OPERATIONAL TECHNOLOGY BRANCH
NATIONAL TRAINING AND OPERATIONAL TECHNOLOGY CENTER
UPDATED SUMMARY
of the
OPERATIONAL CONTROL PROCEDURES
for the
ACTIVATED SLUDGE PROCESS
by
Alfred W. West, P.E.
Chief, Operational Technology Branch
JANUARY 1978
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WATER AND HAZARDOUS MATERIALS
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OBJECTIVES
This summary provides a current digest of the coordinated activated sludge plant control
procedures that have been found most useful by the Operational Technology Branch during on-
site plant operations and operator training programs.
NEW SECTIONS
Certain new sections summarize most of what will be presented in the proposed Part IV -
Sludge Quality and Part V - Operational Control issues of the pamphlet series. The objective is
to present this information now, even though on-going assessment of unique field operations
experiences continues to delay final preparation of Parts IV and V.
RETURN SLUDGE FLOW CONTROL UPDATE
It also updates the original September 1973 Return Sludge Flow Control pamphlet. The
procedures described in the original pamphlet have proven successful when applied to activated
sludges of fair to good quality. But they led to unsatisfactory over-control when applied directly
to extremely fast settling sludges that compacted to very high concentrations and to extremely
slow settling sludges that compacted hardly at all. This update identifies sludge quality and
presents a modification that makes the return sludge flow control procedures effective for most
all types of sludges encountered in actual practice.
PAST AND FUTURE
A brief digest of the previously distributed pamphlets on Observation and Control Tests is also
included for continuity of the discussions.
And finally, this Summary and the previously distributed Parts of the pamphlet series may be
modified and expanded when additional useful process control procedures are developed and
confirmed by on-site plant operation experiences.
BASIC CONCEPTS
These operational control procedures were developed to help activated sludge plant operators
satisfy all significant interrelated process requirements simultaneously. Information provided
by simple control tests reveal the coordinated control adjustments needed to maintain or restore
optimum process balance, sludge quality and final effluent quality. Conversely, the process is
not controlled by attempting to achieve preconceived levels of individual variables such as,
mixed liquor sludge concentration, mean cell residence time and food to microorganism ratios,
etc.
Perceptive observations of aeration tank foams and final clarifier surface characteristics usually
indicate long-term control requirements. Control tests such as, final clarifier sludge blanket
depth determinations, mixed liquor and return sludge concentration (by centrifuge) and sludge
settleability are used to define sludge quality and process status. Control adjustment
requirements are determined from the control test data. Trend charts of sludge settleability
(SSV) and compactability (SSC), mixed liquor and return sludge concentrations, sludge
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wasting, final effluent turbidity, and other informative parameters are maintained to reveal
process responses to the control adjustments.
Conventional parameters such as food to microorganism rations (F/M) are calculated and
sludges are observed under the microscope for monitoring and comparative purposes, but are
not used directly for process control. Thirty-minute sludge volume indices (SVI) or the
corresponding sludge density indices (SDI), which do not reveal as much about sludge quality
as do the more descriptive SSC curves, are not used at all.
The control tests (depth of blanket, settlometer and centrifuge, etc.), nomenclature and many
process relationship calculations are those proposed by E. B. Mallory in the 1940's. The
subsequent procedures to determine the required interrelated process control requirements
were evolved by the author.
Best process performance is achieved by satisfying all interrelated process
requirements simultaneously; not by exclusive dependence upon any single or
preconceived factor.
Preparation of the EPA pamphlet series entitled Operational Control Procedures for the
Activated Sludge Process, describing these procedures used by the Operational Technology
Branch (formerly NFIC-C) of the Municipal Operations and Training Division, continues. A
listing of the completed pamphlets presently available is appended.
OBSERVATIONS
(Discussed in "Part I")
The type of foam observed on the aeration tank surface and the characteristic type of sludge
solids, if any, observed rising to the final clarifier water surface usually reveal the long-term
activated sludge wasting requirements. Conclusions reached from such observations must,
however, be confirmed by the results from the other control tests before the wasting rates are
actually changed. Under certain circumstances, discussed later under Sludge Wasting for Slow
Settling Sludge, temporary short-term wasting adjustments that differ from the overall long-
term needs must be imposed to satisfy the immediate process requirements.
