Waste Water Flow Rates Section

Faculty of Engineering, University of Ruhuna

Unit Title: Water and Wastewater Engineering

Unit Code: CE 4305

Chapter 2 –Wastewater Characteristics and treatment

Secton 2.1:Wastewater flow Rates

1.0 Introduction

The accurate assessment of wastewater flow rates and pollutant loadingsis a fundamental step in

initiating the conceptual process design of wastewater treatment facilities. Reliable data for

existing and projected flow rates must be available for the design considerations such as

hydraulic characteristics, sizing and operational considerations of the treatment system

components. Furthermore, availability of such data set is required to minimize the associated

costs and to share the facilities equitably when facilities serve more than one community or

district.

The assumptions used in determining the wastewater flow have a permanent effect on planning

outcomes and subsequent planning decisions.Therefore, without a thorough analysis of flow,

premature or excessive investment in capital works may result causing unnecessary additional

financial impacts on customers. This lecture provides guidance on the assessment of wastewater

flows and methods of calculating wastewater flow rates and appliances used in wastewater flow

reductions.

1.1 Wastewater Flow Rates

1.1.1 Components of wastewater flows

In situations where wastewater flow rate data are limited or unavailable, wastewater flow rate

estimates have to be developed from water consumption records and other information. If field

measurements of wastewater flow rates are not possible and actual wastewater flow rate data are

not available, water supply records can often be used as an aid to estimate wastewater flow rates.

Therefore, it is in vital importance to understand the components of wastewater flow and its

characteristics. The components that make up the wastewater flow from a community depend on

the type of collection system used and may include;

Domestic wastewater;

Industrial;

Public services

infiltration/inflow; and

unaccounted loses.

Domestic (Also called Sanitary)

Generally, over one-third of the water used in a municipal water supply system is for domestic

purposes such as washing, bathing, culinary, waste removal, land yard watering. The volume of

wastewater from a community varies from 50 to 250 gal per capita per depending on sewer uses.

Principal sources of wastewater generated in a community include residential areas, commercial

districts, institutional facilities, and recreational facilities.

Residential Areas -For many residential areas, wastewater flow rates are commonly

determined on the basis of population density and the average per capita contribution of

wastewater. For large residential areas, it is often advisable to develop flow rates on the basis of

land use areas and anticipated population densities.

Commercial Districts -Commercial wastewater flow rates are generally expressed in

gal/acre.d (m3/ha.d) and are based on existing or anticipated future development or comparative

data. Average unit-flow rate allowances for commercial developments normally range from 800

to 1500 gal/acre. d (7.5 to 14 m3/ha.d). As unit flowrates can vary widely for commercial

facilities, every effort should be made to obtain records from similar facilities.

Institutional Facilities –Flow rates vary with the region, climate and type of facility. The

actual records of institutions are the best sources of flow data for design purposes.

Recreational Facilities -Wastewater flow rates from many recreational facilities are highly

seasonal.

Estimated average wastewater flows from residential dwellings and other establishments are

listed in Table 2.1. As seen in the Table 2.1 mobile homes, trailer parks, day schools and hotels

generate relatively less amount of wastewater than residences since they have fewer appliances.

The quantity and strength of schools, offices, factories and other commercial establishments

depends on hours of operation and presence of eating facilities. Although cafeterias do not

provide a great deal of flow, the wastewater strength is increased materially by food preparation

and cleanup activities.

Table 1- Approximate wastewater flows for various kinds of Establishments and Services

Industrial (nondomestic purposes)

The amount of water supplied by municipal agencies to industries for process (non domestic)

purposes is highly variable.

Other industries such as those involved in "high technology", which have more modest process

water requirements, may depend wholly on municipal supplies. As industrial water use varies

widely, it is therefore desirable in practical design work to inspect the plant concerned and to

make careful estimates of the quantities of both water used from all sources and the wastes

produced. Typical design values for estimating the flows from industrial areas that have no or

little wet-process type industries are 1000 to 1500 gal/acre.d (9 to 14 m3/ha .d ) for light

industrial developments and 1500 to 3000 gal/acre.d for medium industrial developments. For

industries without internal recycling or reuse programs, it can be assumed that about 85 to 95

percent of the water used in the various operations and processes will become wastewater. For

large industries with internal water-reuse programs, separate estimates must be made.

