatment plants.rr Provide the head required to raise the flow of wastewaterfrom the sewer system to a level which allows the flow to proceed through the treat-ment processes and be delivered to the receiving body of water.

The above requires knowledge of open-channel, closed-conduit, and hydraulicmachine flow principles. It also requires an understanding of the interaction between theseelements and their impact on the overall plant (site) hydraulics. Head is either availablethrough the difference in elevation (gravity) or it has to be converted from mechanicalenergy using hydraulic machinery. Distribution of flows using open channels or closedconduit is critical for proper hydraulic loading and process performance.

This chapter brings together information on commonly used hydraulic elements andspecific applications to water treatment plants and wastewater treatment plants. The devel-opment of hydraulic profiles through the entire treatment process with examples for watertreatment plants and wastewater treatment is also presented.

Many processes and flow control devices are similar in both water treatment plants andwastewater treatment plants. Both types of plants require flow distribution devices, gatesand valves, and flowmeters. These devices are discussed in Section 22.2. The developmentof water treatment plant hydraulics, including examples from in-place facilities, are pre-sented in Section 22.3. Wastewater treatment plant hydraulics are discussed in Section22.4, and Section. 22.5 is devoted to non-Newtonian flow principles.

CHAPTER 22WATER AND WASTEWATER

TREATMENT PLANTHYDRAULICS

Federico E. MaischSharon L. ColeDavid V. Hobbs

Frank J. TantoneWilliam L. JudyGreeley and Hansen

Richmond, VA

22.1

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Source: HYDRAULIC DESIGN HANDBOOK

22.2 GENERAL

22.2.1 Introduction

This section addresses some elements which are common to both water treatment plantsand wastewater treatment plants including:

• Flow distribution–manifolds

• Gates and valves

• Flowmeters

• Local losses

22.2.2 Flow distribution–manifolds

In the design of water and wastewater treatment plants, proper flow distribution can be ascritical as process design considerations, which typically receive much more attention.Plant failures resulting from unequal and unmanageable flow distribution are possibly ascommon and as serious as those resulting from errors in process design.

Flow distribution devices, such as distribution channels, pipe manifolds or distributionboxes, are commonly used to distribute or equalize flow to parallel treatment units, suchas flocculation tanks, sedimentation basins, aeration tanks, or filters.

22.2.2.1 Distribution boxes. The simplest of these devices, the distribution box, typical-ly consists of a structure arranged to provide a common water surface as the supply to twoor more outlets. The outlets are typically over weirs and the key to equal flow distributionis to provide independent hydraulic characteristics between the downstream system andthe water level in the distribution box. In other words, provide a free discharge weir (non-submerged under all conditions) for each outlet to eliminate the impact of downstreamphysical system differences on the flow distribution. Velocity gradients across the distrib-ution box must be nearly zero to equalize flow conditions over each outfall weir. Weirsclearly should be of uniform design in terms of physical arrangement length and materi-als of construction. They should also be adjustable to account for any minor flow differ-ences noted in actual operation. The same principles apply if the designer wishes to dis-tribute flows in specific proportions which are not necessarily equal. In this case thedesigner could control the proportions of flow distribution by varying the relative geom-etry of the weirs (i.e., change the width or invert of each weir to achieve a desired flowdistibution). The specifics of weir hydraulics are covered in various texts in the literature.Attention should always be paid to the selection of the proper coefficients to model thespecific weir geometry and the geometry of the approach flow.

22.2.2.2 Distribution channels and pipe manifolds. Distribution channels and manifoldsare also common in plant design but a bit more complex in their function and design. Thedistribution of flow in these devices is impacted by the flow distribution itself. Since a por-tion of the flow leaves the channel or manifold along the length of the device, the veloci-ty of flow and, therefore, the relationship of energy grade line, velocity head and hydraulicgrade line varies along the length of the device. This is more clearly visible in a distribu-tion channel of uniform cross section, using side weirs along its length for flow distribu-tion. At each weir, flow leaves the channel, resulting in less velocity head in the channel

22.2 Chapter Twenty-Two



WATER AND WASTEWATER TREATMENT PLANT HYDRAULICS

and possibly a higher water surface at each ensuing weir. Chao and Trussell (1980), Campand Graber (1968), and Yao (1972) have presented comprehensive approaches for thedesign of distribution channels and manifolds and should be reviewed for details ofdesign.

As in distribution boxes, the most important consideration to achieving equalized flowdistribution is to minimize the effects of unequal hydraulic conditions relative to eachpoint of distribution. In channels this can be accomplished by tapering the channel crosssection, varying weir elevations, making the channel large enough to cause velocity headchanges to be insignificant or a combination of these. Similar considerations may beapplied to manifolds with submerged orifice outlets. A reliable approach here is to pro-vide a large enough manifold, resulting in a total headloss along the length of the distrib-ution of less than one tenth the loss through any individual orifice. This approach essen-tially results in the orifices becoming the only hydraulic control and the accuracy of theflow distribution is then dependent on the uniformity of the orifices themselves.

22.2.3 Gates and Valves

Gates and valves generally serve to either control the rate of flow or to start/stop flow.Gates and valves in treatment plants are typically subjected to much lower pressuresthan those in water distribution systems or sewage force mains and can be of lighter construction.

22.2.3.1 Gates. Gates are typically used in channels or in structures to start and stop flowor to provide a hydraulic control point which is seldom adjusted. Because of the time andeffort required to operate gates, they are not suited for controlling flow when rapidresponse, frequent variation, or delicate adjustments are needed. Primary design consid-erations when using gates are the type of gate fabrication and the installation conditionsduring construction.

There are many fabrication details including materials used, bottom arrangement, andstem arrangement. For instance, for solids bearing flows, a flush bottom, rising stem gatecan be used to avoid creating a point of solids deposition and to minimize solids contactwith the threaded stem. Gate manufacturers are a good source of information for gate fab-rication details and can assist with advice regarding specific applications.

Most commonly used gates are designed to stop flow in a single direction. They mayuse upstream water pressure to assist in achieving a seal (seating head), but typically alsomust be designed to resist static water pressure from downstream (unseating head). Bothseating and unseating heads must be evaluated in design of a gate application. For mostmanufacturers, the seating or unseating head is expressed as the pressure relative to thecenter line of the gate.

22.2.3.2 Valves. Table 22.1 provides a summary of several types of valves and theirapplications. Valves are used to either throttle (control) flow or start/stop flow.Start/stop valves are intended to be fully open or fully closed and nonthrottling. Theyshould present minimum resistance to flow when fully open and should be intended forinfrequent operation.

Gate valves, plug valves, cone valves, ball valves, and butterfly valves are the mostcommon start/stop valve selections. Butterfly valves have a center stem, are most commonin clean water applications and should not be used in applications including materials thatcould hang-up on the stem. Therefore, they are seldom used at wastewater plants prior toachieving a filter effluent water quality.

Water and Wastewater Treatment Plant Hydraulics 22.3




Check valves are a special case of a start/stop valve application. Check valves offerquick, automatic reaction to flow changes and are intended to stop flow direction rever-sal. Typical configurations include swing check, lift check, ball check and spring loaded.These valves are typically used on pump discharge piping and are opened by the pressureof the flowing liquid and close automatically if pressure drops and flow attempts toreverse direction. The rapid closure of these valves can result in unacceptable “water-hammer” pressures with the potential to damage the system. A detailed surge analysis maybe required for many check valve applications (see Chapter. 12). At times, mechanicallyoperating check valves should be avoided in favor of electrically or pneumatically operat-ed valves (typically plug, ball, or cone valves) to provide a mechanism to control time ofclosing and reduce surge pressure peaks.

Throttling valves are used to control rate of flow and are designed for frequent or near-ly continuous operation depending on whether they are manually operated or electroni-cally controlled. Typical throttling valve types include globe valves, needle valves, andangle valves in smaller sizes, and ball, plug, cone, butterfly, and pinch/diaphragm valvesin larger sizes. Throttling valves are typically most effective in the mid-range of loose lineopen/close travel and for best flow control should not be routinely operated nearly fullyclosed or nearly fully open.

22.2.4 Flow meters

The most common types of flow meters used in water and wastewater treatment plants aresummarized in Table 22.2 and fall into the following categories:


TABLE 22.1 Typical Valves and Their Application*

Type Open/Close Throttling Water Wastewater

Sluice gate X X X

Slide gate X X X

Gate valve X X X

Plug valve X X X X

Cone valve X X X X

Ball valve X X X X

Butterfly valve X X X

Swing check X X X

Lift check X X

Ball check X X X

Spring check X X X

Globe valve X X

Needle valve X X

Angle valve X X

Pinch/diaphragm X X X X

*Typical applications–exceptions are possible, but consultation with valve manufacturers is recommended.




• Pressure differential/pressure measuring meters (e.g., Venturi, orifice plate, pitot tube,and Parshall flume meters)

• Magnetic meters

• Doppler (ultrasonic) meters

• Mechanical meters (e.g., propeller and turbine meters)

Accurate flow measurements require uniform flow patterns. Most meters aresignificantly impacted by adjacent piping configurations. Typically a specific number ofstraight pipe diameters is required both upstream and downstream of a meter to obtainreliable measurements. In some cases, 15 straight pipe diameters upstream and 5 straightpipe diameters downstream are recommended. However, different types of meters havevarying levels of susceptibility to the uniformity of the flow pattern. Meter manufacturersshould be consulted.

22.2.4.1 Pressure differential/pressure measuring meters. Pressure differential/pressuremeasuring flow meters include Venturi meters, orifice plates, averaging pitot meters, andParshall flumes. These meters measure the change in pressure through a known flow crosssection–or in the case of the pitot meter, measure the difference in pressure at a point inthe flow versus static pressure just downstream in a uniform section of pipe.

Venturi meters and orifice plates are commonly used in water and wastewater. Solidsin wastewater could plug the openings of a pitot tube meter-limiting their use to relative-ly clean liquids. The Venturi meter and orifice plate meter use pressure taps at the wall ofthe device and can be arranged to minimize potential for debris from clogging the taps.The Parshall flume can be arranged with a side stilling well and level measuring float sys-tem or an ultrasonic level sensing device to measure water level.

22.2.4.2 Magnetic meters. In a magnetic flowmeter, a magnetic field is generated arounda section of pipe. Water passing through the field induces a small electric current propor-tional to the velocity of flow. Because a magnetic meter imposes no obstruction to theflow, it is well suited to measuring solids bearing liquids as well as clean liquids and pro-duces no headloss in addition to the normal pipe loss. Magnetic meters are among the leastsusceptible to the uniformity of the stream lines in the approaching flow.


TABLE 22.2 Common Types of Flow Meters

Type Typical Accuracy Size Range Headloss Cost W WW

Venturi �0.75% of rate 1–120 in Low Medium X X

Orifice plate �2% of scale Any size Medium Low X X

Pitot tube �0.5–5% of scale 1/2–96 in Low Low X

Parshall flume �5% of rate Wide range Low Medium X X

Magnetic �0.5% of rate 1/10–120 in None High X X

Doppler �1–2.5% of rate 1/8–120 in None High X X

Propeller �2% of rate Up to 24 in High High X

Turbine �0.5–2% of rate Up to 24 in High High X




22.2.4.3 Ultrasonic meters. In an ultrasonic flow meter, a pair of transceivers (transmit-ter/receiver) are positioned diagonally across from each other on the pipe wall. Thetransmitter sends out a signal which is affected by the speed of the flow. The receiver mea-sures the difference between the speed of the signal when directed counter to the flow(slowed by the flow) and when directed with the flow (speeded up by the flow). The timedifference is a function of fluid velocity, which is used to compute the flow. As with mag-netic meters, no flow obstruction is imposed resulting in no headloss in addition to thenormal pipe loss. Ultrasonic meters are also available for open-channel applications.

22.2.4.4 Mechanical meters. Mechanical meters include propeller and turbine-typeequipment. The two meters are similar in function in that in each a device is inserted intothe flowpath. The device is rotated by the flow and the speed of rotation is used to com-pute rate of flow. These devices impose an obstruction to flow, are recommended for cleanwater only, and generally result in significant headloss.

22.2.5 Local Losses

In any piping system as flow travels along the pipe, pressure drops as a result of headlossdue to friction along the pipe and local losses at bends, fittings, and valves. The locallosses at bends, fittings, and valves are least significant in long, straight piping systemsand most significant at treatment plants where the length of straight pipe is relatively shortand therefore, the frictional pipe losses comprise a smaller fraction of the total losseswhen compared to the summation of all local losses. A term often used to refer to locallosses is “minor losses,” however, because of the later consideration the term “minor loss-es” can be misleading.

Traditionally, local losses have been computed in terms of “equivalent length” ofstraight pipe or in terms of multiples of velocity head. The “equivalent length” or loss fac-tor K methods attempt to estimate the local losses based on the characteristic of the spe-cific bend, fitting or valve. The K loss factor method is discussed here. Essentially, a localloss is computed as follows:

hL � K�KK2Vg

2VV� (22.1)

where hL � local loss, K � loss factor, V � velocity, g � gravitational acceleration.

The values for K reported by various sources vary considerably for some local lossesand are relatively consistent for others. See references. There are many literature sourcesfor K values. The Bureau of Reclamation (1992) is one such source of information regard-ing energy loss equations. Table 22.3 shows a range of K factors from additional sourcesas well as a typically used value for each. Judgment must be applied in computing locallosses, taking into account any unique system conditions. Throughout this chapter K val-ues were obtained from equipment manufacturers when available. Values from Table 22.3were used only as an approximation when more specific data were unavailable. The read-er is cautioned that there are application-specific characteristics which have significantinfluence on the K factors. One of these characteristics, for example, is size. A K value of0.6 is often encountered in literature to characterize the losses associated with flowthrough the run of a tee. However, for flow past tees in large pipes this factor can be verysmall and nearly zero.






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TABLE 22.3 Typical K Factors for Computing Local Losses

Valve and Fitting Types

Gate valve100% open 0.39 0.19 0.19 0.1–0.3 0.2 0.275% open 1.1 1.15 1.2 1.250% open 4.8 5.6 5.6 5.625% open 27 24 24 25

Globe valve–open 10 10 10 4.0–6.0 10 10Angle valve–open 4.3 5 2.1–3.1 5 1.8–2.9 2.5 5Check valve–ball 4.5 65–70 5Swing check 0.6–2.3 06–2.2 0.6–2.5 2.5Butterfly valve–open 1.2 0.16–0.35 0.5Foot valve–hinged 2.2 1.0–1.4 2.2Foot valve–poppet 12.5 5.0–14.0 14Elbows

45° regular 0.30–0.42 0.42 0.4245° long radius 0.18–0.20 0.18 0.5 0.290° regular 0.21–0.3 0.25 0.7 0.2590° long radius 0.14–0.23 0.18 0.6 0.19180° regular 0.38 0.38180° long radius (flanged) 0.25 0.25

TeesStd. teee–flowthrough run 0.6 0.6 0.6 0.3 1.8 0.6Std. teee–flow-through branch1.8 1.8 1.8 1.8 0.75 1.8

Return bend 1.5 2.2 2.2 0.4 2.2Mitre bend

90° 1.8 1.129–1.265 0.8 1.360° 0.75 0.471–0.684 0.35 0.630° 0.25 0.130–0.165 0.1 0.16

Expansiond/D = 0.75 0.18 0.19 0.2 0.2d/D = 0.5 0.55 0.56 0.6 0.6d/D = 0.25 0.88 0.92 0.9 0.9

Contractiond/D = 0.75 0.18 0.19 0.2 0.2d/D = 0.5 0.33 0.33 0.3 0.33d/D = 0.25 0.43 0.42 0.4 0.43

Entrancee–projecting 0.78 0.78 0.83 0.8 0.8 0.78 0.8Entrancee–sharp 0.5 0.5 0.5 0.5 0.5 0.5 0.5Entrancee–well rounded 0.04 0.04 0.04 0.04 0.25 0.04 0.04Exit 1.0 1.0 1.0 1.0 1.0




22.3 HYDRAULICS OF WATER TREATMENT PLANTS

22.3.1 Introduction

Water treatment comprises the withdrawal of water from a source of supply and thetreatment of raw water through a series of unit processes for the beneficial use of thesystem customers. Raw water quality can vary widely. The ultimate uses of water by thesystem customer (e.g., drinking, fire protection, irrigation, aquifer recharge, etc.) can alsovary and be subject to different treatment level requirements and regulations. Therefore,the selected treatment processes vary widely over a multitude of treatment technologies inuse. Water treatment consists of a series of chemical, biological, and physical processesconnected by channels and pipelines. Figures 22.1 and 22.2 illustrate processflow diagrams (flowsheets) for typical surface water and groundwater treatment plants,respectively. The designer of the water treatment process must carefully evaluate sourcewater characteristics and desired water quality characteristics of the treated water todesign treatment processes capable of purifying the source water to water suitable for thesystem customers. The objective of this chapter is to review the hydraulic considerationsrequired to convey water through the treatment process.

Design of a plant’s treatment process is closely linked with the hydraulic design of thetreatment plant. This chapter presumes that the designer has evaluated and selected treat-ment processes for the water treatment plant. Although design flows are discussed below,we have also assumed that the designer has chosen a design flow requirement for the treat-ment process. For municipal treatment plants, design flows are based on the service area


FIGURE 22.1 Typical surface water treatment plant process flow diagram.

FIGURE 22.2 Typical ground water treatment plant process flowdiagram with dual trains (#1 and #2).




population and the per capita use of water by the population served. The per capita use ofwater can be obtained from literature sources as an initial approximation. However, theseinitial estimations must be corroborated with actual site specific population counts andwater usage. For nonmunicipal treatment facilities, treated water needs of the service areamust be individually evaluated.

22.3.1.1 Sources of supply. Natural sources of supply include groundwater and surfacewater supplies. Groundwater supplies typically are smaller in daily delivery but servemore systems than surface water supplies. Groundwater supplies normally come fromwells, springs, or infiltration galleries.

Wells constitute the largest source of groundwater. Except in rare circumstances ofartesian wells (wells under the influence of a confined aquifer) and springs, groundwatercollection involves pumping facilities. Hydraulics of groundwater treatment plants are fre-quently based on hydraulics of conduits under pressure, such as pipelines, pressurefilters, and pressure tanks. Raw water characteristics of groundwaters are uniform inquality compared with surface supplies.

Surface water supplies are normally larger in daily delivery. Surface supplies are usedto service larger population centers and industrial centers. In areas where groundwatersupplies are limited in yield or where groundwater supplies contain undesirable chemicalcharacteristics, smaller surface water treatment plants may be utilized. Surface watersources of supply include rivers, lakes, impoundments, streams, and ponds. The treatmentprocesses chosen in plants treating surface water favor nonpressurized systems such asgravity sedimentation. The larger flow volumes characteristic of surface water suppliesalso favor open channel hydraulic structures for conveying water through the treatmentprocess. Raw water characteristics of surface supplies can vary rapidly over short periodsof time and also experience seasonal variation.

22.3.1.2 Treatment requirements. Treatment requirements for municipal water treat-ment plants are normally defined by regulatory agencies having authority over the plant’sservice area. In the United States, regulatory agencies include national government regu-lations promulgated through the Environmental Protection Agency and state governmentregulations. Water treatment plants are designed to meet these regulations. Treatment reg-ulations change through improved knowledge of health effects of water constituents andthrough identification of possible new water-borne threats. The designer therefore shouldattempt to select treatment processes which will also meet treatment requirements whichare expected to be promulgated over the next few years. To the extent possible, treatmentplant process design should provide flexibility for future plant expansions or for possibleadditional treatment processes. Because hydraulic design of plants must go hand-in-handwith the process selection, plant hydraulic design should provide for the flexibility to addfuture plant facilities.

Treatment requirements for industrial water treatment plants are dictated by processneeds of the industry and less by regulatory agency requirements. Industrial water treat-ment plants that result in contact between or ingestion of the treated water by humansmust conform to the local regulatory requirements.

22.3.1.3 General design philosophy. Effective design of water treatment plant hydraulicsrequires that the hydraulic designer have a thorough knowledge of all aspects of the watersystem. The overall treatment system hydraulic design must be integrated and coordinat-ed including the treatment plant, the raw water intake and pumping facilities, the treatedwater storage, and treated water pressure/head requirements. The design within the watertreatment plant must also be integrated between the various treatment processes.





Additionally, design considerations must address the availability of operating personneland hours of operation such that the process and hydraulic requirements conform to avail-able resources.

22.3.2 Hydraulic Design Considerations in Process Selection

Water treatment plant process selections are controlled principally by characteristics ofthe raw water and by the desired water quality characteristics of the finished water. Flowthrough each unit process and each conduit connecting processes results in loss ofhydraulic head. Most treatment plants have limited head available.

The selection of a particular unit process will include evaluation of numerous criteriaincluding costs, operability, performance, energy use and similar items. One criteriawhich must also be evaluated for each process is the hydraulic head requirements of theprocess.

22.3.2.1 Head available. For the design flow to pass through a water treatment plant, thetotal available head must exceed the head requirements of the unit processes and con-necting conduits. The head available is the difference in energy grade line (EGL) in thehydraulic profile between the head works of the plant and the end of the plant. Additionalhead may be provided by pumping or by lowering the elevation of treatment units at theend of the plant. See Figure 22.3 for a typical water treatment plant hydraulic profile.

For most surface water plants, the hydraulic profile at the head of the plant iscontrolled by raw water pumps pumping from the intake facilities. The hydraulic profileat the head of a plant in a groundwater system is typically determined by the well pumpsserving the plant.

22.3.2.2 Typical unit process head requirements. Following below is a table of typicalhead requirements for water treatment plant processes. This table may be used for initialevaluation of unit processes. More detailed hydraulic evaluations must be performed afterplant operating modes and design flows are determined. Detailed hydraulic evaluationsmust also include headlosses in connecting conduits.

Head RequirementUnit Process at Rated Capacity, m (ft)

Intakes, including bar screens 0.3–0.9 (1–3)

Rapid mixing 0.15–0.30 (0.5–1)

Flocculation 0.06–0.15 (0.2–0.5)

Sedimentation 0.6–2.4 (2–8)

Filtration – Gravity 3–4.6 (10–15)

– Pressure 3–7.6 (10–25)

Disinfection 0.15–0.6 (0.5–2)

Aeration – Spray 3–4.6 (10–15)

– Cascade 3–4.6 (10–15)

– Compressed air 0.15–0.6 (0.5–2)

Softening 0.15–0.6 (0.5–2)

Ion exchange softening 0.6–1.5 (2–5)

Iron and manganese removal 0.6–1.5 (2–5)






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22.3.3 Hydraulic Design Considerations in Plant Siting

Plant sites are normally selected before the hydraulic designer initiates design of the treat-ment system. If a plant site has not been selected, the designer should be aware ofhydraulic considerations which may influence site selection.

Site elevation has the most significant impact on plant hydraulics. A plant site locatedabove the service area will eliminate or reduce pumping requirements from the plant tothe service areas. Typical municipal distribution system pressures are 40–70 psi, thereforethe elevation of the treatment plant should be at least 100 ft above the service area to elim-inate finished water pumping. Similarly, plant sites which permit gravity intake of thesource water may reduce or eliminate raw water pumping. Few plants are able to meetthese optimal conditions.

The typical surface water plant must pump both raw and finished water. Raw water(low-lift) pumps are used to pump water from the water source into the treatment facilitiesand finished water (high-lift) pumps are used to pump from the treatment plant into theservice area distribution system.

22.3.4 Hydraulic Design Consideration in Plant Layout

After the plant site has been identified, the plant design may be arranged for optimalhydraulic benefit. In particular, arrangement of treatment processes to allow flow to movedown gradient minimizes excavation needs for structures. Arrangements which aredesigned for future expansion should consider the hydraulic needs of the expanded plantas well as the process needs. Grouping of processes together facilitates movement ofwater through the treatment process train.

The designer should also consider secondary hydraulic systems for optimal design.Chemical feed systems and dewatering systems are examples of secondary hydraulicsystems which must be coordinated with the treatment flow system. Normally it isdesirable to minimize the length of chemical piping systems. Dewatering systems are usu-ally based on gravity drainage of basins and conduits.

22.3.5 Bases for Design

After evaluation and selection of a source of supply and development of the treatmentplant process train, the designer is prepared to develop the plant Bases for Design. TheBases for Design is a summary of design flow and capacity, and proposed treatmentprocesses, including the chemical storage and feed facilities.

22.3.5.1 Design flows. Design flows for water treatment plants serving municipalitiesare typically based on the projected population within the water service area for thedesign life of the treatment facilities. Population data is normally determined fromcensus records, land use zoning information, and studies of existing and projectedpopulation densities. Service area per capita demands are affected by the mix ofdomestic, commercial, and industrial water users which are unique to each servicearea.Typically water consumption records are available for water service areas. For newfacilities, the use of generalized water consumption data may be needed. In the UnitedStates, water consumption varies widely but generally ranges between 100–200 gal-lons per capita per day.





From studies of projected population and per capita demand, planned design flows forthe water treatment facilities may be developed. These demands include the following:

• Annual average demand. The average daily water consumption for the water serviceareas, generally computed by multiplying the average daily consumption (gallons percapita) by the projected population of the service area.

• Maximum demand. Maximum demand experienced by the water plant throughout itsservice life. The maximum hour demand is generally 200 to 300 percent of the aver-age demand but numerous factors affect the peak demand experienced by water treat-ment plants. These factors include seasonal demands (particularly for plants where ser-vice areas are located in extremes of hot and cold temperatures), normal daily flowvariations, the community size, industrial usage, and system storage. Normally systemstorage is provided to service peak hour demands, allowing the treatment facilities tobe designed on peak day demands. Peak day demands generally range between 125and 200 percent of the average demand.

• Minimum flow. As the name suggests, the minimum flow expected to be processedthrough the treatment facilities. Minimum flow depends upon system operations. Ingeneral, minimum flows for municipal plants may be estimated as 50 percent of theaverage demand, but range between 25 and 75 percent of the average demand.

22.3.5.2 Rated treatment capacity. The rated treatment capacity of a plant is that capac-ity for which each of the unit processes are designed. For municipal treatment plants withadequate system storage, the rated treatment capacity is the system’s maximum daydemand. Where storage is limited, the rated treatment capacity may be greater, for exam-ple, the system maximum hour demand or greater. Smaller systems may be designed toproduce the rated treatment capacity in one or two 8-h shifts rather than over the entire24-h day.

22.3.5.3 Hydraulic treatment capacity. Treatment plants are normally designed for ahydraulic capacity greater than the rated treatment capacity. Hydraulic treatment capaci-ties are normally equal to 125 to 150 percent of the rated treatment capacity. The hydraulictreatment capacity provides flexibility for future process changes or alternative flow rout-ings through the plant. Hydraulic capacities in excess of the rated treatment capacity pro-vide some margin of safety for operations which may not be optimal (e.g., control gatesinadvertently left partially open).

22.3.5.4 Treatment process bases for design. The development of the water treatmentplant’s “Bases for Design” is a key step in establishing the criteria to which the plant willbe designed. This document must be reviewed carefully with the water treatment plantowner representatives and understood and agreed to by all before the final design pro-ceeds. The Bases for Design presents a summary of each treatment process includingdesign flows (minimum, average, rated capacity), specification of dimension of major ele-ments (e.g., tanks, pumps), both hydraulic and process loading characteristics, requiredperformance, and design data for the chemical storage and feed system. Table 22.4 pre-sents an example of the bases for design for sedimentation basins (one of the many unitprocesses in a water treatment plant).

22.3.6 Plant Hydraulic Design

As noted above, a water treatment plant consists of a series of treatment processesconnected by free surface flow channels and pipelines. During development of the plant’s






TABLE 22.4 Treatment Process Bases for Design—Sedimentation Basins

Item Stage I Stage II StageIII

Maxi– Maxi– Maxi-Annual mum Annual mum Annual mumAverage Day Average Day Average Day

Number of basins 4 4 8 8 12 12

Basin characteristics

Plan–75 ft � 230–6 in

Nominal side water

depth–12 ft (SWD)

Surface area/basin–17,288 ft2

Volume/basin–207,456 f3ff

Channels/basin–2

L:W ratio–6.1:1

Displacement time (h) 3.17 1.99 3.17 1.99 3.17 1.99

Surface loading [(gal�m)/ft2] 0.47 0.75 0.47 0.75 0.47 0.75

Flowthrough velocity (ft/min) 1.21 1.93 1.21 1.93 1.21 1.93

Sludge collectors

Longitudinal collectors

Type: chain flight

Number per basin 8 8 8 8 8 8

Cross collectors

Type: chain flight

Number per basin 1 1 1 1 1 1

Settled sludge pumps

Type: progressive cavity

Number:

100 gal/min capacity 4 4 4 4 4 4

400 gal/min capacity 4 4 4 4 4 4

200 gal/min capacity — — 8 8 16 16

Capacity (gal/min)

Installed 2000 2000 3600 3600 5200 5200

Firm 1600 1600 3200 3200 4800 4800

Bases for Design, the designer determines the rated treatment capacity, average flow,minimum flow and hydraulic capacity of the plant.

Following development of the Bases for Design, the designer must evaluate plantoperating modes to develop a detailed plant flow diagram and hydraulic profilethrough the plant.




22.3.6.1 Plant operating modes. Operating modes describe the sequence of treatmentprocesses the water goes through to achieve the required level of purification. Operationalmodes are normally presented in the form of simplified block diagrams which illustratethe flow path through the plant from one process to the next. These operational modeblock diagrams are useful in visualizing stages during construction, future planned plantexpansions or simply alternative operating modes.

Figures 22.4 through 22.9 show an example of a sequence of plant operating modes fora surface water treatment plant which illustrate three stages of a plant expansion programwith alternatives for the flocculation and sedimentation basins to work in series or in par-allel. Plant processes proposed include raw water control chambers, rapid mix chambers,flocculation/sedimentation basins, ozone contact chambers, and filters. In this example,the raw water control chambers are used to split flow between plant process groups andalso as a rapid mix chamber for chemical addition.


FIGURE 22.4 Stage I—operational mode diagram.

FIGURE 22.5 Stage II—parallel operational mode diagram.




The Stage I facilities including raw water control chamber, flocculation/sedimentationbasins and filters are depicted in Fig. 22.4. Operational modes for a proposed plant expan-sion to double the plant capacity (Stage II) are shown in Figs. 22.5 through 22.7 and oper-ating modes for a second plant expansion to triple the plant capacity (Stage III) are shownin Figs. 22.8 and 22.9. Settled water ozone contact chambers were added to the expandedplant, which illustrates treatment upgrades.

Operational modes for the Stage II treatment plant include parallel and series floc-culation/sedimentation. When the plant is operated in the parallel mode, influent rawwater for each set of sedimentation basins flows by gravity from the raw water controlchamber serving the basin set. Raw water flow is divided between each sedimentationbasin in service at the raw water control chamber. Settled water from each set of basinsis routed to an ozone contact chamber. Ozonated settled water is then combined prior toflowing to the filters.


FIGURE 22.6 Stage II—series flocculation/sedimentation basinoperational mode diagram.

FIGURE 22.7 Stage II—split parallel operational mode diagram.




The Stage III split parallel operational mode is similar to the parallel operational modeexcept that the ozonated settled water from each set of basins is not combined prior toflowing to the filters. Side-by-side plant scale treatment studies are possible with thefuture split parallel mode since part of the flocculation/ sedimentation/filtration processescan be operated as a “control” while the remainder of the plant can be operated in acontrolled experimental mode.

The series flocculation/sedimentation operational mode is designed to permit opera-tion of the sedimentation basins in two stages in lieu of the single–stage parallel mode.Under certain raw water conditions, operation of the basins in series may enhance perfor-mance of the basins. Chemical feed for the first and second sedimentation stages may beadjusted to respond to raw water conditions and settled water quality after the first–stagesedimentation. Series flocculation/sedimentation increases hydraulic losses through the


FIGURE 22.8 Stage III—parallel operational mode diagram.

FIGURE 22.9 Stage III—split parallel operational mode diagram.




plant. Under this mode, twice as much flow is routed to each basin and the flow pattern islonger, since the settled water from the first sedimentation stage must be returned to theinfluent of the second sedimentation stage.

Operational mode block diagrams are also a convenient means to illustrate the effectof side stream flows which may impact the overall plant flow. For example, removal ofsludge from the sedimentation basins is accompanied by a decrease in flow leaving thebasins compared with flow entering the basins. In a similar manner, filter backwash waterremoves a certain amount of flow. A plant designed to produce a certain rated capacitymay have to treat more than the rated capacity through certain processes. The impact ofthese side stream flows must be evaluated on an individual basis. In many treatmentplants, backwash water treatment facilities are installed to recycle backwash water to thehead of the plant.

22.3.6.2 Plant flow diagrams. After establishing plant operating modes, more detailedflow diagrams are developed by the designer. The diagrams normally start with possiblevalving and gating arrangements and are then expanded with tentative valve, sluice gate,pipeline, and conduit sizes.

Valving arrangements are designed to enable any of the major operational units (e.g.,sedimentation basin, ozone contact chamber) to be removed from service. The arrange-ment may include design of temporary flow stop devices, such as stop logs (sectional bar-riers which were originally constructed of logs but are now commonly metal plates). Thearrangement should be designed to permit maintenance work on major valves and sluicegates while minimizing the impact on plant process. Major channel sections should bedesigned so they can be removed from service and dewatered while minimizing impactson the rest of the plant.

The designer should distinguish between units taken out of service frequently (suchas filters), periodically (such as sedimentation basins), or rarely (such as conduits).Filter backwashing occurs so frequently that the rated treatment capacity can be metwith one filter out for backwashing. Sedimentation basins may be removed from serviceonce or twice per year for equipment maintenance. Since the basins outages occur atwidely scattered intervals, it is reasonable to design the units to be removed from ser-vice during lower flow periods. Conduits and pipelines are rarely removed from service,but the hydraulic impacts can be significant. Depending on the conduit location,removal of a conduit can remove a portion of the plant from service. Effective designwill provide redundant conduits so that a portion of the plant can remain in service dur-ing conduit dewatering.

The focus of this section has been on the main plant hydraulics, but the hydraulicdesigner must also design for hydraulic subsystems. An important group of these subsys-tems include dewatering of all basins and conduits. Where plant elevations will allow,gravity dewatering is recommended. In most cases, dewatering pumps are necessary.These pumps may be located in the unit being dewatered or may be located in a separatestructure connected to the process unit by dewatering pipelines.