AERATION TANK FOAM
Voluminous, white billowing foam reveals that the sludge is too young and indicates that sludge
wasting should be reduced (to increase sludge age) on a Jong-term basis within the next few
weeks to one month. Such a reduction should not be started, however, until final clarifier
performance and results from the control tests confirm the ability to do so without forcing
sludge solids out over the clarifier effluent weirs.
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Dark brown, scummy foam usually accompanies old sludge that should be wasted from the
system. If confirmed by final clarifier performance and control test data, sludge wasting should
be increased about 15 to 20 percent per day until acceptable sludge quality is produced.
FINAL CLARIFIER SURFACE
Here again, the process control requirements indicated by the appearance of the final clarifiers
must be checked with and confirmed by the control tests before any significant changes are
made.
Clean Surface, Clear Effluent
Operators should not argue with success while producing an excellent final effluent. Except for
some minor fine tuning, they should continue their proven control strategy unless and until the
sludge blanket starts moving up towards the tank surface. The control tests, which will warn of
impending danger long before serious problems occur, will indicate the corrective control
modifications that may be needed if, and when, the process starts to deteriorate.
Ashing and Clumping
Small sludge particles (called "ashing") or large sludge globs (called "clumping"), that break
away from the settled sludge blanket and rise to the clarifier water surface almost always
indicate that the sludge is too old. This condition, which usually occurs when a fully, or overly,
oxidized sludge denitrifies in the final clarifier, indicates the need for increased sludge wasting.
Sludge Solids Washout (with good sludge quality)
Sludge solids being drawn over the effluent weirs from a high sludge blanket may indicate a
number of causes and possible solutions. Excessive raw waste flows, unequal mixed liquor flow
into, or return sludge flow out of multiple clarifiers, and improperly located overflow weirs can
induce this problem even with excellent sludge quality. Solution of these hydraulic problems
may require infiltration-inflow elimination, installation of suitable meters and control devices,
and possibly plant modifications and additions.
If wastewater flows are within reasonable limits, the control tests will show whether the solids
washout could have been caused by too high or too low return sludge pumping rates.
Bulking Sludge
True bulking (where slow settling, usually light tan colored, sludge is drawn out over the
effluent weirs) practically always indicates that the sludge is too young. Sludge wasting should
be reduced on a long-termbasis and the control tests will indicate the immediate and day-to-day
process control requirements.
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CONTROL TESTS
(Discussed in "Part II")
The clarifier sludge blanket depth observations, the centrifuge test of mixed liquor and return
sludge concentrations, and the settlometer test for mixed liquor sludge settleability define
sludge quality and process status. Control adjustment requirements are determined from the
results of these control tests. Mixed liquor D.O. is determined conventionally, and is normally
maintained in the 2 to 4 mg/1 range. Final effluent quality is monitored by turbidity tests to
reveal plant performance responses to the control adjustments. Other important monitoring
tests such as, BOD and suspended solids are also run, but are not used directly for control
purposes.
The settlometer test should be run at least once per day, and preferably once per shift. The
centrifuge, depth of sludge blanket and final effluent turbidity tests should be run at least once
per 8-hour shift, and more frequently during troublesome periods.
SLUDGE QUALITY
(To be discussed in a forthcoming "Part IV" of the pamphlet series.)
Sludge quality is defined by the shape and endpoint concentration of the settled sludge
concentration curve that is developed from the settlometer and centrifuge control test data.
Figure 1 displays a family of Settled Sludge Volume (SSV) and Settled Sludge Concentration
(SSC) curves for the three different generalized types of activated sludge at similar mixed liquor
concentrations.
The upper set of Settled Sludge Volume curves are graphic presentations of the settlometer test
data. In actual practice, settlometer tests are usually terminated after one hour for "Rapid" to
"Normal" sludges and continued longer only for "Slow" sludges.
Activated sludge quality, however, is defined by the shape of the Settled Sludge Concentration
curves that are calculated from the SSV and the ATC control test data.
The Aeration Tank mixed liquor Concentration (ATC) and the Return Sludge Concentration
(RSC) are determined by a 15-minute centrifuge test.