Public service

Public service water represents the smallest component of municipal water use. Public service

water uses include water used for public buildings, fire fighting, irrigating public parks and

greenbelts, and system maintenance. System maintenance water uses include water for

disinfecting, new water lines and storage reservoirs, and hydraulic flushing of sewers. Only small

amounts of water used for these purposes reach the sanitary sewer system, except that from

public buildings.

Infiltration / Inflow Extraneous flows in sewers, described as infiltration and inflow, are illustrated in Fig. 2-1 and

are defined as follows:

Figure 1- Graphical Identification of Infiltration/Inflow

Infiltration

Infiltration is defined as the component of wastewater which is extraneous water entering the

sewer system from the ground through sources such as defective pipes, pipe joints, connections,

and manhole walls. Some quantity of infiltration is generally expected to be present in

wastewater flow throughout the year. Because infiltration is directly influenced by groundwater

fluctuations, the volume of infiltration entering a sewer system is generally expected to fluctuate

from season to season with typically larger volumes anticipated in the spring and smaller

volumes anticipated in the summer. Sewer systems physically located in or near coastal areas

may be subject to tidal infiltration.

Inflow

Inflow is defined as the component of wastewater which is extraneous water discharged into a

sewer system from sources such as sump pumps, roof leaders, cellar/foundation drains, surface

drains, drains from springs and swampy areas, manhole covers, catch basins, cross-connections

from storm drains, cooling water discharges, leaking tide gates, and other inlets. Inflow differs

from infiltration in that it is the result of direct connections of extraneous flow sources into the

collection system and, generally, is not linked to fluctuations in the groundwater table. The

methodology used to quantify inflowdoes not allow for the independent quantification of

rainfall-induced infiltration; therefore, the quantification of inflow also is likely to include some

portion of rainfall-induced infiltration.

Inflow is largely the result of wet weather (stormwater) influenced on the sewer system. During dry

weather, the quantity of inflow is generally expected to approach zero. During storm events, inflow

may rapidly impact the sewer system causing the wastewater flow to increase. The increase in

wastewater flow due to inflow may terminate a short time after the storm event or it may influence

the sewer system for a prolonged period depending on the type of inflow sources which exist in the

system. It is not uncommon for inflow to elevate wastewater flows for a number of days. Sewer

systems physically located in or near coastal areas may also be subject to tidal inflow. Separate

components of inflow are briefly described below.

Total Inflow volume - is defined as the total volumeof inflow generated from a single storm event

including both direct and delayed inflow.

Direct Inflow Volume - is defined as the portion of total inflow which is generated from direct

connections to the collection system such as catch basins, roof leaders, manhole covers, combined

sewers etc. These inflow sources allow stormwater runoff to rapidly impact the collection system

by causing immediate increase in wastewater flows. Delayed Inflow volume - is defined as the portion of total inflow which is generated from indirect

connections to the collection system or connections which produce inflow after a significant time

delay from the beginning of a storm. Delayed inflow sources include: sump pumps, foundation

drains, indirect sewer/drain cross-connections, etc.

Rainfall-Induced Infiltration Volume - is defined as the short-term increase in infiltration which is

the direct result of stormwater percolation into the ground and through collection system defects

in pipes, joints, connections, or manhole walls which lie near or are readily reached from the

ground surface. Rainfall-induced infiltration (RII) may also be commonly referred to as “rainfall

dependent infiltration (RDI)” or “rainfall dependent I/I (RDI/I)” By definition, rainfall-induced

infiltration is a portion of delayed inflow.

Peak Inflow Rate - is defined as the largest inflow rate determined over a one hour period which is

generated from a single storm event. The peak inflow rate is the largest rate difference between

the storm event hydrograph and the dry weather hydrograph.

Total Annual Inflow Volume - is defined as the total inflow volumedetermined over the entire

calendar year.

Annual Average Inflow Rate - is defined as the average daily inflow rate determined over the

entire calendar year. It may be estimated by subtracting the average daily dry weather wastewater

flow from the average daily total wastewater flow.

Tidal Inflow - is defined as seawater entering the sewer system through direct connections such as

leaking tide gates and defects in manholes located in salt marshes or other areas subject to tidal

influence. In cases where it is not possible to distinguish tidal inflow from tidal infiltration, the

combination may be reported as tidal I/I.

