am-pro-wwt-wwt 4.6 activated sludge-sec3 issue 1.1

8/11/2019 AM-PRO-WWT-WWT 4.6 Activated Sludge-SEC3 Issue 1.1

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THAMES WATER Asset Management

Internal Company and Partners Asset Standards

Technical Lead: Colin Pickersgill Issue 1.1

AM-PRO-WWT-WWT 4.6 Activated Sludge-SEC3 Page 1 of 26

UNCONTROLLED WHEN PRINTED

Asset Management Asset StandardSection 3 of 5Operating Standard

Wastewater Non Infrastructure

WWT 4.6 Activated Sludge

Document Reference AM-PRO-WWT-WWT 4.6 Activated Sludge-SEC3

Version Issue 1.1

Data Owner Sarah Shipsey

Data Steward Kevin Kelleher

Technical Lead Colin Pickersgill

Document Author Tim Wheatley

Authorised by Asset Standards

Document Location AM Lotus Notes Best Practice Database on LNAPP01

Date of Issue December 2012

Reason for Issue Conversion and validation of old BOP into new Standard format

Next Review December 2014


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3.0 Operating Standard

This document defines the Performance Measures for the Activated Sludge Process together withkey operator activities, and a troubleshooting guide. Background information on the Activated Sludgeprocess with types of plant together details of the Waiver and Asset Risk Certificates (ARCs),processes is detailed in Section 1.

3.1 Act ivated Sludge Performance Measures

Measure Why Frequency Method ofCollection

AerationPlant FeedSewage

Determination of the aeration plant feed sewageBOD, allowing calculation of organic loading andthe F:M ratio. This provides an understanding ofcurrent plant performance.

Works PE< 1000> 1000< 5000

5,000 - 25,00025,000-100,000

100,000-250,000250,000-1,000,000

>1,000,000

6 monthlyQuarterlyQuarterlyQuarterlyMonthlyMonthlyWeeklyWeekly

Sampling /Laboratory

Analysis

AerationPlant FeedSewageFlow

Settled Sewage Flow required for ProcessCalculations.

Continuous SCADA,Onsite flowreading, Dailylog

MLSS MLSS must be maintained within therecommended range for the site to ensure BODand ammonia compliance and to avoid excessivecosts in the aeration of unnecessarily high MLSSconcentrations.

< 1000> 1000

1 x week3 x week

Fixed orPortableMeter, On siteworkstation,Laboratory

RASS To ensure sufficient concentration of biomass is

returned to the aeration lane(s) to maintaintreatment. It is also required to calculate SSVI tooptimise performance.

< 1000 -1 x week

> 1000 -3 x week

Fixed or

PortableMeter, On siteworkstation,Laboratory

SSVI To quantify the settling characteristics of theactivated sludge. The SSVI gives an indication ofthe ease with which the mixed liquors will settle inthe final settlement tank, (FST), so thatperformance of the FSTs can be optimised.

< 1000 - 1 xweek> 1000 -3 x week

On siteworkstation


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Measure Why Frequency Method ofCollection

RAS Rate Control of RAS rate to ensure sufficientconcentration of biomass in the aeration tanks toprovide robust treatment and compliance.

Adequate return rates will cater for appropriatelevels of thickening in the Final Settlement Tanksensuring optimum performance.

Package Plant PE< 1000> 1000

Continuous

1 x week5 x week (Note 1)

RAS PumpControl.SCADA,MLSS / RASSconcentration.

SAS Rate Control of Surplus Activated Sludge Rates: Theamount of solids wasted from the system willdetermine the amount of solids retained in thesystem and therefore the sludge age. Sludge age

is an important factor in optimising treatment andcosts, controlling nitrification (low sludge age),and the growth of filamentous micro-organisms.

Package Plant PE< 1000> 1000

Continuous


SAS PumpControl.SCADA

FSTBlanketLevel

Control of Sludge Blanket levels ensures thatgross solids are not lost or discharged in the finaleffluent.

Package Plant PE< 1000

> 1000

Daily orContinuous


SCADA, Fixedor PortableBlanketDetectors.

Energy Control of aeration costs.