22.3.6.3 Hydraulic Profile. One of the most important tools in the hydraulic design of awater treatment plant is the development of a hydraulic profile. The hydraulic profile is adiagram showing the energy grade line (EGL) at each unit process. For open tanks withflows at minimal velocities, which is the case in most water treatment plants, the velocityhead is negligible and the hydraulic grade line (HGL) or water surface elevation (WSEL)provide an adequate representation of the EGL. Profiles normally include critical struc-tural elevations of processes and conduits. The profile may also include ground surfaceprofiles and other site information.





Hydraulic profiles are developed for each of the design flows. In the case of watertreatment plants, the design flows may include rated treatment capacity, hydraulic capac-ity, average flow, and minimum flow. Hydraulic profiles should also take into considera-tion unit processes or conduits which may be taken out of service. Hydraulic profiles arevaluable design and operational tools to assist in scheduling routine maintenance activi-ties and for evaluating the impact to the treatment plant capacity during outages of processunits or conduits.

Computations of hydraulic profiles begin at control points where there is a definiterelationship between the plant flow and water surface depth. For gravity flow plants, themost common forms of control points are weirs and tank water surface elevations (e.g.,clear well water surface elevations), but other types of control points may be used. Fromeach control point, head losses associated with local losses, plant piping, and open chan-nel flow are added to the control water surface. Since flows in water treatment plant’s aremostly in the subcritical regime (Froude number � 1), most hydraulic designers will workupstream from the control point. For pressure plants, control points are typically pressureregulating or pressure control points, frequently in the service area distribution system.From these control points and knowledge of the flow velocity, both the EGL and HGLmay be computed back to the treatment facilities.

Hydraulic profiles are valuable design tools to identify overall losses through the plant.Profiles are also valuable to identify units with excessive losses. Since total head availableis normally limited, units with excessive losses should be considered for redesign toreduce local loss coefficients or to reduce velocities.

Figure 22.3 is an example hydraulic profile for a gravity surface water treatment plantwith conventional treatment processes. The method of computing headlosses is presentedin Section 22.3.7.

22.3.7 Water Treatment Plant Process Hydraulics

In this section calculations required to establish the WSEL through a medium-sized watertreatment plant will be presented. A schematic of the water treatment plant is shown inFig. 22.10. Notice that future growth has been considered in the initial design. Threeexamples are included which illustrate typical hydraulic calculations. The first examplecalculates the WSEL from the sedimentation basin effluent chamber back through theflocculation/sedimentation basins to the Raw Water Control Chamber. The second followsthe flow from the clear well back through the filters. Filter hydraulics are illustrated in thethird example. All examples are presented in a spreadsheet format which is designed tofacilitate calculating the EGL, HGL, and WSEL at various points through the treatmentprocess and for multiple flow rates (i.e., minimum, daily average, peak hour, futureconditions).

22.3.7.1 Coagulation. Process criteria and key hydraulic design parameters. The coag-ulation process, used to reduce particulates and turbidity, is carried out in three steps: mix-ing (often referred to as rapid or flash mixing), flocculation, and sedimentation. Each ofthese steps is briefly discussed below.

Rapid mixing. The mixing process imparts energy to increase contact betweenexisting solids and added coagulants. Possible mixer types include turbine, propeller,pneumatic, and hydraulic. Headloss that occurs in mixing chambers depends on the cho-sen mixing device. Most mechanical mixers do not create significant head losses. Theheadloss coefficient (K) associated with a specific mixer can be obtained from the manu-KKfacturer. Pneumatic mixing, which is not common, has associated losses similar to those





for aeration (see table in Section 22.3.2.2, above). Hydraulic mixing takes place usingweirs, swirl chambers, throttled valves, Parshall flumes, or other devices to induce turbu-lence. Head loss coefficients for these devices can be obtained from the manufacturer.Important considerations during the initial design of a mixing chamber include:

• Velocity gradient. This is mixer—specific information and can be obtained from themanufacturer. The system should be designed to provide a velocity gradient that isoptimal for the coagulation process taking place.

• Dead spots and short circuiting. An ideal mixing system will have minimal dead spotsand short circuiting. These can be avoided with proper sizing and placement of mixers.

Flocculation. Coagulated particles form larger particles (flocs) during the gentle mix-ing of flocculation, where the flow travels slowly through a series of flocculator paddles,baffles, or conduits. Inlets and weirs are designed to provide low turbulence for protectionof the flocs. The energy provided to the system by the flocculators (manufacturer-specif-ic) or baffling is decreased as the flow approaches the sedimentation basins.

Sedimentation. Gravity sedimentation removes coagulated solids prior to filtration.There are four zones in a clarifier as shown in Fig. 22.11 and listed below:

• Inlet zone—where upstream flow conditions transition smoothly to uniform flow set-tling conditions

• Sedimentation zone—where sedimentation takes place

• Sludge zone—where solids collect and are removed

• Outlet zone—where settling conditions smoothly transition to downstream flowconditions


FIGURE 22.10 Schematiz of a water treatment plant.




Each of the zones is designed to minimize turbulence and avoid short circuiting. Thevelocity in the sedimentation zone is limited to 0.3 m/s (1 ft/s) for average flow. Sludgeremoval equipment moves slowly so that settling patterns are not disturbed. Because theprocess is designed for smooth flow and minimal turbulence, very little head loss occursin sedimentation basins. Ports at the inlet and outlet produce the greatest head losses inthis process.

Hydraulic design example. Table 22.5 illustrates the calculation of the WSEL, usingmetric units, through the coagulation process at the medium-sized water treatment plantshown in Fig. 22.10. Figs. 22.12 through 22.14 show plan views and details of the


FIGURE 22.11 Hypothetical zones in a rectangular sedimentation basin.

TABLE 22.5 Hydraulic Calculations of a Typical Coagulation Process, SI Units

Initial Operation Design Operation

Parameter Min. Day. Avg. Day Avg Day Max. Hour

1. Plant Flow (m3/s) 2.19 3.06 3.28 4.38Note: For Points 1 through 8, see Fig. 22.12

2. WSEL at Point 1 (Calculation done in Table 22.6) (m) 109.73 109.73 109.74 109.74

3. Point 1 to Point 2Average flow � 21Q/32 (m3/s) 1.44 2.01 2.15 2.87Flow depth � WSEL @ 1 – invert (106.60 m) (m) 3.13 3.13 3.13 3.14Flow area � 5.13 m width � depth (m2) 16.05 16.06 16.07 16.10Velocity � flow/area (m/s) 0.09 0.13 0.13 0.18Hydraulic Radius r � A/P/ (P � w � 2d) (m) 1.41 1.41 1.41 1.41Conduit loss � [(V � n)/(r2/3rr )]2 � L (m)

where Manning’s n � 0.014 and L � 28.96 m 0.00 0.00 0.00 0.00WSEL at Point 2 (m) 109.73 109.73 109.74 109.74

4. Point 2 to Point 3Average Flow � 5Q/16 (m3/s) 0.68 0.96 1.03 1.37Flow depth � WSEL @ 2 � invert (106.60 m) (m) 3.13 3.13 3.13 3.14Flow area � 5.13 m width � depth (m3) 16.05 16.06 16.07 16.10Velocity � flow/area (m/s) 0.04 0.06 0.06 0.08r � A/P// (P � w � 2d) (m) 1.41 1.41 1.41 1.41Conduit loss � [(V � n)/(r2/3rr )]2 � L (m)

where Manning’s n � 0.014 and L � 14.63 m 0.00 0.00 0.00 0.00WSEL at Point 3 (m) 109.73 109.73 109.74 109.74

5. Point 3 to Point 4Average flow � Q/8 (m3/s) 0.27 0.38 0.41 0.55Flow depth � WSEL @ 3—invert (106.60 m) (m) 3.13 3.13 3.13 3.14Flow area � 5.13 m width � depth (m3) 16.05 16.06 16.07 16.10





TABLE 22.5 (Continued)


Parameter Min. Day. Avg. Day Avg. Day Max. Hour

Velocity � flow/area (m/s) 0.02 0.02 0.03 0.03r = A/P// (P � w � 2d) (m) 1.41 1.41 1.41 1.41Conduit loss � [(V � n)/(r2/3rr )]2 � L (m)

where n � 0.014 and L � 21.95 m 0.00 0.00 0.00 0.00WSEL at Point 4 (m) 109.73 109.73 109.74 109.74

6. Point 4 to Point 5Flow � Q/32 (m3/s) 0.07 0.10 0.10 0.14Port area � 0.30 m deep � 0.76 m wide (m2) 0.23 0.23 0.23 0.23Velocity � flow/area (m/s) 0.29 0.41 0.44 0.59Submerged entrance loss � 0.8 V2VV /2g (m) 0.00 0.01 0.01 0.01WSEL at Point 5 (in Sedimentation Tank) (m) 109.73 109.74 109.74 109.76

7. Point 5 to Point 6Width of sedimentation basin (W) (m) 23.16 23.16 23.16 23.16WWFlow (Q/4) (m3/s) 0.55 0.77 0.82 1.09Invert elevation of sedimentation baffles (m) 105.97 105.97 105.97 105.97Flow depth (H) (WSEL at Point 5—baffle invert) (m) 3.76 3.77 3.77 3.79HHArea downstreams of baffle (W � H) (mHH 2) 87.21 87.36 87.41 87.68Horizontal openings in baffle, 2.54 cm wide

spaced every 22.86 cm. Area ofopenings � A � W � .0254 � H/.2286 (m2) 9.69 9.71 9.71 9.74Velocity of downstream baffle (V downstream) 0.01 0.01 0.01 0.01

(Q/A) (m/s)Velocity of 2.54 cm opening section (V1) (Q/A// ) (m/s) 0.06 0.08 0.08 0.11Local losses � sudden expansion (1.0 � (V downstream)2/2g)

� sudden contraction (0.36 � (VI)VV 2/ 2g) (m) 0.00 0.00 0.00 0.00WSEL at Point 6 (Upstream of sedimentation baffles) (m) 109.73 109.74 109.74 109.76

8. Point 6 to Point 7Loss per stage (provided by flocculator manufacturer) (m) 0.01 0.01 0.03 0.05Total loss (three stages) (m) 0.04 0.04 0.09 0.15WSEL at Point 7 (m) 109.77 109.78 109.83 109.91

9. Point 7 to Point 8Flow � Q/24 (m3/s) 0.09 0.13 0.14 0.18Port area � 0.30 m deep � 0.46 m wide (m2) 0.14 0.14 0.14 0.14Velocity � flow / area (m/s) 0.65 0.92 0.98 1.31Entrance loss � 1.25 V2VV /2g (m) 0.03 0.05 0.06 0.11WSEL at Point 8 (inlet port) (m) 109.80 109.83 109.89 110.02

Note: For Points 8 through 14, see Fig. 22.13

10. Point 8 to Point 9Average flow � Q/24 (m3/s) 0.09 0.13 0.14 0.18Flow depth � WSEL @ 8 – invert (109.12 m) (m) 0.68 0.72 0.77 0.90Flow area � 0.91 m width � depth (m2) 0.62 0.65 0.71 0.82Velocity � flow/area (m/s) 0.15 0.19 0.19 0.22r = A/P (P � w � 2d) (m) 0.27 0.28 0.29 0.30Conduit loss [(V � n)/(r2/3rr )]2� L (m)


11. Point 9 to Point 10Average flow � Q/12 (m3/s) 0.18 0.26 0.27 0.36







Flow depth � WSEL @ 9 – invert (109.12 m) (m) 0.68 0.72 0.77 0.90Flow area � 0.91 m width � depth (m2) 0.62 0.65 0.71 0.82Velocity � flow/area (m/s) 0.29 0.39 0.39 0.44r = A/P// (P � w � 2d) (m) 0.27 0.28 0.29 0.30Conduit loss � [(V � n)/(r2/3rr )]2 � L (m)


12. Point 10 to Point 11Flow � Q/8, m3/s 0.27 0.38 0.41 0.55Flow depth � WSEL @ 10 � invert (109.12 m) (m) 97.34 97.38 97.44 97.56Flow area � 0.91 width � depth (m2) 89.01 89.04 89.09 89.21Velocity � flow/area (m/s) 0.00 0.00 0.00 0.01Loss at two 45° bends � 2 � 0.2 V 2/2g (m) 0.00 0.00 0.00 0.00WSEL at Point 11 (m) 109.80 109.84 109.89 110.02

13. Point 11 to Point 12Flow � Q/4 (m3/s) 0.55 0.77 0.82 1.09Flow depth � WSEL @ 11 � invert (109.12 m) (m) 0.68 0.72 0.78 0.90Flow area � 1.52 m width � depth (m2) 1.04 1.09 1.18 1.37Velocity � flow/area (m/s) 0.52 0.70 0.69 0.80Loss at two 45° bends � 2 � 0.2 V 2/2g (m) 0.00 0.00 0.00 0.00r = A/P// (P � w � 2d) (m) 0.36 0.37 0.38 0.41Conduit loss � [(V � n)/(r2/3rr )]2 � L (m)


14. Point 12 to Point 13Flow � Q/4, (m3/s) 0.55 0.77 0.82 1.09Flow depth � WSEL @ 12 � invert (109.12 m) (m) 0.69 0.72 0.78 0.91Inlet area � 1.52 m width � depth (m2) 1.05 1.10 1.19 1.38Velocity � flow/area (m/s) 0.52 0.69 0.69 0.79Inlet loss � 1 V 2/2g (m) 0.01 0.02 0.02 0.03WSEL at Point 13 (Mixing Chamber No. 2 outlet) (m) 109.82 109.87 109.92 110.06

15. Point 13 to Point 14Note: Mixers provide negligible head lossFlow � Q/4 (m3/s) 0.55 0.77 0.82 1.09Chamber area � 1.83 m � 1.83 m (m2) 3.34 3.34 3.34 3.34Velocity � flow/area (m/s) 0.16 0.23 0.25 0.33Losses � Mixer (1 V 2/2g) � Sharp bend (1.8 V 2/2g) (m) 0.00 0.01 0.01 0.02WSEL at Point 14 (Mixing Chamber No. 2 inlet) (m) 109.82 109.87 109.93 110.07


16. Point 14 to Point 15Flow � Q/2 (m3/s) 1.09 1.53 1.64 2.19Conduit area � 2.29 m wide � 1.22 m deep (m2) 2.79 2.79 2.79 2.79Velocity � flow/area ( m/s) 0.39 0.55 0.59 0.78R = A/P// (P � 2w � 2d) (m) 0.40 0.40 0.40 0.40Conduit losses � L � [V/(0.849VV � C � R0.63)] 1/0.54 (m)

where L � 47.24 m and Hazen-Williams C � 120 0.00 0.01 0.01 0.02Local losses � flow split (0.6 V 2/2g) � contraction

(0.07 V 2/2g) � 0.67 V 2/2g (m) 0.01 0.01 0.01 0.02WSEL at Point 15 (at Mixing Chamber No. 1) (m) 109.83 109.89 109.95 110.11








17. The above calculations (for Points 1 through 15) havebeen for flow routed through Tank No. 4. When theflow is routed through Tank No. 1. the WSEL (m) is: 109.82 109.88 109.94 110.08

In reality, the headloss through each basin is equal.The flow through the basin naturally adusts toequalize headlosses, i. e. flow through Tank No. 1 is greater than Q/4 and flow through Tank No. 4 is less than Q/4. The actual headloss througheach basin can be estimated as the average of: Lossesthrough Tank No’s. 1 and 4

and the WSEL (m) at Point 15 is: 109.83 109.89 109.95 110.10

18. Point 15 to Point 16Flow � Q (m3/s) 2.19 3.06 3.28 4.38Conduit area � 2.29 m wide � 1.22 m deep (m2) 2.79 2.79 2.79 2.79Velocity � flow/area (m/s) 0.78 1.10 1.18 1.57R � A/P// (P � 2w � 2d) (m) 0.40 0.40 0.40 0.40Conduit losses � L � [V/(0.849VV � C � R0.63)]

1/0.54 (m) where L � 125.58 m andHazen-Williams C � 120 0.04 0.08 0.10 0.16

WSEL at Point 16 (m) 109.87 109.97 110.04 110.26

19. Point 16 to Point 17Flow � Q (m3/s) 2.19 3.06 3.28 4.38Conduit area @ 16 � 2.29 m wide � 1.22 m deep (m2) 2.79 2.79 2.79 2.79Conduit area @ 17 � 1.68 m wide � 1.68 m deep (m2) 2.81 2.81 2.81 2.81Average area (m2) 2.80 2.80 2.80 2.80Velocity � flow / Area (m/s) 0.78 1.09 1.17 1.56R @ 16 � A16/ (2 � (2.29 m � 1.22 m)) (m) 0.40 0.40 0.40 0.40R @ 17 � A17/ (2 � (1.68 m � 1.68 m)) (m) 0.42 0.42 0.42 0.42Average R, (m) 0.41 0.41 0.41 0.41Conduit losses � L � [V/(0.849VV � C �

R0.63)]1/0.54 (m) where L � 9.14 mand Hazen-Williams C � 120 0.00 0.01 0.01 0.01

WSEL at Point 17 (m) 109.88 109.98 110.05 110.27

20. Point 17 to Point 18Flow � Q (m3/s) 2.19 3.06 3.28 4.38Conduit area @ 17 � 1.68 m wide � 1.68 m

deep (m2) 2.81 2.81 2.81 2.81Velocity 17 � flow/area 17 (m/s) 0.78 1.09 1.17 1.56Pipe area @ 18 � (�D

4�)2

� � (m) where D � 1.68 m 2.21 2.21 2.21 2.21Velocity 18 � flow/area 18 (m) 0.99 1.39 1.49 1.98Exit losses � V182/2g – V172/2g (m/s) 0.02 0.04 0.04 0.8WSEL at Point 18 (m) 109.90 110.01 110.09 110.35

21. Point 18 to Point 19R = A/P// (P � d � �) (m) 0.42 0.42 0.42 0.42�

Local losses � 3 elbows (3 � 0.25V 2/2g) �

entrance (0.5 � V 2/2g) � 1.25 � V 2/2g (m) 0.06 0.12 0.14 0.25Conduit losses � L � [V/(0.849VV � C �

R0.63)]1/0.54 (m) where L � 138.68 mand Hazen-Williams C � 120 0.07 0.13 0.15 0.26

WSEL at Point 19 (exit of Control Chamber) (m) 110.03 110.27 110.39 110.86








22. Point 19 to Point 20Weir elevation (m) 109.73 109.73 109.73 109.73Depth of flow over weir � (WSEL @

19 – weir elevation), (m) 0.30 0.54 0.66 1.13Length of weir, L, (m) 2.74 2.74 2.74 2.74Flow over weir � q � 1.71 � h3/2 � [ 1 � (d / n)3/2 ]0.385

� LNote: Rather than solve for h, find an h by trial

and error that gives a q equal to the flowfor the given flow scenarios (given in Item 1)

assume h (m) � 0.60 0.90 0.95 1.35then q (m3/s) � 1.84 3.14 3.12 4.21

assume h (m) � 0.66 0.89 0.97 1.37then q (m3/s) � 2.18 3.07 3.27 4.42

Note: These q’s equal the flows for the givenscerios (Item 1)

h (m) 0.66 0.89 0.97 1.37WSEL at Point 20 (h � WSEL @ Point 19) (m) 110.39 110.62 110.70 111.10

23. Point 20 to Point 21Flow � Q (m3/s) 2.19 3.06 3.28 4.38Sluice gate area � 1.37 m � 1.37 m (m2) 1.88 1.88 1.88 1.88Velocity � Flow/Area (m/s) 1.16 1.63 1.74 2.33Gate Losses � 1.5 � V 2/2g (m) 0.10 0.20 0.23 0.41WSEL at Point 21 (Raw Water Control

Chamber) (m) 110.49 110.82 110.93 111.51The overflow weir in the Raw Water ControlChamber is 3.05 m long and is sharp crested

Q = 1.82 � L � h3/2 so h � (Q/1.82L)2/3 (m) 0.54 0.67 0.70 0.85The water surface must not rise above elevation 112.78 mThe overflow weir elevation may be safely set at 111.86 m


hydraulic reaches analyzed in the example. The circled numbers indicate points at whichthe WSEL is calculated. Hydraulic calculations start downstream of the sedimentationbasins (Fig. 22.12) and proceed upstream through the mixing chamber (Fig. 22.13) andthe Raw Water Control Chamber (Fig. 22.14). Mechanical mixers and mechanical floccu-lators are used. Conduit losses between the rapid mix chambers and the Raw WaterControl Chamber are also calculated in the example. Three different flow rates (i.e., min-imum day, average day, and, maximum hour) are used in the calculations. This is a rangeof design flow conditions that a design engineer would typically take into consideration.

The longest path through the flocculation and sedimentation processes, through BasinNo. 4, is followed (Points 1 through 15). Although not shown, losses along the shortestpath have also been calculated. As would be expected, the calculated head loss is smallerfor the shorter path. The actual losses are equal for each path. The flows through each pathnaturally adjust to equalize losses. The flow through the longest path is slightly smallerthan the flow through the shortest path. In the example, the WSEL at Point 15 is adjustedto reflect the average losses through the basins. The WSEL calculations upstream of Point




15 are based on the adjusted WSEL. Alternatively the weirs or ports feeding flow into eachbasin may be adjusted to create an equal distribution of flows in all basins as discussed inSec. 22.2.1.

22.3.7.2.2 Filtration. Process criteria. Suspended solids are removed from the water asit passes through a porous medium during filtration. Filters operate under either gravity orpressure. Filters also differ in the type and distribution of the media used (fine, course,uniformly graded, graded coarse to fine, etc.) and the direction of flow through the media(upflow, downflow, and biflow). Pressure filter hydraulics information is very productspecific and should be obtained from the manufacturer. The design engineer using pres-sure filters should then apply this information to the project using project–specifichydraulic considerations. This section presents information on gravity filters.

Key hydraulic design parameters. The headloss through a filter increases with use asthe voids become filled with solid particles. When the headloss reaches a certain point(terminal headloss), the filter is backwashed to remove the solids. The rate of headlossbuildup is dependent on several factors, including how the filter is graded (the arrange-ment of media particle sizes). The rate of headloss buildup is reduced (and filtration ismore effective) when the flow first goes through the coarse media and then the fine media.


FIGURE 22.12 Flocculation/ sedimentation basin





FIGURE 22.13 Mixing chamber

FIGURE 22.14 Raw water control chamber





However, during backwash, the high rate of flow expands the filter bed and, over time, themedia are regraded so that the more coarsely graded grains are located at the bottom andthe fines are located at the top. To benefit from the coarse-to-fine grading, an upward flowpattern can be used, but is very uncommon. More often the filter media are selected suchthat the fine media have a higher specific gravity than the coarse media to maintain thecourse-to-fine gradation during backwash. The most commonly used filter media are nat-ural silica sand and crushed anthracite coal; however garnet and ilmenite are used inmixed media beds. Granular carbon is often used if taste and odor control is desired.

The terminal headloss is determined by a combination of factors including filter break-through (when the filter bed loses its adsorptive capacity), available static head, and out-let pressure required. The filter should be designed so that the headloss in any level of thefilter bed does not exceed the static pressure. A negative head can result in air binding inthe filter which will, in turn, further increase headloss.

Filter influent piping is sized to limit velocities to about (0.6 m/s). Wash-water andeffluent piping flow velocities are kept below (1.8 m/s) so that hydraulictransients(waterhammer) and excessive headlosses are minimized and controlled towithin tolerable limits.

Hydraulic design example. Table 22.6 illustrates the calculation of the WSEL from theclear well back upstream to the Sedimentation Basin effluent at the medium-sized watertreatment plant shown in Fig. 22.10. Figures 22.15 and 22.16 show details of the hydraulicreaches analyzed in the example. Table 22.7 illustrates the filter hydraulic calculation, thedetails of which are shown in Figs. 22.17 and 22.18.

The hydraulic profile of the plant (based on hydraulic calculations done in Tables 22.5,22.6 and 22.7) is shown in Figure 22.3.

TABLE 22.6 Hydraulic Calculations in a Medium–Sized Water Treatment Plant from the FilterEffluent to the Effluent Clearwell


Parameter Min Day Avg Day Avg Day Max Hour

1. Flow (m3s) 2.19 3.06 3.28 4.38Note: for Points 22 through 28, see Figure 22.15

2. Point 22 to Point 23Maximum water level in Clearwell (Point 22) (m) 105.16 105.16 105.16 105.16Invert in Clearwell (m) 101.50 101.50 101.50 101.50Flow � Q/2 (m3/s) 1.09 1.53 1.64 2.19Stop logs @ A

Flow area (2 openings, 1.52 m wide,3.66 m deep) (m2) 11.15 11.15 11.15 11.15

Velocity � flow/area (m/s) 0.20 0.27 0.29 0.39Loss � 0.5 V 2/2g (m) 0.00 0.00 0.00 0.00Baffles

Flow area (3.05 m wide, 3.66 m deep) (m2) 11.15 11.15 11.15 11.15Velocity � flow/area (m/s) 0.20 0.27 0.29 0.39Loss � 1.0 V 2/2g (m) 0.00 0.00 0.00 0.01

Stop logs @ B and CSame as the losses @ A, times 2 (m) 0.00 0.00 0.00 0.01








WSEL at Point 23 (m) 105.16 105.17 105.17 105.18

3. Point 23 to Point 24Flow � Q/2 (m3/s) 1.09 1.53 1.64 2.191.68 (m) diameter pipeFlow area � d 2/4 � � (m2) 2.21 2.21 2.21 2.21Velocity � flow/area (m/s) 0.50 0.69 0.74 0.99Exit loss @ clearwell � V 2/2g (m) 0.01 0.02 0.03 0.05Loss @ 2 - 90o bends � (0.25 V 2/2g) � 2 (m) 0.01 0.01 0.01 0.03Entrance loss @ Filter Building � 0.5 V 2/2g (m) 0.01 0.01 0.01 0.03Pipe loss � (3.022 � V 1.85 � L)/

(C 1.85 � D1.165) where C � 120 andL � 57.91 m (m) 0.00 0.00 0.00 0.00

WSEL at Point 24 (m) 105.19 105.22 105.23 105.28

4. Point 24 to Point 25Flow � Q/4 (m3/s) 0.55 0.77 0.82 1.09Flow area � 1.52 m � 1.52 m2 2.32 2.32 2.32 2.32Velocity � Q/A (m/s) 0.24 0.33 0.35 0.47Loss as flows merge � 1.0 V 2/2g (m) 0.00 0.01 0.01 0.01Conduit loss � [(V � n)/(R2/3)]2 � L (m)

where n � 0.013, L � 16.76 m and R � A/P//(P � 6.10 m) 0.00 0.00 0.00 0.00

WSEL at Point 25 (m)

5. Point 25 to Point 26Sluice Gate No. 1 flow area � 1.22 m � 0.91 m (m2) 1.11 1.11 1.11 1.11Velocity � Q/A// (m/s) 0.49 0.69 0.74 0.98Loss � 0.5 V 2/2g (m) 0.01 0.01 0.01 0.02WSEL at Point 26 (m) 105.20 105.24 105.24 105.32

6. Point 26 to Point 27Sluice Gate No. 2 Loss � 0.8 V 2/2g (m) 0.01 0.02 0.02 0.04WSEL at Point 27 (m) 105.21 105.25 105.27 105.36

7. Point 27 to Point 28Port to Filter Clearwell: Calculate losses through port

as if were a weir when depth of flow is below topof port. Port dimmensions � 2.74 m wideby 0.813 m deep. Flow � Q/4 (m3s) 0.55 0.77 0.82 1.09

Weir (bottom of port) elevation (m) 104.85 104.85 104.85 104.85Depth of flow over weir �

(WSEL @ 27 – weir elevation) (m) 0.36 0.40 0.42 0.51Flow over submergedweir � q � 1.71 � h3/2

� [1 - (d/dd h)3/2]0.385 � LNote: Rather than solve for h, find an h, by trial

and error, that gives a q equal to the flow for thegiven flow scenario

assume h (m) � 0.40 0.45 0.50 0.60then q (m3/s) � 0.59 0.69 0.95 1.23assume h (m) � 0.39 0.46 0.48 0.58then q (m3/s) � 0.52 0.76 0.82 1.09Note: These q’s equal the flows for the givenscenariosh (m) 0.39 0.46 0.48 0.58








WSEL at Point 28 (m) 105.24 105.31 105.33 105.43Filters—See Filter Hydraulics in Table 22.7Note: for Points 29 through 33, see Fig. 22.16

8. Point 29WSEL above filters (m) 109.73 109.73 109.73 109.73

9. Point 29 to Point 30Entrance to Filter #4Flow, Q/8 (m3/s) 0.27 0.38 0.41 0.55Channel velocity = flow/area

(area � 1.22 m � 1.22 m) (m/s) 0.18 0.26 0.28 0.37Submerged entrance loss � 0.8 V 2/2g (m) 0.00 0.00 0.00 0.011.22 m pipe velocity � flow/area

(area � d 2/4 � �) (m/s) 0.23 0.33 0.35 0.47�

Butterfly valve loss � 0.25 V 2/2g (m) 0.00 0.00 0.00 0.00Sudden enlargement loss � 0.25 V 2/2g (m) 0.00 0.00 0.00 0.00WSEL in influent channel (Point 30) (m) 109.73 109.73 109.73 109.74

10. Point 30 to Point 31Flow depth � WSEL @ 30 � invert (107.29 m) (m) 2.44 2.44 2.44 2.45Flow area � 1.83 m width � depth (m2) 4.46 4.47 4.47 4.48Velocity � flow/area (m/s) 0.06 0.09 0.09 0.12R = A/P// (P � w � 2d) (m) 0.67 0.67 0.67 0.67Conduit Loss � [(V � n)/(r2/3rr )]2 � L

where n � 0.014 and L � 10.77 m (m) 0.00 0.00 0.00 0.00WSEL at Point 31 (m) 109.73 109.73 109.73 109.74

11. Point 31 to Point 32Flow � Q/4 (m3/s) 0.55 0.77 0.82 1.09Flow depth � WSEL @ 31 - invert (107.29 m) (m) 2.44 2.44 2.44 2.45Flow area � 1.83 m width � depth (m2) 4.46 4.47 4.47 4.48Velocity � flow/area (m/s) 0.12 0.17 0.18 0.24R = A/P// (P � w � 2d) (m) 0.67 0.67 0.67 0.67Conduit loss � [(V � n)/(r 2/3)]2 � L (m)


12. Point 32 to Point 33Flow � 3Q/8 (m3/s) 0.82 1.15 1.23 1.64Flow depth � WSEL @ 32 – invert (107.29 m) (m) 2.44 2.44 2.44 2.45Flow area � 1.83 m width � depth (m2) 4.46 4.47 4.47 4.48Velocity � flow/area (m/s) 0.18 0.26 0.28 0.37R � A/P// (P � w � 2d) (m) 0.67 0.67 0.67 0.67Conduit loss � [(V � n)/(r2/3r )]2 � L (m)


13. Point 33 to Point 1Flow � Q/2 (m3/s) 1.09 1.53 1.64 2.19Flow depth � WSEL @ 33 – invert (107.29 m) (m) 2.44 2.44 2.45 2.45Flow area � 1.83 m width � depth (m2) 4.46 4.47 4.47 4.48Velocity � flow/area (m/s) 0.24 0.34 0.37 0.49R = A/P// (P � w � 2d) (m) 0.67 0.67 0.67 0.67Conduit loss � [(V � n)/(r 2/3)]2 � L (m)






FIGURE 22.15 Clearwell to filter effluent

FIGURE 22.16 Filter effluent





TABLE 22.7 Example Hydraulic Calculation of a Typical Filter


Parameter Min. Day Avg. Day. Avg. Day. Max. Hour.

Plant flow (m3/s) 2.19 3.06 3.28 4.38

Filter loading, [(m3 � m)/m2] 0.083 0.167 0.250 0.334

Filter area per filter—seven (7) out of eight (8) 115 115 115 115

filters in operation (m2)

Flow � loading � area (m3/s) 0.16 0.32 0.48 0.64

Losses through filter effluent piping (Fig. 22.17)

0.51 m piping (Q):

Pipe velocity � Q/A// (m/s) 0.79 1.58 2.37 3.16

Local losses � Exit (0.5) � butterfly

valves (2 � 0.25) � 90o elbows (2 � 0.4)

� tee (1.8) � 3.6 V 2/2g (m) 0.11 0.46 1.03 1.83

R � A/P// � (d 2/4 � p)/(d � p) � d/4 (m) 0.13 0.13 0.13 0.13dd

Conduit losses � L � [V/(0.849VV � C � R0.63)]

1/0.54 where L � 6.10 m and Hazen-

Williams C � 120 (m) 0.01 0.03 0.06 0.11

0.51 m piping (Q/2):

Pipe velocity � Q/A (m/s) 0.40 0.79 1.19 1.58

Local Losses � Butterfly Valve (0.25) (m) 0.00 0.01 0.02 0.03

R � A/P// � (d 2/4 � p)/(d � p) � d/4 (m) 0.13 0.13 0.13 0.13dd

Conduit losses � L � [V/(0.849VV � C � R0.63)]

1/0.54 where L � 3.05 m and Hazen-

Williams C � 120 (m) 0.00 0.00 0.01 0.02

0.61 m piping (Q/2):

Pipe velocity � Q/A// (m/s) 0.27 0.55 0.82 1.10

Local losses � entrance (1.0) � tee (1.8)

� 2.8 V 2/2g (m) 0.01 0.04 0.10 0.17

Filter (clean) and underdrain losses (obtain from

manufacturer) (m) 0.09 0.15 0.23 0.34

Total losses (effluent pipe and clean filters) (m) 0.23 0.70 1.45 2.50

Assume that headloss will be allowed to increase 2.44 m before the filters are backwashed. A rate controller

will be used to maintain a constant flow through the filters. Determine the ranges of available head over

which the rate controller will operate.

Static Head (Fig. 22.18)

WSEL above filters (m) 109.73 109.73 109.73 109.73

WSEL in filter effluent conduit, Point 29

(see Example 22.2) break Maximum (m) 105.61 105.61 105.61 105.61

Minimum (m) 105.16 105.16 105.16 105.16

Static head � WSEL above filters—WSEL at

Point 29 (Filter effluent conduit-2)

Maximum (m) 4.57 4.57 4.57 4.57

Minimum (m) 4.11 4.11 4.11 4.11

Available head � static head �2.44 m

Maximum (m) 2.13 2.13 2.13 2.13

Minimum (m) 1.68 1.68 1.68 1.68





FIGURE 22.17 Filter effluent piping

FIGURE 22.18 Available head over which filter effluent rate controller operates—metric units.

