Operating Philosophy

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Operating Philosophy and Control Description for SaudiAramco Bulk PlantsY00002 and Y00003

ByAaron Bergstreser

11/20/07

FOR INTERNAL USE ONLY


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List of Abbreviations

CIP: Clean-In-PlaceDO: Dissolved OxygenMC: Maintenance Clean

MLSS: Mixed Liquor Suspended SolidsMOS: Membrane Operating SystemRAS: Return Activated SludgeTMP: Trans Membrane PressureWAS: Waste Activated Sludge

Internal Use Only Page 3 6/30/2011


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Introduction

Recent advances in membrane separation technology coupled with reduced membranecosts have made Membrane Bioreactors (MBRs) an economically viable wastewater treatment method. As the name implies, MBR technology marries a biological activated

sludge process with downstream solids removal using membranes. This treatmentmethod offers several advantages over traditional wastewater treatment system such asreduced footprint and higher effluent quality. In many cases, the effluent from a MBR issuitable for reuse with minimal downstream processing.

The Rothschild, WI office of Siemens Water Technologies (SWT) has been contracted bytwo engineering firms (Saud Consult and Sofcon) to provide MBRs for sanitarywastewater treatment at six sites owned by Saudi Arabian Oil Company (Aramco). EachMBR system is comprised of two 75% treatment trains. Five of the sites have relativelysignificant lower average daily flows than the sixth site and have thus been termedSmall Bulk Plants. These plants are located in Al Hasa, Duba, North Riyadh, Tabuk,

and Turaif. The sixth plant is referred to as the Large Bulk Plant and is located in North Jeddah.

In an effort to reduce overall project costs, the five Small Bulk Plants share a commonsystem design. Furthermore, the feed concentrations of key contaminants (BOD, TSS,TKN, Ammonia) have been standardized across all of the MBR plants. Although theaverage daily flows vary from plant to plant and the design loadings vary between theSmall Bulk Plants and the Large Bulk Plant, the overall plant design is universal (i.e.same style of equipment and same unit operations). Therefore, the following processdescription will be general and will apply to all six MBR plants.

Process Description

Feed SystemSanitary wastewater will enter the MBR system battery limits and flow through ascreening and grit removal system designed to remove debris and grit from the wastestream that can potentially damage the membranes. Solid material caught in the screenwill be automatically conveyed to a dumpster through action of the supplied motor-drivenscrew conveyor. Grit that has settled in the grit basin will also be removed in a dumpster

but will require the opening of a manual valve for grit drainage.

De-gritted wastewater then flows to the Equalization/Emergency Sump where both flowand composition are stabilized. Mixing is accomplished by bubbling air through theliquid contents of the sump. Each MBR system is designed to handle four times itsaverage daily flow for a period of one day through a combination of flow through the

plant and storage capacity in the Equalization/Emergency Sump. For example, if theaverage daily flow of influent sanitary wastewater to the treatment plant is 24 m 3/day andthe total hydraulic capacity of the plant is 48 m 3/day, the storage volume in theEqualization and Emergency Sump has to be 48 m 3 (this example is taken from thecommon design basis of the Small Bulk Plants). This storage volume is provided in



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addition to the maximum accumulated volume in the sump due to the site diurnal flow pattern.

MBR Feed Pumps transfer wastewater from the Equalization/Emergency Sump to aSplitter Box. Here, flow is divided equally into two streams by overflowing two equal-

length weirs. Each stream will be directed to separate treatment trains within the MBR treatment plant. A recycle line is also provided on the MBR Feed Pump discharge linefor purposes of precise and accurate feed flow control given the common design for theSmall Bulk Plants. The goal is to operate the MBR Feed Pumps on a controllable regionof their pump curves given the comparatively significant differences in average dailyflow rates between the Small Bulk Plants. See the Control Description below for adiscussion of the considerations regarding the operation of the automatic recycle valve.

Before wastewater is introduced to the biological reactors, it must be screened again toremove fine fibrous particles (i.e. hair) that could not be removed in the upstreamscreening and grit removal system (coarse vs. fine screen) . Perforated plate drum screens

are used for this purpose. Wastewater enters the drum screens via gravity flow from thesplitter box. Since the splitter box and drum screens are located above the biologicalreactors, effluent from the drum screens also flows by gravity to downstream treatmentequipment.

Screened solids from the drum screens are collected in dumpsters for ease of removal. Acommon dumpster is used for both treatment trains. The drum screens also require a

periodic washing to remove any solids that may have plugged the perforated plates.

Biological TreatmentThe biological treatment portion of each train includes an anoxic zone followed by anaeration zone to facilitate both nitrogen removal via nitrification/denitrification and BODremoval. Nitrification is the process of producing nitrate from ammonia whiledenitrification generates nitrogen gas from nitrate. Both zones are housed in a singlecylindrical Anoxic/Aeration Tank and are separated by an internal baffle. Mixed liquor from the anoxic zone flows over the baffle to the aeration zone.

Heterotrophic bacteria in the aeration zone provide the bulk of the BOD removal throughcellular respiration. Nitrification reactions also occur in the aeration zone throughautotrophic bacteria. These processes require free oxygen (O 2) which is supplied throughthe Aeration/MOS Blowers and an in-tank air distribution system. The supply of air through the mixed liquor in the aeration zone also provides mixing to the contents of thatzone.

Heterotrophic bacteria in the anoxic zone also remove BOD in conjunction withdenitrification reactions. Since oxygen in the form of nitrate serves as the oxygen sourcefor denitrification, nitrates that are formed in the aeration zone must be recycled back tothe anoxic zone. MOS Feed/Recirculation Pumps are supplied for this purpose. Nitrateis reduced in the process and leaves as N 2. The anoxic zone is located ahead of theaeration zone to ensure a sufficient carbon source (BOD) is available to support the



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denitrification reactions. Mixing in the anoxic zone is accomplished using motor drivenAnoxic Mixers.

Since biological activity and vitality is sensitive to pH, each MBR plant is provided witha pH control system. Sodium hydroxide (20 wt%) and dilute sulfuric acid (20 wt%) are

utilized to control pH and are injected at the bottom of the anoxic zone in theAnoxic/Aeration Tank. Control of pH in the anoxic zone will produce a controlled pH inthe aeration zone. The anoxic zone will operate between a pH range of 7.5-8.5 (typical).Once outside of this range, the appropriate type and quantity of chemical will be doseduntil pH is again within the desired range. Since pH adjustment is a batch operation, pHswings within the desired operating range may be common, depending on the quality of the feed.