Package Plant Log kW HoursPE < 1000PE > 1000

Continuous


SCADA, Sub-metering

Note 1: Performance measure may be completed per visit where site visit frequencies are less than 5times per week,Note 2: DO concentration is likely to be continuously measured for all plants.


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3.2.3 Process Checklis t KEY

OP = OPERA TOR

1W/2D = 2 TIMES PER WEEK 1W = 1 TIME PER WEEK1W/3D = 3 TIMES PER WEEK 1M = 1 TIME EVERY MONTH1W/5D = 5 TIMES PER WEEK (EVERY WEEKDAY)1W/7D = 7 TIMES PER WEEK (EVERYDAY)

3.2.4 Process Checklis t Notes

What to check Action / What to check for on deviation.Bubble pattern and size ofbubbles

Check for uneven patterns / bubble size as a symptom of broken orbadly fitted diffusers.

Blowers are operatingcorrectly (where used)

Check discharge flow and pressure. Align with that required byaeration systemCheck drawn current and confirm this is consistent withmanufacturers dataThe air volumes from the blowers are usually controlled by means ofdissolved oxygen probes and/or timers to ensure that the optimumvolume of air is supplied.

Surface aerators are running(where used)

Check that the aerators are operating.

Check that surface aeratorsand automatic switching (ifapplicable), are operatingcorrectly.

Check for excessive noise, heat, vibration etc..Check that automatic switching (where appropriate), is operatingcorrectly.

Dissolved oxygen (DO)reading (where probes areinstalled)

Check operation of actuated valves in aeration systemCheck for changes in the strength of the incoming sewageCheck for specific operations impacting load to ASP, liquors return,tanker discharge, etc.Check the DO probe for fouling and clean if necessary.Check calibration of DO probe.

Check that mixers in anoxic(unaerated) zones areoperating.

Arrange for repair if not operational or inactive.The activated sludge plant may or may not have anoxic zones which

utilise the oxygen from the nitrates present in the returned activatedsludge (RAS), and thus reduce oxygen requirements. They tend toproduce a more stable sludge with a lower SSVI.

Calibration of MLSS meter ifrequired

On-line MLSS monitors should be regularly checked and calibratedagainst a sample of mixed liquor sent to the Laboratory forgravimetric determination of MLSS concentration.If the correct equipment is available, a gravimetric determination ofMLSS concentration may be carried out on site.

Cockpit Data Collection MLSS, RAS suspended solids, dissolved oxygen, SSVI, power and flow readings should be logged and captured on the Site Cockpit /Dashboard.


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3.3 Troubleshooting Guide

WWT 4.6 Troubleshooting Guide Act ivated Sludge Process

Symptom Possible Cause Action Risk / ConsequenceMLSSconcentrationchanges(Increase ordecrease)

SAS rate not sufficientto keep pace with thesludge growth rate (orSAS rate too high)

Changes in settledsewage make-up.

Insufficient air to keep

solids in suspension

Solids lost fromsettlement tank

Change the SAS ratelittle by little to recoverfrom theincrease/decrease

Check quality ofsewage/settled sewagefeed

Adjust rate of aeration

Solids loading toohigh for the final tanksor insufficientbiomass to treatsewage

Increased costs Insufficient air to

sustain sludgegrowth.

Deterioration ineffluentquality/possibleconsent failure

Blanket spillages

Poordistribution ofsewage and/orRAS

Distribution channel/pipework wholly orpartially blocked

Investigate distributionchannels & pipeworkand clear any blockages

Check RAS return rate

Plant not correctlyutilised withoverloading ofprocess

Inferior qualityeffluent,

Possible consent

failurePoor bubblepattern

Foam or mousse onsurface of aerationlane.

Pipe fracture orblockage

Condensate in airmain

Sample foam and sendto the laboratory forfilamentousidentification.