Hea

d,m

eter

s





22.3.8 Membrane Technology

Membranes are synthetic filtering media manufactured from a variety of materials includ-ing polypropylene, polyamide, polysulfone, and cellulose acetate. The membrane materi-al can be arranged in various configurations, including the following:

• Spiral wound

• Hollow fiber

• Tubular

• Plate frame

Examples of these configurations are presented in Fig. 22.19. In water and wastewatertreatment applications, the most common configurations are spiral wound and hollow fiber.

In general, there are four classes of membranes: microfilters (MF), ultrafilters (UF),nanofilters (NF), and hyperfilters. Treatment through hyperfilters is referred to as hyper-ffiltration, or reverse osmosis (RO).

The hydraulics associated with membranes are membrane-specific and can be obtainedfrom the manufacturer. This section presents general considerations pertinent to flowthrough membranes.

As with natural particle media filters, clean membranes have a specific headloss and,over time, as the membranes become covered with a cake buildup, the effectiveness of themembrane decreases and headloss increases. Fouling (excessive buildup) may damage themembrane.

The need for pretreatment ahead of membranes is determined by the raw water qual-ity and the membrane type. In general, microfilters and ultrafilters do not require pre-treatment for treating surface or groundwater. Nanofilters and reverse osmosis mem-branes may require pretreatment depending on the type of fouling. Membrane foulingcan result from particulate blocking, chemical scaling, and biological growth within themembranes.

An estimate of particulate blocking can be made using indices such as the Silt DensityIndex (SDI) and the Modified Fouling Index (MFI). These fouling indices are determinedfrom simple bench membrane tests using 0.45 micron Millipore filters and monitoringflow through the filter at a given pressure, usually 30 psig. Approximate values of suitableSDIs for nanofiltration are 0–3 units, and for reverse osmosis, 0–2 units. Correspondingvalues of MFI are, for nanofiltration 0 to 10 s/L2, and for RO, 0–2 s/L2.

Scaling control is essential in RO and nanofilter membrane filtration, especially whenthe filtration provides water softening. Controlling precipitation or scaling within themembrane element requires identification of limiting salt, acid addition for prevention ofcalcium carbonate precipitation within the membrane, and/or the addition of anantiscalant. The amount of antiscalant or acid addition is determined by the limiting salt.A diffusion controlled membrane process will naturally concentrate salts on the feed sideof the membrane. As water is passed through the membrane, this concentration processwill continue until a salt precipitates and scaling occurs. Scaling will reduce membraneproductivity and, consequently, recovery is limited by the allowable recovery just beforethe limiting salt precipitates. The limiting salt can be determined from the solubility prod-ucts of potential limiting salts and the actual feed stream water quality. Ionic strength mustalso be considered in these calculations as the natural concentration of the feed streamduring the membrane process increases the ionic strength, allowable solubility and recov-ery. Calcium carbonate scaling is commonly controlled by sulfuric acid addition, althoughsulfate salts, such as barium sulfate and strontium sulfate, are often the limiting salt.Commercially available antiscalants can be used to control scaling by complexing the




metal ions in the feed stream and preventing precipitation. Equilibrium constants for theseantiscalants are not available which prohibits direct calculation. However, some manufac-turers provide computer programs for estimating the required antiscalant dose for a givenrecovery, water quality, and membrane.

Biological fouling is controlled with some membranes such, as cellulose acetate, bymaintaining a free chlorine residual of not more than 1 mg/L. Other membranes, such asthe thin-film composites, are not chlorine tolerant and must rely on upstream disinfectionby, for example, ultraviolet disinfection or chlorination-dechlorination. The extent of foul-ing for a specific application and its influence in the design of nanofiltration and RO mem-brane systems is best determined by pilot studies.

It has been suggested that some buildup on the membrane may be beneficial to treat-ment by providing an additional filtering layer. At facilities operated by the MetropolitanWater District of Southern California (MWD), removal rates of 1.7–2.9 logs were


FIGURE 22.19 Membrane configurations. (a) Spiral wound, (b) hollow fiber, (c) tubular, (d) plateand frame.




observed for seeded virus MS2 bacteriophage through microfilters that had a pore size anorder of magnitude larger than the nominal size of MS2 (1).

The microfiltration system used by MWD utilizes an air backwash procedure where-by compressed air at 90–100 psig is introduced into the filtrate side of the hollow fibermembranes. Accumulated particulates dislodged by the compressed air are swept awayby raw water introduced to the feed side of the membranes. The backwash sequence iscarried out automatically at preset time intervals. MWD found the best interval to beevery 18 minutes. The total volume of backwash represents approximately 5–7 percentof influent flow.

The difference between influent and effluent pressure across the membrane is termedthe transmembrane pressure (TMP). Despite the frequent air and water backwashes, theTMP gradually increases over time. Generally, when the TMP reaches approximately15 psig, chemical cleaning of the membranes is carried out. If the TMP is allowedto increase beyond 15 psig, particulates can become deeply lodged within the lattice struc-ture of the membranes and will not be removed, even by chemical cleaning. Chemicalcleaning typically lasts 2–3 hours and involves circulating a solution of sodium hydroxideand a surfactant through the membranes, and soaking them in the solution.

The membranes at the MWD microfilter plants have a surface loading rate of 40–67ft2. The lower flux rate of 40 ft2 has the advantage that the rate of increase of TMP isreduced and the interval between chemical cleanings is increased. A possible explanationfor this is that particulates are not forced as deeply into the lattice structure of the mem-branes, thereby allowing the air-water backwash to clean the membranes more effective-ly. By reducing the flux rate from 67–40 ft2, the interval between chemical cleanings wasincreased from 2 to 3 weeks to almost 20 weeks. However, MWD has instituted a maxi-mum run time of 3 months between chemical cleanings to ensure the long-term integrityof the membranes.

Nanofiltration is widely used for softening groundwaters in Florida. A typical nanofil-tration plant would include antiscalant for scale control added to the raw water. Cartridgefilters, usually rated at 5 microns, remove particles that may foul the membrane system.Feed water pumps boost the pretreated water pressure to about 90–130 pounds per squareinch (psi) before entering the membrane system. The membranes typically are spiralwound nanofiltration membranes generally with molecular weight cutoff values in the200–500 dalton range.

22.4 WASTEWATER TREATMENT

Many factors and considerations influence the hydraulic design of a wastewater treatmentplant. This section describes typical phases of wastewater treatment planning required fordesign of new plants or additions to existing plants, and then presents typical unit processhydraulic computations.

22.4.1 Wastewater Treatment Planning

Hydraulic decision making for a new wastewater treatment plant or expansion of an exist-ing plant involves several planning phases. Typical planning phases are presented belowin their common order of consideration.

22.4.1.1 Service area and flows. More than 15,000 municipal wastewater treatment plantsare in operation in the United States today. The plants are designed to treat a total of about





140 million m3 of flow each day. Flow quantities requiring treatment change over time basedon a number of factors related to service area. These factors include the following:

Changes in service area size. Most often the service area will increase in size duringthe wastewater treatment plant service life. However, service area size may decrease, suchas when wastewater in larger metropolitan areas is diverted to an alternate wastewatertreatment plant. Information about anticipated changes in the size of a wastewater treat-ment plant service area can sometimes be found in “regional planning” documents.

Changes in service area land use. Changes in the type of land use in the servicearea, such as from residential to industrial, will impact the flow rates to be served bythe treatment plant. Also, the development of impervious areas within the wastewatertreatment plant service area will reduce infiltration and increase runoff volume and rate.If this runoff then enters the sewer system it will impact the flow rate to the plant. Acombined sewer system will be more susceptible to this type of change than a separatesewer system.

Changes in service area density. Wastewater treatment plant flows are a function ofthe number of inhabitants and industries which generate the wastewater. An understand-ing of the regional planning issues which may affect the wastewater treatment plant ser-vice area assists in estimating future increases in flow and making appropriate provisionsfor future plant expansions. Such flow increases will likely be partially offset by increasedwater conservation in water-limited areas.

Changes in service area infiltration/inflow. Most often the rates of infiltration/inflowwill increase as the collection system becomes older. Such flow increases can generallybe offset by periodic sewer rehabilitation, manhole rehabilitation, and enforcement ofinflow control ordinances.

The quantity of wastewater to be handled by a wastewater treatment plant is affectedprimarily by the type of wastewater produced in the service area and type of wastewatercollection system used. The four types of wastewater which may be produced in a givensewer system service area include sanitary wastewater, industrial wastewater, stormwater,and infiltration/inflow. The three types of sewer systems used to collect some or all ofthese flow types include sanitary, storm, and combined-sewer systems. The types ofwastewater are defined as follows:

Sanitary flow. Wastewater discharged from residences and from institutional, com-mercial and similar facilities. Quantities of sanitary flow can be estimated on a per capitabasis for each type and size of residence or facility producing the flow.

Industrial flow. Wastewater discharged from industrial facilities. In a heavily industri-alized area, industrial flow can make up a majority of a wastewater plant’s influent flow.Industrial wastewater quantities produced by a given facility can be estimated based onfacility type, size, and rate of production.

Stormwater. Stormwater is precipitation runoff. Stormwater enters storm or combinedcollection systems as surface or subsurface inflow. The rate of stormwater entering astorm or combined sewer system as inflow mirrors the intensity and quantity of the pre-cipitation event, although if the precipitation is frozen the runoff will be delayed untilmelting occurs.

Infiltration Water (including stormwater) that seeps into a wastewater collection sys-tem through the ground, usually through cracks or leaks in the collection system.Accordingly, infiltration rates typically vary both annually and seasonally. The age of the





collection system should be considered when estimating infiltration rates because oldercollection systems are prone to higher infiltration rates. If the amount of infiltration is sig-nificant enough to affect plant influent water quality, the treatment processes must beselected accordingly.

Inflow. Surface and subsurface stormwater discharging directly into a wastewatercollection system. Precipitation events significantly impact inflow rates and can alsoimpact infiltration rates by surcharging the groundwater table. The elevation of thegroundwater table relative to the sewer elevation directly affects the infiltration flow rate.

The types of sewer systems include sanitary-sewer systems which collect sanitarywastewater, industrial wastewater (if present in service area), and infiltration/inflow.Storm-sewer systems collect stormwater and infiltration/inflow, and combined-sewer sys-tems collect sanitary wastewater, industrial wastewater (if present in service area),stormwater, and infiltration/inflow. Flows to wastewater treatment plants are conveyed byseparate-sewer systems and, in some older systems, combined-sewer systems. Hydraulicdesign guidelines for sanitary-sewer systems have been compiled by the American Societyof Civil Engineers and the Water Environment Federation (1982).

22.4.1.2 Effluent requirements. Treated wastewater can be discharged to rivers, lakes,oceans, and groundwater. There is also increasing re-use of wastewater for nonpotableapplications, such as irrigation and industrial processing. Effluent quality requirements forwastewater treatment plants are generally established by regulatory agencies in the plant’sNPDES permit. Those minimum acceptable effluent characteristics and the anticipatedinfluent characteristics determine what level of treatment is required and, thereby, deter-mines to some degree what treatment processes are needed. Because each process typerequires a different amount of head, the influent characteristics and effluent requirementsalso indirectly affect the plant head requirements.

22.4.1.3 Process selection. Each unit process in a wastewater treatment plant flow traintreats the wastewater physically, chemically or biologically, or in some combination there-of. Because various combinations of unit processes are generally available to produce thedesired effluent quality, the designer must choose among the options to select the opti-mum combination. In anticipation of future requirements, potential changes in effluentrequirements and corresponding treatment train modifications should also be considered.Typical unit treatment processes for new wastewater treatment plants include screening,grit removal, primary sedimentation, aeration, secondary sedimentation, granular mediafiltration, disinfection, dechlorination, and postaeration. Figure 22.20 is a flow diagramshowing how the typical processes are interconnected.

Unless only one treatment process combination is capable of adequately treating thewastewater, pertinent factors must be used to select the process train. Typical factorsinclude capital and operating costs, environmental impacts, aesthetics, and public accep-tance. Process head requirements can directly affect capital costs, as those processes withhigher head requirements are more likely to necessitate costly pumping facilities and deepstructure excavation.

22.4.1.4 Hydraulic bases for design. Flow rates for the wastewater treatment plant mustbe established for the hydraulic design. Design year flow projections are often based onestimated conditions 15–20 years in the future. Providing sufficient treatment capacity toaccommodate new development can be an important municipal commodity for expandingthe municipal tax base. The design should also provide allowances for the initial plantoperation when flow may be significantly less than the design flow, as well as expansionor rehabilitation to handle flows reasonably anticipated beyond the design year.





Peak flow is used for hydraulic design, whereas average flow is used for treatmentprocess design. Peak flow is defined as the maximum hour flow experienced by the waste-water treatment plant throughout its service life. The maximum hour flow is generally twoto five times the average daily flow. Plants serving combined collection systems can expe-rience even greater flow variations. Treatment plant unit processes must convey the max-imum flow unless this flow would cause a hydraulic washout of the treatment plant. In thissituation, the designer should consider the use of equalization basins to minimize negativeimpact on the treatment process. In addition, the plant must also be able to fully processminimum flow without undesirable settling of solids throughout the treatment train. Plantsnormally encounter diurnal fluctuation of pollutant loadings, as well as flow loadings.Fluctuation in pollutant loadings may impact treatment process selection and consequent-ly impact process hydraulics.

22.4.1.5 Flow diagram. A flow diagram should be prepared to depict the results ofprocess selection and hydraulic bases of design. Details in a flow diagram should includethe type of unit processes, number of basins for process redundancy, flow distribution andjunction chambers, piping, and conduits for interconnecting the unit processes and majorrecycle streams such as return-activated sludge (RAS). Figure 22.20, which was men-tioned above, shows a typical flow diagram.

22.4.1.6 Plant siting. Several factors affect the plant site selection process, including siteelevation, topography, geology, and hydrology; site access; utility availability; seismicactivity; surrounding land use and future availability; noise, odor and air quality require-ments at and near the site; existing collection system and receiving water proximity; andother environmental considerations.

A site’s hydraulic suitability for a wastewater treatment plant is determined primarilyby site elevation and topography. The typical site elevation is low-lying, which facilitatesthe flow of wastewater from the service area by gravity and minimizes costly pumping inthe collection system. Such a site, however, may require flood protection. The differencein head between the plant influent sewer and the receiving water body is the head avail-able for the treatment plant. If available head does not exceed the plant’s head require-ments, additional head can be provided by pumping the wastewater. Selecting processeswith lower head requirements can also reduce the need for pumping. Pumping of waste-water, especially untreated wastewater, should be avoided when possible due to potentialoperational difficulties of handling the associated rags, grit, stringy material and otherlarge solids. A mild, continuous slope usually provides optimal gravity flow conditions.Relatively flat sites often necessitate higher pumping heads. Sites on a severe, unevenslope or slopes can require costly hydraulic and structural features, and should be avoid-ed when possible.

22.4.1.7 Plant layout. The selected treatment processes establish the major space andhydraulic requirements needed to develop initial plant layouts. Also, provisions for futureunit process additions and plant capacity expansions should be included both spatially andhydraulically. Support facilities, such as maintenance, laboratory and administrativefacilities, must also be considered.

Arranging process elevations to generally follow plant site topography minimizes theamount of structural excavation. Site geology constraints may limit the practical depth andelevation of the processes. In such cases, additional pumping facilities may be necessaryto provide sufficient head for the required water surface elevation.

When arranging treatment processes, a preliminary hydraulic profile should be devel-oped as discussed below. The plant hydraulic profile and site topography and geologyinformation together determine the location having the optimal elevation for each process.






FIG

UR

E 2

2.20

Sche

mat

ic f

low

dia

gram

of

typi

cal w

aste

wat

er tr

eatm

ent p

lant

.




Other objectives when developing a plant layout at a selected site include: close proxim-ity of processes to associated facilities; structure grouping according to process;transportation equipment and staff traffic pattern efficiency; minimization of processpiping; and safe, isolated hazardous chemical and material locations.

When preparing layouts for addition of a new process to an existing plant, the existingplant hydraulic profile should be consulted to determine the amount of head available forthe new process. If adequate hydraulic head is not available for the new process, newpumping facilities will be necessary.

22.4.1.8 Hydraulic profile and calculations. A hydraulic profile should be prepared forthe flow train to graphically depict the results of hydraulic calculations and site layouts.Details in a profile should include free water surface elevations throughout the flow train,including unit treatment processes, interconnecting piping and channels, junction cham-bers, flowmeters and flow control devices, as well as structural profiles. Figure 22.21shows a typical hydraulic profile. Both high and low water levels are shown to illustratethe range of liquid levels anticipated at each structure. Sufficient freeboard must be pro-vided to prevent liquid or floating material from splashing over the sides under conditionsof high water level. Low water levels are important when designing devices requiring amimimum amount of submergence, such as surface skimmers or baffles.

In addition to normal high and low water levels, hydraulic calculations should addressother potential conditions. For example, for each process having redundant structures, thelargest capacity unit should be assumed to be out of service during maximum flow forconsideration of a “worst case”. The process structure should always be hydraulicallycapable of accommodating the change in elevation due to the “worst case.” head require-ments without liquid overtopping the walls.

The process head requirement is the amount of head lost by the wastewater as it pass-es through a process at maximum flow. The head requirement for a specific process canvary with flow rate, influent water quality, process equipment size, process equipment lay-out, process equipment components included, and process equipment manufacturer.

22.4.2 Typical Unit Process Hydraulics

22.4.2.1 Bar screens. Process criteria. The first unit operation typically encountered ina wastewater treatment plant is screening. A schematic diagram of a typical bar screen sys-tem is shown in Fig. 22.22. A screen is comprised of a screening element with circular orrectangular openings designed to retain coarse sewage solids. The screens are designatedas hand cleaned or mechanically cleaned based on the method of cleaning. Based on thesize of the openings, screens are designated as coarse or fine. The general dividing linebetween coarse and fine screens is an opening size of 6 mm (1/4 in). A bar screen is acoarse screen designed to remove large solids or trash that could otherwise damage orinterfere with the downstream operations of treatment equipment, such as pumps, valves,mechanical aerators, and biological filters. The bar screens are oriented vertically or at aslope varying from 30°– 80° with the horizontal.

Key hydraulic design parameters. The key hydraulic design parameters for barscreens include the approach channel, effective bar opening, and operating head loss.

Approach channel. Velocity distribution in the approach channel is an important fac-tor in successful bar screen operation. A straight channel ahead of the channel providesgood velocity distribution across the screen and promotes effectiveness of the device. Useof a configuration other than a straight approach channel has often resulted in uneven flow






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distribution within the channel and accumulation of debris on one side of the screen. Thevelocity in the approach channel should be maintained at a self-cleaning value to dislodgedeposits of grit or screenings. Ideally, the velocity in the screen chamber should exceed0.4 m/s (1.3 ft/s) at minimum flows to avoid grit deposition if grit chambers follow barscreens. However, this is not always practical with the typical diurnal and seasonal fluc-tuation in wastewater flows. In general, common design practice provides velocities of0.6–1.2 m/s (2–4 ft/s) for mechanically cleaned bar screens and 0.3–0.6 m/s(1–2 ft/s) with a velocity of 0.9 m/s (3 ft/s) at peak instantaneous velocity for manuallycleaned bar screens.

Effective bar opening. Various types of bar screens, including trash racks, manualscreens and mechanically cleaned bar screens, employ a wide range of openings from6 to 150 mm (�

14

�–6 in). The smaller screen openings collect larger quantities of screeningsand generally produce higher head losses. The effective area of the screen openings equalsthe sum of the vertical projections of the screen openings.

Operating head loss. As the screenings are collected, the openings in the screenbecome partially clogged and head losses increase. The maximum design allowance forheadloss through the clogged screens is generally limited to 0.8 m (2.5 ft). Curves andtables for head loss through the screening device are usually available from the equipmentmanufacturer. To prevent flooding of the screening area caused by severe blinding of thescreen during a power failure or similar disruption to cleaning, the design should providefor an overflow weir or gate and a parallel channel allowing overflows to flow aroundthe screen.

Hydraulic design example. The wastewater influent transported through the inletsewer passes the bar screens prior to discharge into the pump well. Three bar screens areprovided to handle hydraulic loadings varying from 1.0 m3/s (23 mgd) for minimum dayflow during initial operation to 3.2 m3/s (73 mgd) for maximum hour flow during designoperation. Sluice gates and stop logs are provided as part of the bar screen design so thatany bar screen can be isolated for maintenance as required.

Design hydraulic calculations for the bar screens are shown in Table 22.8. The WSELat the pump well provides a downstream control point for the bar screens and channels.The WSEL at the pump well normally fluctuates between the pump control high waterlevel and low water level. A high water level (HWL) of 100.60 m at the pump well isassumed. The channel bottom elevation of 99.50 m is determined to provide channel flowvelocities in a range of 0.2–1.3 m/s for the flow range between the minimum and maxi-mum day flow rates. The head requirements for the sample bar screen system is in therange of 0.17–0.36 m (0.56–1.2 ft) when the pump wet well level is at the maximum ele-vation of 100.60.

22.4.2.2 Grit tanks. Process criteria. Grit, consisting of sand, gravel, cinders, and otherheavy solid materials, is present in wastewater conveyed by either separate or combinedsewer systems, with far more in the latter. Grit removal prevents unnecessary abrasion andwear of mechanical equipment, grit deposition in pipelines and channels, and accumula-tion of grit in primary sedimentation basins or aeration basins and anaerobic digesters.Traditionally removal of 95 percent of grit particles larger than 0.21 mm (0.008 in or 65mesh) has been the target of grit equipment design. Modern designs are now capable ofremoving up to 75 percent of 0.15 mm (0.006 in or 100 mesh) to avoid adverse effects ondownstream processes.

A variety of grit removal devices have been applied over the years. The basic types ofgrit removal processes include aerated grit chambers, vortex-type, detritus tank, horizon-tal flow type and hydroclone. Vortex systems are increasingly being selected. Detritus






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tanks and aerated grit chambers are still popular. Depending on the type of grit removalprocess used, the removed grit is often further concentrated in a cyclone, classified, andthen washed to remove light organic material captured with the grit.

Key hydraulic design parameters. The key hydraulic design parameters for grit tanksinclude the inlet channel or inlet baffle, and effluent weir.

Inlet channel/inlet baffle. For aerated grit chambers, the tank inlet and outlet should bepositioned so that the flow through the tank is perpendicular to the roll pattern created by thediffused air. Inlet and outlet baffles serve to dissipate energy and minimize short circuiting.

For vortex tanks, the flow into the vortex tank should be straight, smooth and stream-lined. As a good practice, the straight inlet channel length should be seven times the widthof the inlet channel or 15 ft, whichever is greater. The ideal velocity in the influent chan-nel ranges from 0.6 to 0.9 m/s (2–3 ft/s) and should be used for flows between 40 and 80percent of the peak flow. The minimum acceptable velocity for low flow is 0.15 m/s (0.5ft/s). A baffle, located at the entrance, helps control the flow system in the tank and alsoforces the grit downward as it enters the tank.

For detritus tanks, the performance relies on well-distributed flow into the settlingbasin. Allowances for inlet and outlet turbulence, as well as short circuiting, are necessaryto determine the total tank area required.

For horizontal flow grit chambers, velocity control throughout the chamber at approx-imately 0.3 m/s (1 ft/s) is important. An allowance for inlet and outlet turbulence is nec-essary to determine the actual length of the channel.


TABLE 22.8 Example Hydraulic Calculation of a Typical Bar Screen System


Parameter Min Day Avg.Day Avg.Day Max Hour Max Hour

1. Wastewater flow rate, Q (m3/s) 1.0 1.6 2.0 3.2 3.2(mgd) 23 36 46 73 73

Bar screensTotal number of units 3 3 3 3 3Number of units in operation 2 2 2 2 2Number of units on standby 1 1 1 1 1Flow rate per screen in operation, q (m3/s) 0.5 0.8 1.0 1.1 1.6Width of each bar screen, w (m) 2.5 2.5 2.5 2.5 2.52.At point 8Pump wetwell HGL at high water level, HGL7 (m) 100.60 100.60 100.60 100.60 100.60(pump starts at EL 100.60 and stops at EL 100.00)Pump well bottom EL (m) 99.00 99.00 99.00 99.00 99.00Critical depth in a rectangular channel,

Yc=(q2/g/w2 2)1/3 0.16 0.22 0.25 0.26 0.35Bar screen channel depth= 1.10 1.10 1.10 1.10 1.10

pump WW HGL - channel bottom EL (m)(Water level at pump well controls upstreamhydraulics if bar screen channel depth is higherthan Yc)Is bar screen channel depth higher than Yc? yes yes yes yes yes3.Point 8 to point 7Channel bottom EL (m) 99.50 99.50 99.50 99.50 99.50








Depth in channel, y7 (m) 1.10 1.10 1.10 1.10 1.10Velocity, V7 (m/s) 0.18 0.29 0.36 0.39 0.58Exit loss from channel to pump well

Exit loss coeficient, KexitK 1.0 1.0 1.0 1.0 1.0Headloss, Hle7=KexitK .� V7277 /2g (m) 0.00 0.00 0.01 0.01 0.02

HGL at point 7, HGL7 � HGL8+Hle7 (m) 100.60 100.60 100.61 100.61 100.624.Point 7 to Point 6

Friction headloss through channelLength of approach channel, L6 (m) 7 7 7 7 7Manning’s number for concrete channel, n 0.013 0.013 0.013 0.013 0.013Channel width, w6 (m) 2.50 2.50 2.50 2.50 2.50Water depth, h6 (m) 1.10 1.10 1.11 1.11 1.12Velocity, V6 (m/s) 0.18 0.29 0.36 0.39 0.57

Hydraulic radius, R6 � (h6 � w6)/(2 � h6 � w6) 0.59 0.59 0.59 0.59 0.59Headloss, Hlf6 � (V6 � n/r62/366 )2 � L6 (m) 0.00 0.00 0.00 0.00 0.00

HGL at Point 6, HGL6 � HGL7 + Hlf6 (m) 100.60 100.60 100.61 100.61 100.625. Point 6 to Point 5

Bar width (m) 0.010 0.010 0.010 0.010 0.010Bar shape factor, bsf 2.42 2.42 2.42 2.42 2.42Cross-sectional width of bars, w (m) 0.89 0.89 0.89 0.89 0.89Clear spacing of bars, b (m) 1.61 1.61 1.61 1.61 1.61Upstream velocity head, h (m) 0.0041 0.0104 0.0163 0.0186 0.0418Angle of bar screen with horizontal, p (degrees) 60 60 60 60 60(Kirschmer’s eq),. Hls � bsf � w/b

1.33 � h � sin p (m) 0.01 0.02 0.03 0.03 0.06Allow 0.15 m head for blinding

by screenings, Ha (m) 0.15 0.15 0.15 0.15 0.15

HGL upstream of bar screen, HGL5 �

HGL6 � Hls � Ha (m) 100.76 100.77 100.78 100.79 100.83

6. Point 5 to Point 4Friction headloss through channel

Length of approach channel, L4 (m) 7.00 7.00 7.00 7.00 7.00Manning’s number for concrete channel n 0.013 0.013 0.013 0.013 0.013Channel width, w4 (m) 2.50 2.50 2.50 2.50 2.50Channel bottom elevation (m) 99.65 99.65 99.65 99.65 99.65Water depth, h4 (m) 1.11 1.12 1.13 1.14 1.18Channel velocity, V4 (m/s) 0.18 0.29 0.35 0.38 0.54VVHydraulic radius R4 � h4 �

w4/(2 � h4 � w4) 0.59 0.59 0.59 0.60 0.61Headloss , Hlf4ff � (V4*VV n/R// 4 (2/3)

2 �L4 (m) 0.00 0.00 0.00 0.00 0.00

HGL at Point 4, HGL4 � HGL5 + Hlf4 (m) 100.76 100.77 100.78 100.79 100.83ff

7. Point 4 to Point 3Headloss at sluice gate contraction

KgateK 1.0 1.0 1.0 1.0 1.0Sluice gate width (m) 1.2 1.2 1.2 1.2 1.2Sluice gate height (m) 0.9 0.9 0.9 0.9 0.9Velocity through sluice gate, Vs (m/s) 0.38 0.59 0.74 0.78 1.13







Sluice gate headloss,Hls � KgateK � Vs2 /2g (m) 0.01 0.02 0.03 0.03 0.06

HGL at Point 3, HGL3 (m) 100.77 100.79 100.81 100.82 100.90

8. Point 3 to Point 2Water depth at point 2, h2 (m) 1.12 1.14 1.16 1.17 1.25Channel width, w2 (m) 2.00 2.00 2.00 2.00 2.00Channel velocity, V2 (m/s) 0.22 0.35 0.43 0.46 0.64VVFitting headloss through 45° bend KbendKK � 0.2 0.20 0.20 0.20 0.20 0.20

Headloss, Hlb2 = KbendK � V 2 2/2g (m) 0.0005 0.0013 0.0019 0.0021 0.0042Friction headloss through channel

Length of approach channel, L2 (m) 4.00 4.00 4.00 4.00 4.00Manning’s, number for concrete channel n 0.013 0.013 0.013 0.013 0.013Hydraulic radius

R2 � h2 � w2/(2 � f2ff �w2) (m) 0.53 0.53 0.54 0.54 0.56Headloss Hlf2ff � (V2VV � n/R// 2(2/3)

2 � L2, (m) 0.0001 0.0002 0.0003 0.0003 0.0006Entrance loss

KentK � 0.5 n 0.50 0.50 0.50 0.50 0.50Headloss, Hle2 = KentK � V 2 2/2g(m) 0.0013 0.0031 0.0047 0.0053 0.0105

HGL at Point 2, HGL2 � HGL3 �

Hlb2� Hlf2ff � Hle2 (m) 100.77 100.79 100.82 100.83 100.91

9. Point 2 to Point 1HGL at point 1, HGL 1 � HGL2 (m) 100.77 100.79 100.82 100.83 100.91Invert EL of inlet sewer, INV1 (m) 99.50 99.50 99.50 99.50 99.50Crown EL of inlet sewer, CWN1 (m) 101.65 101.65 101.65 101.65 101.65Surcharge to inlet sewer? No No No No No


Effluent weir. The effluent weir of the grit chamber provides the hydraulic controlpoint of this process. With a free fall at the weir, critical depth occurs upstream near theweir and it affects the water surface profile upstream if the flow is subcritical. The efflu-ent weir should be designed to keep the velocity below 0.3 m/s (1 ft/s) and to minimizeturbulence in the outlet.

Hydraulic design example. A schematic diagram of a typical vortex grit tank systemis shown in Fig. 22.23. The effluent from the bar screen is pumped to the grit tank influ-ent channel. The influent is distributed to three grit tanks. The hydraulic loadingconditions are the same as those for the bar screens.

Design hydraulic calculations for the vortex grit tank system is shown in Table 22.9.The head requirements for the sample grit tank system are in the range of 0.30–0.69 m(1.0–2.3 ft).

22.4.2.3 Sedimentation tanks. Process criteria. A typical municipal wastewater treat-ment system consists of primary sedimentation and secondary (or final) sedimentationtanks. The purpose of both type of sedimentation tanks is to separate the settleable solidsfrom the liquid stream by gravity settling.




The primary sedimentation tank receives the wastewater passed through bar screensand/or grit tanks. The objectives of primary sedimentation are to produce a liquid effluentsuitable for downstream biological treatment and to achieve solids separation. The solidsresult in a sludge that can be conveniently and economically treated before ultimatedisposal. On an average basis, the primary sedimentation tank removes approximately60 and 30 percent of influent total suspended solids (TSS) and 5-day biological oxygendemand (BOD5), respectively.

The secondary sedimentation tank receives mixed liquor from the aeration tank. Mixedliquor is a suspended biological growth stream containing microorganisms and treatedwastewater. The microorganisms settle with other settleable solids and the clear water is dis-charged from the sedimentation tank as an effluent. The sedimentation process also thickensthe settled solids, a major part of which is returned to the aeration tank and the remainder iswasted as secondary sludge. Sedimentation tank performance is critical for meeting effluentlimits for TSS and BOD5. The secondary sedimentation effluents are usually designed toproduce 30 mg/L or lower for TSS or BOD5, depending on the effluent requirement.

Both primary and secondary sedimentation tanks are commonly arranged in either rectangular or circular shape. Key design parameters include surface overflow rate (SOR),tank water depth, hydraulic detention time, and weir loading rate. Solids loading rate is anoth-er important parameter for the secondary sedimentation tank. A properly designed sedimen-tation tank will provide similar performance for both rectangular and circular shapes. Choiceof the shape depends on the site constraints, construction cost, and designer preference.

Key hydraulic design parameters. The key hydraulic design parameters for sedimen-tation tanks include the inlet conditions, inlet channel, inlet flow distribution, inlet baffle,outlet conditions, overflow weir, and effluent launder.

Inlet conditions. Inlets should be designed to dissipate the inlet port velocity, distrib-ute flow and solids equally across the cross-sectional area of the tank, and prevent shortcircuiting in the sedimentation tank. The minimum distance between the inlet and outletshould be 3 m (10 ft) unless the tank includes special provisions to prevent shortcircuiting.

Inlet channel. Inlet channels should be designed to maintain velocities high enough toprevent solids deposition. The minimum channel velocity is typically 0.3 m/s (1 ft/s).Alternatively, inlet channel aeration or water jet nozzles can be designed to prevent solidsdeposition.

Inlet flow distribution. Inlet flow can be distributed by inlet weirs, submerged ports,or orifices with velocities between 0.05 and 0.15 m/s (0.15–0.5 ft/s), and sluice gates orgate valves. Uniform flow to the sedimentation tanks can be achieved by locating inletports away from sides, adding partitions or baffles in the inlet zone to redirect the influ-ent, and creating a higher head loss in the inlet ports relative to that in the inlet channel.Alternatively, splitter boxes are used for equally splitting the flow as well as solids con-tained in the liquid into multiple sedimentation tanks.