Foaming is not an uncommon phenomenon in activated sludge processes and can lead tooverflowing of the Anoxic/Aeration Tank if not properly managed. A spray nozzle is

provided in each Anoxic/Aeration Tank for foam control purposes. The nozzle is fed

from a side stream of the anoxic recirculation line, which itself is being fed from theMOS Feed/Recirculation Pumps. With this design, mixed liquor from the aeration zoneis being used to control foaming and no additional foam control chemicals are required.

Membrane Operating System (MOS)The purpose of the membranes in an MBR system is to separate the suspended solids,consisting of bacteria used for treatment, from the treated water. Membranes are physical

barriers which only allow particles of a certain size to pass through. The size of such particles depends upon the nominal pore size of the membrane in use.

The style of membranes used in both the Small and Large Bulk Plants are hollow fiber.The membranes are submerged in mixed liquor in atmospheric MOS Tanks, and filtrate

passes from the outside of the membranes to the inside (lumens) over a pressure gradient.This pressure gradient is an important operating parameter and is known as TMP (trans-membrane pressure). TMP must always be below 50 kPa to avoid membrane damage.

Since the MOS Tank is atmospheric, the lumens of the membrane are under vacuum.The magnitude of the required vacuum is dependent upon the extent to which themembranes are fouled. Membrane fouling, therefore, is another important operating

parameter and is qualitatively assessed through membrane permeability.

Permeability is calculated as membrane flux (l/m 2h or LMH) divided by TMP (overallunits of l/m 2h bar). Permeability varies with TMP and TMP varies with the viscosity of water. Since the viscosity of water varies with temperature, TMP and permeability alsovary with temperature. Therefore, operating permeability is adjusted to a standardtemperature from which conclusions regarding membrane fouling can be made.

The MOS Tanks are fed from the MOS Feed/Recirculation Pumps. Filtrate is drawnthrough the membranes by means of filtrate pumps, and mixed liquor is returned to theaeration zone via an overflow on the MOS Tank.



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There are several design considerations and operating procedures focused on membranefouling management. These include providing a reasonable membrane flux, control of solids concentration in the MOS Tank, membrane scouring, relaxation, maintenanceclean, and clean-in-place (CIP). The first five devices are used continually in attempts to

sustain membrane permeability while the last is used for permeability recovery.

Flux is defined as a volume or mass flow rate divided by a unit area. Generally, a lowmembrane flux produces a longer run time between cleaning. Of course, the term lowhas to be assessed for the particular wastewater being treated, as different wastewatershave different fouling characteristics.

It should also be mentioned that there are two different flux terms. The net flux (whether it be average daily net flux, peak daily net flux, etc.) is simply the net filtrate flow ratedivided by membrane area. Conversely, the instantaneous flux is the actual flux of theMBR system due to downtime. In this context, downtime refers to any event causing the

Filtrate Pumps to shut off or function at a flow rate lower than the average daily influentflow rate. An example of this is a cleaning procedure where the filtrate pumps arestopped, causing influent flow to accumulate somewhere in the system. Therefore, sincethe total accumulated flow in a given time period needs to be processed through thesystem, and since the system is not functional for that entire time period, the filtrate

pumps need to operate at a flow rate slightly higher than the average influent flow rate.

The instantaneous flux is always larger than the average daily net flux. However, thetotal supplied membrane area should always be determined by the highest allowable flux.For instance, if a plant has a hydraulic rating that is much larger than the average dailyflow rate, the hydraulic flux may be larger than the instantaneous flux. More membranearea may be supplied, but the frequency of the hydraulic flow scenario must also beconsidered before deciding upon a reasonable membrane area.

The Small Bulk Plants were designed with a design net flux of 10.0 l/m 2h and the LargeBulk Plant was designed with a design net flux of 14.5 l/m 2h.

Solids concentration in the MOS Tank also affects the membrane fouling rate. Anincrease of the solids concentration in the aeration zone directly increases the solidsconcentration in the MOS Tank since mixed liquor is fed from the former to the latter.Therefore, solids concentration must be controlled in the aeration zone in order to controlthe solids concentration in the MOS Tank. Bacterial growth and reproduction occur as

biological activity for wastewater treatment proceeds. In order for the solidsconcentration in the aeration zone to remain constant, solids must be wasted from thesystem. This is accomplished through a wasting line piped from the discharge of theMOS Feed/Recirculation Pumps to the Sludge Holding Tank for further processing.

A second method of solids concentration control in the MOS Tank is through operationof the MOS Feed/Recirculation Pumps. The mixed liquor solids in the MOS Tank arecontinually concentrated as the Filtrate Pumps pull treated water through the membranes.



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Consequently, the MOS Feed/Recirculation Pumps must supply a sufficiently higher flowto the MOS Tanks than the filtrate flow from the MOS Tanks such that the mixed liquor concentration in the MOS Tanks does not exceed a certain value. Of course, this flowrate depends upon the solids concentration in the aeration zone. Typical solidsconcentrations are 10,000 mg/l in the aeration zone and 12,500 mg/l in the MOS Tank.

This gives a typical MOS feed rate of 5 times the filtrate flow rate. The solidsconcentration in the MOS Tank is allowed to reach 14,500 mg/l during plant peak dailyflow events.

Another technique used to reduce fouling rates is membrane scouring. Air, supplied fromthe Aeration/MOS Blowers, is distributed through the membrane modules specifically for scouring purposes. Mixed liquor is also distributed through the modules, but for thespecific reason of maintaining an equal flux across all membranes instead of scouring.

Nonetheless, distribution of air and mixed liquor in this fashion provides a two phasescouring action. The scouring energy must be vigorous enough to remove bulk solidsfrom membrane surfaces but sufficiently gentle to not damage the membranes.

A third method for managing membrane fouling is relaxation. Relaxation is a procedure programmed in the logic controller which shuts down the Filtrate Pumps associated withthe MOS Tank undergoing relaxation for one minute. Scouring air and mixed liquor flowto the MOS Tank continues during relaxation to dislodge solids from the membranesurface while the continual pull from the Filtrate Pumps is no longer present. Relaxationoccurs once every twelve minutes.

Cleaning procedures are performed as a final means of managing membrane fouling.Two types of cleaning procedures are employed for this purpose. A Maintenance Clean(MC) is recommended once or twice per week and utilizes a 300 ppm chlorine solution toremove organic fouling. Flow is directed through the lumens of the membranes and intothe MOS Tank. There are two backwash steps during a MC with intermediate relaxation.A complete MC procedure lasts approximately 50 minutes.