Unaccounted for system losses and leakage: Unaccounted system losses include

unauthorized use, incorrect meter calibration or readings, improper meter sizing, and inadequate

system controls. Leakage is may be due to system age, materials of construction, and lack of

system maintenance. Unaccounted system losses and leakage may range from 10 to 12 percent of

production for newer distribution systems (less than 25 years old ) and from 15 to 30 percent for

older systems. In small water systems, unaccounted losses and leakage may account for as much

as 50 percent of production. As much as 40 to 60 percent of the unaccounted water may be

attributed to meter error. Therefore, while water records may be useful in forecasting wastewater

flowrates, the accuracy of the records must be checked carefully.

Water records of various types are kept by water supply agencies. These records usually include

information on the amount of water produced or withdrawn and discharged to the water supply

system and the amount of water actually used (consumed). The distinction is important because

more water is produced than is actually used by the consumer. The difference between these two

values is the amount of water lost or unaccounted for in the distribution system plus the amount

used for various public services that may be un-metered. Therefore, in using water supply

records to estimate wastewater flowrates, it is necessary to determine the amount of water

actually used by the customers. Unaccounted water and losses do not reach the wastewater

system and have to be excluded in making flow estimates.

EXAMPLE 1

Convert to SI units for the construction allowable infiltration rate of 500 gal/(d.mile) per inch of

pipe diameter.

EXAMPLE 2

A large city has measured high flow rates during the wet season of the year. The flow rates

during the dry period of the year, when rainfall is rare and groundwater infiltration is negligible,

averages 128,000 m3/d. During the wet period when groundwater levels are elevated, the flow

rate averaged 240,000 m3/d excluding those days during and following any significant rainfall

events. During a recent storm, hourly flow rates were recorded during the peak flow period as

well as several days following the storm. The flowrate plots are shown in the accompanying

figure. Compute the infiltration and inflow and determine if the infiltration is excessive.

Excessive infiltration is defined by the local regulatory agency as rates over 0.752 m3/d. mm-km

of the sewer. The composite diameter-length of the sewer system is 270,000 mm-km.

Figure 2- Variation of the Flow in the City

EXAMPLE 3:

A small community water supply agency furnishes water to 147 customers from a well supply.

Water records are kept showing the amount of water pumped to the system. The agency recently

installed meters for all customers and total water sales records are also kept. The following data

are obtained. Table 2.2

Month Production (gal/month) Sales (gal/month)

May

June

July

Total

1,414,100

1,421,000

1,407,600

4,242,700

1,033,600

1,104,300

1,086,300

3,224,200

Average (gal/d) 46,116 35,046

Determine the amount of water consumed (gal/capita.d) and the amount of water that is

unaccounted system loss (as a percent of production). The average household size as determined

by the local planning agency is 2.43 persons per service.

1.2 Variation of wastewater Flow

1.2.1 Proportion of Municipal Water Supply Recharging the Collection System as Wastewater

As wastewater consists primarily of used water, the portion of the water supplied that reaches the

collection system must be estimated. A considerable portion of the water produced does not

reach the sanitary sewer system and includes;

(1) Product water used by manufacturing establishments,

(2) Water used for landscape irrigation, system maintenance, and extinguishing fires,

(3) Water used by consumers whose facilities are not connected to sewers, and

(4) Leakage from water mains and service pipes (unaccounted for losses).

About 60 to 85 percent of the per capita consumption of water becomes wastewater. Application

of appropriate percentages to records from metered water use generally can be used to obtain a

reasonable estimate of wastewater flowrates, excluding infiltration/inflow.However, in some

cases, excessive infiltration, roof water, and water used by industries that is obtained from

privately owned water supplies make the quantity of wastewater larger than the water

consumption from the public supply. If a community has well-constructed sewers and if

stormwater and snowmelt drainage are excluded and there is no substantial change in the

industrial uses of water, the variation from year to year in the ratio of wastewater to water supply

will not be great.

1.2.2 Variations in Water Use

Although it is important to know the average rates of water use, it is equally important to have

data on the fluctuations in rates of use. Representative data on the typical fluctuations in water

use are reported in Table 2.3.