Inlet baffle. Inlet baffles are designed to dissipate the energy of the inlet velocities.Baffles are usually installed 0.6–0.9 m (2–3 ft) downstream of the inlet port andsubmerged 0.45–0.6 (1.5–2 ft), depending on tank depth. The top of the baffle should befar enough below the water surface to allow scum to pass over the top. Circular tanks typ-ically have a feed well with a diameter 15 to 20 percent of the tank diameter. Thesubmergence varies depending on the manufacturer.

Outlet conditions. Effluent should be uniformly withdrawn to prevent localized highvelocity zones and short circuiting. Typically, effluent is withdrawn from a sedimentation






FIGURE 22.23 Schematic diagram of vortex grit tank system.

TABLE 22.9 Example Hydraulic Calculation of a Typical Vortex Grit Tank System


Parameter Min Day Avg.Day Avg.Day Max Hour Peak


2. Vortex grit tanksTotal number of units 3 3 3 3 3Number of units in operation 2 2 2 3 2Number of units on standby 1 1 1 0 1Flow rate per vortex grit tank in

operation (m3/s) 0.5 0.8 1.0 1.1 1.6

Control point is located at Point 8(effluent channel weir)

Hydraulic calculations upstream of control point

3. At Point 8Headloss over sharp-crested weir

Sharp-crested weir EL, weir EL (m) 106.00 106.00 106.00 106.00 106.00Effluent channel bottom EL (m) 105.00 105.00 105.00 105.00 105.00Flow rate over weir, q (m3/s) 0.5 0.8 1.0 1.1 1.6








Length of weir, L (m) 3.00 3.00 3.00 3.00 3.00Head over end contracted weir,

He (assumed) 0.20 0.28 0.32 0.34 0.45Headloss, He8 � (q/1.84 (L – 0.2He)(2/3) (m) 0.20 0.28 0.32 0.34 0.45Hle8 – He (must be zero) 0.00 0.00 0.00 0.00 0.00

HGL at Point 8, HGL8 � weirEL � Hle8 (m) 106.20 106.28 106.32 106.34 106.45

4. Point 8 to Point 7Channel width, w7 (m) 3.00 3.00 3.00 3.00 3.00Channel bottom EL (m) 105.00 105.00 105.00 105.00 105.00Water depth, h7 (m) 1.20 1.28 1.32 1.34 1.45Velocity, V 7 (m/s)Exit headloss from channel to effluent weir

Exit headloss coefficient KexitK � 1.0 1.0 1.0 1.0 1.0 1.0Headloss, Hle7 � Kexit KK � V72/2g (m) 0.0010 0.0022 0.0032 0.0036 0.0069

HGL at Point 7, HGL7 � HGL8 � Hle7 (m) 106.20 106.28 106.33 106.34 106.45

5. Point 7 to Point 6Channel width, w6 (m) 2.50 2.50 2.50 2.50 2.50Channel bottom EL (m) 105.00 105.00 105.00 105.00 105.00Water depth, h6 (m) 1.20 1.28 1.33 1.34 1.45Velocity, V6 (m/s) 0.17 0.25 0.30 0.32 0.44VVFriction headloss through channel

Length of approach channel, L6 (m) 10.00 10.00 10.00 10.00 10.00Manning’s number for concrete channel n 0.013 0.013 0.013 0.013 0.013Hydraulic radius, R6 � (h6 � w6)/

(2 x h6 � w6) (m) 0.61 0.63 0.64 0.65 0.67Headloss Hlf6ff �[(V6VV �n/R// 6(2/3)]2

�L6(m)0.0001 0.0002 0.0003 0.0003 0.0006Fitting headloss through 90º bendFitting headloss coefficientKbendK � 1.0 1.0 1.0 1.0 1.0 1.0Headloss, Hlb6 � KbendKK � V6VV 2/2g(m) 0.0014 0.0032 0.0046 0.0051 0.0099


Hlf6ff � Hlb6 (m) 106.21 106.28 106.33 106.35 106.46

6. Point 6 to Point 5Headloss through sluice gate

Sluice gate headloss coefficient KgateK � 1.0 1.0 1..0 1.0 1.0 1.0Sluice gate width (m) 1.5 1.5 1.5 1.5 1.5Sluice gate height (m) 1.0 1.0 1.0 1.0 1.0Water depth, h5 (m) 1.20 1.28 1.33 1.34 1.45Sluice gate height or h5,

whichever is smaller (m) 1.0 1.0 1.0 1.0 1.0Velocity through sluice gate,

V5 (m/s) 0.33 0.53 0.67 0.71 1.07VV







Headloss, Hls5 � KgateK � V5VV 2/2g (m) 0.0057 0.0145 0.0227 0.0258 0.0580HGL at Point 5, HGL5 �

HGL6 � Hls5 (m) 106.21 106.30 106.36 106.37 106.52

7. Point 5 to Point 4Channel width, w4 (m) 2.50 2.50 2.50 2.50 2.50Bottom of channel EL (m) 105.20 105.20 105.20 105.20 105.20Water depth, h4 (m) 1.01 1.01 1.16 1.17 1.32Channel velocity, V4 (m/s) 0.20 0.29 0.35 0.36 0.48VVFitting headloss through a 90º bend

Fitting headloss coefficientKbendK � 1.0 1.0 1.0 1.0 1.0 1.0

Headloss, Hlb4 � KbendKK � V4VV 2/2g (m) 0.0020 0.0043 0.0061 0.0067 0.0120Friction headloss through channel

Length of channel, L4 (m) 10.00 10.00 10.00 10.00 10.00Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013Hydraulic radius, R4 � h4 � w4/

(2 � h4 � w4) (m) 0.56 0.58 0.60 0.61 0.64Headloss, Hlf4ff � [(V4VV �n/R// 4(2/3)]2� L4 (m)0.0001 0.0003 0.0004 0.0004 0.0007


Hlb4 � Hlf4 (m) 106.21 106.30 106.36 106.38 106.54ff

8. Point 4 to Point 3Headloss across vortex grit

tank, Hltank (m) 0.06 0.06 0.06 0.06 0.06(per manufacturer recommendations)


Hltank (m) 106.27 106.36 106.42 106.44 106.60

9. Point 3 to Point 2Channel width, w2 (m) 2.00 2.00 2.00 2.00 2.00Bottom of channel EL (m) 105.60 105.60 105.60 105.60 105.60Water depth, h2 (m) 0.67 0.76 0.82 0.84 1.00Channel velocity, V2 (m/s) 0.37 0.52 0.61 0.63 0.80VVFriction headloss through channel

Length of approach channel,L2 (m) 14.00 14.00 14.00 14.00 14.00

Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013Hydraulic radius, R2 � h2 � w2/

(2 � h2 � w2) (m) 0.40 0.43 0.45 0.46 0.50Headloss, Hlf2ff � [(V2VV � n/R// 2(2/3)]2

� L2 (m) 0.0011 0.0020 0.0025 0.0027 0.0039Headloss through sluice gate

Sluice gate headloss coefficient KgateK � 1.0 1.0 1..0 1.0 1.0 1.0

Sluice gate width (m) 1.5 1.5 1.5 1.5 1.5Sluice gate height (m) 1.0 1.0 1.0 1.0 1.0









Water depth, h2 (m) 0.67 0.76 0.82 0.84 1.00Sluice gate height or h2,

whichever is smaller 0.67 0.76 0.82 0.84 1.00Velocity through sluice gate,V2VV (m/s) 0.49 0.70 0.81 0.85 1.07Headloss, Hls2 � KgateK � V2VV 2/2g (m) 0.0125 0.0249 0.0335 0.0364 0.0586


Hlf2ff � Hls2 (m) 106.29 106.39 106.46 106.48 106.66

10. Point 2 to Point 1

Channel width, w1 (m) 2.00 2.00 2.00 2.00 2.00Bottom of channel EL (m) 105.65 105.65 105.65 105.65 105.65Water depth, h1 (m) 0.64 0.74 0.81 0.83 1.01Channel velocity, V1 (m/s) 0.39 0.54 0.62 0.64 0.79Fitting headloss through a 90º bend

Fitting headloss coefficientKbendK � 1.0 1.0 1.0 1.0 1.0 1.0

Headloss, Hlb1 � Kbend �

V12/2g (m) 0.0078 0.0149 0.0195 0.0210 0.0322Friction headloss through channel

Length of approach channel, L1 (m) 5.00 5.00 5.00 5.00 5.00Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013Hydraulic radius, R1 � h1 � w1/

(2 � h1 � w1) (m) 0.39 0.43 0.45 0.45 0.50Headloss, Hlf1ff � (V1 � n/R// 1(2/3)2

� L1 (m) 0.0005 0.0008 0.0009 0.0010 0.0013

(Influent channel may be aeratedusing diffused air to prevent solidssettling or odor problem)HGL at Point 1, HGL1 � HGL2 �

Hlc1 � Hlf1 (m) 106.30 106.41 106.48 106.50 106.69ff

tank over an effluent weir into a trough and/or effluent channel. Clarifier performance canoften be improved by installation of interior baffles. For circular tanks, particularly forsecondary sedimentation tanks, a baffle mounted on the wall beneath the effluent weir candeflect solids rising along the wall. Alternatively, mid-radius baffles supported by thesludge removal mechanism are also available.

Overflow weir. The overflow weir must be level to promote uniform effluent with-drawal. Weirs may be either straight edged or “V”-notched. “V”-notched weirs have high-er headloss, but provide better lateral distribution than straight-edged weirs that are imper-fectly leveled.

Effluent launder (or trough). Effluent launders may be designed with submerged ori-fices or free discharge into the collection chamber or channel from which the effluentflows to the effluent pipe. Disadvantage of the submerged launder is that it is not effective





in varying flow rates. Disadvantage of the free fall launder is potential release of odorousgases. Two principal approaches to weir and launder design are the long-launder andshort-launder options. Long launders control the head loss over the weir within a narrowrange. In cold regions, fluctuating water levels with short launders would minimize iceattachment to launders and basin walls.

Hydraulic design example for primary sedimentation. A schematic diagram of typical cir-cular primary sedimentation tank system is shown in Fig. 22.24. The primary sedimenta-tion tanks receive the grit tank effluent and hydraulic loading conditions are the same asthose of the grit tanks. A single primary sedimentation tank is shown for simplicity.Design hydraulic calculations for the primary sedimentation tank system is shown inTable 22.10. Note that the design locates Points 5 and 6 at elevations such that down-stream flow conditions will not impact flow conditions in the effluent channel or overflowweir. The head requirements for the sample primary sedimentation tanks are in the rangeof 1.1–1.5 m (3.6–4.9 ft).

Hydraulic design example for secondary sedimentation. A schematic diagram of typicalrectangular secondary sedimentation tank system is shown in Figure 22.25. The secondarysedimentation tanks receive flows from the aeration tanks and hydraulic loading condi-tions are same as those of the aeration tanks. A single secondary sedimentation tank isshown for simplicity. Design hydraulic calculations for the secondary sedimentation tanksystem is shown in Table 22.11. The head requirements for the sample secondary sedi-mentation tanks are in the range of 1.6–1.7 m (5.2–6.2 ft).

22.4.2.4 Aeration tanks Process criteria. The most common aerobic suspendedgrowth treatment system for municipal wastewater is the activated sludge system.Wastewater and biological solids (mixed–liquor suspended solids or MLSS) are com-bined, mixed, and aerated in the aeration tank. The biological MLSS solids take up theorganics and nutrients contained in the wastewater and convert them into more biosolidsand gaseous by-products. After sufficient time for biological reactions, the mixed liquoris transferred to the following secondary sedimentation tanks where biosolids are separat-ed from the wastewater. The separated wastewater is discharged as an effluent. The sepa-rated biosolids are returned to the aeration tank (return activated sludge or RAS) while apredetermined amount of the separated biosolids is wasted as waste activated sludge(WAS).

Factors that must be considered in the design of the activated sludge process includeloading criteria, selection of reactor type, sludge production, oxygen requirements andtransfer, nutrient requirements, environmental requirements, solid-liquid separation, andeffluent characteristics.

Sizing of aeration basins is based on two key factors: providing sufficient time foroxidation of organics or ammonia nitrogen; and maintaining of a flocculent, well-set-tling MLSS that can be effectively removed by gravity settling. Solids residence time(SRT) or mean cell residence time (MCRT) is often used to relate substrate removaltime requirements to biological growth and biosolids production. Once an SRT is select-ed, calculation of aeration tank volume requires an estimation of biosolids productionand selection of proper MLSS concentration. The selected MLSS concentration alongwith the solids settling characteristics is important to the final sedimentation tank per-formance. Therefore, sizing of the aeration tank is always optimized with the final sed-imentation tank design.

The aeration tank should be provided with sufficient oxygen required for the biologi-cal reaction and sufficient power required for thorough mixing of the biomass with theincoming wastewater stream. Although a variety of diffused aeration and mechanical aer-ation systems are available, diffused aeration systems are more popular in the municipalwastewater treatment.





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Aeration basin configurations.. Common aeration basins include various process con-figurations, physical configurations and designs for process selectors. A schematic dia-gram of a typical rectangular aeration tank system is shown in Fig. 22.26.

Process configuration Various aeration process configurations can be used dependingon the range of loading conditions, design effluent quality, aeration system design require-ments and flexibility of operation. Configurations often encountered include completemix, plug flow, oxidation ditch, and a combination of these. For smaller plants, oxidation

TABLE 22.10 Example Hydraulic Calculations of a Typical Primary SedimentationTank System




2. Primary sedimentation tanks (PSTs)Total number of units 3 3 3 3 3Number of units in operation 2 2 3 3 2Number of units on standby 1 1 0 0 1Flow rate per PTS in

operation, q (m3/s) 0.5 0.8 0.7 1.1 1.6

Control points are located at Points5 and 6 so that back up fromdown stream does not flood effluentchannel or overflow weir.

Hydraulic Calculations beginningat Point 7

1. At Point 7HGL7 must be equal to HGL1 of

aeration tank (m) 104.46 104.46 104.46 104.46 104.46

2. At Point 6Allowance of 0.10 m from HGL at

pipe entrance to bottom of PSTeffluent trough at discharge end (m) 0.10 0.10 0.10 0.10 0.10

Elevation of PTS trough bottomat discharge end, ELdcb (m) 104.56 104.56 104.56 104.56 104.56

Calculation of water depth inPST effluent troughTank diameter, Dt (m) 45.0 45.0 45.0 45.0 45.0Number of channels per tank, nc 2 2 2 2 2Total flow through tank, q (m3/s) 0.50 0.80 0.67 1.07 1.60Flow per channel, qc � q/nc (m3/s) 0.25 0.40 0.33 0.53 0.80Channel slope, Sc, (selected

to prevent solids settling) 0.20 0.20 0.20 0.20 0.20Channel width, w6 (m) 1.00 1.00 1.00 1.00 1.00Channel length, Lc � 3.14 �

(Dt � (w6/2))/nc (m) 69.87 69.87 69.87 69.87 69.87Change in channel EL,

EL dif � Sc � Lc (m) 0.14 0.14 0.14 0.14 0.14








Critical depth, yc � (qc2/(g � w62)0.33 (m) 0.19 0.26 0.23 0.31 0.41Water depth at upstream end

of channel, yu � 2 � (yc)2 � (yc �

(S*L/3)LL 2]0.5 � (2 � Sc � L/3) (m) 0.21 0.33 0.28 0.42 0.58LLChannel bottom El at upstream

end of trough, 104.70 104.70 104.70 104.70 104.70ELucb � ELdcb � ELdif (m)

HGL at trough downstream,HGL6d � ELdcb � yc (m) 104.75 104.82 104.79 104.87 104.97

HGL at trough upstream,HGL6u � ELucb � yu (m) 104.91 105.03 104.98 105.12 105.28

3. Point 6 to Point 5Allowance to Weir from

high trough HGL (m) 0.10 0.10 0.10 .010 0.10Weir elevation, Elwe, max.

HGL6u � allowance (m) 105.38 105.38 105.38 105.38 105.38Headloss over V–notch weirsVV

Number of weirs per tank, Nw 1 1 1 1 1Tank diameter, Dt, (m) 45.00 45.00 45.00 45.00 45.00Weir length, Lw � (Dt) � 3.14 (m) 141.30 141.30 141.30 141.30 141.30Hydraulic load, So � q/Lw// , [(m3·/s)/m] 0.0035 0.0057 0.0047 0.0075 0.0113Weir angle, A, (degrees) 90.00 90.00 90.00 90.00 90.00V-notch height, Vh (m) 0.10 0.10 0.10 0.10 0.10V-notch width, Vw � 2 �

(TAN(A(( /2) � Vh (m) 0.20 0.20 0.20 0.20 0.20Space between notches, Esv (m) 0.03 0.03 0.03 0.03 0.03Number of notches per weir,

nv � Lw/(Ew � Esv) 614 614 614 614 614Flow per notch, Qcw � q/nv (m3/s) 0.0008 0.0013 0.0011 0.0017 0.0026Weir coefficient for 90º notch, Cw 1.34 1.34 1.34 1.34 1.34Water depth over the weir, hle5

� (Qcw/Cw)(1/2.48) 0.05 0.06 0.06 0.07 0.08hle5 < Vh? (If not, need to

readjust calculations) Yes Yes Yes Yes YesHGL at Point 5, HGL5 �

ELwe � hle5 (m) 105.44 105.45 105.44 105.45 105.47

4. Point 5 to Point 4Headloss through primary

sedimentation tanksNumber of tanks, Nt 2 2 3 3 2Flow per tank, q (m3/s) 0.50 0.80 0.67 1.07 1.60Tank diameter, Dt (m) 45.00 45.00 45.00 45.00 45.00Side water depth, Dsw (m) 4.30 4.30 4.30 4.30 4.30Tank bottom elevation,

ELt � HGL5 � Dsw (m) 101.14 101.14 101.14 101.14 101.14Tank floor slope, St (%) 8.33 8.33 8.33 8.33 8.33








Minimum floor tank elevation, ELtf 99.27 99.27 99.27 99.27 99.27� 0.0833 � (Dt/2)tt � EL (m)

Headloss through tank, hlt4 (m) 0.05 0.05 0.05 0.05 0.05tt(available from equipment

manufacturer)

HGL at Point 4, HGL4 �

HGL5 � hlt4 (m) 105.49 105.50 105.49 105.50 105.52tt


Headloss through PST influent pierPier diameter, Dp � 1.07 m 1.07 1.07 1.07 1.07 1.07Pier length, Lp (m) 6.50 6.50 6.50 6.50 6.50Velocity, V3VV � Q/(3.14 �

(Dp/2)2) (m/s) 0.56 0.89 0.74 1.19 1.78Hazen-Williams coefficient, Cp 120 120 120 120 120Hydraulic radius, Rp � Dp/4 (m) 0.27 0.27 0.27 0.27 0.27Slope, Sp � [V3/(0.85VV � Cp � Rp(0.63)](1/0.54)

(%) 0.03 0.07 0.05 0.12 0.26Headloss, Hlf3ff � Lp � Sp (m) 0.0020 0.0047 0.0033 0.0079 0.0168

Exit headloss from pierExit headloss coefficient

KexitK � 1.0 1 1 1 1 1Headloss, hle3 � K � V3VV 2/2g (m) 0.0158 0.0404 0.0281 0.0719 0.1617


HGL4 � Hlf3ff � hle3 (m) 105.50 105.54 105.52 105.58 105.69

6. Point 3 to Point 2Total number of pipes 3 3 3 3 3Number of pipes per primary

sedimentation tank 1 1 1 1 1Pipe diameter, Dp (m) 1.20 1.20 1.20 1.20 1.20Flow per pipe, q (m3/s) 0.50 0.80 0.67 1.07 1.60Velocity, V2 0.44 0.71 0.59 0.94 1.42VVFriction headloss through primary

sedimentation tank influent pipeHazen-Williams coefficient, Cp 120 120 120 120 120Hydraulic radius, Rp � Dp/4 (m) 0.30 0.30 0.30 0.30 0.30Length of pipe, Lp (m) 70.0 70.0 70.0 70.0 70.0Slope, Sp � [V2/(0.85VV � Cp � Rp(0.63)](1/0.54)

(%) 0.02 0.04 0.03 0.07 0.15Headloss, hlf2ff � Lp � Sp (m) 0.0120 0.0287 0.0205 0.0490 0.1037

Fitting headloss through two 45º bendsFitting headloss coefficientKbendKK � 0.5 0.05 0.05 0.05 0.05 0.05Headloss, hlb2 � K � V2VV 2/2g (m) 0.0050 0.0128 0.0089 0.0227 0.0511


HGL3 � hlb2 � hlf2 (m) 105.52 105.58 105.55 105.65 105.85ff

7. At Point 1








Entrance headloss fromprimary sedimentationtank influent distribution box

to influent pipePipe diameter, Dp (m) 1.20 1.20 1.20 1.20 1.20Flow per pipe, q (m3/s) 0.50 0.80 0.67 1.07 1.60Velocity, V1 (m/s) 0.44 0.71 0.59 0.94 1.42Entrance headloss coefficient

KentranceK � 0.5 0.50 0.50 0.50 0.50 0.50Headloss, Hle1 �

KentranceK � V12/2g (m) 0.0050 0.0128 0.0089 0.0227 0.0511


HGL2 � Hle1 (m) 105.52 105.60 105.56 105.68 105.90

Allowance to grit tank effluent weir from maximum 0.10 0.10 0.10 0.10 0.10HGL1, Hall (m)

Grit tank effluent elevation, ELgr �

HGL1 � Hall (m) 106.00 106.00 106.00 106.00 106.00

ditches are more popular and for larger plants, plug flow is favored. Various modificationsof plug flow systems include conventional, tapered aeration, step aeration, modified aera-tion, and contact stabilization.

Physical configuration. Various physical configurations are used in the aeration tankdesign, including rectangular, circular, oval, and octagonal shapes.

Selector design. Selectors are small compartments for aerobic, anoxic or anaerobicprocessing usually located in the front end of the aeration tank. The purpose of the selec-tors is to promote the growth of floc-forming microorganisms by providing a favorableffood to microorganisms (F:M) ratio while suppressing filamentous growth. Typicallyselectors are designed with low HRTs and high F:M ratio.

Key hydraulic design parameters. The key hydraulic design parameters for aerationtanks include the distribution box, inlet channel, inlet flow distribution, inlet baffles, aer-ation equipment, RAS, effluent weir, and effluent channel.

Distribution box. Sluice gates, weirs, gate valves or orifices installed in a distributionbox are often used to distribute the upstream flow to multiple aeration tanks and to a sec-ondary treatment bypass line. Design should provide the desired rate of flow distributionat all flow conditions with minimum headloss. Provisions to minimize solids depositionin the distribution box and appurtenances should be considered.

Inlet channel. Inlet channels should be designed to maintain velocities high enough toprevent solids deposition but low enough to minimize headloss. A velocity of 0.3 m/s(1 ft/s) is typically used to keep organic solids in suspension. Alternatively, inlet channelaeration with diffused air, fed at a rate of 0.5–0.8 m3/min (20–30 scfm), is often used.

Inlet flow distribution. Inlet flow can be distributed by inlet weirs, submerged ports ororifices, and sluice gates or gate valves. Return activated sludge may be introduced prior





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TABLE 22.11 Example Hydraulic Calculation of a Typical Final Sedimentation Tank System


PARAMETER Min Day Avg.Day Avg.Day Max Hour Peak


RAS flow, Qras (% of average day flow) 20 50 50 100 100RAS flow, Qras

/100, (m3/s) 0.32 0.80 1.00 2.00 2.00Final sedimentation tank influent

flow, Qin, (m3/s) 1.32 2.40 3.00 5.20 5.20Final sedimentation tank effluent

flow, Qeff, (mff 3/s) 1.00 1.60 2.00 3.20 3.20Final sedimentation tanks

Total number of units 4 4 4 4 4Number of units in operation 3 3 3 4 3Number of units on standby 1 1 1 0 1Tank width (m) 16 16 16 16 16Influent per operating tank,

qin, (m3/s) 0.44 0.80 1.00 1.30 1.73Effluent per operating tank,

qeff, (mff 3/s) 0.33 0.53 0.67 0.80 1.07

2. Select control point at Point 3(where effluent wiers are located)Hydraulic calculations downstreamof control pointAt Point 3V-notch weir

Number per tank, Nw 20 20 20 20 20Individual weir length, Lw (m) 7.0 7.0 7.0 7.0 7.0Total weir length, Lwt � Lw � Nw (m) 140.0 140.0 140.0 140.0 140.0Weir angle, A degrees 90.0 90.0 90.0 90.0 90.0V-notch height, Vh (m) 0.10 0.10 0.10 0.10 0.10V-notch width, Vw � 2 �

(TAN(NN A(( /2) � Vh (m) 0.20 0.20 0.20 0.20 0.20Space between notches, Esv (m) 0.03 0.03 0.03 0.03 0.03Total number of notches per

tank, nv � Lwt/(tt Vw � Esv) 608 608 608 608 608Flow per notch, Qcw � qeff/ff nv 0.0005 0.0009 0.0011 0.0013 0.0018Weir coefficient for 90º notch, Cw 1.34 1.34 1.34 1.34 1.34Water depth over the weir,

hle3 � (Qcw/Cw)(1/2.48) (m) 0.04 0.05 0.06 0.06 0.07hle3 � Vh? (If not, need toreadjust calculations) yes yes yes yes yesWeir EL (m) (Select weirelevation so that HGL1 103.37 103.37 103.37 103.37 103.37equals aeration tank’s HGL6)

EGL at Point 3, EGL3 �

Weir EL � hle3 (m) 103.41 103.42 103.43 103.43 103.44Velocity head, HV3 � 0

(assume V3VV � 0) (m) 0.00 0.00 0.00 0.00 0.00






Initial Operation Initial Operation


HGL at point 3, HGL3 �

Weir EL � hle3 (m) 103.41 103.42 103.43 103.43 103.44

3. Point 3 to Point 4Effluent troughs

Number of troughs, nt 10 10 10 10 10Flow per trough, qt � qeff/ff nt (m3/s) 0.03 0.05 0.07 0.08 0.11Trough slope, St (%) (select

to prevent solids settling) 0.20 0.20 0.20 0.20 0.20Trough width, w6 (m) 0.5 0.5 0.5 0.5 0.5Approximate trough length, Lt (m) 7.0 7.0 7.0 7.0 7.0Change in trough EL due to slope

difEL4 � St* Lt (m) 0.01 0.01 0.01 0.01 0.01Critical depth at downstream end, yc �

(qt2/(gw62)0.33 (m) 0.08 0.11 0.12 0.14 0.17Water depth at upstream end

of trough for free fall 0.12 0.17 0.20 0.23 0.28from trough into finaleffluent channel

yu4 � [2 � (yc)2 � (yc � (S*L/3)LL 2].5

� (2 � S � L/3) (m)LLMax water EL downstream of weir

(occurring at max. hourly flow 103.27with one tank out of service)Elmax4 � weir EL � 0.1 (m)

(see Point 3 for weirEL)Trough bottom EL at upstream

end of trough, TbuEL4 (m) 102.99 102.99 102.99 102.99 102.99Tbu EL4 � EL max4 � yu for max

hour flow with one tank out of serviceHGL at upstream end,

HGL4u � Tbu EL4 � yu4 (m) 103.11 103.16 103.19 103.22 103.27Velocity head, HV4VV u � 0

(assume V � 0) (m) 0.00 0.00 0.00 0.00 0.00EGL at upstream end, EGL4u �

HGL4u � HV4VV u (m) 103.11 103.16 103.19 103.22 103.27

Trough bottom EL at downstreamend of trough 102.97 102.97 102.97 102.97 102.97Tbd EL4 �Tbu EL4 � dif EL4 (m)


TbdEL4 � yc (m) 103.05 103.08 103.10 103.11 103.14Velocity head, HV4VV d � Vc2/2g (m) 0.04 0.05 0.06 0.07 0.08








EGL at downstream end,EGL4d � HGL4d � HV4VV d (m) 103.09 103.13 103.16 103.18 103.22

4. Point 4 to Point 5Effluent channel upstream

Max. water surface level at upstreamend of effluent channel, ELmax5 �

TbdEL4 � 0.1 (m) 102.87 102.87 102.87 102.87 102.87

HGL maximum at Point 5, HGL5�ELmax5(m)102.87 102.87 102.87 102.87 102.87Velocity head, HV5VV � 0

(assume V � 0) (m) 0.00 0.00 0.00 0.00 0.00EGL maximum at Point 5,EGL5m � HGL5m � HV5 (m) 102.87 102.87 102.87 102.87 102.87VV

5. Point 5 to Point 6Effluent channel downstream

Flow through channel, Qeff (m3/s) 1.00 1.60 2.00 3.20 3.20Channel slope, Sc (%) (select

to prevent solids settling) 0.20 0.20 0.20 0.20 0.20Channel width, w6 (m) 3.0 3.0 3.0 3.0 3.0Approximate channel length, Lch (m) 64.0 64.0 64.0 64.0 64.0Change in channel EL,

difEL6 � Sc � Lch (m) 0.13 0.13 0.13 0.13 0.13Critical depth, yc �

(q2/(gw62)0.33 (m) 0.23 0.31 0.36 0.49 0.49Water depth at upstream end of channel, 0.29 0.43 0.52 0.74 0.74

yu6 � [2 � (yc)2 � (yc � (S � L/3)LL 2].5

� (2 � S � L/3) (m)LLChannel bottom EL at upstream

end of channel, 102.13 102.13 102.13 102.13 102.13cbuEL6 � HGL5- maximum yu6 (m)

HGL at upstream end of channel,HGL5 � cbuEL6 � yu6 (m) 102.42 102.56 102.65 102.87 102.87

Velocity head, HV5VV � 0(assume V � 0) (m) 0.00 0.00 0.00 0.00 0.00

EGL at upstream end of channel,EGL5 � HGL5 � HV5 (m) 102.42 102.56 102.65 102.87 102.87VV

Channel bottom EL atdownstream end of channel, 102.00 102.00 102.00 102.00 102.00cbdEL6 � cbuEL6 � difEL6 (m)

HGL at Point 6, HGL6 � cbdEL6 �yc (m) 102.23 102.31 102.36 102.50 102.50Velocity head, HV6VV � Vc2/2g (m) 0.11 0.15 0.17 0.24 0.24









HGL6 � HV6 (m) 102.34 102.47 102.54 102.74 102.74VV

6. At Point 7Max water EL downstream of

channel end free-fall 101.90 101.90 101.90 101.90 101.90HGL at Point 7, HGL7 � cbdEL6 � 0.1 (m)

(This must be the same asmaximum elevation at Point 1of multimedia filter)

Hydraulic Calculations Upstreamof Control Point

7. At Point 2Final sedimentation tanks (Gould type)

Number of tanks in operation, nt 3 3 3 4 3Flow per tank upstream of sludge

collection, qin (m3/s) 0.44 0.80 1.00 1.30 1.73Tank width, Wt (m) 16.0 16.0 16.0 16.0 16.0Tank length, Lt (m) 120.0 120.0 120.0 120.0 120.0Tank bottom elevation at

influent end (m) 99.2 99.2 99.2 99.2 99.2Side water depth (m) 4.24 4.25 4.26 4.26 4.27Assume friction losses, Hlf2,ff

through tank are negligible 0.0 0.0 0.0 0.0 0.0


EGL3 � Hlf2 (m) 103.41 103.42 103.43 103.43 103.44ffVelocity head, HV2VV � 0

(assume V � 0) (m) 0.00 0.00 0.00 0.00 0.00HGL at Point 2, HGL2 �

EGL3 � HV2 (m) 103.41 103.42 103.43 103.43 103.44VV

8. Point 2 to Point 1Tank influent sluice gates

Height (m) 1.0 1.0 1.0 1.0 1.0Width, Ws (m) 1.0 1.0 1.0 1.0 1.0Area (m2) 1.0 1.0 1.0 1.0 1.0Number of sluice gates per tank, Nsg 4 4 4 4 4Flow per sluice gate,

qsg � qin/Nsg// (m3/s) 0.11 0.20 0.25 0.33 0.43Upstream head over weir, Du �

(select so Qsub � qsg � D) (m) 0.21 0.31 0.36 0.44 0.53Effective sluice gate width, Ws' � 1.0 1.0 0.9 0.9 0.9

Ws � (0.1)(2 contractions)(Dd) (m)Downstream head over weir,

Dd � (qsg/1.84/Ws')(2/3) (m) 0.16 0.24 0.27 0.33 0.40Free–fall flow, Qfree � 1.84 �

Ws' � Du(3/2), (m3/s) 0.17 0.31 0.38 0.49 0.66Submerged flow, Qsub � Qfree

(1 � (Dd/dd Du// )3/2)0.385 (m3/s) 0.11 0.20 0.25 0.33 0.44








Difference, (Qsub � qsg) (m3/s)(should be zero) 0.00 0.00 0.00 0.00 0.00

Head difference between tank and channel, Hl 1 � Du � Dd (m) 0.051 0.077 0.090 0.106 0.130

Top of sluice gate set elevation,Els � HGL2 � Dd (m) 103.26 103.19 103.15 103.10 103.04

HGL at Point 1 (upstream of sluicegate), HGL1 � HGL2 �Hl1 (m) 103.46 103.50 103.52 103.54 103.57

Velocity head, HV1 � 0(assume V � 0) (m) 0.00 0.00 0.00 0.00 0.00

EGL at Point 1, EGL1 � HGL1 � HV1 (m) 103.46 103.50 103.52 103.54 103.57

Maximum HGL1 (m) 103.57

Max HGL1 should equal HGL6for aeration tank

to or after the inlet flow distribution. Good mixing should be provided to promote uniformdistribution of the influent flow and RAS flow. Wastewater flow split inlet design with arelatively high headloss is often used to provide reasonably equal distribution of flow tomultiple aeration tanks or to multiple inlets in each aeration tank operating in a step feedmode. Sometimes influent distribution piping which is extended to and having an inletport at each step feed point is used.

Inlet baffles. Depending on the aeration tank configuration, inlet baffles are used todissipate the energy from the inlet velocities. Inlet baffles are designed to direct uniformdistribution of MLSS along the width of the aeration tank.