The second cleaning procedure is termed Clean-In-Place (CIP). Solutions of 1500 ppmchlorine and 2 wt% citric acid can both be used during a CIP, the former used to removeorganic fouling while the latter is used to remove inorganic scale. A CIP lastsapproximately 5-7 hours per chemical and can be thought of as a permeability recovery

procedure rather than a fouling prevention mechanism like a MC. The cleaning solutionsare prepared in the MOS Tank which requires initial draining of the mixed liquor in thetank. Valving is arranged such that the filtrate pumps draw the cleaning solution throughthe membranes and recirculate it back to the MOS Tank. There are a number of intermediate steps associated with a CIP which are detailed in other documents. A CIP isrecommended whenever the permeability drops below a certain value or every 3 months,whichever comes first.

An important clarification regarding the chlorine concentration specified for the cleaningsolution should be made here. The hypochlorite ion (OCl-) actually provides disinfection

power in liquid (the hypochlorite ion concentration will be referred to as disinfection



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power in the following discussion). Therefore, any chemical or solution that can provide hypochlorite ions in solution can be used instead of chlorine. It is common touse sodium hypochlorite solutions (bleach) instead of chlorine because of the risksassociated with storing and using compressed chlorine gas.

Since chlorine was commonly used in the past, a set of vocabulary related to chlorinedisinfection has become the vernacular when discussing disinfection. This lexicon has been preserved in the age of hypochlorite solutions. Consequently, the specification of a1500 ppm chlorine solution during CIP, for example, means that the disinfection power of the cleaning solution should be the same as a 1500 ppm chlorine solution, regardless of what ultimate form of hypochlorite ion is used to make the solution.

Continuing with sodium hypochlorite as the common solution used for MBR systems,one mole of sodium hypochlorite has the same disinfecting power as one mole of chlorine. This analysis assumes sodium hypochlorite fully dissociates into the sodiumand hypochlorite ions, and that chlorine fully dissociates to HOCl, hydrogen ion, and

chlorine ion in water. The latter assumption is based on chlorine equilibrium in water at pH greater than 4, which should always be the case in MBR applications. Once insolution, HOCl establishes equilibrium with hydrogen ion and hypochlorite ion.Therefore, a 1500 ppm chlorine solution produces a specific concentration of hypochlorite ion for disinfection. This hypochlorite concentration is the same if equalmolar amounts of chlorine or sodium hypochlorite are added to solution because of theequilibrium between HOCl, hydrogen ion, and hypochlorite ion.

As a final point regarding the lexicon that has been established surrounding chlorinedisinfection, the listed or advertised concentrations of sodium hypochlorite solutionsoften times reflect a trade percent available chlorine. For example, a twelve percentsodium hypochlorite solution may actually contain 120 grams per liter of chlorineequivalent disinfection power (120 gpl/1000 gpl [density of water] = 0.12). Thissolution, then, is approximately 10.76 per cent sodium hypochlorite by weight.

This distinction, although technically accurate, has produced little difference in suppliedCIP equipment thus far. The recommendation for sodium hypochlorite transfer pumpsused during CIP is to dose the required chemical in the MOS Tank in 20-30 minutes. Asa quick estimate, one can use the advertised sodium hypochlorite concentration as theweight per cent sodium hypochlorite and assume a 20 minute fill time to size thesetransfer pumps. In most cases, if not all, using this pump with the more accurate weight

per cent sodium hypochlorite will produce a fill time of less than 30 minutes. However,the more accurate calculation should always be used to calculate the sodium hypochloritemetering pumps used during MC as this dosing rate must be matched with the continuouswater flow rate used during the backwash (MC cleaning solution is made continuouslyin-line while CIP cleaning solution is made batch-wise in the MOS Tank before beingused).

The cleaning philosophy of the Small Bulk Plants and the Large Bulk Plants is that of CIP only. Since all of the plants are treating sanitary wastewater and the net flux rates



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are low, the membrane fouling rate is not anticipated to be high. Sodium HypochloriteChemical Feed Pumps are used in conjunction with the Filtrate Pump of the onlinetreatment train to create the cleaning solution in the MOS Tank of the train to be cleaned.

Effluent System

As mentioned above, the Filtrate Pumps are charged with the task of pulling treated water through the membranes and pumping the filtrate to the final discharge location. Twomain considerations when deciding upon a filtrate system are pump type and pumplocation. It may be that settling on one category dictates the other, but that is not alwaysthe case.

Two common filtrate pump types used in MBR applications are centrifugal and rotary(gear or lobe). Centrifugal pumps offer an economical advantage over rotary pumps butare often times more restrictive in suction capabilities. Rotary pumps offer the additionaladvantage of reversing the flow through the pump. This significantly simplifies the

piping required to accommodate various membrane cleaning operations.

Wet suction lift is an important criterion for the filtrate pumps because these pumps mustovercome membrane pressure losses (max 50 kPa), piping pressure losses, and,

potentially, elevation lift on the suction side. This is where the physical location of thefiltrate pumps becomes important. If the filtrate pump is located above the membranes,not only does a structure need to be provided to support and provide access to the pumps,

but the pump must provide enough wet suction lift to overcome the aforementionedlosses. A filtrate pump that is located on the ground, on the other hand, can takeadvantage of the siphon effect as the suction of the pump is at a lower elevation than themembrane filtrate header. Therefore, rotary pumps are a better choice of filtrate pumpwhen located above the MOS Tank.

As a further note, the location of the filtrate pump does not have a significant impact onthe required total developed head (TDH) of the pump. If the final discharge point is at alower elevation than the liquid contents of the MOS Tank, the siphon effect will occur onthe suction side of the pump if elevated or the discharge side of the pump if at grade. Inother words, it takes the same amount of energy to pump from one elevation to any other elevation (ignoring differences that may occur in pipe routing of course).

Also, the dry suction lift capability of the chosen filtrate pump has little impact on theselection of the pump. The reason for this is there is an eductor located at the high pointof the suction piping. The eductor is used to prime the pump any time it is stopped. Asecondary function of the eductor is to provide a temporary solution to vacuum leaks inthe filtrate pump suction piping. Such leaks will cause the filtrate pump to gas bind.Prolonged operation with a leaky vacuum system will cause a loss of production andoperational headaches.

It should be noted that the operation of a grade-located filtrate pump may requireadditional attention as the extra elbows in the piping act as potential collection points for gas or vapor which could break pump suction. However, leaking eductor isolation valves



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are expected to be more problematic. For this reason, solenoid valves, or any other valves that perform well under vacuum and frequent actuation, are used for this service.