Investigate airflows andpressures

Repair fractures Check air control valves Drain condensate traps

Poor effluent quality Possible consent

failure


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WWT 4.6 Troubleshooting Guide Act ivated Sludge Process

Symptom Possible Cause Action Risk / Consequence

Turbid &

Cloudyeffluent and/orHigh ammoniain FE

Very Low MLSS

High DO / dead MLSS

Low DO / high MLSS

Strong incoming flowor liquors. (Low DO /normal MLSS)

Problems withchemical dosing

Check RAS pumps in

working order anddelivering correct flowfrom the FSTs..

Reduce SAS rate toincrease MLSS

Potential toxic discharge.to the site

Check DO probes areoperating correctly.

Consider re-seedingMLSS

Check DO probes areoperating and clean.

Increase aeration ifpossible. DO NOToverpressure aerationpipework or domes.

Increase SAS rate toreduce MLSS.

Visually check incomingsewage.

Check DO probes areoperating and clean.

Increase aeration ifpossible. DO NOToverpressure aerationpipework or domes.

Process check of sludgestream

Check that the pump isoperating correctly andfor chemical overdose.

.

Loss of nitrification.

Loss of treatment(ammonia and BOD)

Probable consentfailure

Loss of nitrification. Inferior quality effluent

quality.

Loss of nitrification. Possi ble consent

failure

Possible consentfailure


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Internal Company and Partners Asset Strategy & Standards




Appendices

3.4 Notes for Operation and Control

3.4.1 Act ivated Sludge Process Effic iencyTo maintain high levels of treatment performance over a wide range of operating conditions it isfundamental to apply adequate control of the Activated Sludge Process. Sludge Age and MLSS arethe most commonly used parameters for ASP control. The main strategies for the management of

ASP systems are:

Control of Dissolved Oxygen concentration in the aeration lanes: Oxygen is used by micro-organismsduring the breakdown of organic matter and the oxidation of ammonia. Oxygen levels in the aerationlanes are therefore an indication of the microbial activity taking place. In feeding, micro-organismsuse more air so if we dont supply sufficient air the DO concentrations in the lane will decrease. Onthe other hand, sharp increases in DO levels could reflect inhibition of the microbial activity or even itsdestruction due to a toxic feed.

DO levels are frequently used for aeration control. Probes are attached to SCADA and haveoperational set points assigned to them. The aeration system will pump more or less air as requiredto maintain a minimum DO concentration, guaranteeing enough air is available to support wastedegradation. There is littl e benefit , in terms of ammonia removal, in going beyond DO concentrationshigher than 2.5mg/l. DO levels in aeration lanes are usually maintained in the 0.5 to 2 mg/l range.

Control of Returned Activated Sludge rates: The purpose of the RAS is to maintain a sufficientconcentration of biomass in the aeration tanks to provide adequate degradation of the waste in thewater. Adequate return rates will cater for appropriate levels of thickening in the Final SettlementTanks. As the retention times in the settlement tanks will be reduced during peak flows and thethickness of the sludge will therefore be reduced, RAS pumping systems must be sized to guaranteethat enough solids are returned to maintain stable MLSS levels.

Control of Surplus Activated Sludge Rates: The amount of solids wasted from the system will

determine the amount of solids retained in the system and therefore the sludge age.

Sludge Age: Is an indication of whether efficient nitrification is likely to be taking place.

MLSS must be maintained within the recommended range for the site so as to guarantee BOD andammonia compliance and to avoid excessive costs in the aeration of unnecessary high solidsconcentrations.


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3.4.2 Impact of Poor Operation of Act ivated Sludge Process As the activated sludge process is the means by which contaminants are biodegraded/oxidised andtherefore removed from the wastewater, poor performance of the process has a direct impact on theworks ability to meet its consent. Inadequate aeration would risk BOD and/or ammonia breaches

while poor operation of the Final Settlement Tanks, usually as a consequence of poor control of theoverall process will jeopardise solids removal and therefore Suspended Solids compliance.

Symptom ConsequenceHigh MLSSconcentration

High MLSS may result in low dissolved oxygen levels and consequently, risingammonia levels in the final effluent. If the Mixed Liquor Suspended Solids (MLSS) riseeven further, then (depending upon the specific plant), then there may be problemsremoving enough Activated Sludge from the final settlement tanks, leading to increasedsolids within the final effluent and even sludge blanket loss.