Aeration equipment. Diffused aeration systems are predominantly used in the munic-ipal treatment plants. Although the air bubbles dispersed in the wastewater occupy approx-imately 1 percent of the volume, no allowance is made in aeration tank sizing. The vol-ume occupied by submerged piping and diffusers is usually negligible. If spiral-flow mix-ing with coarse bubble diffusers is used, the width-to-depth ratios vary from 1:1 to 2.2:1.The tank depth, most commonly 4–5 m (13–16 ft), is usually determined by desired oxy-gen transfer efficiency of various aeration equipment. Freeboard from 0.3 to 0.6 m (1 to 2ft) above the water surface is normally provided. If surface mechanical aerators are used,a freeboard of more than 0.6 m (2 ft) may be required depending on the power input forthe aeration and mixing. Freezing during the winter due to the mist should also be con-sidered in the design.

Return activated sludge (RAS). The rate of RAS is normally 30 to 50 percent of thewastewater flow. Peak rate of RAS may go up to 100 percent of the wastewater flow forlarge plants and up to 150 percent of the wastewater flow for small plants. Design shouldprovide adequate mixing, hydraulic capacity, and uniform distribution where RAS isintroduced to the incoming wastewater.

Effluent weir. The effluent weir provides a fixed control elevation of hydraulics in theaeration tank. Sometimes effluent ports instead of effluent weir are used to minimizeheadloss.





Effluent channel. The design considerations described in the inlet channel also applyto the design of the effluent channel. Often the effluent channel from the aeration tanks isthe same as the influent to the final sedimentation tanks.

Hydraulic design example. The aeration tanks receive the primary sedimentation tankeffluent and hydraulic loading conditions are the same as those of the primary sedimenta-tion tanks. Design hydraulic calculations for the aeration tank system is shown in Table22.12. The head requirements for the sample aeration tanks are in the range of 0.4–1.0 m(1.3–3.3 ft).

22.4.2.5 Granular media filter. Process criteria. Granular media filtration is usuallyused where the plant suspended solids effluent limit is equal to or less than 10 mg/L. Itmay also be applied following secondary biological treatment to remove particulate car-bonaceous BOD5 and residual insolubilized phosphorus. The degree of suspended solids

FIGURE 22.26 Schematic diagram of aeration tank system. (AT = aeration tank; PST = primarysedimentation tank).




removal when filtering secondary effluents without the use of chemical coagulationdepends on the degree of bioflocculation achieved during secondary treatment. The pres-ence of significant amounts of algae impedes filtration of lagoon effluents. Pretreatmentwith a coagulant is considered to be a good practice for such cases.

There are many types of proprietary granular filters available. However, granularmedia filters are generally classified according to direction of flow, type, and number ofmedia comprising the bed, the driving force, and method of flow control. Most wastewaterfilters are downflow units while some proprietary filters use various combinations ofupflow and downflow. The driving force for filtration may be either gravity or pressure.Gravity filters are commonly used in large municipal treatment plants while pressure fil-ters are often used in smaller plants.

Gravity filters are generally sized for a filtration rate of 1.4–4 L/(m2�s)/ (2–6gal/(ft2�min) and terminal headlosses of 2.4–3.0 m (8–10 ft). Multiple units are used toallow continuous filtration during backwash or maintenance. Typical length to width ratioof gravity filters vary from 1:1 to 4:1.

Key hydraulic design parameters. The key hydraulic design parameters for granularmedia filters include headlosses, filter operation, collection and distribution systems, andbackwash requirements.

Head losses. The head losses includes the losses associated with piping, valves,meters, bends, constrictions, filter media, underdrains, and collection systems. All lossesvary with the square of the velocity. Clean water headloss for the filter media is influencedby media type, size, uniformity, and depth. As filtration rate increases within the terminalhead loss range, less headloss capacity is available for solids storage. The head requiredfor the filter is the sum of all headlosses including the terminal head loss of the filtermedia. If sufficient head is not available, pumping of filter influent is required.

Filter operation. Three basic methods of filter operation are constant pressure, con-stant rate and variable declining rate. The constant pressure system requires a largeupstream storage and is seldom used with gravity filters. The constant rate system requiresa relatively costly rate control system and true constant-rate filtration is seldom used. Indeclining-rate filtration, the filtration rate may be kept constant using influent or effluentcontrol weirs during the initial period of operation and, thereafter, declining rate of filtra-tion. Generally, declining-rate filters are the best mode of gravity filter operation unlessthe design terminal headloss exceeds 3 m.

Collection and distribution systems. (underdrain). In conventional downflow filters,the underdrain system serves to both collect the filtrate and distribute the backwash water.Traditional systems using gravel layers with perforated pipe are no longer commonlyused. More popular underdrain materials include precast channels, poured-in-place con-crete, or steel pipe with built-in nozzles and orifices. Porous plates made of aluminumoxide or stainless steel are also available but they are susceptible to clogging.

Backwash requirements. Backwash is the cleaning of the filter by reversing the flowthrough the filter media at a controlled flow rate. Backwashing causes an expansion of thebed, normally no more than 10 percent of the depth, by allowing abrasive action amongparticles. The quantity of backwash water will generally be about 3000–4000 L/m2

(75–100 gal/ft2). Bachwashed water is collected in the wash-trough which is located about0.9 m (3 ft) above the filter media. Biological solids in secondary effluent are stronglyattached to the media and air scour before or during backwash is often required to pro-mote successful cleaning. Air requirements for the air scour are on the order of0.015–0.025 (m3/m2)/s [3–5 (ft3�ft�� 2)/min].






TABLE 22.12 Example Hydraulic Calculation of a Typical Final Aeration Tank System


PARAMETER Min Day Avg. Day Avg. Day Max Hour Peak


RAS flow. (% of average flow)(added downstream of aeration tank 20 50 50 100 100influent sluice gates)

RAS flow, Qras (m3/s) 0.32 0.80 1.00 2.00 2.00

2. Aeration tanksTotal of nunber of units 3 3 3 3 3Number of units in operation 2 2 3 3 2Number of units on standby 1 1 0 0 1Flow rate per aeration tank in

operation, q (m3/s) 0.50 0.80 0.67 1.07 1.60Flow rate per aeration tank in operation

including RAS flow (downstreamof influent sluice gate), qras (m3/s) 0.66 1.20 1.00 1.73 2.60

Control point is located at Point 5(aeration tank effluent weir).

3. At Point 6

Set maximum HGL6 � effluentweir elevation � 0.10 (m) 103.57 103.57 103.57 103.57 103.57

Hydraulic Calculations Upstreamof Control Point


Headloss over sharp-crested weirSharp-crested weir EL (m) 103.67 103.67 103.67 103.67 103.67Effluent channel bottom EL (m) 100.67 100.67 100.67 100.67 100.67Flow rate over weir, qras (m3/s) 0.66 1.20 1.00 1.73 2.60Length of weir L (m) 6.00 6.00 6.00 6.00 6.00Headloss, Hle5 � (q/1.84L)(2/3) (m) 0.15 0.23 0.20 0.29 0.38


weir EL � Hle5 (m) 103.82 103.90 103.87 103.96 104.05Velocity head, HV5VV � (qras/

Wp/Hle// 5)2/2g (m) 0.03 0.04 0.03 0.05 0.07EGL at Point 5, EGL5 �

HGL5 � HV5 (m) 103.85 103.94 103.91 104.01 104.12VV


Flow rate per aeration tank inoperation, qras (m3/s) 0.66 1.20 1.00 1.73 2.60

Pass width, Wp (m) 6.0 6.0 6.0 6.0 6.0Tank length, Lt (m) 60.0 60.0 60.0 60.0 60.0Tank bottom elevation, ELtb �

Avg. day WSEL - 6 (m) 97.87 97.87 97.87 97.87 97.87Water depth in tank at design

average flow, Dt (m) 5.95 6.03 6.00 6.09 6.18Number of passes per tank, Np 5 5 5 5 5








Effective length of tank,L � (Lt)(Np) (m) 300.0 300.0 300.0 300.0 300.0

Velocity, V4 (m/s) 0.02 0.03 0.03 0.05 0.07VVCritical depth, yc � ((q2/g/ /Wp2)(0.333) (m) 0.11 0.16 0.14 0.20 0.27Friction headloss through aeration

tank channelManning’s number for concrete channel n 0.013 0.013 0.013 0.013 0.013Hydraulic radius, R � (Dt � Wp)/

(2 � Dt � Wp) (m) 1.99 2.00 2.00 2.01 2.02Headloss, Hlf4ff � (V4VV � n/R// (2/3))2

� L (m) 0.0000 0.0000 0.0000 0.0000 0.0001Fitting headloss through 90º

Fitting headloss coefficient Kbend 1.0 1.0 1.0 1.0Number of bends, Nb 8 8 8 8 8Headloss, Hlb4 � Kbend*

� V4VV 2/2gNb (m) 0.0001 0.0004 0.0003 0.0009 0.0020Velocity head, Hvsd

(see below at Point 3) 0.07 0.10 0.08 0.12 0.16


EGL5 � Hlf4ff � Hvsd (m) 103.92 104.03 103.99 104.13 104.28Velocity head, Hvsd

(see below at Point 3) 0.07 0.10 0.08 0.12 0.16HGL at Point 4, HGL4 �

EGL4 � HV4 (m) 103.85 103.94 103.91 104.01 104.12VV

6. Point 4 to Point 3Headloss over aeration tank influent

sluice gatesSluice gate width, Ws (m) 1.2 1.2 1.2 1.2 1.2Sluice gate height (m) 1.0 1.0 1.0 1.0 1.0Flow per sluice gate, q (m3/s) 0.50 0.80 0.67 1.07 1.60Upstream head over weir,

Du � (select so ZsubZ � q � 0) (m) 0.52 0.73 0.64 0.91 1.24Effective sluice gate width, Ws' � Ws

� (0.1)(2 contractions) (Du) (m) 1.10 1.05 1.07 1.02 0.95Downstream head over weir,

Dd � (q/1.84/Ws') (2/3) (m) 0.39 0.55 0.49 0.69 0.94Free–fall flow, QfreeQ � 1.84 �

Ws'Du(3/2), (m3s) 0.76 1.21 1.01 1.62 2.43Submerged flow, Qsub � QfreeQ

(1 � (Dd/dd Du// )3/2)0.385, (m3/s) 0.50 0.80 0.67 1.07 1.60







Difference, (Qsub � q) (m3/s)(should be zero) 0.00 0.00 0.00 0.00 0.00

Head difference between tank and channel, Hl4 � Du � Dd (m) 0.13 0.18 0.16 0.22 0.30

Velocity head downstream of sluicegate, HVsd � (q/Ws'/Dd// )2/2g, (m) 0.07 0.10 0.08 0.12 0.16Velocity head upstream of sluicegate, HVsu � (q/Ws'/' Du// )2/2g (m) 0.04 0.05 0.05 0.07 0.09Top of sluice gate elevation,

Els � HGL4 � Dd (m) 103.45 103.38 103.42 103.33 103.18HGL upstream of sluice gate,

HGLsu � HGL4 � Hl4 (m) 103.98 104.12 104.06 104.23 104.42EGL upstream of sluice gate,

EGLsu � HGLsu � HVsu (m) 104.02 104.17 104.11 104.30 104.51Friction headloss through influent

channel to tank #3Average length of influentchannel per tank, L3 31.5 31.5 31.5 31.5 31.5

� Np � Wp � 3 tanks1/2 (m)Influent channel width, W3 (m) 4.0 4.0 4.0 4.0 4.0WWManning’s number n for concrete channel n 0.013 0.013 0.013 0.013 0.013Influent channel bottom elevation,

Elb � avg. EGLsu � 3 (m) 101.1 101.1 101.1 101.1 101.1Water depth in influent channel,

h3 � HGLs � Elb (m) 2.87 3.00 2.95 3.12 3.31Hydraulic radius, R � (h3 � w3)/

(2 � h3 � w3) (m) 1.18 1.20 1.19 1.22 1.25Velocity, V3VV � q/w3/h3 (m/s) 0.04 0.07 0.06 0.09 0.12Headloss, Hlf3ff � (V3VV � n/R// (2/3))2

� L3 (m) 0.0000 0.0000 0.0000 0.0000 0.0001Friction headloss through influent

channel to tank #2Flow rate, q2 � 2 � q (m3/s) 1.00 1.60 1.33 2.13 3.20Velocity, V2VV � q/w2/h2 (m/s) 0.09 0.13 0.11 0.17 0.24Headloss, Hlf2ff � (V2VV � n/R// (2/3))2

� L3 (m) 0.0000 0.0001 0.0001 0.0001 0.0002Friction headloss through influent

channel to tank #1Flow rate, q1 � 3 � q (m3/s) 1.00 1.60 2.00 3.20 3.20Velocity, V1 � q/w1/h1 (m/s) 0.09 0.13 0.17 0.26 0.24Headloss, Hlf1ff � (V1 � n/R// (2/3))2

� L3 (m) 0.0000 0.0001 0.0001 0.0003 0.0002










HGLs � Hlf3ff � Hlf2ff � Hlf1 (m) 103.98 104.12 104.06 104.23 104.42ff


Sluice gate headlosscoefficient KgateKK 1.0 1.0 1.0 1.0 1.0

Sluice gate width, W2 (m) 1.80 1.80 1.80 1.80 1.80WWSluice gate height, Hg (m) 1.80 1.80 1.80 1.80 1.80Channel water depth, Dc (m) 2.87 3.00 2.95 3.12 3.31Gate opening depth, Hg or

Dc, whichever is smaller (m) 1.80 1.80 1.80 1.80 1.80Velocity through sluice gate,

V5VV � Q/W2 (m/s) 0.31 0.49 0.62 0.99 0.99WWHeadloss, Hls2 � KgateKK �

V5VV 2/2g (m) 0.0049 0.0124 0.0194 0.0498 0.0498HGL at Point 2, HGL2 �

HGL3 � Hls2 (m) 103.98 104.13 104.08 104.28 104.47


Exit headloss from primary sed. tank effluent pipe to aeration tank influent channelPrimary effluent pipe diameter, Dp (m) 2.00 2.00 2.00 2.00 2.00All PST effluent flow, Q (m3/s) 1.00 1.60 2.00 3.20 3.20Velocity, V1 (m/s) 0.32 0.51 0.64 1.02 1.02Exit headloss coefficient KexitKK 1.0 1.0 1.0 1.0 1.0Exit headloss, hle1 � (V12)/

2g � KexitKK (m) 0.0052 0.0132 0.0207 0.0529 0.0529Friction headloss through PST effluent

pipe section 2Flow per pipe, q (m3/s) 1.00 1.60 2.00 3.20 3.20Pipe diameter, (Dp2) (m) 2.00 2.00 2.00 2.00 2.00Velocity, V12 (m/s) 0.32 0.51 0.64 1.02 1.02Hazen-Williams coefficient, Cp 120.00 120.00 120.00 120.00 120.00Hydraulic radius, Rp2 � (Dp2)/4 (m) 0.50 0.50 0.50 0.50 0.50Length of pipe, Lp2 (m) 50.00 50.00 50.00 50.00 50.00Slope Sp2�[V12/(0.85�Cp�Rp2(0.63)](1/0.54)

(%) 0.0001 0.0001 0.0002 0.0004 0.0004Headloss, hlf2ff � Lp2 � Sp2 (m) 0.0026 0.0061 0.0093 0.0221 0.0221

Friction headloss through PSTeffluent pipe section 1Flow per pipe, q (m3/s) 0.50 0.80 0.67 1.07 1.60Pipe diameter, Dp1 (m) 1.50 1.50 1.50 1.50 1.50Velocity, V11 (m/s) 0.28 0.45 0.38 0.61 0.91Hazen-Williams coefficient, Cp 120.00 120.00 120.00 120.00 120.00Hydraulic radius, Rp1 � (Dp1)/4 (m) 0.38 0.38 0.38 0.38 0.38Length of pipe, Lp1 (m) 50.00 50.00 50.00 50.00 50.00Slope,Sp1�[V11/(0.85�Cp�Rp1(0.63)](1/0.54)

(%) 0.0001 0.0001 0.0001 0.0002 0.0005







PARAMETER Min Day Avg Day Avg Day Max Hour Peak

Pipe entrance headlossKe 0.50 0.50 0.50 0.50 0.50Headloss, hen1 � Ke � V112/2g (m) 0.0020 0.0052 0.0037 0.0094 0.0209

HGL at upstream of PSTeffluent pipe, HGL1 � HGL2 �

hle1 � hlf2ff � hlf1ff � hen1 (m) 103.99 104.16 104.12 104.38 104.59HGL7 of PST must be maximum

of HGL1 (m) 104.59 104.59 104.59 104.59 104.59

Hydraulic design example. A schematic diagram of a typical granular media filter sys-tem is shown in Fig. 22.27. The granular media filters receive the secondary effluent eitherbefore or after chlorination and hydraulic loading conditions are the same as those of thesecondary effluent. A single granular media filter is shown for simplicity. Designhydraulic calculations for the granular media filter system is shown in Table 22.13.The head requirements for the granular media filters are in the range of 2.8–3.2 m(9.3–10.6 ft).

22.4.2.6 Mixing and contact chambers. Process criteria. Physical and chemical waste-water treatment processes involve mixing, coagulation, flocculation, and sedimentation.Chemical coagulation is often used for enhanced treatment in primary sedimentation andfor tertiary treatment after secondary treatment, and before or after filtration. Advantagesof coagulation include greater removal efficiencies of total suspended solids, organicmaterials, phosphorus, and other pollutants. Disadvantages include an increased produc-tion of chemical sludge and an increased operating cost.

Chemical coagulants are mixed with wastewater during rapid mix which is the firststep of the coagulation process. The coagulants destabilize the colloidal particles whichallows their agglomeration. Velocity gradients (G) or a mixing intensity of 300 (m�mm m)/s��are generally sufficient for rapid mix. The rapid mix can be accomplished with mechani-cal mixers, in-line blenders, pumps, or air mixers.

Following the rapid mixing, flocculation takes place through gentle prolonged mixingwhich promotes the destabilized particles to grow and agglomerate. Typical detentiontimes for flocculation range between 20 and 30 minutes. During this period, velocity gra-dients of 50–80 (m�mm m)/s should be maintained. Following flocculation, the settleable��solids are settled in the following sedimentation tank.

Key hydraulic design parameters. The key hydraulic design parameters for mixingand contact chambers include the inlet channel, inlet baffles, mixing equipment, andoutlet channel

Inlet channel. Inlet channels should be designed to maintain velocities high enough toprevent solids deposition and to promote equal distribution of flow if multiple tanks areused.

Inlet baffles. Inlet baffles should be designed to dissipate the energy from the veloci-ties and to prevent short circuiting.




Mixing equipment. A sufficient freeboard should be provided to prevent liquidspillage over the walls due to intense mixing. Provisions for easily removing the mixingequipment for repair and maintenance should be considered. Tank geometry should beconfigured to minimize areas with inadequate mixing.

Outlet channel. Velocity in the outlet channel which leads to the sedimentation tankshould be high enough to prevent solids from settling but not too high to cause breakdownof flocculated solids.

22.4.2.7 Cascade aerators Process criteria. Cascade aeration is a physical unit processtypically used for effluent aeration. The system employs a series of steps or weirs overwhich the effluent is discharged. The system is configured to maximize turbulence inorder to increase oxygen transfer. The head requirements vary depending on the initial dis-solved oxygen (DO) and the desired final DO. If the necessary head is not available, efflu-ent pumping or mechanical aeration is required.

Although cascade aeration is not a new concept, its application to wastewater treatmentis relatively new. Design criteria for an efficient cascade aeration system design include afall height at each step equal to or less than 1.2 m (4 ft); a flow rate equal to or less than235 (m3�h)/m�� [315(gal/min)/ft] of width; and a pool depth after each fall equal to or less than0.28 m(0.9 ft).

Hydraulic design example. A schematic diagram of a typical cascade aeration systemis shown in Figure 22.28. Cascade aerators normally receive the secondary treatmenteffluent and hydraulic loading conditions are the same as those of the secondary treatmenteffluent. Design hydraulic calculations for the cascade aeration system is shown in Table22.14. The head requirements for this example of the cascade aerators is 4.6 m (15.1 ft).

22.4.2.8 Effluent outfall. Process Criteria. The treatment plant accomplishes as muchpollutant removal as required to produce effluent meeting the criteria established by theregulatory agencies. Ultimate disposal of wastewater effluents are by dilution in receivingwaters, by discharge on land, seepage into the ground, or reclamation and reuse. Of these,disposal into the receiving waters is the most common practice. The receiving watersinclude rivers, lakes, estuaries, and oceans.

The outfall size is determined by the velocity, headloss, structural considerations, andthe economics of the situation. Velocities of 0.6–0.9 m/s (2–3 ft/s) at average flow are nor-mally recommended in pipeline design to avoid excessive head loss. If the effluentreceived preliminary treatment, lower velocities can be used. However, velocities higherthan 2.4–3.0 m/s (8–10 ft/s) should be avoided due to excessive headloss.

Key hydraulic design parameters. The key hydraulic design parameters for effluentoutfalls include available head, mixing and dispersion, submerged discharge, and dif-fusers.

Available head. Sufficient head for gravity flow from the point of plant effluent discharge to the receiving stream is not always possible. If sufficient head is not avail-able, effluent pumping is required to prevent flooding of the plant area. Some plantsrequire effluent pumping during storm events or where tidal waves cause salt waterintrusion.

Mixing and dispersion. The outfall should be designed to operate at an adequate veloc-ity to promote rapid dispersion and mixing of the effluent with the receiving stream. Thiswill minimize localized deposits of settleable solids and stratification of the residual organ-ics and nutrients in the localized area, which may cause a DO deficit and algae growth.






Submerged discharge. An effluent discharge pipe terminated at the bank of a streamusually leads to development of foam under low-flow conditions. The problem of foamcan be overcome simply by submerging the pipe discharge below the low-water levelwhen physical conditions in the stream allows such an arrangement.

Diffusers. Certain outfalls, such as an ocean disposal, are typically accomplished bysubmarine outfall that consists of a long section of pipe to transport effluent and a diffusersection to dilute the effluent with the receiving stream. When the effluent water isdischarged from a single- or multiport diffuser, the exit velocity will provide turbulentmixing with the surrounding water.

FIGURE 22.27 Schematic diagram of multimedia filter system. (HWL = .)





TABLE 22.13 Example Hydraulic Calculation of a Typical Multimedia Filter System


PARAMETER Min Day Avg Day Avg Day Max Hour Peak


2. Multimedia filtersTotal number of units 6 6 6 6 6Number of units in operation 4 5 5 6 5Number of units on standby 2 1 1 0 1Flow rate per operating multimedia

filter, q (m3/s) 0.25 0.32 0.40 0.53 0.64

Hydraulic Calculations at Filter Effluent3. At Point 7

Max. HGL in filtered waterstorage tank, HGL7 (m) 98.67 98.67 98.67 98.67 98.67Velocity in storage tank, V7 (m/s) 0.00 0.00 0.00 0.00 0.00VVMax. EGL in storage tank, EGL7 �

HGL7 � V7VV 2/2g (m) 98.67 98.67 98.67 98.67 97.67

4. At Point 6

Filtered water effluent channel weirSharp-crested weir EL, Wel6 �

HGL7 � 0.1 (m) 98.77 98.77 98.77 98.77 97.77Flow rate over weir � Q (m3/s) 1.00 1.60 2.00 3.20 3.20Length of weir (m) 7.00 7.00 7.00 7.00 7.00Headloss, Hlw6 � (q/1.84L)(2/3) (m) 0.18 0.25 0.29 0.40 0.40

HGL at Point 6, HGL6 � Wel6 � Hlw6 (m) 98.95 99.02 99.06 99.17 99.17Velocity in weir box,

V6 (assumeVV V � 0) (m) 0.00 0.00 0.00 0.00 0.00EGL at Point 6, EGL6 �

HGL6 � V6VV 2/2g (m) 98.95 99.02 99.06 99.17 99.17


Loss through effluent concrete conduitFlow rate, Q (m3/s) 1.00 1.60 2.00 3.20 3.20Width of conduit, Wc (m) 3.00 3.00 3.00 3.00 3.00Depth of conduit, Dc (m) 2.00 2.00 2.00 2.00 2.00Length of conduit, Lc (m) 10.00 10.00 10.00 10.00 10.00Velocity, Vc (m/s) 0.17 0.27 0.33 0.53 0.53Hydraulic radius, R � (Wc � Dc/2)

/(Wc � Dc) (m) 0.60 0.60 0.60 0.60 0.60Manning’s n 0.013 0.013 0.013 0.013 0.013Headloss, Hlc5 � (Vc � n/R// (2/3))2

� Lc (m) 0.0001 00002 0.004 0.0009 0.0009Exit loss from pipe to concrete conduit

Effluent pipe diameter, Dp (m) 1.0 1.0 1.0 1.0 1.0Pipe flow (for each filter) (m3/s) 0.25 0.32 0.40 0.53 0.64Velocity, Vp (m/s) 0.32 0.41 0.51 0.68 0.82Hle5 � Vp2/2g for sharp

concrete outlet (m) 0.0052 0.0085 0.0132 0.0236 0.0339EGL at Point 5, EGL5 �

EGL6 � Hlc5 � Hle6 (m) 98.96 99.03 99.07 99.19 99.20








Velocity head at Point 5,HV5VV � Vp2/2g (m) 0.01 0.01 0.01 0.02 0.03


EGL5 � HV5 (m) 98.95 99.02 99.06 99.17 99.17VV


Filter effluent pipe lossPipe diameter, Dp (m) 0.90 0.90 0.90 0.90 0.90Max. flow through filter

effluent pipe � q (m3/s) 0.25 0.32 0.40 0.53 0.64Velocity of flow through

pipe, Vp (m/s) 0.39 0.50 0.63 0.84 1.01Hazen-Williams coefficient, Cp 120 120 120 120 120Hydraulic radius, Rp � Dp/4 (m) 0.23 0.23 0.23 0.23 0.23Length of pipe, Lp (m) 15.00 15.00 15.00 15.00 15.00Slope, Sp�[Vp/(0.85 � Cp � Rp0.63)](1/0.54)

(%) 0.0193 0.0305 0.0461 0.0785 0.1100Head loss, Hlf4ff � Lp � Sp (m) 0.0029 0.0046 0.0069 0.0118 0.0165

Headloss through butterfly valveKvalveKK (fully open) 0.30 0.30 0.30 0.30 0.30Valve diameter (m) 0.90 0.90 0.90 0.90 0.90Headloss, Hval4 � KvalveKK �

(Vp2/2g) (m) 0.0024 0.0039 0.0061 0.0108 0.0155Flow rate controller

Venturi throat-to-inlet ratiofor long tube, KrateKK 1.20 1.20 1.20 1.20 1.20Inlet velocity, Vi � Vp (m/s) 0.32 0.41 0.51 0.68 0.82Headloss, hrate = KrateKK � (Vi2/2g) (m) 0.0062 0.0102 0.0159 0.0283 0.0407

(minimum headloss when controlvalve is fully open)

Pipe entrance lossKent 0.50 0.50 0.50 0.50 0.50Headloss, Hlent � KentKK � (Vp2/2g) (m) 0.0026 0.0042 0.0066 0.0118 0.0170

EGL at Point 4, EGL4 � EGL5 �

Hlf4ff � Hval4 � Hrate � Hlent (m) 98.97 99.05 99.11 99.25 99.29Velocity head, HV4VV � V4VV 2/2g

(assume V � 0) (m) 0.00 0.00 0.00 0.00 0.00HGL at Point 4, HGL4 �

EGL4 � HV4 (m) 98.97 99.05 99.11 99.25 99.29VV


Dirty filter head requirement,Hldf (m) (assumed) 2.5 2.5 2.5 2.5 2.5(consult with filter manufacturer)

Dirty filter EGL, EGLdf �

HGL4 � Hldf (m) 101.47 101.55 101.61 101.75 101.79Velocity head, HV3VV � 0 (m)

(assume V3VV � 0) (m) 0.00 0.00 0.00 0.00 0.00







Dirty filter HGL, HGLdf �

EGLdf � HV3 (m) 101.47 101.55 101.61 101.75 101.79VVClean filter headloss

Filter bed area (m2) 160 160 160 160 160Flow per filter, q (m3/s) 0.25 0.32 0.40 0.53 0.64Filter rate, qfilt m3(min � m2) 0.094 0.120 0.150 0.200 0.240Media depth, Dm (m) 1.00 1.00 1.00 1.00 1.00Effective media size, Md (mm) 0.50 0.50 0.50 0.50 0.50Headloss through filter,Hlf � 2.32 m loss per m3(min � m2)

(consultant with manufacturer) 0.2175 0.2784 0.3480 0.4640 0.5568Entrance headloss through underdrain

flume, Hlu � 0.45 m m3(min � m2) 0.0422 0.0540 0.0675 0.0900 0.1080(consult with filter manufacturer)

Clean filter EGL, EGLcf �

EGL4 � Hlf � Hlu (m) 99.23 99.38 99.52 99.81 99.95Velocity head, HV3VV � 0 (assume

V3VV � 0) (m)Clean filter HGL, HGLcf � EGLcf � HV3 (m)VVEGL required at Point 3, EGL3 � EGLdf (m) 101.47 101.55 101.61 101.75 101.79HGL required at Point 3, HGL3 � HGLdf (m)101.47 101.55 101.61 101.75 101.79

(Head required for dirty filter controls)8. Point 3 to Point 2Filter inlet discharge lossKeffKK 1.0 1.0 1.0 1.0 1.0Flow rate, q (m3/s) 0.25 0.32 0.40 0.53 0.64Pipe diameter, Dp 2 (m) 0.9 0.9 0.9 0.9 0.9Velocity, Vp2 (m/s) 0.39 0.50 0.63 0.84 1.01Headloss, Hld2 � KeffKK � (Vp22/2g) (m) 0.0079 0.0129 0.0202 0.0359 0.0517EGL at Point 2, EGL2 � EGL3 � Hld2 (m) 101.48 101.56 101.63 101.79 101.084Velocity head, HV2VV � Vp22/g/ (m) 0.01 0.01 0.02 0.04 0.05HGL at Point 2, HGL2 � EGL2� HV2 (m) 101.47 101.55 101.61 101.75 101.79VV

9. Point 2 to Point 1Headloss through butterfly valve

KvalKK (fully open) 0.3 0.3 0.3 0.3 0.3Headloss, Hlv1 � KvalKK � (Vp22/2g) 0.0024 0.0039 0.0061 0.0108 1.0155Headloss through inlet pipeLength of pipe, Lp1 (m) 20.0 20.0 20.0 20.0 20.0Hazen-Williams coefficient, Cp 120 120 120 120 120Hydraulic radius, Rp � Dp2/4 (m) 0.23 0.23 0.23 0.23 0.23Headloss, Hlf1ff � (Vp2/(0.85 � Cp

� Rp1.63)(1/0.54) � Lp (m) 0.0039 0.0061 0.0092 0.0157 0.0220Headloss through entrance to pipe

Kent 0.50 0.50 0.50 0.50 0.50Headloss, Hlent � KentKK � Vp2/2g (m) 0.0039 0.0065 0.0101 0.0179 0.0258










EGL2 � Hlv1 � Hlf � Hlent (m) 101.49 101.58 101.65 101.83 101.90Velocity head, HV1 � 0

(assume V1 � 0) (m) 0.00 0.00 0.00 0.00 0.00HGL at Point 1, HGL1 �

EGL1 � HV1 (m) 101.49 101.58 101.65 101.83 101.90

Minimum required controlHGL at Point 1 (m) 101.90 101.90 101.90 101.90 101.90(Max. HGL1 must equal HGL7of final sedimentation tank)

22.4.2.9 Slurry and chemical pumping. Sludge solids. Typical needs for sludge pump-ing involve transporting sludge from primary and secondary clarifiers to and betweenthickening, conditioning, digestion or dewatering facilities, and from biological process-es for recycle or further treatment. Several different types of sludge pumps are used sincevarious types of sludge require a wide range of service conditions.

The flow characteristics (rheology) of wastewater sludges vary widely from process toprocess and from plant to plant. Because rheological properties directly influence pipelinefriction losses of pumped sludges, head loss characteristics of wastewater sludges alsovary extensively. Minimizing pumping distance and applying a conservative multiplier toheadlosses calculated for equivalent flows of water is the traditional approach to thedesign of sludge pumping and piping systems. However, this approach is often inade-quate. As a result of past research of non Newtonian fluid characteristics of sludges,sludge pumping system design data based on specific measured rheological characteris-

FIGURE 22.28 Schematic diagram of cascade aeration system.





tics of sludge and the characteristics on piping systems are now available. These data arepresented in Section 22.5.

Scum. Scum is collected from the surface of primary sedimentation tank or sec-ondary sedimentation tank. Scum from the secondary treatment is more dilute and is usu-ally returned to the head of the treatment plant or thickened prior to combining the thick-ened scum with that from primary treatment. The scum is collected to a scum wet welland pumped to another location for processing. Progressive cavity pumps, pneumatic ejec-tors, and recessed impeller centrifugal pumps are used to pump scum. Key design ele-ments for the scum collection and handling system include sloping the bottom of the scum

TABLE 22.14 Example Hydraulic Calculation of a Typical Cascade Aeration System




2. Cascade aeratorTotal number of units 1 1 1 1 1Flow rate through aerator, Q (m3/s) 1.00 1.60 2.00 3.20 3.20Optimal flow rate per m width over

step, q (m3/s) 0.0653 0.0653 0.0653 0.0653 0.0653DO concentration of postaeration

influent, CO (mg/L) 0.00 0.00 0.00 0.00 0.00Desired DO concentration of postaeration 5.00 5.00 5.00 5.00 5.00

effluent, Cu (mg/L)Calculation of aerator dimensionswith predetermined weir length

3. Weir length, W (m) 5.0 5.0 5.0 5.0 5.0

Flow over weir, q � Q/W, (mWW 3/s) 0.20 0.32 0.40 0.64 0.64Critical depth at upstream step edge,

hc � (q2/g/ )1/3 (m) 0.160 1.219 0.254 0.347 0.347Optimal fall height of nappe, h, 1.2 1.2 1.2 1.2 1.2Length of downstream bubble cushion,

Lo � 0.0629(h0.134)(q0.666) (m) 5.16 7.05 8.18 11.19 11.19Length of downstream receiving

channel, L � 0.8Lo (m) 4.12 5.64 6.54 8.95 8.95Optimal tailwater depth,

H' � 0.236h (m) for h 1.2 m 0.28 0.28 0.28 0.28 0.28Deficit ratio log at 20º C, In(r20)

� 5.39(h1.31)(q�0.363)(H0.31HH ) 0.42 0.36 0.33 0.28 0.28Deficit ratio, r20 1.53 1.43 1.39 1.32 1.32

Calculate concentration of dissolvedoxygen downstream of step. If concentration is less than desireddownstream concentration, add anotherstep and again calculate DO downstreamconcentration. Continue adding stepsuntil the desired DO concentration isachieved.