Rotary gear pumps were chosen as the Filtrate Pumps for the Small and Large Bulk Plants because the pumps are located above the MOS Tank. The design on the Bulk

Plants, however, does not take advantage of the reduced complexity of the filtrate pipingrequired for cleaning. The reason for this is that the original design utilized centrifugal pumps, and the change to rotary gear pumps did not come until the project was wellalong its schedule. Also, since MC is not provided on the Bulk Plant design, only oneline could be removed for piping simplification if desired.

A common means of system protection, when utilizing positive displacement pumps, isthe supply of relief valves. Relief valves offer the advantages of overpressure protectionand overheating protection by providing an open flow path at a given set pressure.Overpressure protection is not a concern for the Bulk Plants because the total pressurethat can be developed by the chosen filtrate pumps is less than maximum allowable

pressure of the piping and pump casing. Also, the pump motor is protected through themotor control center (MCC).

The only failure mechanism associated with these pumps, therefore, is overheating andseizing. Overheating will occur if there is inadequate flow through the pump to carryheat away. A low-low filtrate flow trip function is provided for this purpose instead of relief valves.

The reasons for providing a low-low flow trip function for pump protection instead of arelief valve for the Bulk Plants are numerous. The first reason is the failure mechanismof the Filtrate Pumps as discussed above. Second, a VFD is provided on the FiltratePumps to adjust pump speed to meet a desired filtrate flow rate. The only way for the

pump to experience low flow, other than a mechanical failure of the pump, is through aclosure of a downstream block valve. If this happens, the normal flow control schemewill increase pump speed. Given the selected motor horsepower for the Small Bulk Plants, the total differential pressure that can be developed by the pump is approximately38 psi at the maximum speed (approximately the same for the Large Bulk Plant). This

presents a difficulty in designing the inlet and discharge piping for a relief valve tocomply with the 3% and 10% (of set pressure) pressure losses, respectively. Third, thesuction losses have to be subtracted from 38 psi to determine a set point since this valueis given as a developed pressure. This drives the relief valve set point even lower,especially when one considers the maximum pressure drop across the membrane.Finally, with such a low set point, the normal pump discharge pressure may exceed therelief valve set point or be close enough to cause excessive valve chatter.

Often times, filtrate is disinfected before leaving the MBR system battery limits as finaleffluent. In the case of the Small and Large Bulk Plants, disinfection is accomplishedthrough tablet chlorination. Filtrate flows through the Tablet Chlorinator and contactscalcium hypochlorite which dissolves in the liquid. The hypochlorite ion (OCl -) provides



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the disinfecting power. A static mixer is also supplied downstream of the TabletChlorinator to ensure proper mixing.

The residual chlorine concentration can be varied by adjusting the cap of the TabletChlorinator. The initial cap setting will be determined in the field. The Tablet

Chlorinators are provided as one operating and one spare to facilitate switch over whenthe tablet inventory is depleted in the online unit. Switch over will require operator attention as there is no automatic means of detecting when the tablets have beenexhausted.

Other disinfection methods include UV light and sodium hypochlorite. In the authorsexperience thus far, disinfection using sodium hypochlorite is by far the most common

practice. This method involves the injection of a sodium hypochlorite solution into thefiltrate stream. A static mixer is often placed downstream of the injection point but may

be disregarded in some installations. This method offers the advantage of utilizing (or partially utilizing) existing equipment as sodium hypochlorite is often used to prepare the

chlorinated membrane cleaning solutions described above.Waste Sludge Handling SystemThe waste sludge handling system designed for the Bulk Plants consists of a sludgeholding tank and drying beds. Waste activated sludge is directed to the Sludge HoldingTank from the MOS Feed/Recirculation Pumps. The Sludge Holding Tank is an aerated,atmospheric tank with a sludge retention time that satisfies the sludge stabilizationrequirements of project specification SAES-A-104 (USEPA Class B sludgerequirements) prior to sludge dewatering and final disposal. The specific sludge retentiontime is country/project specific and other countries/projects may have differentrequirements. Aeration is provided to keep the contents of the tank well mixed and to

prevent septic conditions. Essentially, the Sludge Holding Tank functions as an aeratedsludge digester.

Digested sludge flows by gravity from the Sludge Holding Tank to the Drying Beds for dewatering. To assure the aforementioned sludge stabilization requirements areachieved, discharge from the Sludge Holding Tank will only occur when the liquid levelin the tank reaches the 20 + 10 days of wasted volume level and will continue until the 20day level is reached (approximately 2.5m drained to 1.7m for the Small Bulk Plants andapproximately 3.2m drained to 2.1m for the Large Bulk Plant). Draining the SludgeHolding Tank is a manual operation. Free water flows through the Drying Beds, iscollected in an under drain, and is returned to the Equalization/Emergency Sump viagravity. The dried sludge is physically scooped from the drying beds and can be used for land application or sent to a landfill.

Control Description

Equalization/Emergency Sump and Feed Flow ControlThe purpose of the Equalization/Emergency Sump is to provide a buffering volume for the site diurnal flow patterns and a collection volume to accommodate an emergency



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flow volume, for one day, of four times the average daily flow less the hydraulic capacityof the MBR system. As the influent flow to the MBR system is pumped from theEqualization/Emergency Sump via the MBR Feed Pumps, the forward feed flow controlis closely associated with the level control in this sump.

The MBR Feed Pumps are supplied as one operating and one spare and will have aremote manual start function. Startup of the operating pump will require a level in theEqualization/Emergency Sump. If at any point the level in the Equalization/EmergencySump drops below a low-low level, a shutdown signal will be sent to both pumps.Additionally, a fault signal from the operating pump will automatically enable the start of the spare pump, provided the aforementioned level condition is met. The stop functionwill be a manual operation and will be provided remotely through the DCS.

The forward feed flow to the MBR system will be automatically controlled to a user defined set point through use of a flow meter and control valve. When both treatmenttrains are operating, the set point of the flow controller should be equal to the site average

daily flow. The level in the Equalization and Emergency Sump will be allowed to vary between high and low levels, based on the site diurnal flow pattern. The level controller for the Equalization/Emergency Sump will provide a remote set point to the feed flowcontroller when level eclipses or falls below the level band established by the diurnalflow pattern. As level increases, the remote set point will gradually increase until thehydraulic capacity of the two treatment trains is reached. If level continues to rise, ahigh-high alarm will be given to alert the operators that system capacity has beenreached. As level decreases, the remote set point will gradually decrease in an attempt to

prevent a low-low level tripshutdown of the MBR Feed Pumps. If level continues todecrease, the forward flow set point will be set to zero, the automatic block valvesupstream of the Drum Screens will close, and the recycle line will open. Forward flowwill continue when the Equalization/Emergency Sump level reaches its normal highlevel.