Low MLSSconcentration

Low MLSS result in shorter sludge ages that are potentially insufficient to carry outeffective nitrification, increasing the risk of ammonia breakthrough in the final effluent.If the Mixed Liquor Suspended Solids (MLSS) are lower than target levels, the amountof biomass available for BOD removal may also be insufficient risking, a breach of theBOD consent.

Excessively lowF:M levels

The relationship between the Biochemical Oxygen Demand (BOD) loading and theamount of biomass (or Mixed Liquor Suspended Solids), (MLSS), within aeration plantis quantified as the food to mass ratio (F: M).Excessively low F: M levels will reduce the food available to the organisms tometabolise, which will result in reduced growth which in extreme cases can lead to themortality rate exceeding the reproductive rate, resulting in a loss of treatment.

High or low F:Mratio

Further problems with either high or low F: M ratio is the increased potential to cause afilamentous bloom, which can detrimentally affect the treatment process and settlementin the FSTs, especially when combined with high MLSS and high flows.

High DOConcentration

Causes pin flocs and poor settlement. Inefficient operation leading to high power costs.

Low DOConcentration

Poor performance including possible loss of nitrification and high ammonia levels in thefinal effluent.Potential for formation of filamentous micro-organisms resulting in high SSVI values andloss of solids from the FSTs especially when combined with high MLSS and highflows.

Too Long aSludge Age

This may be a result of supplying high organic loads to the aeration lanes. It is also aconsequence of operating with too low a surplus activated sludge (SAS) rate. Longsludge ages will be reflected in high MLSS, excessive oxygen requirements, powerusage and growth of filamentous micro-organisms

Too Low a Sludge Age

This may be due to operating higher than required surplus sludge (SAS) wastage rates leading to low MLSS in the plant. Low sludge ages result in inadequate nitrification

and the potential for high ammonias in the final effluent.Too high RASrates

A reasonably clear effluent might be produced but It would affect the ability of the FinalSettlement Tanks to adequately thicken the sludge, as a result of no sludge blanketsfine solids will be noticed in the final effluent.The level of MLSS in the aeration basins will rise due to the low Suspended Solidsconcentration of the SAS.

Too low RASrates

Not enough nitrifiers returned to aeration lanes for ammonia removal, risk of ammoniabreakthrough, denitrification in the FSTs and blanket spills.

Too shorthydraulicretention time

Treatment may be inadequate. An increase in ammonia, Suspended Solids and BODwould be likely.


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3.4.3 Settlement Problems Floc Shear: This effect is caused by the breaking of flocs by mechanical means, mostly due

to over aeration and the use of surface aerators. During a SSVI jar pin flocs have a powderyappearance. They sink to the bottom of the jar but no clear water/sludge interface is formed.

Dispersed Growth: Tension active substances in the sewage (i.e. certain detergents) canprevent the flocs from aggregating and therefore gaining the size and weight required for theirsettlement. One indication is high SSVI however there are other causes of high SSVI whichwould need to be considered.

Non-Filamentous Bulking: This phenomenon is caused by food imbalances in the biologicalreactor. When fed purely with BOD and COD, but with insufficient nutrients such asPhosphorus (P) or Nitrogen (N), bacteria tend to generate only waste as they are unable tosynthesise new cells. This waste is sticky, it doesnt compact and because it is highly water-retentive it does not settle either. The condition is exacerbated by very old sludge. It results infoaming on aeration tanks and Final Settlement Tanks and could lead to blanket spills.

Bubbles of gas in Final Settlement Tanks: The presence of gas bubbles in the FinalSettlement Tanks is problematic because in their upward movement to escape to the

atmosphere the bubbles lift bits of flocs. Bubbles in the settlement tanks can be either air ornitrogen . The former are caused by over aeration. The latter by anoxic conditions developingin the Final Settlement Tanks sludge when bacteria reduce nitrate to nitrogen gas. Thecondition is promoted by the lack of an anoxic zone in the aeration tanks and by holdingsludge for too long in the Final Settlement Tanks. It results in small sludge flocs in the finaleffluent and on bubbles visible on the surface of the Final Settlement Tanks and in the SSVItest jar.