Select cascade aerator dimensionscorresponding to those

calculated for average flow.

4. Calculation of number of steps to obtain desired DO

Desired DO concentration

at average flow, Cu (mg/L) 5.00

Step 1 effluent DO, C1 � 9.07

(1 � (1/r20)) � CO/r20) (mg/L) 3.14 2.73 2.55 2.21 2.21


(1 � (1/r20)) � C1/r20) (mg/L) 4.81 4.51 4.39 4.14 4.14


(1 � (1/r20)) � C2/r20) (mg/L) 6.01 5.80 5.70 5.52 5.52

In this example, the desired downstream DO

concentration for average flow is achieved

after three steps.

5. Calculation of HGL at each step

Head loss

from filtered water

storage tank to point 1 (m) 1.00 1.00 1.00 1.00 1.00

Cascade fall height, h (m) 1.20 1.20 1.20 1.20 1.20

HGL at Point 1, HGL1 (m) 97.53 97.53 97.53 97.53 97.53

HGL at Point 2, HGL2 � HGL1 � h (m) 96.33 96.33 96.33 96.33 96.33HGL at Point 3, HGL3 � HGL2 � h (m) 95.13 95.13 95.13 95.13 95.13HGL at Point 4, HGL4 � HGL3 � h (m) 93.93 93.93 93.93 93.93 93.93

tank, use of smooth pipe such as glass-lined pipe, providing flushing connections, piggingstations and cleanouts.

Grit slurry. Removal and conveyance of grit from the grit chamber can be accom-plished with varying degrees of success by a number of different methods, includinginclined screw or tubular conveyers, chain and bucket elevators, clamshell buckets, andpumping. Of these methods, pumping of grit from hoppers in the form of slurry offers dis-tinct advantages over other methods but also has some disadvantages. The advantagesinclude small space requirement and flexibility of service by any grit pump from any grittank to any grit handling system with simple valve operation. A disadvantage is frequentmaintenance required for piping and valves due to the abrasive grit. Considerations to begiven in piping design include minimization of bends, providing cleanouts at criticalbends, providing redundant piping at the location of likely clogging, and maintaining avelocity of 1–2 m/s (3–6 ft/s).

Vortex or recessed impeller pumps and air lift pumps normally handle grit slurries.Frequent pumping and applying waterjets or compressed air to loosen the compacted gritin the hopper prior to pumping is a good practice for grit pumping.

Chemical solutions. Chemicals used in municipal treatment plants are received ineither liquid or solid form. The chemicals in solid form generally are converted to solu-





tion or slurry prior to feeding although dry feeding is also practiced. Design of solutionfeed systems mainly depends on liquid volume and viscosity.

Liquid feed units include piston, positive-displacement, and diaphragm pumps, as wellas liquid gravity feeders. The unit best suitable for a particular application depends on therequired head, chemical corrosiveness, application rate, other liquid properties, and thetype of control.

22.5 NON-NEWTONIAN FLOW CONSIDERATIONS

This section addresses pipe transport of mixtures of solids in a liquid media. This is rele-vant to us for the analysis of wastewater sludge transport. When a fluid motion beginswithin a pipe, the velocities of flow at all points along the cross section of the pipe areequal. Over time, velocity gradients are established, beginning at the wall of the pipe dueto the resistance forces developed at the fluid-solid interface. Eventually the velocitygradients extend throughout the cross section of the flow. The velocity gradients resultfrom the relative movement between fluid layers and the resultant shear. Fluids resistshear and, therefore, shear stresses are caused within a fluid in motion in a pipe. For waterand other newtonian fluids, the shear stress is directly proportional to the velocitygradient.

Many suspensions behave in non-newtonian fashion, as plastic fluids. In thin suspen-sions, the suspended particles are not in contact and the suspension will exhibit the new-tonian properties of water. When the concentration becomes sufficiently great to force theparticles into contact with each other, a measurable stress is needed to produce motion.

Experiments by Bingham (1922) and Babbitt and Caldwell (1939) demonstrated thatsewage sludges exhibit both types of flow characteristics depending on the type of solidsand the moisture content. At low solid concentrations, the solid particles are generally notin contact with one another. In this case the presence of the solids has negligible impacton the density and the viscosity of the liquid. As the solids concentration increases, thesuspended particles come into contact with each other and the resultant shearing stressmust be overcome before any movement can start. Under such conditions, the flowassumes plastic characteristics and the headloss varies almost directly with the reductionof moisture M. The headlosses associated with the two types of flow are different. Thedividing point between these two is called the limiting moisture content MLM , which isdefined as the moisture content in percent where a measurable yield stress, SySS , first occurs.As described by Chou (1958), below MLM , the flow is plastic, and, above it, the flow is insuspension only.

Furthermore, it is generally recognized that in sludge flow, as in other fluid flow, thereis a critical velocity and, consequently, the Reynolds number, which divides the flow intolaminar and turbulent stages.

With flow in suspension there is no yield stress value and the Reynolds numbertakes the form of

Re � �ρV

µDVV� (22.2)

where Re � Reynolds number ρ � specific weight, V � velocity, D � pipe diameter, µ� coefficient of viscosity similar to that for water.

In plastic flow the apparent viscosity decreases with the increase in velocity, as dis-cussed by Hatfield (1938) and, in a given range, it may be expressed as

µ� η � �16

3

S

VySS D� (22.3)





where SySS � yield stress η � coefficient of rigidity and the corresponding Reynolds num-ber becomes

R � �ρV

µDVV� � �

3ηv3�

ρ��

D1v6

2

SySS D� (22.4)

Using the Babbitt and Caldwell (1939) recommendation, and taking 2000 as the lowerand 3000 as the upper limits of R, the critical velocities are:

For flow in suspension, where M MLM (flow in suspension);

VLCVV � �20

ρ0D0µ� (22.5)

VUCVV � �30

ρ0D0µ� (22.6)

For plastic flow, where M � MLM (plastic flow):

VLCVV � (22.7)

VUCVV � (22.8)

where VLCVV � lower critical velocity VUCVV � upper critical velocity

The yield stress value SySS , the coefficient of rigidity η, and the specific weight are thebasic variables required in computing critical velocities and headlosses. These propertiesvary from sludge to sludge depending on characteristics such as moisture M, nature of thesuspended particles, temperature, and extent of turbulence. These factors also influenceeach other making it difficult to develop a useful equation for engineering practice. Toresolve this issue, Chou presented an approach using moisture content M as the principalindex of sludge, while placing all other parameters into the general term “origin or kindof sludge,” such as “”primary,” “digested,” and “digested from Imhoff tank,” and so on.

The following development was presented by Chou (1958). The graphical values weretaken from Babbitt and Caldwell (1938) and Keefer (1940).

G related to M. Specific gravity, G, is primarily used in computing specific weightas in ρ � 62.4 G. Specific gravity for activated sludge was shown to be, G � 1.007, at atypical moisture content of about 98 to 99 percent. Primary sludges are more variable, butthe curve in Fig. 22.29 indicates a reasonable mean. In Figure 22.29, digested sludgeshave a cluster of points near G � 1.025, but the curve shows the general tendency. Thethree points from Imhoff tanks are on a smooth curve.

SySS related to M. Yield stress, SySS , has an important role to play in calculating headlossand critical velocities. In Fig. 22.30 the two curves marked with “Imhoff Tank” and “GoodDigestion” were considered to be representative of true conditions. The “Primary” valuesbased on two points are clearly an approximation. The rest of the points varied consider-

1500η � 127 �1�� 4�� 0�� η�� 2�� D�� 2S�� ySS ρ��ρD

1000η � 103 �9�� 4�� η�� 2�� D�� 2S�� ySS ρ��ρD




ably and were designated as the curve of “Poor Digestion” in an attempt to represent theupper limit of the range. The limiting moisture MLM can be determined from Fig. 22.30 asthe point where SySS � 0 cuts the curve.

η related to M. The experimental determination of the coefficient of rigidity indicat-ed its variation with moisture M to be less pronounced than SySS . Accordingly, the plots aremore scattered. The two lines (shown in Fig. 22.31) of “High” and “Mean” aresuggested for design purposes.

Case 1—Suspension/Laminar Stage. For flow in suspension, the solid particles are freeto move past one another and there is consequently no yield value to overcome. Reductionof moisture content only slightly increases the specific weight ρ (ρ � 62.4 G) and the vis-cosity µ. Both remain close to the values for water. The yield stress, SySS , is zero for flow insuspension.

The equation for headloss for laminar stage flow in suspension becomes

�HL

� � �62.4

ηGV

D2� (22.9)

where G � specific gravity

in which both G and η for the corresponding M can be determined from the Figs. 22.29and 22.31.

Case 2—Suspension/Turbulent Stage. Streck (1950) and Winkel (1943) reported theheadloss of turbulent flow in suspension may be computed as follows:

HSH � G2HWHH (22.10)

where HSH � the headloss of flow in suspension with moisture M HWHH � the correspondingheadloss of pure water

G � the specific gravity of the suspension (from Fig. 22.29)

The headloss of flow in suspension for both laminar and turbulent conditions is not sig-nificantly greater than the corresponding headloss for water.

Case 3—Plastic Flow/Laminar Stage. Plastic flow in the laminar stage is the most com-mon case in sludge flow. As discussed above, the headloss is partly due to yield value andpartly due to coefficient of rigidity, both of which are affected by the moisture M. Babbittand Caldwell (1939) reported headloss for this case as follows:

�HL� � �

136ρSDρρ

ySS�� ρ

ηDV

2�� (22.11)

in which the values of ρ, SySS , and η may be determined from Figs. 22.29, 22.30 and 22.31,respectively. For any moisture below the limiting value, plastic flow conditions mean SySS 0 and a headloss occurs due to yield value, SyS , alone. As motion begins,headloss increases with the first power of velocity in the laminar stage. Hence, as soonas the applied head is greater than SySS , relatively little additional head is required toaccelerate the flow to critical velocity. Therefore, it may be concluded that the mosteconomical velocity of sludge flow is the critical velocity, above which the headlossincreases rapidly with the velocity.

Case 4—Plastic Flow/Turbulent Stage. Published data for turbulent plastic flow headlossare variable and inconsistent. Due to variation of sludge characteristics, the velocities, theresults are extremely unpredictable.






For fully turbulent flow, it seems reasonable that the headloss results primarily fromkinetics and is proportional to v2/2g and the specific weight ρ and, therefore, will differfrom that of water only slightly by the effect of ρ. This ideal condition of full turbulencerarely occurs for plastic flows. As the moisture drops below MLM , the critical velocitiesincrease and the thickness of the boundary layers is increased in proportion to moisturereduction. The velocity distribution in a cross section and the impacts of the boundary lay-ers are not the same as the regular patterns of homogeneous liquids. Due to the compli-cated and variable phenomena occurring during turbulent plastic flow, it is difficult, if notimpossible, to accurately anticipate headloss for flow in this condition. Designing for this

FIGURE 22.29 Specific gravity G of sludge (From Chou, 1958)

SPECIFIC GRAVITY G





condition is uncertain and not recommended. However, some experimental data are avail-able for guidance when turbulent plastic flow is unavoidable. Brisbin (1957) compiledheadloss data for raw, thickened sludge. Thus, from such complicated phenomena, uni-form results can hardly be expected.

The corresponding C in the Hazen-Williams formula

V� 1.318Cr0.63 s0.54 (22.12)

where r � hydraulic radius and s � H/HH L// � hydraulic slope

was computed from the observed headlosses. These C ' values are tabulated in Table 22.15along with the ratio to water headloss.

FIGURE 22.30 Yield value of Sy of sewage sludges (From Chou, 1958)

Yield Stress, Sy





22.5.1 Headloss Computation

With the source and M of the sludge known or assumed, the first step is to determine ifthe flow is a suspension or plastic. Empirically this can be done by the curves in Fig.22.30. Values for G, SySS and η are then chosen from curves in Figs. 22.29, 22.30, and 22.31.

Example. Given primary sludge, M � 95. The flow is plastic since M � MLM (MLM � 99.8percent at point in Fig. 22.30 where SySS � 0).

From Figs. 22.29, 22.30 and 22.31,

G � 1.022, � � 1.022 � 62.4 � 63.77 lb/ft2

SySS � 0.065 lb/ft/s

η � 0.0127 (lb�ft)/s

Critical velocities

VLCVV � 12.7 � (22.13)103�0�� .0�� 1�� 5�� 1�� 3�� 4�� .1�� 4�� 5�� D�� 2��

63.77D

FIGURE 22.31 Coefficient of rigidity n of sludge (From Chou, 1958)

PE

RC

EN

TAG

E O

F M

OIS

TU

RE

BY

WE

IGH

T




VUCVV � 19.1 � (22.14)

The values are given in Table 22.16.

Laminar stage

�HL

� � �0.00

D555� � 0.000204 �

DV

2�� (22.15)

The values are tabulated against the pipe diameter D for a range of laminar flow veloc-ities in Table 22.17.

Turbulent stage: Assume C � 100 for M � 100, and from a plot of Table 1 C' values,the corresponding C' � 54.7 for M � 95.

V � 72.09r 0.63s0.54 (22.16)

s � �HL� � �72.0

V91

1VV.

.

8

8

5

5

r1.165� � �CoVn

1VVs

.8

ta

5

nt�

The headlosses are computed in Table 22.18.It is useful to plot results as shown in Figs. 22.32 and 22.33 with critical velocities

indicated. For laminar flow, values are taken from the left of VLCVV , and for turbulent flow,they are taken from right of VLCV . It is also useful to tabulate results as shown in Table22.19, including the minimum headloss to account for SyS as well as the operatingheadloss.

Head losscomputations for solids bearing flows are not an exact science. Where thephysical properties of the sludge cannot be measured, use of the data reproduced here inFigs. 22.29, through 22.31 and the methodology developed by Chou et al. (1958) andsummarized here should provide reasonable results.

127�0�� .0�� 2�� 2�� 6�� 4�� .1�� 4�� 5�� D�� 2��

63.77D


TABLE 22.15 C’ Values for Raw, Thickened Sludge

M C’ Ratio to Water Headloss

Moisture Content (%) Percentage of C at M � 100%

�1C0’0

�

1.85

100 100 100

98 80.5 1.49

97 — —

96 62.8 2.37

95 — —

94 50.5 3.54

91.5 37.6 6.11

90 33.6 7.54





FIGURE 22.32 Results of Headloss computation examples–laminar flow





FIGURE 22.33 Head loss for turbulent flow (m � 95%)





TABLE 22.19 Summary of Results

Pipes Q Headloss, feetL ---------- D gal/m ft3/s ft/s Minimum(1) Operating

16 ft ------- 0 in 2000 4.46 8.20 0.11 1.78

5 ft ------- 8 in 2000 4046 12.80 0.04 1.65

11 ft ------- 14 in 4000 8.91 8.36 0.05 0.86

20 ft ------- 20 in 4000 8.91 4.08 0.08 0.28

20 ft ------- 10 in 600 1.34 2.44 0.13 0.15

50 ft ------- 8 in 500 1.11 3.21 0.42 0.49

30 ft ------- 14 in 1600 3.57 3.34 0.14 0.16

40 ft ------- 20 in 3300 7.35 3.37 0.13 0.14

Minimum is the headloss required to overcome SySS and initiate flow.

TABLE 22.16 Example of Critical Velocities

D 8 in 10 in 14 in 20 in

VLCVV (ft/s) 3.58 3.55 3.45 3.40

VUCVV (ft/s) 4.52 4.42 4.31 4.23

TABLE 22.17 Example Hydraulic Slope for Laminar Stage

D 8 in 10 in 14 in 20 in

�HL�

�f

f

tfftff��

�0.000458v �0.000293v �0.000149v �0.000073v

V � 0, �HL� 0.00833 0.00666 0.00476 0.00333

V � 3, �HL� 0.00970 0.00754 0.00521 0.00355

VLCVV , �HL� 0.00997 0.00770 0.00527 0.00358

Varies (see Table 22.16)

TABLE 22.18 Example Hydraulic Slope for Turbulent Stage

D:8 in D:10 in D:14 in D:20 in

V V1.85VV �3V3

1VV8

.8

.

5

9� �

4V3

1VV9

.8

.

5

6� �

6V566

1VV0

.8

.

5

5� �

9V8

1VV6

.8

.

5

5�

fps

VUC Varies Table 22.16 0.0481 0.0356 0.0229 0.01465 19.64 0.0580 0.0447 0.0302 0.01996 27.51 0.0813 0.0625 0.0423 0.02797 36.60 0.108 0.0832 0.0562 0.03718 46.85 0.138 0.106 0.072 0.04749 58.25 0.172 0.133 0.0896 0.0591

10 70.80 0.209 0.161 0.109 0.0718





REFERENCES

American Society of Civil Engineers, and Water Environment Federation, Gravity Sanitary SewerDesign and Construction, American Society Civil Engineers Manuals and Reports onEngineering Practice No. 60 and Water Environment Federation Manual of Practice No. FD-5,1982.

Babbitt, H. E., and Caldwell, David H., Laminar Flow of Sludges in Pipes with Special Referenceto Sewage Sludge, University of Illinois, Bulletin 319, 1939.

Bingham, E. C., Fluidity and Plasticity, McGraw-Hill, New York, 1922.

Brisbin, S. G., “Flow of Concentrated Raw Sewage Sludges in Pipes,” Proceedings Paper 1274,American Society Civil Engineers 1957.

Bulletin No. 2552 University of Wisconsin.

Bureau of Reclamation, Design Standards No.3, Water Conveyance Systems, Chapter 11 GeneralHydraulic Considerations (Draft), (7-2071) (6-84), Sept. 30, 1992.

Camp, T. R., and Graber, S. D., Dispersion Conduits, Journal of the Sanitary Engineering Division,American Society of Civil Engineer, 94(SA1), February 1968.

Chao, J.–L., and Trussell, R. R., “Hydraulic Design of Flow in Distribution Channels,” Journal ofEnvironmental Engineering Division, ASCE, 6(EE2), April 1980.

Chou, T.–L., “Resistance of Sewage Sludge to Flow in Pipes,” Journal of Sanitary EngineeringDiv., American Society of Civil Engineer, Paper 1780, September 1958.

Committee on Pipeline Planning, Pipeline Division, Pipeline Design for Water and Wastewater,American Society of Civil Engineers, New York, 1975.

Crane Co., “Flow of Fluids Through Valves, Fittings, and Pipe”, Technical Paper No. 410-C, 23rded., Banford, Ontario, 1987.

Daugherty, R. L., and J. B. Franzini, Fluid Mechanics with Engineering Applications, 7th ed.,McGraw-Hill, New York, 1977.

Hatfield, W. D., “Viscosity or Psendo-Plastic Properties of Sewage Sludges,” Sewage WorksJournal, 10, 1938.

Ito, H., and Imani, K., “Energy Losses at 90o Pipe Junctions.” Journal of the Hydraulics Division,American Society of Civil Engineer, HY9, 1973.

Keefer, C. E., Sewage Treatment Works, McGraw-Hill, New York, 1940.

Sanks, R. L., Pumping Station Design, Butterworths, Stoneham, MA, 1989.

Shaw, G. V., and A. W. Loomis, eds., Cameron Hydraulic Data, Ingersoll-Rand Co., CameronPump Division, 14th Ed., 1970.

Simon, A. L., Hydraulics, 3rd ed., John Wiley & Sons, New York, 1986.

Streck, O., Grund und Wasserbrau in Praktischen Biespielen, Springer-Verlag, Berlin, 1950.

Ten-State Standards, Recommended Standards for Sewage Works, Great Lakes–Upper MississippiBoard of Sanitary Engineers, Health Education Service, Inc., Albany, NY, 1978.

Walski, T. M., Analysis of Water Distribution Systems, Krieger, Malabar, FL, 1992.

Williamson, J. V., and Rhone, T. J., ßDividing Flow in Branches and Wyes,” Journal of theHydraulics Division, American Society of Civil Engineer, No. HY5, 1973.

Winkel, R., Angwandte Hydromechanik im Wasserbau, Ernst & Sohn, Berlin, 1943.

Yao, K. M., Hydraulic Control for Flow Distribution, Journal of the Sanitary Engineering Division,American Society of Civil Engineer, 98 (SA2), April 1972





TABLE 22.5 Hydraulic Calculations of a Typical Coagulation Process



1. Plant Flow (mgd) 50 70 75 100

(ft3/s) 77.36 108.31 116.04 154.72

Note: For Points 1 through 8, see Fig. 22.12.

2. WSEL at Point 1 (calculation done in Table 22.6) (ft) 360.1 360.01 360.02 360.04


Average Flow � 21Q/32 (ft3/s) 50.77 71.08 76.15 101.54

Flow depth � WSEL @ 1 � invert (349 ft 9 in) (ft) 10.26 10.27 10.27 10.29

Flow area � 16ft � 10in width � depth (ft2) 172.72 172.89 172.94 173.25

Velocity � flow/area (ft/s) 0.29 0.41 0.44 0.59

R = A/P// (P � w + 2d) (ft) 4.62 4.63 4.63 4.63

Condiut loss � [(V � n )/(1.486 � R2/3)]2� L (ft)

where n � 0.014 and L � 95 ft 0.00 0.00 0.00 0.00

WSEL at Point 2 (ft) 360.01 360.02 360.02 360.04


Average flow � 5Q/16 (ft3/s) 24.18 33.85 36.26 48.35

Flow depth = WSEL @ 2 – invert (349 ft 9 in) (ft) 10.26 10.27 10.27 10.29

Flow area � 16 ft – 10 in width � depth (ft2) 172.72 172.89 172.94 173.25


R = A/P// (P � w � 2d) (ft) 4.62 4.63 4.63 4.63

Condiut loss � [(V � n)/(1.486b � R2/3)] 2� L (ft)

x L where n � 0.014 and L � 48 ft 0.00 0.00 0.00 0.00

WSEL at Point 3 (ft) 360.01 360.02 360.02 360.04


Average flow � Q/8 (ft3/s) 9.67 13.54 14.51 19.34

Flow depth � WSEL @ 3 – invert (349 ft – 9 in) (ft) 10.26 10.27 10.27 10.29

Flow area � 16 – ft 10 in width � depth (ft2) 172.72 172.89 172.94 173.25


R= A/P// (P � w � 2d) (ft) 4.62 4.63 4.63 4.63

APPENDIXWATER AND WASTEWATER

TREATMENT PLANTHYDRAULICS

ENGLISH UNITS EXAMPLES








Condiut Loss � [(V � n)/(1.486 � R2/3)]2 � L (ft)

where n � 0.014 and L � 72 ft 0.00 0.00 0.00 0.00

WSEL at Point 4 (ft) 360.01 360.02 360.02 360.04


Flow � Q/32 (ft3/s) 2.42 3.38 3.63 4.84

Port area � 1 ft deep � 2.5 ft wide (ft2) 2.50 2.50 2.50 2.50


Submerged entrance loss � 0.8 V 2/2g (ft) 0.01 0.02 0.03 0.05

WSEL at Point 5 (in sedimentation tank) (ft) 360.02 360.04 360.05 360.09


Width of sedimentation basin (W) (ft) 76.00 76.00 76.00 76.00WW

Flow (Q/4) (ft3/s) 19.34 27.08 29.01 38.68

Invert elevation of sedimentation baffles (ft) 347.67 347.67 347.67 347.67

Fow depth (H) (WSEL at Point 5 – baffle invert) (ft) 12.35 12.37 12.38 12.42HH

Area downstreams of baffle (W � H) (ftHH 2) 938.77 940.39 940.88 943.83

Horizontal openings in baffles, 1 in

wide, every 9 inches Area of

openings, A � W � H/ 9 (ftHH 2) 104.31 104.49 104.54 104.87

Velocity of downstream baffle (V downstream)

(Q/A) (ft/s) 0.02 0.03 0.03 0.04

Velocity of 1 in opening section

(V1) (Q/A// ) (ft/s) 0.19 0.26 0.28 0.37

Local losses � Sudden expansion

(1.0 � V downstream2/2g) � sudden contraction

(0.36 V12/2g) (ft) 0.00 0.00 0.00 0.00

WSEL at Point 6 (Upstream of

sedimentation baffles) (ft) 360.02 360.04 360.05 360.09


Loss per stage (provided by flocculator

manufacturer) (ft) 0.04 0.04 0.10 0.17

Total loss (three stages) (ft) 0.13 0.13 0.29 0.50

WSEL at Point 7 (ft) 360.15 360.17 360.34 360.59


Flow � Q/24 (ft3/s) 3.22 4.51 4.84 6.45

Port area � 1 in deep � 1 ft – 6 in wide (ft2) 1.50 1.50 1.50 1.50


Entrance loss � 1.25 V 2/2g (ft) 0.09 0.18 0.20 0.36

WSEL at Point 8 (inlet port) (ft) 360.24 360.35 360.54 360.95



Average Flow � Q/24 (ft3/s) 3.22 4.51 4.84 6.45

Flow depth � WSEL @ 8 - invert (358 ft) (ft) 2.24 2.35 2.54 2.95

Flow area � 3 ft width � depth (ft2) 6.73 7.05 7.63 8.84


R = A/P// (P � w � 2d) (ft) 0.90 0.92 0.94 0.99







Condiut loss � [(V � n)/(1.486 � R2/3)]2� L

where n = 0.014 and L � 12 ft – 8in (ft) 0.00 0.00 0.00 0.00

WSEL at Point 9 (ft) 360.24 360.35 360.54 360.95


Average flow � Q/12 (ft3s) 6.45 9.03 9.67 12.89

Flow depth � WSEL @ 9 – invert (358 ft) (ft) 2.24 2.35 2.54 2.95



R = A/P// (P � w � 2d) (ft) 0.90 0.92 0.94 0.99

Condiut loss � [(V � n)/(1.486 � R2/3)]2 � L (ft)

where n � 0.014 and L � 12 ft – 8 in (ft) 0.00 0.00 0.00 0.00

WSEL at Point 10 (ft) 360.24 360.35 360.54 360.95


Flow � Q/8 (ft3/s) 9.67 13.54 14.51 19.34




Loss at two 45o bends � 2 � 0.2 V 2/2g (ft) 0.01 0.02 0.02 0.03

WSEL at Point 11 (ft) 360.26 360.38 360.57 360.98


Flow � Q/4 (ft3/s) 19.34 27.08 29.01 38.68




Loss at two 45o bends � 2 � 0.2 V2/2VV g (ft) 0.02 0.03 0.03 0.04

R � A/P// (P � w � 2d) (ft) 1.19 1.22 1.27 1.36

Condiut Loss � [(V � n)/(1.486 � R2/3)]2 � L (ft)

where n � 0.014 and L � 32 ft 0.01 0.01 0.01 0.01

WSEL at Point 12 (ft) 360.28 360.42 360.61 361.04


Flow � Q/4 (ft3/s) 19.34 27.08 29.01 38.68


Inlet area � 5 ft width � depth (ft2) 11.41 12.09 13.05 15.18


Inlet loss � 1 V 2/2g (ft) 0.04 0.08 0.08 0.10

WSEL at Point 13 (Mixing chamber No. 2 outlet) (ft) 360.33 360.50 360.69 361.14


Note: Mixers provide negligible head loss

Flow � Q/4 (ft3/s) 19.34 27.08 29.01 38.68

Chamber area � 6 ft � 6 ft (ft2) 36.00 36.00 36.00 36.00


Losses � Mixer (1 V 2/2g) � Sharp

bend (1.8 V 2/2g) (ft) 0.01 0.02 0.03 0.05

WSEL at Point 14 (Mixing

Chamber No. 2 inlet) (ft) 360.34 360.52 360.71 361.19











Flow � Q/4 (ft3/s) 38.68 54.15 58.02 77.36

Condiut area � 7.5 ft wide � 4 ft deep (ft3) 30.00 30.00 30.00 30.00


R � A/P// (P � 2w � 2d) (ft) 1.30 1.30 1.30 1.30

Condiut losses � L � [V/(1.318VV �

C � R0.63)]1/0.54 (ft)

where L � 155 ft and Hazen-Williams C � 120 0.02 0.03 0.03 0.06

Local losses � Flow split (0.6 V 2/2g) � contraction

(0.07 V 2/2g) � 0.67 V 2/2g (ft) 0.02 0.03 0.04 0.07

WSEL at Point 15 (at Mixing Chamber No. 1) (ft) 360.37 360.58 360.79 361.31

17. The above calculations (for Points 1 through 15)

have been routed through Tank No. 4

When the flow isrouted through Tank No. 1,

the WSEL (ft) is: 360.31 360.51 360.70 361.17

In reality, the headloss through each basin is

equal. The flow through the basin naturally

adjusts to equalize headlosses, that is flow through

Tank No. 1 is greater than Q/4 and flow

through Tank No. 4 is less than Q/4. The actual

headloss through each basin is the average of

Tank #’s 1 and 4 and the WSEL (ft) at Point 15 is: 360.34 360.54 360.74 361.24


Flow � Q (ft3/s) 77.36 108.31 116.04 152.72

Condiut area � 7.5 ft wide � 4 ft deep (ft2) 30.00 30.00 30.00 30.00


R � A/P// (P � 2w � 2d) (ft) 1.30 1.30 1.30 1.30

Condiut losses � L � [V/(1.318VV �

C � R0.63)]1/0.54 (ft)


WSEL at Point 16 (ft) 360.49 360.82 361.06 361.77


Flow � Q (ft3/s) 77.36 108.31 116.04 154.72

Condiut area @ 16 � 7.5 ft wide � 4 ft deep (ft2) 30.00 30.00 30.00 30.00

Condiut area @ 17 � 5.5 ft wide � 5.5 ft deep (ft2) 30.25 30.25 30.25 30.25

Average area (ft2) 30.13 30.13 30.13 30.13


R @ 16 � A16/ (2 � (7.5 ft � 4 ft) (ft) 1.30 1.30 1.30 1.30

R @ 17 � A17/ (2 � (5.5 ft � 5.5 ft) (ft) 1.38 1.38 1.38 1.38

Average R (ft) 1.34 1.34 1.34 1.34

Condiut Losses � L � [V/(1.318VV �

C � R0.63)]1/0.54 (ft)


WSEL at Point 17 (ft) 360.50 360.84 361.08 361.81







20. Point 17 to Point 18Flow � Q (ft3/s) 77.36 108.31 116.04 154.72Condiut area @ 17 � 5.5 ft wide � 5.5 ft deep (ft2) 30.25 30.25 30.25 30.25Velocity 17 � flow/area 17 (ft/s) 2.56 3.58 3.84 5.11Pipe area @ 18 � 5.5 ft2/4 � � (ft2) 23.76 23.76 23.76 23.76

Velocity 18 � flow/area 18 (ft/s) 3.26 4.56 4.88 6.51

Exit Losses � V182/2g � V172/2g (ft) 0.06 0.12 0.14 0.25

WSEL at Point 18 (ft) 360.56 360.96 361.22 362.06


R � A/P// (P � d � �) (ft) 1.38 1.38 1.38 1.38

Local losses � 3 elbows (3 � 0.25V 2/2g)

� entrance (0.5 � V 2/2g ) � 1.25 � V 2/2g (ft) 0.21 0.40 0.46 0.82

Condiut losses � L � [V/(1.318VV � C � R0.63)]1/0.54

where L � 455 ft and Hazen-Williams C � 120 (ft) 0.24 0.44 0.50 0.85

WSEL at Point 19 (exit of Control Chamber) (ft) 361.00 361.81 362.18 363.74


Weir elevation (ft) 360.00 360.00 360.00 360.00

Depth of flow over weir � (WSEL @

19 - weir elevation) (ft) 1.00 1.81 2.18 3.74

Length of weir, L (ft) 9.00 9.00 9.00 9.00

Flow over weir � q � 3.1 � h 3/2 x [1–(d/h)3/2]0.385 � L

Note: Rather than solve for h, find an h by trial

and error that gives a q equal to the flow

for the given flow scenario (given in Item 1)

First Iteration assume h (ft) � 2 3 3 4

then q (ft3/s) � 66.63 113.72 99.77 90.40

Second Iteration assume h (ft) � 2.17 2.93 3.2 4.5

then q (ft3/s) � 77.11 108.41 116.04 154.28

Note: These q’s equal the flows for the given

scerios (Item 1)

h (ft) 2.17 2.93 3.2 4.5

WSEL at Point 20 (h + WSEL @ Point 19) (ft) 362.17 362.93 363.20 364.50


Flow � Q (ft3/s) 77.36 108.31 116.04 154.72

Sluice gate area � 54 in � 54 in (ft2) 20.25 20.25 20.25 20.25

Velocity = flow/area (ft/s) 3.82 5.35 5.73 7.64

Gate losses � 1.5 � V 2/2g (ft) 0.34 0.67 0.76 1.36

WSEL at Point 21 (Raw Water Control Chamber) (ft) 362.51 363.60 363.96 365.86

The overflow weir in the Raw Water Control Chamber

is 10 ft long and is sharp crested.

Q � 3.3 � L � h3/2 so � h � (Q/3.3/L// )2/3 (ft) 1.76 2.21 2.31 2.80

The water surface must not rise above

elevation 370 ft – 0 in.

The overflow weir elevation may be

safely set at 367 ft – 0 in.