During an automatic shutdown of one treatment train, the forward flow set point will beautomatically reduced to the design flow rate of one treatment train if the previous user defined set point exceeded this value. The only automatic shutdown of a treatment trainoccurs from a high-high level in the aeration zone of the Anoxic/Aeration Tank.Accordingly, the remote set point of the flow controller from the level controller in theEqualization/Emergency Sump will now becontinue to adjust ed the remote set point of the flow controller based on the normal flow, design flow, andut will not be allowed toeclipse the hydraulic capacity of one treatment train.

A flow recirculation line is also supplied on the MBR Feed Pump discharge line for purposes of accurate and precise forward feed flow control. Since the Small Bulk Plantsshare a common design and are supplied with the same equipment, and since the averagedaily flow variation between each plant is comparatively significant, the recycle line is

provided so the MBR Feed Pumps will operate on a controllable portion of their pumpcurves. The normal position of the automatic valve on the recycle line will be sitespecific. The pump characteristics for the larger flow Small Bulk Plants may be such that



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recycle may not be needed for adequate flow control. For the plants that do requirerecycle, the valve should normally be open with closure on a low flow measurementlywhen the feed pump cannot produce the desired forward feed flow . This scheme shouldnot be impacted by the Equalization/Emergency Sump level control of the forward flowset point because the entire flow control scheme will be set up to handle the range of

flows from normal to hydraulic capacity. The feed flow and Equalization/EmergencySump level control schemes should also be customized for each location such that theresidence time in the Equalization/Emergency Sump does not cause an appreciableincrease in water temperature (i.e. some plants may be run at a lower sump level) .

The feed flow controller is also active in the anoxic recirculation, MOS feed, and filtrateflow control loops. This interaction will be discussed in more detail during thedescription of those individual control schemes.

Drum ScreenBackwashing of the Drum Screens is required whenever the screen openings become

plugged with solids. This is accomplished with a controller which opens a solenoid valveon the utility water flush line at regular intervals. Another control shuts the solenoidvalve after a flush has occurred for pre-set duration.

Automatic block valves are also supplied upstream of the Drum Screens. These valvesare used to stop feed flow to the associated treatment train, and may be actuated manuallythrough the DCS or automatically through a high-high aeration zone level trip function or a CIP procedure. Limit switches are also provided on these valves for indication of whether the valve is opened or closed. If one of the valves is closed and the forward feedflow exceeds the capacity of the online filtrate pump, level will build in the onlineaeration zone which may completely stop feed flow.

Aeration/MOS BlowersAir is required to support the biological reactions occurring in the aeration zone and to

provide scouring to the membranes to manage fouling. One positive displacementAeration/MOS Blower is supplied per treatment train to provide air for both functions. Acommon in-line spare is also provided. The spare blower automatically starts on a faultsignal from one of the normally operating blowers. The discharge of the spare blower is

piped into the air distribution line of both treatment trains. Automatic block valves,normally used to isolate the spare blower from the air distribution lines of both trains,automatically open to the appropriate treatment train when the spare blower becomesoperational.

A VFD is supplied with each Aeration/MOS Blower to manage the total air demand for the MBR system. The two processes requiring air have different needs, and a unique air control scheme is in place to handle both requirements.

The aeration zone in the Anoxic/Aeration Tank requires enough air for the biology todestroy BOD and nitrify. This air demand varies as the BOD and ammonia load to thesystem varies. To handle this variable air demand situation, dissolved oxygen (DO)



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control is utilized. The DO probe in the aeration zone measures the concentration of dissolved oxygen in the mixed liquor. A DO controller compares this measurement to auser-defined set point (2 mg/l DO is recommended) and manipulates the travel of thecontrol valve to maintain the set point.

Unlike the varying air demand of the aeration zone, the MOS Tank requires a constantscouring air flow rate. Since both services are the responsibility of one centrifugal blower, any fluctuation in air demand in the aeration zone affects the air flow rate to theMOS Tank. Therefore, a flow controller is utilized to maintain a constant air flow to theMOS Tanks. A flow meter in the air line to the MOS Tanks measures the air flow rateand communicates with the flow controller. The flow controller manipulates the VFD onthe blowers to maintain flow. A low-low air flow interlock is also utilized to protect themembranes in the event that scouring air cannot be delivered to the MOS Tank. Under this scenario, the block valve on the mixed liquor feed line to the MOS Tank isautomatically closed and the filtrate pumps are tripped for the individual trainexperiencing the condition.

Certain steps in the CIP procedure requires air flow to the MOS Tank to be stopped.During these times, the block valve on the air line to the MOS Tank undergoing CIP isautomatically closed. To maintain proper air flow to the associated aeration zone, thecontrol valve is locked in a predetermined position through a DCS stopits last positionand the signal from the DO controller manipulates the VFD on the associated blower.Operation in this manner will also prevent a blower relief event.

Sludge Tank/Equalization Sump BlowersAir is supplied to the Sludge Holding Tank and Equalization/Emergency Sump for the

purpose of mixing. Individual positive displacement blowers are provided for eachservice. A common spare blower is also provided. The automatic start of the spare

blower and the operation of the automatic discharge block valves are enacted exactly thesame as those for the spare Aeration/MOS Blower. In fact, the automatic start of anyspare equipment follows the same philosophy. When the spare equipment is notconnected to two lines, automatic block valves are not used. Rather, check valves arerelied upon to prevent flow in the reverse direction.

Since the purpose of these blowers is to provide mixing, no control logic is required. The blowers have been sized to provide adequate mixing for the maximum liquid volumes of the Sludge Holding Tank and Equalization/Emergency Sump. Manual motor speedadjustment will be required if less mixing air is desired.

pH ControlSodium hydroxide and sulfuric acid chemical feed packages are supplied to controlsystem pH. Each chemical feed package consists of a day tank and individual dosing

pumps for each treatment train with a common spare. The day tanks will be equippedwith level instruments and a low-low level trip of the dosing pumps for equipment

protection.



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The pH probes in the anoxic zone of the Anoxic/Aeration Tanks continually monitor system pH. When the pH falls outside of a predetermined range, the pH controller willautomatically start the Caustic Dosing Pump or Acid Dosing Pump associated with thetreatment train experiencing a low or high pH, respectively. To avoid on/off cycling of the dosing pumps, chemical will be continually injected until the pH reaches the opposite

limit of the initial fault (i.e. if the pH is low, caustic will be dosed until the pH reaches thehigh limit of the operating range).