The SSC value for any SSV value is:
1000 ATC
SSC =
SSV
A set of settled sludge concentration (SSC) curves for "Rapid", "Normal" and "Slow" settling
activated sludges, developed from the upper SSV curve data at ATC = 4.0, is illustrated in the
lower portion of Figure 1.
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SLUDGE SETTLING TIMES SST (MRS)
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1 2 3
SLUDGE SETTLING TIME SST (HRSj
FIGURE 1.- SSV and SSC CURVES for Slow, Normal and Rapid
settling and concentrating activated sludges.
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12 to 18 percent in
NORMAL SETTLING SLUDGE
A "Normal" settlir g good quality activated sludge will concentrate to an SSC range from about
one hour and will reach ultimate compaction (i.e., won't settle any more)
somewhere between one and two hours.
\
RAPID SETTLING SLUDGE
A "Rapid" settling sludge, concentraing to an SSC of more than 20 percent in one hour and
reaching its endpoint concentration in less than one hour is usually an old over oxidized sludge.
SLOW SETTLING SLUDGE
An extremely "Slow" settling sludge, concentrating to an SSC of less than 10 percent is usually
a young sludge. Such a sludge may not settle at all during the first 5 or 10 minutes and may only
reach between 700 and 900 cc/1 during the first hour of the settlometer test. Furthermore,
settling and compaction continues, though at a slow rate, for 3 to 4 hours or longer.
PROCESS CONTROL
(To be discussed in a forthcoming "Part V" of the pamphlet series.)
AERATION
Except for clumping or bulking conditions (when DO's should be maintained below 1.0 mg/1),
blowers or aerators should normally be set to maintain about 2 to 4 mg/1 DO in the aeration
tanks. The aeration tank contents must be mixed thoroughly and continuously from top to
bottom.
It may be necessary to change air diffuser types and locations and modify or replace mechanical
aerators if the aeration and mixing requirements cannot be met.
CLARIFIER SLUDGE FLOW CONTROL
This section updates the September 1973 Return Sludge Flow Control Pamphlet. Here, process
control is discussed in terms of Clarifier Sludge Flow Percentages (CSP expressed as a decimal
fraction) rather than in terms of Return Sludge FlowsQRSF).
The Clarifier Sludge Flow Percentage Demand (CSPD) equations actually reflect a mass
balance around the clarifier, rather than around the aeration tank. CSPD is the new CSP setting
needed to force RSC towards the desired SSC. while keeping the sludge units entering the
clarifier equal to the sum of the return and waste sludge units leaving the clarifier.
Furthermore, the control demands are calculated in terms of clarifier sludge flow percentages
(CSP) instead of the clarifier sludge /7ow(CSF) rates. The CSP demand can be used directly as
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the set-point value for flow-pacing, automatic return sludge flow controllers. The CSP demands
must obviously be converted to sludge flow rates (based on wastewater flows) for manually-
controlled return sludge pumps.
An optimum Clarifier Sludge Flow Percentage (CSP) can practically always be determined for
the specific circumstances prevailing at any individual plant.
csp _ Return Sludge Flow + Waste Sludge Flow
Wastewater Flow
The optimum CSP range will be governed by sludge quality; but the practical application ranges
will be limited by the final clarifier size, equipment and design features.
Clarifier sludge flow percentages should be adjusted to optimize the time sludge remains in the
anaerobic final clarifier environment and minimize the amount of water pumped with the
sludge. At normal to high wastewater flow rates (especially with slowly settling sludges) proper
clarifier sludge control will hold the sludge blanket as low as possible in the final clarifier. It will
also help induce proper sludge oxidation rates, overall plant performance and final effluent
quality.
Sludge quality will be degraded if CSP is adjusted either too much lower or too much higher
than the optimum range. Too low a CSP will usually hold excessive sludge quantities in the
clarifier, increase the sludge blanket thickness and probably induce sludge septicity.
Conversely, too high a CSP will increase turbulence within the final clarifier, decrease aeration
tank detention time and (especially for slow settling sludges) also increase the sludge blanket
thickness. Of the two, a CSP that is lower than optimum can degrade plant performance more
than a CSP that is higher than optimum. Since precise optimums can seldom be maintained in
practical operations, CSP should be adjusted towards the high side rather than towards the
more dangerous low side of the acceptable CSP range.