TABLE 22.6 Hydraulic Calculations in a Medium Sized Water Treatment Plant from the FilterEffluent to the Effluent Clearwell



1. Plant flow (mgd) 50 70 75 100(ft3/s) 77.36 108.31 116.04 154.72

Note: for Points 22 through 28, see Fig. 22.15

2. Point 22 to Point 23Maximum water level in clearwell (Point 22) (ft) 345.00 345.00 345.00 345.00Invert in clearwell, (ft) 333.00 333.00 333.00 333.00Flow � Q/2 (ft3/s) 38.68 54.15 58.02 77.36Stop logs @ A

Flow area (2 openings, 5 ft wide, 12 ft deep) (ft2) 120.00 120.00 120.00 120.00Velocity � flow/area (ft/s) 0.64 0.90 0.97 1.29Loss � 0.5 V 2/2g (ft) 0.00 0.01 0.01 0.01

BafflesFlow area (10 ft wide, 12 ft deep) (ft2) 120.00 120.00 120.00 120.00Velocity � flow/area (ft/s) 0.64 0.90 0.97 1.29Loss � 1.0 V2VV /2g (ft) 0.01 0.01 0.01 0.03

Stop logs @ B and CSame as the losses @ A, times 2 (ft) 0.01 0.01 0.01 0.03

WSEL at Point 23 (ft) 345.02 345.03 345.04 345.06

3. Point 23 to Point 24Flow � Q/2 (ft3/s) 38.68 54.15 58.02 77.3666 inch diameter pipeFlow area � d 2/4 � p (ft2) 23.76 23.76 23.76 23.76Velocity � flow/area (ft/s) 1.63 2.28 2.44 3.26Exit loss @ clearwell � V 2/2g (ft) 0.16Loss @ 2 � 90o bends � (0.25 V 2/2g) � 2 (ft) 0.02 0.04 0.05 0.08Entrance loss @ filter building � 0.5 V 2/2g (ft) 0.02 0.04 0.05 0.08Pipe loss � (3.022 � V 1.85 � L)/

(C 1.85 � D1.165) (ft)where C � 120 and L � 190 0.03 0.05 0.06 0.10

WSEL at Point 24 (ft) 345.08 345.16 345.19 345.49

4. Point 24 to Point 25Flow � Q/4 (ft3/s) 19.34 27.08 29.01 38.68Flow area � 5 ft � 5ft (ft2) 25.00 25.00 25.00 25.00Velocity � Q/A// (ft/s) 0.77 1.08 1.16 1.55Loss as flows merge � 1.0 V 2/2g (ft) 0.01 0.02 0.02 0.04Condiut loss � [(V � n)/(1.486 � R2/3)]2 � L (ft)

where n � 0.013, L � 55 ft and R � A/P// (P � 20) 0.00 0.00 0.00 0.01WSEL at Point 25 (ft) 345.10 345.19 345.21 345.54

5. Point 25 to Point 26Sluice Gate No. 1 flow area � 48 in � 36 in (ft2) 12 12 12 12Velocity � Q/A// (ft/s) 1.61 2.26 2.42 3.22Loss � 0.5 V 2/2g (ft) 0.02 0.04 0.05 0.08WSEL at Point 26 (ft) 345.12 345.23 345.26 345.62

6. Point 26 to Point 27Sluice Gate No. 2 Loss � 0.8 V 2/2g (ft) 0.03 0.06 0.07 0.13WSEL at Point 27 (ft) 345.15 345.29 345.33 345.75








7. Point 27 to Point 28Port to filter clearwell. Calculate losses through portas if it were a weir when depth of flow isbelow top of port.Port Dimmensions � 9 ft wide by 2 ft – 8 in feet deepFlow � Q/4 (ft3/s) 19.34 27.08 29.01 38.68Weir (bottom of port) elevation (ft) 344.00 344.00 344.00 344.00Depth of flow over weir � (WSEL @ 27

� weir elevation) (ft) 1.15 1.29 1.33 1.75Flow over submerged weir = q = 3.1 � h3/2 �

[1 � (d/dd h)3/2]0.385 � L

Note: Rather than solve for h, find an hby trial and error that gives a q equal to theflow for the given flow scenario (given in item 1).

assume h (ft) = 1.3 1.4 1.5 2then q (ft3s) = 20.8841 20.2379 25.5883 41.0387assume h (ft) = 1.28 1.49 1.55 1.97then q (ft3/s) = 19.4646 27.0883 29.232 38.485

Note: These q’s equal the flows for the givenscerios (Item 1)

h (feet) 1.28 1.49 1.55 1.97WSEL at Point 28, (ft) 345.28 345.49 345.55 345.97

Filters–See Filter Hydraulics in Table 22.7

Note: For Points 29 thruogh 33, see Fig. 22.16

8. Point 29WSEL above filters (ft) 360.00 360.00 360.00 360.00

9. Point 29 to Point 30Entrance to Filter #4Flow � Q/8 (ft3/s) 9.67 13.54 14.51 19.34Channel VelocityVV � Flow/FF A// rea

(area � 4 ft � 4 ft) (ft/s) 0.60 0.85 0.91 1.21Submerged entrance loss =

0.8 V 2/2g (ft) 0.00 0.01 0.01 0.0248 in Pipe velocity � flow/area

(area � d 2/4 � �) (ft/s) 0.77 1.08 1.15 1.54Butterfly valve loss � 0.25 V 2/2g (ft) 0.00 0.00 0.01 0.01Sudden elargement loss � 0.25 V 2/2g (ft) 0.00 0.00 0.01 0.01WSEL in influent channel (Point 30) (ft) 360.01 360.02 360.02 360.04

10. Point 30 to Point 31Flow depth � WSEL @ 30 � invert (352 ft) (ft) 8.01 8.02 8.02 8.04Flow area � 6 ft width � depth (ft2) 48.05 48.11 48.12 48.22Velocity � flow/area (ft/s) 0.20 0.28 0.30 0.40R � A/P// (P � w � 2d) (ft) 2.18 2.18 2.18 2.18Condiut loss � [(V � n)/(1.486 � R 2/3)] 2 � L

where n � 0.014 and L � 35 ft – 4 in (ft) 0.00 0.00 0.00 0.00WSEL at Point 31 (ft) 360.01 360.02 360.02 360.04









Flow � Q/4 (ft3/s) 19.34 27.08 29.01 38.68Flow depth � WSEL @ 31 � invert (352 ft) (ft) 8.01 8.02 8.02 8.04Flow area � 6 ft width � depth (ft2) 48.06 48.11 48.12 48.22Velocity � flow/area (ft/s) 0.40 0.56 0.60 0.80R � A/P// (P � w � 2d) (ft) 2.18 2.18 2.18 2.18Condiut loss � [(V � n)/(1.486 � R 2/3)] 2 � L (ft)

where n � 0.014 and L � 35ft 4 in 0.00 0.00 0.00 0.00WSEL at Point 32 (ft) 360.01 360.02 360.02 360.04

12. Point 32 to Point 33Flow � 3Q/8 (ft3/s) 29.01 40.61 43.52 58.02Flow depth � WSEL @ 32 � invert (352 ft) (ft) 8.01 8.02 8.02 8.04Flow area 6 – ft width � depth (ft2) 48.06 48.11 48.13 48.22Velocity � flow/area (ft/s) 0.60 0.84 0.90 1.20R � A/P// (P � w � 2d) (ft) 2.18 2.18 2.18 2.18Condiut loss � [(V � n)/(1.486 � R 2/3)] 2 � L (ft)

where n � 0.014 and L � 35 ft – 4 in 0.00 0.00 0.00 0.00WSEL at Point 33 (ft) 360.01 360.02 360.02 360.04

13. Point 33 to Point 1Flow � Q/2 (ft3/s) 38.68 54.15 58.02 77.36Flow depth � WSEL @ 33 � invert (352 ft) (ft) 8.01 8.02 8.02 8.04Flow area � 6 ft width � depth (ft3) 48.06 48.11 48.13 48.23VelocityVV � flow/area (ft/s) 0.80 1.13 1.21 1.60R � A/P// (P � w � 2d) (ft) 2.18 2.18 2.18 2.18Condiut loss � [(V � n)/1.486 � R 2/3)] 2 � L (ft)

where n � 0.014 and L � 36 ft – 4 in 0.00 0.00 0.00 0.00WSEL at Point 1 (ft) 360.01 360.02 360.02 360.04


TABLE 22.7 Example Hydraulic Calculation of a Typical Filter



Plant Flow (mgd) 50 70 75 100Filter loading, gpm/ft2 2 4 6 8Filter area per filter – 7 out of 8 Filters in Operation (ft2) 1240 1240 1240 1240Flow = loading � area

(gal/min) 2480 4960 7440 9920(mgd) 3.57 7.14 10.71 14.29(ft3/s) 5.53 11.05 16.58 22.10

Losses through filter effluent piping (Fig. 22.17)20 in piping (Q):

Pipe velocity � Q/A// (ft/s) 2.53 5.07 7.60 10.13Local losses � Exit (0.5) � butterfly valves (2 � 0.25)

+ 90o Elbows (2 � 0.4) � tee (1.8) � 3.6 V 2/2g (ft) 0.36 1.43 3.23 5.74

R � A/P// � (d 2/4 � �)/(d � �) � d/4 (ft) 0.42 0.42 0.42 0.42dd

Condiut losses � L � [V/(1.318VV � C � R0.63)]1/0.54


20 in piping (Q/2):





TABLE 22.7 (Continued)Initial Operation Design Operation


Pipe velocity � Q/A (ft/s) 1.27 2.53 3.80 5.07

Local losses � butterfly valve (0.25) (ft) 0.01 0.02 0.06 0.10

R � A/P// � (d 2/4 � �)/(d � �) � d/4 (ft) 0.42 0.42 0.42 0.42dd

Condiut losses � L � [V/(1.318VV � C � R 0.63)]1/0.54


24 in piping (Q/2):

Pipe velocity � Q/A, (ft/s) 0.88 1.76 2.64 3.52

Local losses � entrance (1.0) � Tee (1.8)

� 2.8 V 2/2g (ft) 0.03 0.13 0.30 0.54

Filter (clean) and underdrain losses

(obtain from manufacturer) (ft) 0.30 0.50 0.75 1.10

Total losses (effluent pipe and clean filters) (ft) 0.73 2.20 4.57 7.87

Assume that headloss will be allowed to incrase eight ft before the filters are backwashed. A rate controller will

be used to maintain a constant flow through the filter. Determine the ranges of available head over which the rate

controller will operate.

Static head see figure 2.18

WSEL above filters (ft) 360.00 360.00 360.00 360.00

WSEL in filter effluent conduit,

Point 29 (see Example 22–2)

Maximum (ft) 346.50 346.50 346.50 346.50

Minimum (ft) 345.00 345.00 345.00 345.00

Static head � WSEL above filters – WSEL in filter

effluent condiut

Maximum (ft) 15.00 15.00 15.00 15.00

Minimum (ft) 13.50 13.50 13.50 13.50

Available head � static head � 8 ft

Maximum (ft) 7.00 7.00 7.00 7.00

Minimum (ft) 5.50 5.50 5.50 5.50

TABLE 22.8 Example Hydraulic Calculation of a Typical Bar Screen System


Parameter Min Day Avg Day Avg Day Max Hour Max Hour

1. Wastewater flow rate (ft3/s) 35.3 56.5 70.6 113.0 113.0(mgd) 23 37 46 73 73

Bar screensTotal of number of units 3 3 3 3 3Number of units in operation 2 2 2 3 3Number of units in standby 1 1 1 0 1Flow rate per screen in operation, q (ft3/s) 17.1 28.3 35.3 37.7 56.5Width of each bar screen, w (ft) 8.2 8.2 8.2 8.2 8.2

2. At Point 8Pump wetwell HGL at high

water level, HGL7 (ft) 330.05 330.05 330.05 330.05 330.05








(pump starts at EL 330.05 ft andstops at EL 328.08 (ft))Pump well bottom EL (ft) 324.80 324.80 324.80 324.80 324.80

Check for critical depth in barscreen channel: Critical depth ina rectangular channel

Yc � (q2/g/ /w2)(1/3) (ft) 0.52 0.72 0.83 0.87 1.14Bar screen channel depth � 3.61 3.61 3.61 3.61 3.61

pump WW HGL � channel bottom ELl (ft)(water level at pump well controlsupstream hydraulics if normal depthis higher than Yc)

Io bar screen channel depth higher than yc yes yes yes yes yes

3. Point 8 to Point 7Channel bottom EL (ft) 326.44 326.44 326.44 326.44 326.44Depth in channel, y7 (ft) 3.61 3.61 3.61 3.61 3.61Velocity, V7 (ft/s) 0.60 0.95 1.19 1.27 1.91VVExit loss from channel to pump well

Exit loss coefficient KexitKK � 1.0 1.0 1.0 1.0 1.0 1.0Headloss � Kexit K � V7VV 2/2g, Hle7 (ft) 0.01 0.01 0.02 0.03 0.06

HGL at Point 7, HGL7= HGL8 � �le(ft) 330.06 330.07 330.07 330.08 330.11


Length of approach channel, L6 (ft) 23 23 23 23 23Manning's number n for concrete channel 0.013 0.013 0.013 0.013 0.013Channel width, w6 (ft) 8.20 8.20 8.20 8.20 8.20Water depth, h6 (ft) 3.61 3.62 3.63 3.63 3.67Velocity, V6 (fps) 0.60 0.95 1.19 1.26 1.88VVHydraulic radius, R6 �

(h6 � w6)/(2 � h6�w6) (ft) 1.92 1.92 1.93 1.93 1.94Headloss � (V6 xVV n/1.486 � R6(2/3))2

� L6, Hlf6 (ft) 0.00 0.00 0.00 0.00 0.00ff

HGL at Point 6, HGL6 � HGL7 � Hlf6 (ft) 330.06 330.07 330.08 330.08 330.11ff

5. Point 6 to Point 5Calculate headloss through bar screenSpace between bars (ft) 0.06 0.06 0.06 0.06 0.06Bar width (ft) 0.033 0.033 0.033 0.033 0.033

Bar shape factor, bsf 2.42 2.42 2.42 2.42 2.42Cross sectional width of bars, w (ft) 2.93 2.93 2.93 2.93 2.93Clear spacing of bars, b (ft) 5.27 5.27 5.27 5.27 5.27Upstream velocity head, h (ft) 0.0134 0.0342 0.0535 0.0608 0.1369Angle of bar screen withhorizontal, p (degrees) 60 60 60 60 60

Kirschmer’s eq. Hls � bsf �

w/b � 1.33 � h � sin p (ft) 0.02 0.05 0.08 0.09 0.21Allow 6 in head for blinding

by screenings, Ha (ft) 0.5 0.5 0.5 0.5 0.5







HGL upstream of bar screen,HGL5 � HGL6 � Hls�Ha (ft) 330.58 330.62 330.66 330.67 330.82


Length of approach channel, L4 (ft) 22.97 22.97 22.97 22.97 22.97Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013Channel width, w4 (ft) 8.20 8.20 8.20 8.20 8.20Channel bottom elevation (ft) 326.94 326.94 326.94 326.94 326.94Water depth, h4 (ft) 3.64 3.68 3.72 3.74 3.89Channel velocity, V4 (ft/s) 0.59 0.93 1.16 1.23 1.77VVHydraulic radius R4�h4�w4/(2�h4�w4) 1.93 1.94 1.95 1.96 2.00Headloss � (V4VV � n/1.486 � R4(2/3) )2

� L4, Hlf4 (ft) 0.00 0.00 0.00 0.00 0.00ff


HGL5 � Hlf4, ft 330.58 330.62 330.66 330.67 330.83ff

7. Point 4 to Point 3Headloss at sluice gate contraction

KgateK 1.0 1.0 1.0 1.0 1.0Sluice gate width (ft) 3.94 3.94 3.94 3.94 3.94Sluice gate heigth (ft) 2.95 2.95 2.95 2.95 2.95Velocity through sluice gate, Vs (ft/s) 1.23 1.95 2.41 2.56 3.69Sluice gate headloss, Hls �

KgateK � Vs2/2g (ft) 0.02 0.06 0.09 0.10 0.21

HGL at Point 3, HGL3 (ft) 330.60 330.68 330.75 330.75 331.04

8. Point 3 to Point 2Water depth at Point 2, h2 (ft) 3.67 3.74 3.81 3.84 4.10Channel width, w2 (ft) 6.56 6.56 6.56 6.56 6.56Channel velocity, V2 (ft/s) 0.73 1.15 1.41 1.49 2.10VVFitting headloss through a 45° bend,

KbendK � 0.20 0.20 0.20 0.20 0.20Headloss � KbendKK � V2VV 2/2g, Hlb2 (ft) 0.0017 0.0041 0.0062 0.0069 0.0137Friction headloss through channel

Length of approach channel, L2 (ft) 13.12 13.12 13.12 13.12 13.12Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013

Hydraulic radius R2 � h2 � w2/(2 � f 2 � w2) (ft) 1.73 1.75 1.76 1.77 1.82

Headloss � (V � n/1.486 � R2(2/3))2

� L2, Hlf2 (ft) 0.00 0.00 0.00 0.00 0.00ffEntrance loss

KentK � 0.5 0.50 0.50 0.50 0.50 0.50Headloss � KentK � V 22

/2g, Hle2 (ft) 0.0042 0.0103 0.0155 0.0174 0.0342


HGL3 � Hlb2 � Hlf2ff � Hle2 (ft) 330.61 330.69 330.77 330.80 331.09

9. Point 2 to Point 1HGL at Point 1, HGL 1 �

HGL2 (ft) 330.61 330.69 330.77 330.80 331.09Invert EL of inlet sewer, INV1 (ft) 326.44 326.44 326.44 326.44 326.44Crown EL of inlet sewer, CWN1 (ft) 333.50 333.50 333.50 333.50 333.50Surcharge to inlet sewer? No No No No No






TABLE 22.9 Example Hydraulic Calculation of a Typical Vortex Grit Tank System


Parameter Min Day Avg Day Avg Day Max Hour Peak

1. Wastewater flow rate, Q (cfs) 35.3 56.5 70.6 113.0 113.0(mgd) 23 36 46 73 73

2. Vortex grit tankstotal number of units 3 3 3 3 3Number of units in operation 2 2 2 3 2Number of units in standby 1 1 1 0 1Flow rate per vortex grit tank in operation, q (cfs) 17.7 28.3 35.3 37.7 56.5

Control point is located atchannel weir

Hydraulic Calculations Upstream of Control point

3. At Point 8Headloss over sharp-crested weir

Sharp-crested weir EL, weir EL (ft) 347.79 347.79 347.79 347.79 347.79Effluent channel bottom EL (ft) 344.51 344.51 344.51 344.51 344.50Flow rate over weir, q (ft3/s) 17.7 28.3 35.3 37.7 56.5Length of weir, L (ft) 9.84 9.84 9.84 9.84 9.84Head over end contractedweir, He (assumed) 0.67 0.92 1.06 1.11 1.46[q/ 33.3(L–0.2LL He)](2/3) (ft) 0.67 0.92 1.06 1.11 1.46Hle8 – He (must be zero) 0.00 0.00 0.00 0.00 0.00


weir EL � Hle8 (ft) 348.45 348.70 348.85 348.90 349.25

4. Point 8 to Point 7Channel width, w7 (ft) 9.84 9.84 9.84 9.84 9.84Channel bottom EL (ft) 344.49 344.49 344.49 344.49 344.49Water depth, h7 (ft) 3.96 4.21 4.36 4.41 4.46Velocity, V7 (ft/s) 0.45 0.68 0.82 0.87 1.21VVExit headloss from channel to effluent weir

Exit headloss coefficient KexitK � 1.0 1.0 1.0 1.0 1.0 1.0Headloss, Hle7 � Kexit

� V 72/2g (ft) 0.0032 0.0072 0.0105 0.0117 0.0226


HGL8 � Hle7 (ft) 348.46 348.71 348.86 348.91 349.27

5. Point 7 to Point 6Channel width, w6 (ft) 8.20 8.20 8.20 8.20 8.20Channel bottom EL (ft) 344.49 344.49 344.49 344.49 344.49Water depth, h6 (ft) 3.97 4.22 4.37 4.42 4.78Velocity, V6 (ft/s) 0.54 0.82 0.98 1.04 1.44VVFriction headloss through channel

Length of approach channel, L6 (ft) 32.81 32.81 32.81 32.81 32.81Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013Hydraulic radius, R6 � (h6 � w6)/

(2 � h6 � w6) (ft) 2.02 2.08 2.12 2.13 2.21

Headloss � (V6VV � n/R// 6(2/3))2

� L6, Hlf6 (ft) 0.0003 0.0006 0.0009 0.0010 0.0018ff

Fitting headloss through 90° bend

Fitting headloss coefficient KbendK � 1.0 1.0 1.0 1.0 1.0 1.0








Headloss � KbendK � V6VV 2/2g, Hlb6 (ft) 0.0046 0.0103 0.0151 0.0168 0.0322


HGL7 � Hlf6ff � Hlb6 (ft) 348.46 348.72 348.88 348.93 349.31


Headloss through sluice gate

Sluice gate headloss coefficient

KgateK � 1.0 1.0 1.0 1.0 1.0 1.0

Sluice gate width (ft) 4.92 4.92 4.92 4.92 4.92

Sluice gate height (ft) 3.28 3.28 3.28 3.28 3.28

Water depth, h5 (ft) 3.97 4.22 4.37 4.42 4.78

Sluice gate height or h5,

whichever smaller 3.28 3.28 3.28 3.28 3.28

Velocity through sluice gate, V5 (ft/s) 1.09 1.75 2.19 2.33 3.50VV

Headloss, Hls5 � KgateK � V52/2g (ft) 0.0186 0.0475 0.0743 0.0845 0.1902


HGL6 � Hls5 (ft) 348.48 348.77 348.95 349.01 349.50


Channel width, w4 (ft) 8.20 8.20 8.20 8.20 8.20

Bottom of channel EL (ft) 345.14 345.14 345.14 345.14 345.14

Water depth, h4 (ft) 3.34 3.62 3.81 3.87 4.35

Channel velocity, V4 (ft/s) 0.65 0.95 1.13 1.19 1.58VV

Fitting headloss through a 90° bend

Fitting headloss coefficient KbendKK � 1.0 1.0 1.0 1.0 1.0 1.0

Headloss, Hlb4 � Kbend � V4VV 2/2g (ft) 0.0065 0.0140 0.0199 0.0219 0.0389

Friction headloss through channel

Length of approach channel, L4, (ft) 32.81 32.81 32.81 32.81 32.81

Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013

Hydraulic radius R4 � h4 � w4/

(2 � h4 � w4) (ft) 1.84 1.92 1.97 1.99 2.11

Headloss, Hlf4ff � (V4VV � n/1.486 � R4(2/3))2

� L4 (ft) 0.0005 0.0009 0.0013 0.0014 0.0023


HGL5 � Hlb4 � Hlf4 (ft) 348.49 348.78 348.97 349.04 349.54ff


Headloss across vortex grit tank, H1tank (ft) 0.20 0.20 0.20 0.20 0.20

(per manufacturer recommendations)

HGL at Point 3, HGL3 � HGL4 � H1tank (ft) 348.68 348.98 349.17 349.23 349.73


Channel width, w2, (ft) 6.56 6.56 6.56 6.56 6.56

Bottom of channel EL (ft) 346.46 346.46 346.46 346.46 346.46

Water depth, h2 (ft) 2.23 2.52 2.71 2.78 3.28

Channel velocity, V2 (ft/s) 1.21 1.71 1.98 2.07 2.63VV

Friction headloss through channelLength of approach channel, L2 (ft) 45.93 45.93 45.93 45.93 45.93Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013Hydraulic radius R2 � h2R4(2/3))2

w2/(2*h2 � w2) (ft) 1.33 1.43 1.48 1.50 1.64




TABLE 22.10 Example Hydraulic Calculation of a Typical Primary Sedimentation Tank SystemInitial Operation Design Operation


1. Wastewater flow rate, Q (ft3/s) 35.3 56.5 70.6 113.0 113.0(mgd) 23 37 46 73 73

2. Primary sedimentation tanks (PSTs)Total number of units 3 3 3 3 3Number of units in operation 2 2 3 3 2Number of units on standby 1 1 0 0 1Flow rate per PTS in operation, q (ft3/s) 17.7 28.3 23.5 37.7 56.5




Headloss, Hlf2ff � (V2*VV n/1.486 � R(2/3))2

� L2 (ft) 0.0035 0.0064 0.0082 0.0087 0.0125Headloss through sluice gate

Sluice gate headloss coefficient KgateK � 1.0 1.0 1..0 1.0 1.0 1.0

Sluice gate width (ft) 4.9 4.9 4.9 4.9 4.9Sluice gate height (ft) 3.3 3.3 3.3 3.3 3.3

Water depth, h2 (ft) 2.23 2.52 2.71 2.78 3.28Sluice gate height or h2, whichever smaller (ft) 2.23 2.52 2.71 2.78 3.28Velocity through sluice gate (ft/s) 1.61 2.28 2.65 2.76 3.50Headloss, Hls2 � KgateK � V2VV 2/2g (ft) 0.0403 0.0804 0.1087 0.1181 0.1905


HGL3 � Hlf2ff � Hls2 (ft) 348.73 349.07 349.29 349.36 349.94

10. Point 2 to Point 1Channel width, w1 (ft) 6.56 6.56 6.56 6.56 6.56Bottom of channel EL (ft) 346.62 346.62 346.62 346.62 346.62Water depth, h1 (ft) 2.11 2.44 2.67 2.74 3.32Channel velocity, V1 (ft/s) 1.28 1.76 2.02 2.10 2.60Fitting headloss through a 90° deg bendFitting headloss coefficient KbendKK � 1.0 1.0 1.0 1.0 1.0 1.0Headloss, Hlb1 � KbendKK � V12/2g (ft) 0.0253 0.0482 0.0633 0.0683 0.1046

Friction headloss through channelLength of approach channel, L1 (ft) 16.40 16.40 16.40 16.40 16.40Manning’s n for concrete channel 0.013 0.013 0.013 0.013 0.013Hydraulic radius R1 �

h1 � w1/(2 � h1 � w1) (ft) 1.28 1.40 1.47 1.49 1.65Headloss, Hlf1ff � (V1 � n/1.486 � R1(2/3))2

� L1 (ft) 0.0015 0.0025 0.0031 0.0032 0.0043

(Influent channel may be aerated usingdiffused air to prevent solids settlingor odor problem)


HGL2 � Hlb1 � Hlf1 (ft) 348.75 349.12 349.35 349.43 350.05ff









Control points are located at Points 5 and 6so that back up from down stream doesnot flood effluent channel or overflow weir.

Hydraulic Calculations beginningat Point 7

3. At Point 7

HGL7 must be equal to HGL1

of aeration tank (ft) 342.73 342.73 342.73 342.73 342.73

4. At Point 6

Allowance of 0.33 ft from HGL at

pipe entrance to bottom 0.33 0.33 0.33 0.33 0.33

of PST effluent trough at

discharge end (ft)

Elevation of PTS trough bottom

at discharge end, 343.06 343.06 343.06 343.06 343.06

EL dcb (ft)

Calculation of water depth in

PST effluent trough

Tank diameter, Dt (ft) 147.6 147.6 147.6 147.6 147.6

Number of channels per tank nc 2 2 2 2 2

Total flow through tank, q (ft3/s) 17.66 28.25 23.54 37.67 56.50

Flow per channel, qc � q/nc (ft3/s) 8.83 14.13 11.77 18.83 28.25

Channel slope, Sc (selected to

prevent solids setting) 0.20 0.20 0.20 0.20 0.20

Channel width, w6 (ft) 3.28 3.28 3.28 3.28 3.28

Channel length, Lc � 3.14 �

(Dt-(w6/2))/nc (ft) 229.23 229.23 229.23 229.23 229.23

Change in channel EL, ELdif � Sc � Lc (ft) 0.46 0.46 0.46 0.46 0.46

Critical depth, yc � (qc2/(g � w62))0.33 (ft) 0.62 0.84 0.75 1.02 1.33

Water depth at upstream end of channel, yu 0.69 1.07 0.91 1.38 1.92

� [2 � (yc)2 � (yc � (S � L/3)LL 2]0.5

� (2 � S � L/3) (ft)LL

Channel bottom El at upstream

end of trough, 343.52 343.52 343.52 343.52 343.52

ELucb � ELdcb � ELdif (ft)

HGL at trough downstream,

HGL6d � ELdcb � yc (ft) 343.68 343.91 343.81 344.08 344.39

HGL at trough upstream,

HGL6u � ELucb � yu (ft) 344.21 344.59 344.43 344.90 345.44


Allowance to Weir from

high trough HGL (ft) 0.33 0.33 0.33 .033 0.33







Weir elevation, Elwe, max.

HGL6u � allowance (ft) 345.77 345.77 345.77 345.77 345.77

Headloss over V–notch weirs

Number of weirs per tank, Nw 1 1 1 1 1


Weir length, Lw � (Dt) � 3.14 (ft) 463.58 463.58 463.58 463.58 463.58

Hydraulic load, So � q/Lw// ,(ft3/s/s/ft) 0.0381 0.0609 0.0508 0.0813 0.1219

Weir angle, A (°) 90.00 90.00 90.00 90.00 90.00

V–notch height, Vh (ft) 0.33 0.33 0.33 0.33 0.33

V–notch width, Vw � 2 �

(TAN(A(( /2)) � Vh (ft) 0.66 0.66 0.66 0.66 0.66

Space between notches, Esv (ft) 0.10 0.10 0.10 0.10 0.10

Number of notches per

weir, nv � Lw/(Ew � Esv) 614 614 614 614 614

Flow per notch, Qcw � q/nv 0.0288 0.0460 0.0383 0.0614 0.0920

Weir coefficient for 90° notch, Cw 2.43 2.43 2.43 2.43 2.43

Water depth over the weir, hle5 0.17 0.20 0.19 0.23 0.27

� (Qcw/Cw)(1/2.48) (ft)

hle5 � Vh? (if not, need to

readjust calculations) Yes Yes Yes Yes Yes


ELwe � hle5, (ft) 345.93 345.97 345.95 345.99 346.03


Headloss through primary

sedimentation tanks

Number of tanks, Nt 2 2 3 3 2

Flow per tank, q (ft3/s) 17.66 28.25 23.54 37.67 56.50


Side water depth, Dsw (ft) 14.11 14.11 14.11 14.11 14.11

Tank bottom elevation, ELt � HGL5

� Dsw (ft) 331.85 331.85 331.85 331.85 331.85

Tank floor slope, St (%) 8.33 8.33 8.33 8.33 8.33

Minimum floor tank elevation, Eltf 325.70 325.70 325.70 325.70 325.70

� 0.0833 � (Dt/2)tt � ELt (ft)

Headloss through tank, hlt4 (ft) 0.16 0.16 0.16 0.16 0.16tt

(Available from equipment

manufacturer)


HGL5 � hlt4 (ft) 346.10 346.13 346.12 346.16 346.20tt

7. Point 4 to Point3

Headloss through PST influent pier

Pier diameter, Dp � 42 in 3.51 3.51 3.51 3.51 3.51

Pier length, Lp (ft) 21.33 21.33 21.33 21.33 21.33

Velocity, V3VV � Qt/(3.14tt � (Dp/2)2) (ft/s) 1.83 2.92 2.43 3.89 5.84

Hazen-Williams coefficient, Cp 120 120 120 120 120

Hydraulic radius, Rp � Dp/4 (ft) 0.88 0.88 0.88 0.88 0.88








Slope, Sp � [V3/(1.318VV � Cp �

Rp(0.63))](1/0.54) (%) 0.03 0.07 0.05 0.12 0.26

Headloss, Hlf3ff � Lp � Sp (ft) 0.0064 0.0153 0.0109 0.0261 0.0552

Exit headloss from pier

Exit headloss coefficient KexitK � 1.0 1 1 1 1 1

Headloss, hle3 � K � V3VV 2/2g (ft) 0.0517 0.1324 0.0920 0.2354 0.5297


HGL4 � Hlf3ff � hle3 (ft) 346.16 346.28 346.22 346.42 346.78


Total number of pipes 3 3 3 3 3

Number of pipes per primary

sedimentation tank 1 1 1 1 1

Pipe diameter, Dp (ft) 3.94 3.94 3.94 3.94 3.94

Flow per pipe, q (cfs) 17.66 28.25 23.54 37.67 56.50

Velocity, V2 1.45 2.32 1.93 3.10 4.64VV

Friction headloss through primary

sedimentation tank influent pipe

Hazen-Williams coefficient, Cp 120 120 120 120 120

Hydraulic radius, Rp � Dp/4 (ft) 0.98 0.98 0.98 0.98 0.98

Length of pipe, Lp (ft) 229.7 229.7 229.7 229.7 229.7

Slope, Sp � [V2/(1.318VV � Cp �

Rp (0.63))](1/0.54) (%) 0.02 0.04 0.03 0.07 0.15

Headloss, hlf2ff � Lp � Sp (ft) 0.0395 0.0942 0.0672 0.1605 0.3402

Fitting headloss through two 45° bends

Fitting headloss coefficient KbendKK � 0.5 0.05 0.05 0.05 0.05 0.05

Headloss, hlb2 � K � V2VV 2/2g (ft) 0.0164 0.0419 0.0291 0.0744 0.1674


HGL3 � hlb2 � hlf2 (ft) 346.21 346.42 346.32 346.65 347.29ff

9. At Point 1Entrance headloss from primary

sedimentation tank influentdistribution box to influent pipe

Pipe diameter, Dp (ft) 3.94 3.94 3.94 3.94 3.94Flow per pipe, q (ft3/s) 17.66 28.25 23.54 37.67 56.50Velocity, V1 (ft3/s) 1.45 2.32 1.93 3.10 4.64Entrance headloss coefficient

KentranceK � 0.5 0.50 0.50 0.50 0.50 0.50Headloss, Hle1 � KentranceK � V12/2g (ft) 0.0164 0.0419 0.0291 0.0744 0.1674

HGL at point 1, HGL1 � HGL2 � Hle1 (ft) 346.23 346.46 346.35 346.73 347.46

Allowance to grit tank effluent weirfrom maximum 0.33 0.33 0.33 0.33 0.33

HGL1, Hall (ft)Grit tank effluent elevation,ELgr � HGL1 � HallH (ft) 347.79 347.79 347.79 347.79 347.79






TABLE 22.11 Example Hydraulic Calculation of a Typical Final Sedimentation Tank



1. Wastewater flow rate, Q (ft3/s) 35.31 56.50 70.63 113.01 113.01(mdg) 23 37 46 73 73

RAS flow, % of average day flow 20 50 50 100 100RAS flow, Qras � Q � RAS flow/100 (ft3/s) 11.30 28.25 35.31 70.63 70.63Final sedimentation tank

influent flow, Qin (ft3/s) 46.62 84.76 105.94 183.64 183.64

Final sedimentation tank effluentflow, Qeff (ft3/s) 35.31 56.50 70.63 113.01 113.01

Final sedimentation tanksTotal number of units 4 4 4 4 4Number of units in operation 3 3 3 4 3Number of units on standby 1 1 1 0 1Tank width (ft) 52 52 52 52 52Influent per operating tank, qin (ft3/s) 15.54 28.25 35.31 45.91 61.21Effluent per operating tank, qeff (ft3/s) 11.77 18.83 23.54 28.25 37.67