Filamentous Bulking: This condition is caused by the growth of filamentous organisms ororganisms that can grow in a filamentous form. Although filaments can help to prevent flocshear by providing a stronger structure to the floc, when growing in excess the filaments ofsingle-cell organism attach end-to-end forming a floc with a large surface area to mass ratiowith poor settling characteristics. This type of bulking results in the formation of a mousse-type foam on the aeration lanes and increased risk of blanket spilling from the FinalSettlement Tanks and the return of activated sludge with very low suspended solids.

3.4.4 Filamentous OutbreaksEach organism within the Mixed Liquor has its own preferential set of conditions. Under thoseconditions, that particular organism will multiply fastest than the others and be dominant. In sufficientnumbers filamentous bacteria render the flocs hydrophobic (not liking water) and attach to the gasbubbles in the aeration lane, which then floats to the surface.

Filamentous growth


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Filamentous outbreaks occur when certain types of bacteria predominate in the aeration basin due tofavourable influent temperature, F: M ratio, nutrient levels and DO concentrations. It is fundamentalto act fast once symptoms of filamentous bulking have been observed in the aeration lanes, the FinalSettlement Tanks or during the SSVI or MLSS microscopy tests.

Since the outbreak is caused by the predominance of a specific type of bacteria it is necessary toidentify it, to understand the conditions which are favourable for their development and to alter suchconditions.

A Filamentous ID will include the filament identification, filament index (how much of that species is inthe MLSS), floc size, shape and strength.By knowing the filament and what conditions it prefers, decisions can be made on how to control theorganism in the long term. Sometimes it is necessary to increase the MLSS concentrations (notoften), sometimes to decrease it. Sometimes it is necessary to increase the DO, other times toreduce it or change the air balance. In extreme cases chemical dosing may be required.

The two main bacteria responsible for filamentous outbreak are:

Nocardia Microthrix Parvicella

Favourable conditions: Septic conditions pH 6.5

Lives on fats Tolerates most sludge ages Low F:M ratio Uniform DO Encouraged by foam created by

detergents in sewage

Favourable conditions: Low F:M ratio High Sludge Age (>10days)

Low DO Grows on broken solids Crude sewage mixed with RAS prior to

aeration

Response:

Remove fats from system Physical removal of foam Ensure anoxic conditions in anoxic zones

Response:

Reduce sludge age Maintain uniform DO in aeration pockets Ensure Primary Settlement Tanks are not

holding sludge


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SAS must always be increased carefully as a too abrupt change could result i n the wasting oftoo big a portion of ni trifying bacteria species (nitrifiers).

Nitrifiers are very vulnerable to changing conditions in the aeration lanes and as such to shock loads.

Primary Settlement Tanks should be controlled to minimise BOD load to the biological treatmentstage.Some plants might need to run at high MLSS levels particularly during cold weather.

BOD shock loads are detrimental both to BOD and ammonia compliance. As the organic load to theaeration lanes increase, heterotrophic bacteria predominate over a longer section of the lane, leavingpotentially insufficient volume for nitrification. Young or immature activated sludge can reach the FinalSettlement Tanks. These are small and with poor settling characteristics, also risking BOD and TSScompliance.

The main cost associated with biological treatment is due to electric power for aeration. It isfundamental to maintain MLSS levels within the prescribed band to guarantee adequate treatmentwithout incurring the excessive cost of unnecessary aeration.

Minimising Aeration Costs

Maintain blowers, diffusers and air pipework in good working order

Adequately manage Primary Settlement Tanks to maximise BOD removal and minimise loadto aerobic treatment.

Avoid BOD sludge loading to aeration plant (shock load)

Manage liquors return times to prevent shock loads.