The Settled Sludge Concentration (SSC) curve data is used to calculate the CSP adjustments
that are needed to satisfy the actual process requirements. The SSC range for CSP control is
shown on Figure 2. CSP should be controlled to force the measured return sludge
concentrations (RSC) towards the settled sludge concentration (SSC) reached near the upper
portion of the curved segment of the SSC curves for "Rapid" and "Normal" sludges. RSC
should usually be forced to the segment near the inflection point on the SSC curve for "Slow"
sludges. The segments of the SSC curves within the shaded area of Figure 2 identify the CSP
control ranges for the intermediate types of activated sludges.
The optimum CSP range will be dictated by mixed liquor sludge concentrations, sludge quality,
wastewater flows and loadings. Too high a CSP can at times be almost as harmful as too low a
CSP. Arbitrarily increasing CSP whenever clarifier sludge bulking occurs can frequently
intensify rather than cure the problem.
But conversely, CSP adjustments do not significantly alter food to microorganism ratios (F/M)
in a properly balanced system. F/Mis controlled by wasting. Mixed liquor concentrations will
be increased significantly by increasing CSP only if excessive sludge solids had been permitted
to pile up in the final clarifier previously while CSP had been reduced too low.
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In a balanced system, the 24 hour average mixed liquor concentration (ATC) remains fairly
constant from day to day regardless of normal CSP adjustments. ATC is, however, increased by
this reduced early morning wastewater flows and decreased by the increased midday flows. The
return sludge concentration (RSC), however, changes predictably and rapidly, in response to
CSP adjustments.
Normal Settling Sludge
For "Normal" Settling, good quality sludges (SSC in the 12 to 18 percent range) CSP, based
upon the sum of the return sludge and waste sludge flow rates, should be adjusted to force the
observed RSC within the 40-minute to 60-minute SSC range of the SSC curve.
00
oo
oo
I ' ' • ' | ' •
s RSC DEMAND RANGE
FOR CSP CONTROL
SST RANGE FOR
CSP CONTROL
1 2 3
SLUDGE SETTLING TIME SST (HRS)
FIGURE 2. SSC RANGE FOR CSP CONTROL
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The "Normal" sludge illustrated in Figure 2 concentrated to the 13-15 percent SSC range after
40 to 60 minutes settling time. CSP should, therefore, be adjusted to force the return sludge
concentration (RSC) within this 13 to 15 percent range.
Increase CSP to reduce RSC
and
Reduce CSP to increase RSC
The rationale of the following Clarifier Sludge Flow Demand (CSFD) calculation example is
discussed in Part III-A.
After reading the flow meters and performing the control tests, the proper clarifier sludge flow
requirement (CSPD) can be calculated from the following formula:
CSPD = CSP X RSC - ATC
SSCt - ATC
CSPD is the Clarifier Sludge Flow Percentage Demand, CSP is the observed Clarifier Sludge
Flow Percentage and SSC. is the SSC concentration at the selected settling time (t). Other
terms have been identified previously.
To illustrate, assume that:
Waste Water Flow = 1.20 mgd
Metered CSF = 0.48 mgd
Observed RSC = 17.5
Observed ATC = 5.0
Desired SSC6Q = 15.0 @ the 1-hour SST
(for the selected t = 1.0 hr.)
From the mixing formula:
CSP = ATC
RSC - ATC
5.0
= 0.40
17.5 - 5.0
(i.e. 40% of the wastewater flow)
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From the CSPD formula:
CSPD = CSP X RSC ~ ATC
SSCt - ATC
= 0.40 X 17"5 ~ 5-°
15.0 - 5.0
= 0.40 X 1.25 = 0.50 (i.e., 50%)
In this case the set-point of an automatic sludge flow controller would be increased by a factor
of 1.25 to increase the clarifier sludge percent from its original 40% setting to the new 50%
requirement.
Under manual control, the metered clarifier sludge flow would be increased by the same 1.25
factor from 0.48 to 0.60 mgd for the 1.20 mgd wastewater flow.