Table 3- Typical Fluctuations in water Use in Community Systems

Percentage of average for year

Range Typical

Daily average in maximum month

Daily average in maximum week

Maximum day

Maximum hour

110-140

120-170

160-220

225-320

120

140

180

270

The maximum use usually occurs during warmer seasons. Hourly variations in water

consumption also effect the rate of wastewater flow. In general, the wastewater discharge curve

closely parallels the water-consumption curve, but with a lag of several hours. In some cities,

large quantities of water used by industries and obtained from sources other than the municipal

supply are discharged into sewers during the working hours of the day. When such discharge

occurs, the peak flow is higher than the amount resulting from the normal variation in the draft

on the municipal supply.

1.2.3 Variations in Wastewater flow rates

Variability of wastewater flow is usually characterized by daily and hourly minimum and

maximum flows and instantaneous peak flows that occur during the day. The intermittent

occurrence of individual wastewater-generating activities can create large variations in

wastewater flows from residential or nonresidential establishments. This variability can affect

gravity-fed onsite systems by potentially causing hydraulic overloads of the system during peak

flow conditions. Figure 2.3 illustrates the routine fluctuations in wastewater flows for a typical

residential dwelling.

Figure 3- Daily indoor water use pattern for single-family residence

These variations can be discussed under following topics.

(a) Short Term Variations - The variations in wastewater flow rates observed at treatment

plants tend to follow a somewhat diurnal pattern, as shown in Fig. 2.4 .Minimum flows occur

during the early morning hours when water consumption is lowest and when the base flow

consists of infiltration and small quantities of sanitary wastewater. The first peak flow generally

occurs in the late morning when wastewater from the peak morning water use reaches the

treatment plant. A second peak flow generally occurs in the early evening between 7 and 9 p.m.

but this varies with the size of the community and the length of the sewers. When extraneous

flows are minimal, wastewater-discharge curves closely parallel water- consumption curves, but

with a lag of several hours. A plot of typical weekly flow rates for both wet and dry periods is

shown in Fig 2.5.

Figure 4- Typical hourly variation in domestic w/w flow rates

Figure 5- Typical daily and weekly variation in domestic w/w flow rates

(b) Seasonal Variations - Seasonal variations in domestic wastewater flows are commonly

observed at resort areas, in small communities with college campuses, and in communities with

seasonal commercial and industrial activities. The magnitude of the variations to be expected

depends on both the size of the community and the seasonal activity.

(c) Industrial Variations - Industrial wastewater discharges are difficult to predict. Many

manufacturing facilities generate relatively constant flow rates during production, but the flow

rates change markedly during cleanup and shutdown. Although internal process changes may

lead to reduced discharge rates, plant expansion and increases production may lead to increased

wastewater generation.

1.3Analysis of wastewater flow rates

1.3.1 Key Flow parameters

Key flow parameters which are needed in estimation of wastewater flows are given in Table 2.4.

Table 4- KeySewage flow parameters

The determination of the ADWF, PDWF and PWWF should be based on:

1. Actual system performance;

2. The data based on Sewerage Code in particular catchment;

3. The historical catchment approach where typically

PDWF = C2 x ADWF where C2 = 4.7 x (EP)-0.105

PWWF = (5 x ADWF) or (C1 x ADWF) whichever is the larger and

C1 = 15 x (EP)-0.1587

(Note: the minimum value of C1 = 3.5)

In the above formulae, EP is the total equivalent population in the catchments gravitating to a

pump station.

EXAMPLE 4:

Calculate the PDWF for a given wastewater flow by considering Equivalent population 15 000, and

ADWF =180 L/EP.day.

1.3.2Design Flow parameters

The parameters which are generally used as the basis of design for sewers, lift stations,

wastewater (sewage) treatment plants, treatment units and other wastewater handling facilities

are:

The average daily flow (Volume per unit time);

Maximum daily flow;

Peak hourly flow;

Design peak flow;

Minimum daily and hourly flows.

The average daily flow (Volume per unit time) – The average daily flow is the average

of the daily volumes to be received for a continuous 12 month period of the design year.

The average flow may be used to estimate pumping and chemical costs, sludge generation

and organic loading rates.

The maximum daily flow – Themaximum daily flow is the largest volume of flow to be

received during a continuous 24-hour period. It is employed in the calculation of retention

time for equalization basin and chlorine contact time.

The peak hourly flow – Thepeakhourly flow is the largest volume received during a one

hour period, based on annual data.It isused for the design ofcollection and interceptor

sewers, wet wells, wastewater pumping stations, wastewater flow measurements, grit

chambers, settling basins chlorine contact tanks and pipings.