Anoxic/Aeration TanksThe biological processes used for wastewater treatment occur in the Anoxic/AerationTanks. Any measures not described above for controlling biological treatment willtherefore be described here. This includes control of the anoxic recirculation rate, theMOS feed rate, and control of mixed liquor wasting. Equipment and instruments used inoverall system control functions not directly associated with biological treatment but

physically located near or on the Anoxic/Aeration Tanks are also described here. Thisincludes the MOS feed rate, control of mixed liquor wasting, level control and trip

functions, and temperature measurements for permeability alarms.Control of the recirculation rate from the aeration zone to the anoxic zone is important for nitrogen removal purposes. Control of the MOS feed flow is important for managing thefouling rate of the membranes. Both flows are produced from the MOS Feed/RecircPumps (supplied as one operating and one spare) and are controlled in a similar fashion.The operator defines a set point for each flow rate as a ratio to the influent flow rate.Flow meters are used in each line to measure actual flow and communicate with theassociated flow controllers. The influent flow meter also communicates with each flowcontroller, which creates an actual set point in flow based on the user-defined input ratio.The position of the flow control valves is manipulated based on the deviation between theactual flow rate and the set point.

The amount of waste activated sludge (WAS) is controlled to manage the MLSSconcentration in the Anoxic/Aeration Tanks. If the biomass concentration is too low, theMBR system may not reach its treatment objectives. If the MLSS concentration is toohigh, the MOS Tanks will experience operational problems (see Process Descriptionabove for more details).

The control scheme surrounding wasting is fairly simple and consists of an automaticvalve and a flow totalizer. The operator will define how often the automatic wastingvalve will open and the amount of flow to waste per event. The automatic valve opens atthe start of each interval and will close when the defined volume, as measure by the flowtotalizer, is wasted. A level transmitter and controller is also utilized on the SludgeHolding to prohibit the wasting valve from opening if the level in the Sludge HoldingTank is too high.

Level in the Anoxic/Aeration Tank also plays a role in the operation of the filtrate pumps.If the total daily filtrate volume does not match the total daily influent volume, level inthe Anoxic/Aeration Tank will increase or decrease. Tank level is, however, expected to



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Operator action is required to initiate the CIP procedure which proceeds completelyautomatically.

The inputs to the permeability calculation are filtrate flow rate and temperature. Filtrateflow rate is measured by the flow meter downstream of the associated filtrate pump of a

given MOS Tank. The temperature measurement from the aeration zone is used tocorrect the permeability measurement at the operating temperature to that at a standardtemperature (typically 20 C). This is necessary because the pressure drop through themembranes decreases with increasing temperature due to a decrease in water viscosity.Therefore, the pressure drop through the membranes at 35 C may be low enough to

produce an actual (uncorrected) permeability that does not alarm a CIP at face value.However, this may be a false representation of membrane fouling because the decrease in

pressure drop due to the increased temperature may be masking any increases in pressuredrop due to fouling. Since the clean membrane permeability criteria are defined at 20C, a temperature adjustment of permeability is necessary.

Clean-In-Place (CIP) ProcedureSaudi Aramcos operating philosophy is complete automation. For this reason, severalautomatic valves are provided where manual valves would normally be used for a semi-automated CIP procedure. In an effort to facilitate a comprehensible discussion of theCIP procedure for both the Small Bulk Plants and the Large Bulk Plant, and for a CIP

procedure in general, a list of definitions is given below. The description of whichautomatic valves open or close during the various steps of the CIP procedure willreference the following definitions.

MOS Tank Fill Line: The line from the online filtrate pumps to the MOS Tank undergoing CIP. Sodium hypochlorite is also dosed in this line.

MOS Tank Recirculation Line: The line from the filtrate pumps associated with theMOS Tank undergoing CIP through which the cleaning solution is recirculated. This lineis piped into the MOS Tank fill line.

Mixed Liquor Feed Line: The discharge line from the MOS Feed Pumps of the aerationzone associated with the MOS Tank in question. This line supplies mixed liquor from theaeration zone to the MOS Tank for solids separation.

MOS Tank Air Line: The discharge line from the Aeration/MOS Blowers associatedwith the MOS Tank in question. This line provides scouring air to the membranes.

MOS Tank Drain Line: The drain line for the MOS Tank in question.

Operating Filtrate Pump: The filtrate pump associated with the treatment train that is notundergoing CIP.

Normal Discharge Line: The discharge line of the filtrate pumps that directs filtratetowards the Tablet Chlorinators and the final discharge point.



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The following list describes the steps involved in the CIP procedure with explanation of the control functions required to enact each step. A CIP procedure is initiated through theDCS by operator action. The control system will cycle through the steps described belowand the CIP procedure status will be displayed on the control screen.

Chemical: Sodium HypochloriteChemical Concentration: 1500ppm chlorineChemical Recirculation flowrate: 568L/hr (2.5 gpm) per module - aflux rate of 15 lmh or the lowest flow achievable by the pump.Frequency: 3 monthsDuration: 5-7 hours

Procedure:1. Stop the Filtrate Pump associated with the MOS Tank being

cleaned. Drain Tank stop blower when 1/3 of the module is

exposed . Upon operator initiation of the CIP sequence, theautomatic valve upstream of the Drum Screen associated withthe affected treatment train will close. This will cause theforward feed flow set point to automatically adjust to the designflow of the online treatment train (see description inEqualization/Emergency Sump and Feed Flow Control section above ).Simultaneously, the Filtrate Pump associated with the MOS Tankbeing cleaned will automatically stop. Then, the automaticvalves on the Normal Discharge Line of this filtrate pump and onthe Mixed Liquor Feed Line will also close. The mixed liquor feedflow control set point will be automatically set to zero. The MOS

Tank will then be drained by opening the automatic valve on theMOS Tank Drain Line. The level controller on the MOS Tank willshut the automatic block valve on the MOS Tank Air Line when1/3 of the module is exposed. Recall that when the block valveon the MOS Tank Air Line is closed, the blower VFD ismanipulated from the DO signal in the appropriate aeration zoneinstead of the flow meter in the MOS Tank Air Line.

2. Start ML feed pump with the membrane tank drain open. Run

for 2 minutes to allow sludge on the bottom of the tank to beremoved. The automatic valve on the Mixed Liquor Feed Line

will open and the set point of the mixed liquor feed flowcontroller will reset to its previous value. A timer will close thisautomatic valve after two minutes and will again set the mixedliquor feed flow controller set point to zero.

3. Drain tank and close membrane tank drain. The automatic valve

on the MOS Tank Drain line should still be open at this point and



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will close when the level controller on the MOS Tank indicates anempty tank.