Rapid Settling Sludge
The clarifier sludge flow should be adjusted to force RSC towards the 15 to 20-minute SSC
value when the sludge settles too rapidly, (when SSCxn exceeds 20 percent as shown in Figure
2) 6°
It would be imprudent to try and force RSC to the 1-hour SSC value on the flat portion of the
SSC curve. The curve reveals that the sludge could be piled up, and held, in the clarifier for 2 or
more hours if the return sludge concentration were brought up to, or slightly over, this 60-
minute SSC level of 24 percent.
Slow Settling Sludge
Selection of the proper SSC and CSP target values for return sludge flow control is more critical
for slowly settling sludges than for the other two types.
Additional calculation procedures that the author has developed to help select the appropriate
SSC target value from the SSC curve are described in the "BUT vs CSP" supplement to the
pamphlet series. Additionally, here is an orderly "cut-and-try" procedure that can be used if
sludge quality and ATC do not change significantly while determining the proper CSP value.
Start out by adjusting the CSP for force RSC to some SSC value in the range between the 2-hour
and 3-hour SSC values on the SSC curve. The 2-hour SSC is usually a good starting point.
Then observe the clarifier Depth Of Blanket (DOB), the mixed liquor concentration (ATC) and
the return sludge concentration (RSC) critically. If the sludge blanket is lowered by this
adjustment, you are on the right track.
Then decrease CSP about 15 to 25 percent of its former value per day for the next 2 or 3 days. If
the sludge blanket level is reduced after each CSP adjustment, continue moderate clarifier
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sludge flow reductions until the lowest blanket level is achieved. CSP should have been
optimized at this lowest sludge blanket level. If CSP is reduced too low, however, sludge will
pile up in the clarifier and the blanket level trend will usually reverse and start rising towards
the clarifier surface. A reduction of CSP below the critical level will also be revealed by
significantly reduced ATC values.
Reverse the above procedure if reducing clarifier sludge flow did not help. Start increasing CSP
moderately, day by day until the lowest sludge blanket level is obtained. This should occur at
optimum CSP. Here again, the blanket will also start rising if CSP is increased too high.
This "wiggling-in" procedure should reveal the proper SSC, RSC and CSP needed to minimize
bulking hazards and (with properly coordinated sludge wasting) and improve sludge quality
and settleability.
CSP Control Strategies
The clarifier sludge flow percent demand (CSPD) control strategy is similar for practically all
varieties of sludge quality. It is straightforward for "Normal" and "Rapid" sludges, where
CSPD can be calculated directly from the demand formula but the additional "wiggling-in"
intermediate step may be necessary for "Slow" sludges. The shaded area intersecting the three
SSC curves on Figure 2 shows the probable SST-SSC ranges for sludge qualities intermediate
between rapid and normal, and between normal and slow sludges.
Normal Sludges
An SSC. value within the 40-60 minute SST range can be plugged directly into the clarifier
sludge flow percentage demand formula (CSPD) to determine the CSP needed to achieve
optimum process balance for most normal sludges.
Rapid Sludges
Similarly, calculating CSPD for the 15-20 minute SSC value will be appropriate for the
rapid sludges.
Slow Sludges
The SST - SSC control range, which is not usually revealed as clearly by the "Slow" SSC
curves, must be determined before the SSC target value can be selected for use in the CSPD
formula. The "wiggling-in" procedure, described previously, will establish both the proper
SST - SSC range and the CSP values for the sludge represented by the SSC curve.
Subsequently, as long as sludge quality remains fairly constant, the SSC target determined
previously by the wiggling-in procedure can be plugged into the CSPD formula to
determine the CSP requirements to adjust for changes in mixed liquor concentrations
(ATC). Finally, the CSPD formula can once again be used directly when sludge quality
improves and approaches the "Normal" status.
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General Conditions
Clarifier size and hydraulics may limit the range of practical CSF adjustments. Regardless of
calculated demands, CSF should not be reduced to the level where slowly moving, thick
clarifier sludge will plug the sludge withdrawal pipes. Nor can the CSF be reduced significantly
below the level where the clarifier sludge detention time (CSDT) consistently exceeds the
clarifier detention time (CDT - based on wastewater flow plus clarifier sludge flow) in
hydraulically overloaded small clarifiers. Clarifier Sludge Detention Time (CSDT) calculations
are illustrated in Part III-A of the pamphlet series.