The design peak flow – The design peak flow is theinstantaneous maximum flow rate to

be received. The peak hourly flow is commonly assumed as three times the average daily

flow.

The minimum daily flow – The minimum daily flow is the smallest volume of flow

received during 24-hour period. The minimum daily flow is important in the sizing of

conduits where solids might be deposited at low flow rates.

The minimum hourly flow – The minimum hourly flow is the smallest hourly flow rate

occurring over a 24-hour period based on annual data. It is important to the sizing of

wastewater flowmeters, chemical-feed systems and pumping systems.

EXAMPLE 5:

Estimate the average and maximum hourly flow for a community of 10,000 persons. Assume

average water consumption is 200 L/c.d and assume 80% of water consumption goes to the

sewer.

EXAMPLE 6:

The following data is given:

Sewered population = 50,000.

Average domestic wastewater flow = 100 gal/c.d

Assume infiltration flow rate = 500 gal/(d.mile) per inch of pipe diameter

Sanitary sewer systems for the city:

4-in house sewers = 66.6 miles

6-in building sewers = 13.2 miles

8-in street laterals = 35.2 miles

12-in submains = 9.8 miles

18-in mains = 7.4 miles

Estimate the infiltration flowrate and its percentage of the average daily and peak hourly

domestic wastewater flows.

1.3.3 Statistical Analysis of Flow Rates

The statistical analysis of wastewater flow rates involves the determination of statistical

parameters used to quantify a series of measurements. Commonly used statistical measures

include the mean, median, mode, standard deviation and coefficient variation based on the

assumption that data are distributed normally.

For most practical purposes the type of distribution can be determined by plotting the data on

both arithmetic-probability and log-probability paper and noting the data can be fitted with a

straight line. The three steps which involved in plotting the data are as follows.

1. Arrange the measurements in a data set in order of increasing magnitude and assign a rank

serial number.

2. Compute a corresponding plotting position for each data point using following formula.

Plotting position (%) = m/(n+1) × 100 Where m= rank serial number and n= number of

observations.

The term (n+1) is used to correct for a small sample bias. The plotting position represents the

percent or frequency of observations that are equal or less than the indicated value.

3. Plot the data on arithmetic-probability and log-probability paper. The probability scale is

labeled “ percent of values equal to or less than the indicated value”. If the data plotted on

arithmetic probability paper and if the data set fit with a straight line then the data are

assumed to be normally distributed. If the data is not fitting to a straight line (which is called

as Skewness) the data is re-plotted on the log-probability paper. The implication here is that

the logarithm of the observed values is normally distributed. However, in the scope of this

lecture we are dealing with arithmetic-probability papers, i.e we are only looking at normally

distributed data sets.

EXAMPLE 7:

Using the following weekly flow rate data obtained from an industrial dicharger for a calendar

quarter of operation, determine the statistical characteristics and predict the maximum weekly

flow rate that will occur during a full year’s operation.

Table 5- Weekly flow rate data for a quarter of year Week No. Flow rate, m3/wk

1 2900

2 3040

3 3540

4 3360

5 3770

6 4080

7 4015

8 3675

9 3810

10 3450

11 3265

12 3180

13 3135

1.4 Reduction of wastewater flowrates

Since the importance of conserving both resources and energy, various means for reducing

wastewater flowrates and pollutant loadings from domestic sources are gaining increasing

attention. The reduction of wastewater flowrates from domestic sources results directly from

the reduction in interior water use. The principal devices and appliances that are used to reduce

domestic water use and wastewater flow rates are described in Table 2.6.

Table 2.6- Flow Reduction Devices and Appliances

Device/appliance Description and/or application Faucet aerators Increases the rinsing power of water by adding air and concentrating

flow, thus reducing the amount of wash water use

Limiting-flow shower heads Restricts and concentrates watr passage by means of orifices that limit

and divert shower flow for optimum use by the bather

Low flush toilet Reduces the discharge of lower amounts of water per flush

Pressure reducing valve Maintains home water pressure at a lower level than that of the water

distribution system. Decreases the probability of leaks and dripping

faucets.

Retrofit kits for bathroom

fixtures Kits may consist of shower flow restrictors, toilets dams, or displacement

bags and toilet leak detector tablets.

Water efficient dishwasher Reduces the water used

Water efficient clothes

washer Reduces the water used

Toilet leak detectors Tablets that dissolve in the water closet and release dye to indicate

leakage of the flush valve.

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