4. Fill tank with filtrate, start blower when 1/3 of module is

exposed. Fill to level of headers. Aerate for 30 mins. The MOS

Tank will be filled with filtrate from the Operating Filtrate Pump. The automatic valve on the MOS Tank Fill Line will be opened asthe automatic valve on the Normal Discharge Line of theOperating Filtrate Pump is closed. When 1/3 of the module isexposed, as determined by the level controller on the MOS Tank,the automatic valve on the MOS Tank Air Line will be opened.Similarly, the level controller on the MOS Tank will determinewhen the tank is filled to the level of the headers. At this point,the automatic valve on the MOS Tank Fill Line will be closedwhile the automatic valve on the Normal Discharge Line of theOperating Filtrate Pump is opened.

5. Drain tank stop blower when 1/3 of the module is exposed.After 30 minutes of aeration, a timer will open the automaticvalve on the MOS Tank Drain Line. The level controller on theMOS Tank will again initiate the closure of the automatic valve onthe MOS Tank Air Line when 1/3 of the membrane module isexposed. This controller will also close the automatic valve onthe MOS Tank Drain Line when the tank is empty.

6. Fill with chemical solution to set point level just above the top of

the module. After the MOS Tank is completely drained, the

automatic valve on the MOS Tank Fill Line will open while theautomatic valve on the Normal Discharge Line of the OperatingFiltrate Pump closes. The Sodium Hypochlorite Chemical FeedPumps will start after these valves are appropriately positionedand will operate for a pre-set period of time (determined duringstart up). The appropriate amount of chemical should be dosedin the MOS Tank before the tank is filled to the top of themembrane modules. Therefore, the level controller on the MOS

Tank will open the automatic valve on the Normal Discharge Lineof the Operating Filtrate Pump and close the automatic valve onthe MOS Tank Fill Line when this level is reached. Ensuring that

the Operating Filtrate Pump runs longer than the SodiumHypochlorite Chemical Feed Pumps provides a flush to the MOS

Tank Fill Line.

7. Filtrate Recirculation with filtrate pump - 568L/hr (2.5 gpm) per module or the lowest flow achievable by the pump for 20minutes. After the level mentioned in the previous step isreached, the automatic valve on the MOS Tank Recirculation Line



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will open and the Filtrate Pump associated with the MOS Tankundergoing CIP will be started. The flow rate of this filtrate pumpwill be controlled to the aforementioned flow rates through thefiltrate pump flow controller (set point will be automaticallyadjusted to the above flow rate). After 20 minutes, the Filtrate

Pump will again be stopped.

8. Soak for a. 3 hours if T > 10 Cb. 5 hours if T < 10 C

9. Air Pulse during soak stage a. Chlorine CIP: 30 secs on / 15 mins off. The air pulse is

achieved by opening and closing the automatic valve onthe MOS Tank Air Line. A timer controls the opening andclosing of this valve based on the previously mentioned

schedule. During this step, the blower VFD will still becontrolled by the DO signal in the aeration zone. b. Citric CIP (NOT USED FOR THIS APPLICATION): 15 mins on /

15 mins off: air pulse is achieved by opening the valve for that tank and having the blower run for the 15 min period.

10. Drain tank. After the last 15 minutes of the air-pulsed soakingstep, the automatic valve on the MOS Tank Drain Line is opened.

This valve will close when the level controller senses an emptyMOS Tank.

11. Refill with mixed liquor start blower when 1/3 of the module isexposed. After the MOS Tank has been completely drained, theautomatic valve on the Mixed Liquor Feed Line will open and theset point of the mixed liquor feed flow controller will be adjustedbased on the feed flow to the online treatment train as if thetreatment train undergoing CIP was the only online train.

12. Relaxation: 15 mins with aeration and mixed liquor feed pump

running. The MOS Feed/Recirc Pump is now providing its normalfeed flow the MOS Tank. During this step, the automatic valveon the MOS Tank Air Line will open to provide scouring air to the

membranes. The Filtrate Pump remains off and the MOS Tankwill operate in this relaxation mode for 15 minutes.

13. Prime filtrate suction line with priming eductor. After 15 minutes

of relaxation, the CIP sequence will open the solenoid valves onthe eductor suction line and the eductor motive fluid line. Thesevalves will be closed when the level sensor in the eductor suction



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line senses a liquid level. This indicates that the Filtrate Pumpsuction line is primed.

14. Filtrate Recirculation: recirculation filtrate back to the membranetank for 10 minutes for chemical flush and turbidity spikes

(TURBIDIMETER NOT PROVIDED IN THIS APPLICATION). Theautomatic valve on the Normal Discharge Line of the FiltratePump undergoing CIP should already be closed from step 1.Similarly, the automatic valve on the MOS Tank RecirculationLine should already be open from step 7. After the level sensorin the eductor suction has measured a liquid level and closed thevalves on the eductor lines, the Filtrate Pump of the MOS Tankundergoing cleaning will be started. Recirculation of filtrate backto the MOS Tank will occur for 10 minutes. The VFD on thisFiltrate Pump will now be manipulated by the forward feed flowto the online treatment train as if the treatment train undergoing

CIP was the only online train (now the Filtrate Pump flow rateshould be in sync with the mixed liquor feed flow, much like innormal operation).

15. Open the isolation valve for turbidity meter (NOT IN THIS APPLICATION), start in normal filtration. After 10 minutes of filtrate recirculation, the automatic valve on the NormalDischarge Line of the Filtrate Pump associated with the MOS

Tank undergoing cleaning will open. The automatic valve on theMOS Tank Recirculation Line will also shut at this point. Thismarks the end of the CIP sequence and the entire MBR system

will operate as in normal filtration. The automatic valveupstream of the Drum Screen will open and forward feed flowcontrols will be adjusted for two online trains (feed flow set pointwill reset to the user-defined set point previously applied). TheFiltrate Pumps of both trains will also operate in their normalratio control to the total forward feed flow (flow per Filtrate Pumpshould be half of the total forward feed flow).

Clean-in-Place Permeability- Warning for CIP at 100 lmh/bar (4.12 gfd/psi)- Shutdown at 60 lmh/bar. (2.47 gfd/psi)

- Clean return at 200 lmh/bar. (8.24 gfd/psi)

Maintenance Clean (MC) ProcedureEven though MC is not provided on the Small Bulk Plants or the LargeBulk Plant (due to small size, low membrane flux, and the fact that thewastewater feed is sanitary waste), the procedures for a MC areoutlined here for completeness. However, no control commentary isgiven as this procedure does not apply to this application.



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Chemical: Sodium HypochloriteChemical Concentration: 300ppmChemical Backwash flowrate: 568L/hr (2.5 gpm) per module - aflux rate of 15 lmh

Frequency: 7 daysDuration: 50 minutes total.