2. Select control Point at Point 3(where effluent wiers are located)

Hydraulic calculations downstreamof control point

At Point 3V-notch weir

Number per tank, Nw 20 20 20 20 20Individual weir length, Lw (ft) 23.0 23.0 23.0 23.0 23.0Total weir length, Lwt � Lw � Nw (ft) 459.3 459.3 459.3 459.3 459.3Weir angle, A° 90.0 90.0 90.0 90.0 90.0V-notch height, Vh (ft) 0.33 0.33 0.33 0.33 0.33V-notch width, Vw � 2 �

(TAN(A(( /2)) � Vh (ft) 0.66 0.66 0.66 0.66 0.66Space between notches, Esv (ft) 0.10 0.10 0.10 0.10 0.10Total number of notches per

tank, nv � Lwt/(tt Vw � Esv) 608 608 608 608 608Flow per notch, Qcw � qeff /nv 0.0194 0.0310 0.0387 0.0465 0.0620Weir coefficient for 90° notch, Cw 2.43 2.43 2.43 2.43 2.43Water depth over the weir,

hle3 � (Qcw/Cw)(1/2.48) (ft) 0.14 0.17 0.19 0.20 0.23hle3 � Vh? (if not, need

to readjust calculations) Yes Yes Yes Yes YesWeir EL (ft) (select weir

elevation so that HGL1 339.16 339.16 339.16 339.16 339.16equals aeration tank’s HGL6)

HGL at Point 3, EGL3 �

Weir EL � hle3 (ft) 339.30 339.33 339.35 339.36 339.38Velocity head, HV � 0

(assume V3VV � 0) (ft) 0.00 0.00 0.00 0.00 0.00HGL at point 3, HGL3 �

weir EL � hle3 (ft) 339.30 339.33 339.35 339.36 339.38






3. Point 3 to Point 4Effluent troughs

Number of troughs, nt 10 10 10 10 10Flow per trough, qt � qeff /nt 1.18 1.88 2.35 2.83 3.77Trough slope, St (%) (select to

prevent solids settling) 0.20 0.20 0.20 0.20 0.20Trough width, w6 (ft) 1.6 1.6 1.6 1.6 1.6

Approximate trough length, Lt (ft) 23.0 23.0 23.0 23.0 23.0Change in trough EL, difEL4 � St � Lt (ft) 0.05 0.05 0.05 0.05 0.05Critical depth, yc � (qt 2/(gw62)0.33 (ft) 0.26 0.35 0.41 0.46 0.56Water depth at upstream end of trough

for free fall 0.41 0.57 0.67 0.76 0.93from trough into final effluent channel

yu4 � [2 � (yc)2 � (yc-(S � L/3))LL 2]0.5

� (2 � S � L/3) (ft)LLMax water EL downstream of weir (occuring

at max, hour flow with one 338.83tank out of service),

Elmax4 � weir EL-0.33 ft(see Point 3 for weirEL)

Trough bottom EL at upstreamend of trough, TbuEL4 ft 337.90 337.90 337.90 337.90 337.90

TbuEL4 � ELmax4 � yu for max hourflow with one tank out of service

HGL at upstream end, HGL4u � TbuEL4� yu4 (ft) 338.31 338.47 338.57 338.66 338.83

Velocity head, HV4VV u � 0(assume V � 0) (ft) 0.00 0.00 0.00 0.00 0.00

EGL at upstream end,EGL4u � HGL4u � HV4VV u (ft) 338.31 338.47 338.57 338.66 338.83

Trough bottom EL atdownstream end of trough 337.86 337.86 337.86 337.86 337.86

Tbd EL4 � TbuEL4 � dif EL4 (ft)

HGL at Point 4, HGL4 � TbdEL4 � yc (ft) 338.12 338.21 338.27 338.32 338.41Velocity head, HV4VV d = Vc2/2 2g (ft) 0.39 0.54 0.63 0.71 0.87EGL at upstream end, EGL4u �

HGL4u � HV4VV u, ft 338.51 338.75 338.90 339.03 339.28

4. Point 4 to Point 5Effluent channel upstreamMax. water surface level at upstream end of

effluent channel,ELmax5 = TbdEL4-0.33 (ft) 337.53 337.53 337.53 337.53 337.53

HGL at Point 5, HGL5 � ELmax5 (ft) 337.53 337.53 337.53 337.53 337.53Velocity head, HV5VV � 0 (assume V � 0) (ft) 0.00 0.00 0.00 0.00 0.00EGL maximum at point 5,

EGL5m � HGL5m � HV5 (ft) 337.53 337.53 337.53 337.53 337.53VV

5. Point 5 to Point 6Effluent channel downstream

Flow through channel, Qeff (ft3/s) 35.31 56.50 70.63 113.01 113.01







Channel slope, Sc (%) (selectto prevent solids settling) 0.20 0.20 0.20 0.20 0.20

Channel width, w6 (ft) 9.8 9.8 9.8 9.8 9.8Approximate channel length, Lch (ft) 210.0 210.0 210.0 210.0 210.0

Change in channel EL, difEL6 � Sc � Lch (ft) 0.42 0.42 0.42 0.42 0.42Critical depth, yc � (q2/(gw62)0.33 (ft) 0.75 1.02 1.18 1.61 1.61Water depth at upstream end of channel, 0.94 1.41 1.69 2.43 2.43

yu6 � [2 � (yc)2 � (yc � (S � L/3))2]0.5

� (2 � S � L/3), ftChannel bottom EL at upstreamend of channel, 335.10 335.10 335.10 335.10 335.10cbuEL6 � HGL5- maximum yu6 (ft)

HGL at upstream end of channel,HGL5 � cbuEL6 � yu6 (ft) 336.04 336.51 336.79 337.53 337.53

Velocity head, HV5VV � 0(assume V � 0) (ft) 0.00 0.00 0.00 0.00 0.00

EGL at upstream end of channel,EGL5 � HGL5 � HV5 (ft) 336.04 336.51 336.79 337.53 337.53VV

Channel bottom EL at downstream endof channel, 334.68 334.68 334.68 334.68 334.68cbdEL6 � cbuEL6 � difEL6 (ft)

HGL at Point 6, HGL6 � cbdEL6 � yc (ft) 335.42 335.70 335.86 336.29 336.29Velocity head, HV6VV � Vc2/2g (ft) 1.17 1.62 1.88 2.59 2.59EGL at Point 6, EGL6 � HGL6 � HV6 (ft) 336.60 337.31 337.74 338.88 338.88VV

6. At Point 7Max. water EL downstream of

channel end free-fall 334.35 334.35 334.35 334.35 334.35HGL at Point 7, HGL7 � cbdEL6 �

0.33 (ft) (This must be the same asmaximum elevation at Point 1 of

multi-media filter.)

Hydraulic calculations upstreamof control point

7. At Point 2Final sedimentation tanks (Gould type)

Number of tanks in operation, nt 3 3 3 4 3Flow per tank upstream of sludge

collection, qin (ft3/s) 15.54 28.25 35.31 45.91 61.21Tank width, Wt (ft) 52.5 52.5 52.5 52.5 52.5Tank length, Lt (ft) 393.7 393.7 393.7 393.7 393.7Tank bottom elevation at influent end (ft) 325.4 325.4 325.4 325.4 325.4Side water depth (ft) 13.92 13.95 13.97 13.98 14.01Assume friction losses, Hlf2,ff

through tank are negligible 0.0 0.0 0.0 0.0 0.0

EGL at Point 2, EGL2 � EGL3 � Hlf2 (ft) 339.30 339.33 339.35 333.36 339.38ffVelocity head, HV2VV � 0 (assume V � 0) (ft) 0.00 0.00 0.00 0.00 0.00HGL at Point 2, HGL2 � EGL3 � HV2 (ft) 339.30 339.33 339.35 333.36 339.38VV









Tank influent sluice gatesHeight (ft) 3.3 3.3 3.3 3.3 3.3Width, Ws (ft) 3.3 3.3 3.3 3.3 3.3Area (ft2) 10.8 10.8 10.8 10.8 10.8Number of sluice gates per tank, Nsg 4 4 4 4 4Flow per sluice gate,

qsg � qin /Nsg// , (ft3/s) 3.88 7.06 8.83 11.48 15.30Upstream head over weir,

Du � (select soQsub � qsg � 0) (ft) 0.67 1.02 1.19 1.43 1.76

Downstream head over weir,Dd � (qsg/3.33/Ws')(2/3) (ft) 0.51 0.77 0.90 1.09 1.33

Effective sluice gate width, Ws' � 3.2 3.1 3.1 3.0 3.0Ws � (0.1)(2 contractions)(Dd) (ft)

Free fall flow, Qfree � 3.34 �

Ws' � Du(3/2) (ft3/s) 5.89 10.70 13.38 17.39 23.18Submerged flow, Qsub � Qfree

(1 � (Dd/dd Du// )3/2)0.385 (ft3/s) 3.89 7.07 8.83 11.48 15.30Difference, (Qsub � qsg), ft3/s 0.00 0.00 0.00 0.00 0.00Head difference between tank

and channel, Hl 1 � Du � Dd (ft) 0.163 0.246 0.287 0.347 0.426Top of sluice gate set elevation,

Els � HGL2 � Dd (ft) 338.79 338.56 338.45 338.27 338.05339.46

HGL at Point 1 (upstream of sluice gate),HGL1 � HGL2 � Hll (ft) 339.46 339.58 339.63 339.71 339.81

Velocity head, HV1=0 (assume V � 0) (m) 0.00 0.00 0.00 0.00 0.00EGL at point 1, EGL1 � � HV1 (m) 339.46 339.58 339.63 339.71 339.81

Maximum HGL1 (ft) 339.81Max HGL1 should equal HGL6 for aeration tank.

TABLE 22.12 Example Hydraulic Calculation of a Typical Aereation Tank System




RAS flow, % of average flow(added downstream of 20 50 50 100 100

aeration tank influent sluice gates)RAS flow, Qras � Q � RAS flow/100 (ft3/s) 11.30 28.25 35.31 70.63 70.63

2. Aeration tanksTotal of nunber of units 3 3 3 3 3Number of units in operation 2 2 3 3 2Number of units on standby 1 1 0 0 1







Flow rate per aeration tank in operation, q (ft3/s) 17.66 28.25 23.54 37.67 56.50

Flow rate per aeration tank inoperation including RAS 23.31 42.38 35.31 61.21 91.82

flow (downstream of influentsluice gate), qras, (ft3/s)

Control point is located at Point 5(aeration tank effluent weir).

3. At Point 6Set maximum HGL6 � effluent

weir elevation�0.33 (ft) 339.79 339.79 339.79 337.79 339.79

Hydraulic calculations upstreamof control point

4. Point 6 to Point 5Headloss over sharp-crested weir

Sharp-crested weir EL (ft) 340.12 340.12 340.12 340.12 340.12Effluent channel bottom EL (ft) 330.28 330.28 330.28 330.28 330.28Flow rate over weir, qras (ft3/s) 23.31 42.38 35.31 61.21 91.82Length of weir (ft) 19.69 19.69 19.69 19.69 19.69headloss, Hle5 �

(q/3.33L)(2/3) (ft) 0.50 0.75 0.66 0.95 1.25


weir EL � Hle5 (ft) 340.63 340.87 340.79 341.08 341.37Velocity head, HV5VV �

(gras/Wp/Hle// 5)2/2g (ft) 0.09 0.13 0.11 0.17 0.22EGL at Point 5, EGL5 �

HGL5 � HV5 (ft) 340.71 341.00 340.90 341.24 341.59VV

5. Point 5 to Point 4Flow rate per aeration tank in operation, qras (ft3/s) 23.31 42.38 35.31 61.21 91.82Pass width, Wp (ft) 19.7 19.7 19.7 19.7 19.7Tank length, Lt (ft) 196.9 196.9 196.9 196.9 196.9Tank bottom elevation, ELtb �

avg. day WSEL � 19.69 (ft) 320.94 320.94 320.94 320.94 320.94Water depth in tank at design

average flow, Dt (ft) 19.69 19.94 19.85 20.14 20.44Number of passes per tank, Np 5 5 5 5 5Effective length of tank, L � Lt � Np (ft) 984.3 984.3 984.3 984.3 984.3Velocity, V4 (ft/s) 0.06 0.11 0.09 0.15 0.23VVCritical depth, yc �

((q2/g/ /Wp2))(0.333) (ft) 0.35 0.52 0.46 0.67 0.88Friction headloss thruogh

aeration tank channelManning's n for concrete channel 0.013 0.013 0.013 0.013 0.013Hydraulic radius, R �

(Dt � Wp)/(2 � Dt � Wp) (ft) 6.56 6.59 6.58 6.61 6.64Headloss, Hlf4ff � (V4VV � n/








1.486 � R(2/3))2 � L (ft) 0.0000 0.0001 0.0000 0.0001 0.0003Fitting headloss through a 90° bend

Fitting headloss coefficient KbendKK � 1.0 1.0 1.0 1.0 1.0 1.0Number of bends, Nb 8 8 8 8 8Headloss, Hlb4 � KbendKK �

V4VV 2/2g (ft) 0.0004 0.0014 0.0010 0.0030 0.0065Velocity head, Hvsd

(see below at Point 3) 0.74 1.03 0.90 1.28 1.76

RAS flow, % of average flowEGL at Point 4, EGL4 �

EGL5 � Hlf4ff � Hvsd (ft) 341.45 342.03 341.81 342.53 343.35Velocity head, Hvsd (see below at point 3) 0.74 1.03 0.90 1.28 1.76HGL at point 4, HGL4 � EGL4 � HV4 (ft) 340.71 341.00 340.90 341.25 341.60

6. Point 4 to Point 3Headloss over aeration tank influent

sluice gatesSluice gate width, Ws (ft) 3.9 3.9 3.9 3.9 3.9Sluice gate heigth (ft) 3.3 3.3 3.3 3.3 3.3Flow per sluice gate, q (ft3/s) 17.66 28.25 23.54 37.67 56.50Upstream head over weir,

Du � (select so ZsubZ � q � 0) (ft) 1.71 2.39 2.10 2.97 4.07Downstream head over weir,Dd � (q/3.33/Ws')(2/3) (ft) 1.29 1.82 1.59 2.25 3.08Effective sluice gate width, Ws' �

Ws � (0.33)(2 contractions)(Du) (ft) 3.60 3.46 3.52 3.34 3.12Free fall flow, Qfree �

3.34 � Ws'Du (3/2) (ft3/s) 26.75 42.80 35.67 57.07 85.61Submerged flow, Qsub � Qfree

(1-(Dd/dd Du// )0.385 (ft3/s) 17.66 28.25 23.54 37.67 56.51Difference, (Qsub � q), ft3/s (should de zero) 0.00 0.00 0.00 0.00 0.00Head difference between tank

and channel, Hl4 � Du � Dd (ft) 0.41 0.58 0.51 0.72 0.98Velocity head downstream of sluice

gate, HVsd � (q/ Ws'/Dd// )2/2g, 0.22 0.31 0.28 0.39 0.53Velocity head upstream of sluice

gate, HVsu � (q/ Ws'/Du// )2/2g (ft) 0.13 0.18 0.16 0.22 0.31

Top of sluice gate elevation,Els � HGL4 � Dd (ft) 339.42 339.19 339.31 339.00 338.51

HGL upstream of sluice gate,HGLsu � HGL4 � Hl4 (ft) 341.13 341.58 341.41 341.96 342.58

EGL upstream of sluice gate,EGLsu � HGLsu � HVsu (ft) 341.25 341.76 341.57 342.19 342.89

Friction headloss through influentchannel to tank #3

Average length of influentchannel per tank, L3 103.3 103.3 103.3 103.3 103.3

� Np � Wp � 3 tanks1/2 (ft)Influent channel width, W3 (ft) 13.1 13.1 13.1 13.1 13.1WW








Manning's n for concrete channel 0.013 0.013 0.013 0.013 0.013Influent channel bottom elevation,

Elb � ave.EGLsu � 9.84 (ft) 331.7 331.7 331.7 331.7 331.7Water depth in influent channel,

h3 � HGLs � Elb (ft) 9.40 9.86 9.68 10.24 10.86Hydraulic radius, R �

(h3 � w3)/(2 � h3 � w3) (ft) 3.86 3.94 3.91 4.00 4.09Velocity, V3VV � q/w3/h3 (ft/s) 0.14 0.22 0.19 0.28 0.40Headloss, Hlf3ff � (V3VV � n/1.486 �

R(2/3))2 � L3 (ft) 0.0000 0.0001 0.0000 0.0001 0.0002Friction headloss through influent

channel to tank #2Flow rate, q2 � 2 � q (ft3/s) 35.31 56.50 47.09 75.34 113.01

Velocity, V2VV � q/w2/h2 (ft/s) 0.29 0.44 0.37 0.56 0.79Headloss, Hlf2 � (V2 � n/1.486 �

R(2/3))2 � L3, m 0.0001 0.0002 0.00020. 0004 0.0008Friction headloss through influent

channel to tank #1Flow rate, q1 � 3 � q (ft3/s) 35.31 56.50 70.63 113.01 113.01Velocity, V1 � q/w1/h1 (ft/s) 0.29 0.44 0.56 0.84 0.79Headloss, Hlf1ff � (V1 � n/1.486 �

R(2/3))2 � L3 (ft) 0.0001 0.0002 0.0004 0.0009 0.0008


HGLs � Hlf3ff � Hlf2ff � Hlf1 (ft) 341.13 341.58 341.41 341.97 342.58ff


Sluice gate headloss coefficient KgateK � 1.0 1.0 1.0 1.0 1.0 1.0

RAS flow, % of average flow (addeddownstream of 20 50 50 100 100aeration tank influent sluice gates)

RAS flow, Qras � Q � RAS flow/100, cfs 11.30 28.25 35.31 70.63 70.63

Sluice gate width, W2 (ft) 5.91 5.91 5.91 5.91 5.91WWSluice gate heigth, Hg (ft) 5.91 5.91 5.91 5.91 5.91Channel water depth, Dc (ft) 9.40 9.86 9.69 10.24 10.86Gate opening depth, Hg or

Dc whichever is smaller (ft) 5.91 5.91 5.91 5.91 5.91Velocity through sluice

gate, V5VV � Q/W2 (ft/s) 1.01 1.62 2.03 3.24 3.24WWHeadloss, Hls2 � KgateK

� V5VV 2/2g (ft) 0.0159 0.0408 0.0637 0.1630 0.1630

HGL at point 2, HGL2 � HGL3 � Hls2 (ft) 341.14 341.62 341.47 342.13 342.75

8. Point 2 to Point1AllowanceExit headloss from primary sed, tank

effluent pipe to aeration tankinfluent channel

Primary effluent pipe diameter, Dp (ft) 6.56 6.56 6.56 6.56 6.56All PST effluent flow, Q 9 (ft3/s) 35.31 56.50 70.63 113.01 113.01








Velocity, V1 (ft/s) 1.04 1.67 2.09 3.34 3.34Exit headloss coefficient KexitK � 1.0 1.0 1.0 1.0 1.0 1.0Exit headloss, hle1 �

(V12)/2/g/ � KexitK (ft) 0.0169 0.0434 0.0677 0.1734 0.1734Friction headloss through PST

effluent pipe section 2Flow per pipe, Q (ft3/s) 35.31 56.50 70.63 113.01 113.01Pipe diameter, Dp2 (ft) 6.56 6.56 6.56 6.56 6.56Velocity, V12 (ft/s) 1.04 1.67 2.09 3.34 3.34Hazen–Williams coefficient, Cp 120.00 120.00 120.00 120.00 120.00Hydraulic radius, Rp2 � Dp2/4 (ft) 1.64 1.64 1.64 1.64 1.64Length of pipe, Lp2 (ft) 164.04 164.04 164.04 164.04 164.04Slope, Sp2 � [v12/(1.318 � Cp �

Rp2(0.63))](1/0.54) (%) 0.0001 0.0001 0.0002 0.0004 0.0004Headloss, hlf2ff � Lp2 � Sp2 (ft) 0.0084 0.0202 0.0305 0.0728 0.0728

Friction headloss through PSTeffluent pipe section 1Flow per pipe, q (ft3/s) 17.66 28.25 23.66 37.79 56.50Pipr diameter, Dp1 (ft) 4.92 4.92 4.92 4.92 4.92Velocity, V11 (ft/s) 0.93 1.49 1.24 1.99 2.97Hazen-Williams coefficient, Cp 120.00 120.00 120.00 120.00 120.00Hydraulic radius, Rp1 � Dp1/4 (ft) 1.23 1.23 1.23 1.23 1.23Length of pipe, Lp1 (ft) 164.04 164.04 164.04 164.04 164.04Slope, Sp1 � [v11/(1.318 � Cp �

Rp1(0.63))](1/0.54) (%) 0.0001 0.0001 0.0001 0.0002 0.0005Headloss, hlf1ff � Lp1 � Sp1 (ft) 0.0095 0.0227 0.0163 0.0389 0.0819

Pipe entrance head lossKe 0.50 0.50 0.50 0.50 0.50

Head loss, hen1 � Ke � V112/2g (ft) 0.0067 0.0171 0.0120 0.0306 0.0685

HGL at upstream of PSTeffluent pipe, HGL1 � 341.18 341.73 341.60 342.44 343.14

HGL2 � hle1 � hlf2ff � hlf1ff � hen1 (ft)

HGL7 of PST must be maximumof HGL1 (ft) 343.14 343.14 343.14 343.14 343.14


TABLE 22.13 Example Hydraulic Calculations of a Typical Multimedia Filter System




2. Multimedia filtersTotal number of units 6 6 6 6 6Number of units in operation 4 5 5 6 5Number of units on standby 2 1 1 0 1Flow rate per operating

multimedia filter, q (ft3/s) 8.83 11.30 14.13 18.83 22.60







Hydraulic calculations atfilter effluent

3. At Point 7Max HGL in filtered water

storage tank, HGL7 (ft) 323.72 323.72 323.72 323.72 323.72Velocity in storage tank, V7 (ft/s) 0.00 0.00 0.00 0.00 0.00VVMax EGL in storage tank,EGL7 � HGL7 � V7VV 2/2g (ft) 323.72 323.72 323.72 323.72 323.72

4. At Point 6Filtered water effluent channel weir

Sharp-crested weir EL,Wel6 � HGL7 � 0.33 (ft) 324.05 324.05 324.05 324.05 324.05Flow rate over weir � Q (ft3/s) 35.31 56.50 70.63 113.01 113.01Length of weir (ft) 22.97 22.97 22.97 22.97 22.97Headloss, Hlw6 � (q/3.33L)(2/3) (ft) 0.60 0.82 0.95 1.30 1.30


Wel6 � Hlw6 (ft) 324.65 324.87 325.00 325.35 325.35Velocity in weir box, V6, mVV

(assume V � 0) (ft) 0.00 0.00 0.00 0.00 0.00EGL at Point 6, EGL6 �

HGL6 � V62/2g (ft) 324.65 324.87 325.00 325.35 325.35

5. Point 6 to Point 5Loss through effluent concrete condiut

Flow rate, Q (ft3/s) 35.31 56.50 70.63 113.01 113.01Width of condiut, Wc (ft) 9.84 9.84 9.84 9.84 9.84Depth of condiut, Dc (ft) 6.56 6.56 6.56 6.56 6.56Length of condiut, Lc (ft) 32.81 32.81 32.81 32.81 32.81Velocity, Vc (ft/s) 0.55 0.87 1.09 1.75 1.75Hydraulic radius,

R � Wc � Dc/2/(Wc � Dc) (ft) 1.97 1.97 1.97 1.97 1.97Manning's n 0.013 0.013 0.013 0.013 0.013Headloss, Hlc5 � (Vc � n/1.486 �

R(2/3))2 � Lc (ft) 0.0003 0.0008 0.0012 0.0031 0.0031Exit loss from pipe to concrete conduit

Effluent pipe diameter, Dp (ft) 3.3 3.3 3.3 3.3 3.3Pipe flow (for each filter) (ft3/s) 8.83 11.30 14.13 18.83 22.60Velocity, Vp (ft/s) 1.04 1.34 1.67 2.23 2.67Hle5 � Vp2/2g for sharp

concrete outlet (ft) 0.0170 0.0278 0.0434 0.0772 0.1111


EGL6 � Hlc5 � Hle6 (ft) 324.66 324.89 325.04 325.43 325.46Velocity head at Point 5,

HV5VV � Vp2/2g (ft) 0.02 0.03 0.04 0.08 0.11HGL at Point 5, HGL5 �

EGL5 � HV5 (ft) 324.65 324.87 325.00 325.35 325.35VV

6. Point 5 to Point 4Filter effluent pipe loss

Pipe diameter, Dp (ft) 2.95 2.95 2.95 2.95 2.95








Max flow through filtereffluent pipe � q (ft3/s) 8.83 11.30 14.13 18.83 22.60

Velocity of flow throughpipe, Vp (ft/s) 1.29 1.65 2.06 2.75 3.30

Hazen-Williams coefficient, Cp 120 120 120 120 120Hydraulic radius, Rp � Dp/4 (ft) 0.74 0.74 0.74 0.74 0.74Length of pipe, Lp (ft) 49.21 49.21 49.21 49.21 49.21Slope, Sp � [Vp/(1.318 � Cp �

Rp0.63)](1/0.54) (%) 0.0193 0.0305 0.0461 0.0786 0.1102Head loss, Hlf4ff � Lp � Sp (ft) 0.0095 0.0150 0.0227 0.0387 0.0542

Headloss through butterfly valveKvalveKK (fully open) 0.30 0.30 0.30 0.30 0.30Valve diameter (ft) 2.95 2.95 2.95 2.95 2.95Headloss, hval4 � KvalveKK � (Vp2/2g) (ft) 0.0078 0.0127 0.0198 0.0353 0.0508

Flow rate controllerVenturi throat-to-inlet ration

for long tube, KrateKK 1.20 1.20 1.20 1.20 1.20Inlet velocity, ViVV � Vp (ft/s) 1.04 1.34 1.67 2.23 2.67Headloss, hrate � KrateKK � (Vi 2/2g) (ft) 0.0203 0.0333 0.0521 0.0926 0.1333

(minimum headloss when controlvalve is fully open)

Pipe entrance lossKentK 0.50 0.50 0.50 0.50 0.50

Headloss, HlentH � KentK � (Vp2/2g) (ft) 0.0085 0.0139 0.0217 0.0386 0.0555

EGL at Point 4, EGL4 � EGL5 �

Hlf4ff � Hval4 � HrateHH � HlentH (ft) 324.71 324.97 325.16 325.63 325.75Velocity head, HV4VV �

V4VV 2/2g, (assume V � 0) (ft) 0.00 0.00 0.00 0.00 0.00HGL at Point 4, HGL4 �

EGL4 � HV4 (ft) 324.71 324.97 325.16 325.63 325.75VV

7. Point 4 to Point3Dirty filter head requirement,

Hldf, (ft) (assumed) 8.2 8.2 8.2 8.2 8.2ff(consult with filter manufacturer)

Dirty filter HGL, HGLdf �

HGL4 � Hldf (ft) 332.91 33.17 333.36 333.83 333.96Velocity head, HV3VV � 0

(assume V3VV � 0) (ft) 0.00 0.00 0.00 0.00 0.00Dirty filter HGL, HGLdf �

EGLdf � HV3 (ft) 332.91 33.17 333.36 333.83 333.96VV

Clean filter headlossFilter bed area (ft2) 1722 1722 1722 1722 1722Flow per filter, q (ft3/s) 8.83 11.30 14.13 18.83 22.60Filter rate, qfilt, (ft3/ min/ft2) 0.308 0.394 0.492 0.656 0.787Media depth, Dm (ft) 3.28 3.28 3.28 3.28 3.28Effective media size, Md (in) 0.2 0.2 0.2 0.2 0.2Headloss through filter,

Hlf � 2.32 ft loss per(ft3/ min/ft2)(consultant withmanufacturer) 0.7136 0.9134 1.1417 1.5223 1.8268








Entrance headloss throughunderdrain flume, Hlu � 0.45 ft lossper(ft3/ min/ft2) (ft)(consult with 0.1384 0.1772 0.2215 0.2953 0.3543filter manufacturer)

Clean filter EGL, EGLcf �

EGL4 � Hlf+ff Hlu (ft) 325.56 326.06 326.52 327.45 327.94Velocity head, HV3VV � 0 (m)

(assume V3VV � 0) (ft) 0.00 0.00 0.00 0.00 0.00Clean filter HGL, HGLcf �

EGLcf � HV3 (ft) 325.56 326.06 326.52 327.45 327.94VV

EGL requierd at Point 3,EGL3 � EGLdf (ft) 332.91 33.17 333.36 333.83 333.96

HGL requierd at Point 3,HGL3 � HGLdf (ft) 332.91 33.17 333.36 333.83 333.96

(Head requierd for dirty filter controls)

8. Point 3 to Point 2Filter inlet discharge loss

KeffK 1.0 1.0 1.0 1.0 1.0Flow rate, q (ft3/s) 8.83 11.30 14.13 18.83 22.60Pipe diameter, Dp2 (ft) 3.0 3.0 3.0 3.0 3.0Velocity, Vp2 (ft/s) 1.29 1.65 2.06 2.75 3.30headloss, Hld2 � KeffK � (Vp22/2g) (ft) 0.0258 0.0423 0.0661 0.1176 0.1693

EGLat Point 2, EGL2 � EGL3 � Hld2 (ft) 332.94 333.21 333.43 333.95 333.13Velocity head, HV2VV � Vp22/g/ (ft) 0.03 0.04 0.07 0.12 0.17HGL at Point 2HGL2 � EGL2 � HV2 (ft) 332.91 333.17 333.36 333.83 333.96VV

9. Point 2 to Point 1Head loss through butterfly valve

KvalKK (fully open) 0.3 0.3 0.3 0.3 0.3Headloss, Hlv1 � KvalKK � (Vp22/2g) 0.0078 0.0127 0.0198 0.0353 1.0508Headloss through inlet pipeLength of pipe, Lp1 (ft) 65.6 65.6 65.6 65.6 65.6Hazen-Williams coefficient (Cp) 120 120 120 120 120Hydraulic radius, Rp � Dp2/4 (ft) 0.74 0.74 0.74 0.74 0.74Headloss, Hlf1ff � (Vp2/(1.318 � Cp

� Rp1.63))(1/0.54) � Lp (ft) 0.0127 0.0200 0.0303 0.0516 0.0723Headloss through entrance to pipe

KentK 0.50 0.50 0.50 0.50 0.50Headloss, Hlent � KentK � Vp2/2g (ft) 0.0129 0.0212 0.0331 0.0588 0.0847


EGL2 � Hlv1 � Hlf � Hlent (ft) 332.97 333.27 333.51 334.10 334.33Velocity head, HV1 � 0

(assume V1 � 0) (ft) 0.00 0.00 0.00 0.00 0.00HGL at point 1, HGL � EGL1 � HV1 (ft) 332.97 333.27 333.51 334.10 334.33

Minimum required controlHGL at Point 1 (ft) 334.33 334.33 334.33 334.33 334.33

(max. HGL1 must equal HGL7of final sedimentation tank)






TABLE 22.14 Example Hydraulic Calculation of a Typical Cascade Aeration System




2. Cascade aeratorTotal number of units 1 1 1 1 1Flow rate through aerator, Q (ft3/s) 35.31 56.50 70.63 113.01 113.01Optimal flow rate per ft width

over step, q (ft2/s) 0.7029 0.7029 0.7029 0.7029 0.7029DO concentration of postaeration

influent, Co (mg/L) 0.00 0.00 0.00 0.00 0.00Desired DO concentration of postaeration 5.00 5.00 5.00 5.00 5.00

effluent, Cu (mg/l)

Calculation of aerator dimensions withwith predetermined weir length

3. Weir length, W (ft) 16.4 16.4 16.4 16.4 16.4Flow over weir, q � Q/W (ft3/s/ft) 2.15 3.44 4.31 6.89 6.89Critical depth at upstream stepedge, hc � (q2/g)1/3 (ft) 0.524 0.717 0.832 1.138 1.138Optimal fall height of nappe, h (ft) 3.9 3.9 3.9 3.9 3.9Length of downstream bubble cushion, Lo

� 0.0629(h0.134)(q0.666) (ft) 16.93 23.16 26.87 36.74 36.74Length of downstream receiving

channel, L � 0.8Lo (ft) 13.55 18.53 21.49 29.39 29.39Optimal tailwater depth,

H' � 0.236 h, ft for h 3.9 ft 0.93 0.93 0.93 0.93 0.93Deficit ratio log at 68 F, Inr68

� 1.86(h1.31)(q�0.363)(H0.31HH ) 0.42 0.36 0.33 0.28 0.28Deficit ratio, r20 1.53 1.43 1.39 1.32 1.32

Calculate concentration of dissolvedoxygen downstream of step. If concentration is less than desiredconcentration, add another stepand again calculate DO downstreamconcentration. Continue adding stepsuntil the desired DO oncentrationis achieved.

Select cascade aerator dimensioncorresponding to those calculatedfor average flow.

4. Calculation of number steps toobtain desired DO

Desired DO concentration ataverage flow, Cu (mg/L) 5.00Step 1 effluent DO, C1 �

9.07(1 �(1/r20)) � Co/r20) (mg/L) 3.13 2.73 2.55 2.20 2.20Step 2 effluent DO, C2 �

9.07 � (1 �(1/r20)) � Co/r20) (mg/L) 4.80 4.51 4.38 4.13 4.13







Step 3 effluent DO, C3 � 9.07 �

(1 �(1/r20)) � Co/r20) (mg/L) 6.00 5.79 5.70 5.52 5.52

In this example, the desired downstreamDO concentration for average flow isachieved after three steps.

5. Calculation of HGL at each stepHead loss from filtered water storage

tank to point 1 (ft) 3.28 3.28 3.28 3.28 3.28Cascade fall height, h (ft) 3.94 3.94 3.94 3.94 3.94

HGL at Point 1, HGL1 (ft) 319.98 319.98 319.98 319.98 319.98

HGL at Point 2, HGL2 � HGL1 � h (ft) 316.04 316.04 316.04 316.04 316.04







Chapter 22 - Water and Waste Water Treatment Plant Hydraulics

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