Avoid excessive MLSS levels

Properly locate, calibrate and maintain DO probes

3.4.6 Contingency Planning

BO D shock loadShock BOD loads to the aeration lanes can be caused by:

Return Liquors (sludge dewatering, tanker discharges, cesspool waste, etc) Poor quality settled sewage Accidental discharges from industry of effluents with a high organic load, (dairy, brewery,

abattoir, cattle markets, food processing, etc)

In the case of an unplanned increase in the BOD load to the plant caused by internal events such ashigh strength liquors (digester liquor, septic sludge etc.), then the flow should be diverted if possibleto a spare tank or tankered from site and the source identified and stopped.The increased microbial activity in the aeration lanes triggered by the surge of food will cause a sharpdecrease in DO levels. It is fundamental to monitor these closely and increase aeration levels wherepossible, either by raising blowers speed / vane position or by increasing the immersion depth ofsurface aerators.To maintain enough active biomass in the reactor, sludge withdrawal rates from Final SettlementTanks should be increased and SAS rates decreased. MLSS levels must be closely monitored.


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Toxic DischargeWhenever possible the toxic discharge must be prevented from entering the biological reactor. Followthe Procedure for unknown substance entering STW (below).

Procedure for an Unknown Substance Entering an STW

Control Options Ac tion to take

Notify appropriate personnel Inform line management and or the department controllingeffluent discharges from industry. This may be a site emergencyrequiring immediate action.

Prevent any further toxicdischarge from entering theworks

- divert flow to storm tanks- undertake over-pumping or pumping station tankering- take samples

Keep the surplus activatedsludge separate from othersludges

Divert surplus sludge to a separate holding tank. The sludge isprobably toxic and must not receive further treatment (digestion,pressing, return to works inlet, land disposal). Special disposalmethods must be considered

Mixed liquor suspended solidslevel and sludge settlement

Monitor regularly, recovery may take several days

Reseed the plant withuncontaminated activatedsludge from another works

Balance surplusing the toxic sludge with the reseeding operation.

Final effluent and rivermonitoring (upstream anddownstream)

Monitor (ammonia, turbidity) closely for several days. Furthermaybe required.

If this is not possible, flows should be reduced and MLSS increased in the aeration lanes (bydecreasing SAS rates). Final effluent must be closely monitored and intensive sampling performed.

As Activated Sludge is probably toxic it should be kept separate form other sludge on site and notreceive further treatment (digestion, pressing, return to works inlet, land disposal). Special disposalmethods must be considered.Should the toxic materials consist of matter not suitable for standard treatment and disposal, theactivated sludge will need to be removed and replaced with uncontaminated activated sludge fromother works. The reseeding operation should be balanced with purging of the toxic sludge.

Rising SSVI

As the settling properties of the mixed liquor start to deteriorate and SSVI values to increase, RASrates should be raised to prevent sludge blanket in the FST from spilling. The causes of increasingSSVI should be addressed using the Troubleshooting Guide section 3.5.3.

3.4.7 Plant Release / Taking out of and Bringing into service

Taking out of service Planned

When activated sludge process units are being taken out of service the Plant Release procedureneeds to be followed together with site process risk assessment and plant availability. If an ASP unitcannot be taken out of service due to process risk then this needs to be highlighted in the RiskRegister for potential investment/capital delivery purposes.


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THAMES WATER

Technical Lead: Colin Pickersgill

AM-PRO-WWT-WWT 4.6 Activated Sludge-SEC3


Internal Company and Partners

3.5 Performance Factors

3.5.1 Calculations

Hydraulic Retention Time in Aeration Tank:

The hydraulic retention time is the length of time that the mixed liquors actually spend in the aeration tank. Typically it should be above 4 houdifferent tank configurations. A reduction in the retention period occurs for example during periods of peak flow of when RAS rates are increased. can be obtained:

(A) Aeration volume: Design information SOM(B) Daily Sewage Flow: Onsite flow reading, Daily log(E) RAS flow: Onsite flow reading, Daily Log, SCADA

A B C D E F G

Vol of aerationtank (m 3)

Sewageflow (l/s)

Sewage flow(m 3/hr)

Sewage RTin aeration

(hrs)

RAS flow(l/s)

RAS flow(m 3/hr)

Total flow toaeration (m 3

B x 3.6 A / C E x 3.6 C + F


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Food:Mass Ratio

This is the ratio between the substrate available to the microorganisms and their concentration. The data required to calculate it can be obtained:(A) Daily Sewage Flow: Onsite flow reading, Daily log(B) Sewage BOD: Laboratory(D) MLSS: On site test, Laboratory, Solids Meter(E) Aeration volume: Design information SOM