Procedure

1. Stop Filtration: optional to have ML feed pump off, aerationcontinues.

2. First Backwash:

a. 1 minute line prime: Chemical pump and filtrate pumpoperating

b. 4 minutes chemical backwash: continue with pumpsrunning at required flowrates.

3. Relaxation: 10 mins, optional to have ML feed pump off. 4. Second Backwash:

a. 1 minute line prime: Chemical pump and filtrate pumpoperating

b. 4 minutes chemical backwash: continue with pumpsrunning at required flowrates

c. minute line flush: chemical dosing pump stops and

filtrate is allowed to flush out residual chlorine.

5. Relaxation: 10 mins, optional to have ML feed pump off.

6. Prime filtrate suction line with priming eductor. 7. Filtrate Recirculation: recirculation filtrate back to the membrane

tank for 10 minutes for chemical flush and turbidity spikes. 8. Open the isolation valve for turbidity meter, start in normal

filtration.

Filtrate PumpsThe Filtrate Pumps are supplied as one operating and one inline spare with automaticswitchover to the spare pump on a fault signal from the operating pump. Filtrate flowrate is automatically controlled as a ratio to the MBR system feed flow throughmanipulation of a VFD. A signal from the downstream flow meter is also an input in thecontroller which manipulates the VFD in order to ensure the correct flow rate is achieved.The purpose of this ratio control is to actively manage liquid level in the system. Since



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the Filtrate Pumps are routinely stopped for relaxation, CIP, etc., the time available for pumping filtrate from the system is less than the time over which wastewater enters thesystem. In other words, to prevent accumulation of water, the same volume must be

pumped out during a shorter time period.

The user-defined ratio used in this control scheme can only be changed by operator action. The overall control ratio, on the other hand, is automatically adjusted based onthe number of online treatment trains. Recall that one treatment train is consideredoffline when the block valve upstream of the Drum Screen associated with that train isclosed. For instance, during normal operation when two treatment trains are operating,the input control ratio is multiplied by one-half to determine the required flow rate for each filtrate pump. When one treatment train is operating, the input control ratio is notadjusted and only functions on one filtrate pump.

Several scenarios which cause the block valve upstream of the Drum Screen to close have been addressed above. Another scenario causing the closure of this valve is the lack of a

run signal from the Filtrate Pump. Valve closure cannot be immediate, however, becauseof the automatic switchover to the spare Filtrate Pump.

The exact value of the user-defined ratio must be determined after a CIP frequency has been established. However, since the relaxation step accounts for the majority of systemdowntime, the ratio can initially be set at 13/12 (operate for 12 minutes, relax for 1minute). In fact, the 13/12 ratio may be adequate given the other controls provided onthis system.

The ratio control described above is the primary means of controlling liquid level in thesystem. The secondary means is level control from the aeration zone described in theAnoxic/Aeration Tanks section above. This secondary control is used in the event theoperator-defined ratio is not exactly correct, and to conform to Saudi Aramcos

philosophy of fully automatic operation with little operator action.

The range over which this level controller acts is relatively small. In all of the Bulk Plants, the normal liquid level in the aeration zone is one half foot lower than theoverflow weir separating the anoxic zone from the aeration zone. Also, 3 feet of freeboard is provided in the anoxic zone. Therefore, the level control of the filtrate

pumps will be enabled when the level in the aeration zone approaches the level of theoverflow weir (operator input for exact height) and will continue over the remainder of the total tank height.

Membrane relaxation, as mentioned above, is a regular maintenance procedure initiated by the control system for the purpose of managing membrane fouling. During relaxation,the Filtrate Pumps are shut off while mixed liquor and scouring air continue to flow pastthe membranes. Relaxation occurs once every twelve minutes for one minute. Sincerelaxation is a regular maintenance procedure initiated by the control system and lastingone minute, the block valve upstream of the Drum Screen will not close due to the lack of the Filtrate Pump run signal during relaxation.



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Since the filtrate piping is elevated above the normal liquid level in the MOS Tank, theFiltrate Pump suction piping is not necessarily flooded after pump shutoff. Therefore, a

pump priming sequence must be initiated before the Filtrate Pumps are restarted. Insome cases, such as after certain steps in the CIP or after relaxation, the Filtrate Pumps

are automatically primed. In other instances, such as an operator-initiated shutdown of the Filtrate Pumps, the priming sequence must be started by an operator through the DCS.The priming sequence involves the opening of the automatic valve on the eductor suctionline followed by the opening of the automatic valve on the eductor motive fluid line(instrument air in the case of the Bulk Plants). The eductor will pull filtrate through themembranes and into the suction piping of the Filtrate Pumps. A level sensor in theeductor suction line senses when the piping is liquid-filled. At this point, the block valves on the eductor suction and motive fluid lines are closed and the Filtrate Pumps areallowed to start.

A number of Filtrate Pump trip functions are provided in order to protect equipment.

One such function utilized for membrane protection is the high-high TMP trip discussedin the MOS Tanks Section above. Another Filtrate Pump trip function utilized for membrane protection is the low-low scouring air trip. Unlike the high-high TMP tripfunction which protects against physical fiber damage, the low-low scouring air trip

protects against excessive (and possibly irreversible) fouling.

Two additional trip functions, provided for Filtrate Pump protection, include a low-lowMOS Tank level trip function and a low-low filtrate flow trip function. The low-lowMOS Tank level trip offers protection against cavitation due to low net positive suctionhead whereas the low-low filtrate flow trip affords protection against overheating andseizing. The reasons for providing a low-low filtrate flow trip instead of a relief valve for

pump protection are outlined in the Effluent System section of the Process Descriptionabove. The Filtrate Pumps will also be shutdown on the lack of a MBR Feed Pump runsignal to prevent a low-low MOS Tank level and an excessive solids concentration in theMOS Tank.

Sodium Hypochlorite Chemical Feed PackageThe Sodium Hypochlorite Chemical Feed Package consists of a 100 liter storage tank andtwo Chemical Feed Pumps (one operating, one spare). Automatic switchover to the spare

pump is provided and occurs on a fault signal from the operating pump. A low-lowSodium Hypochlorite Chemical Feed Tank level trip of the Chemical Feed Pumps is also

provided for pump protection.

The Chemical Feed Pumps are used to supply chlorine to the MOS Tank during a CIP.Therefore, these pumps are automatically started and stopped through the CIP controlsequence (as described in the CIP Procedure above). In the event that the MOS Tank isfilled faster than expected, the level controller on the MOS Tank signals a shutdown of these pumps.

Operating Philosophy

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