A B C D E F

Daily sewageflow (m 3/d)

SewageBOD (mg/l)

BOD load(kg/d)

MLSS(mg/l)

Volume ofaeration tank

(m 3)ML Mass (kg) F:M(kgML/kgBO

A x B / 1000 D x E / 1000 C / F




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Sludge Age

This parameter gives an indication of whether effective nitrification is likely to occur in the aeration lanes. The data required to calculate it can be obt(A) MLSS: On site test, Laboratory, Solids Meter(B) Aeration volume: Design information SOM(D) RAS SS: On site test, Laboratory(E)/(F) SAS rate: Current settings, logbook, SCADA(K) FE SS: On site test, Laboratory(L) Sewage Flow: Onsite flow reading, Daily log

A B C D E F H J K L

MLSS(mg/l)

Aerationtank

volume

(m3

)

TotalMLSSmass

(kg)

RASS(mg/l)

SAS flow(l/s)

SAS hrsrun/d

DailySAS

volume

(m3

)

ML lost toSAS (kg/d)

FE sussolids(mg/l)

Sewageflow (l/

A x B /1000

E x 3.6 xF H x D / 1000




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SSVI (Stir red Specific Volume Index)

The Stirred Specific Volume Index (SSVI) is the volume in ml occupied by 1g of sludge after a periodof 30 minutes of settling whilst being slowly stirred. This measure was developed to quantify the

settling characteristics of the activated sludge. It gives an indication of the ease with which the mixedliquor will settle in the final settlement tanks. Because solids concentration is related to settleability,SSVI figures are usually quoted at a MLSS of 3500 mg/l. The SSVI is then designated as SSVI 3.5 .SSVI tests are commonly performed by local operators and the parameters value can be obtainedfrom the collected data using the calculations below:

MLSS

A B C D

MLSS(mg/l)

Height ofsludge incylinder

Settlement% MLSS SSVI (ml/g)

B x 2 C x 10,000 / MLSS

RAS

E F G H

RAS Susp.Solids (mg/l)

Height o fsludge incylinder

Settlement% RAS SSVI (ml/g)

F x 2 G x 10,000 / MLSS

If RAS SSVI is GREATER than the MLSS SSVI

J

Sludge SettlementReduction

SSVI 3.5 (ml/g)

(H-D) x (E-3500) / (E-A) H J

If RAS SSVI is LESS than the MLSS SSVI

J

Sludge SettlementReduction

SSVI 3.5 (ml/g)

(D-H) x (E-3500) / (E-A) H + J


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3.6 Health and Safety

The Thames Water H&S management system has been designed to ensure effective risk control, anda key aspect of this system is risk assessment and the safe undertaking of activities conducted by thebusiness.

External suppliers using this Operational Standard must utilise an H&S process that is equal to theThames Water approach to managing risk

Thames Water managers must identify via the completion of the SHE 1 document activities to beundertaken and how they are to be controlled via the adoption and implementation of relevant keyprocedures (HSPs) and risk assessment guidelines (RAGs).

Thames Water employees must be provided with the necessary information, resources and training toperform these tasks, and prior to completion they must confirm that they can comply with theseprocedures and guidelines. However, if an employee believes that compliance will require additionalresource, the task should be reviewed using the SHE 5 form and if necessary curtailed.

Thames Water managers must be trained in this system, and be in receipt of a controlled copy of anH&S manual which is supported by information posted on the company portal.

Glossary of Terms

TERM DESCRIPTION

ASP Activated Sludge Process

DO Bulk Dissolved Oxygen

F:M Food : Mass ratio

MLSS Mixed Liquor Suspended Solids

SSVI 3.5 Stirred Sludge Volume Index at 3500 mg/l MLSS

RAS Returned Activated Sludge

SAS Surplus Activated Sludge

FST Final Settlement Tank

---- End of section 3 ----

am-pro-wwt-wwt 4.6 activated sludge-sec3 issue 1.1

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