Furthermore, it is usually preferable to make the total calculated clarifier sludge flow
adjustment in two or more intermediate steps, about 1 or 2 hours apart, whenever the demand
calculations call for much more than 25% CSP changes. And finally, it is good practice to
increase the low nighttime clarifier sludge flow rates to approach the anticipated higher CSP
demands before, rather than hours after, the increased daytime wastewater flows actually reach
the plant.
Here are some additional facts that have been found helpful:
CSP has been reduced too low if RSC remains substantially constant while either, or both,
clarifier sludge or wastewater flow rates change significantly. This usually occurs when
excessive sludge, that has reached maximum compaction, is piled up in the final clarifier.
CSP has also been reduced too low if ATC is lowered substantially after the clarifier sludge
flow has been reduced (with fairly "constant" wastewater flow and sludge wasting). In this
case some of the sludge that would normally have remained in the aeration tanks under
balanced conditions has been transferred to and piled up in the clarifiers by reducing CSP
too low.
Clarifier Sludge Flow Control Summary
1. Determine return sludge flow control requirements from the sludge quality and SSC
Concentrations revealed by the SSC curves.
2. Force RSC (by proper return sludge percent adjustments) to approach the following
guideline SSC values:
"Rapid" Settling Sludge 15 to 20 min. SSC
"Normal" Sludge 40 to 60 min. SSC
"Slow" Settling Sludge Start by approaching the 2-hour SSC
and then "wiggle-in" to the best value.
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SLUDGE WASTING CONTROL
Activated sludge wasting practice strongly influences sludge quality and overall plant
performance. It is frequently the "tail that wags the entire dog." Many otherwise satisfactory
activated sludge plants are seriously degraded by an inability to waste excess sludge from the
process.
This section describes an orderly procedure to control sludge wasting by day-to-day evaluation
of the trend charts. In other words, the operator can usually use the basic control test data to
determine whether to hold, increase or decrease wasting. The direction and the rate at which
the sludge quality (SSC) and sludge age have responded to previous wasting (XSU) adjustments
will be revealed by the SSC and XSU trend charts. The progressive trend chart relationships
will then indicate when the corrective wasting schedule should be readjusted.
Proper wasting control depends upon satisfying the true process requirements that
vary with loading changes and are altered by control adjustments.
Properly coordinated wasting control is seldom achieved by attempting to
establish independent preconceived levels of mixed liquor concentration, F/M, or
sludge age.
Normal Settling Sludge
Continue the established wasting rate if the sludge is concentrating to the desirable 14 to 16 SSC
range, the blankets are deep down in the final clarifier and the final effluent is clear and solids
free. The end results have proven that the true process requirements have been satisfied for the
prevailing circumstances.
Wasting schedules should be adjusted to meet the new demands when the trend chart
comparisons show that sludge quality and process balance are starting to shift away from
optimum.
Rapid Settling Old Sludge
Sludge that concentrates too rapidly (i.e., SSC exceeds 20 percent in less than 1 hour) can most
always be slowed down to a lower and more desirable SSC range by increasing the sludge
wasting rate to reduce sludge age. But the wasting rate should be increased gradually (about 15
percent per day) to progressively replace'the older sludge with younger, slower settling sludge.
Two precautions are necessary. If too much sludge is wasted at any one time, the sludge settling
rate will usually increase rather than decrease. In such cases, the mixed liquor concentration is
reduced immediately without inducing a comparable reduction in sludge age, and the thin
sludge will settle faster than a thicker sludge of equal quality. Secondly, the sludge may become
extremely slow settling and bulky if the wasting rate is increased too much for too long. This
will usually occur when the newly developed slow settling young sludge forms too great a
proportion of the total sludge mass.
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Sludge wasting rates should no longer be increased, and most probably should be reduced
slightly, when the SSC trend line starts falling at an accelerated rate.
The "ashing" and "clumping" in the final clarifier and dark scummy foam on the aeration
tanks, associated with old sludge, are practically always eliminated when proper sludge quality
is restored.
Slow Settling Sludge
Sludge Concentration Control
The settling rates of equal quality activated sludges (similar SSCs) vary according to
changes in mixed liquor concentrations (ATCs). Thin, low ATC sludges (of equal quality -
SSC) settle faster than thicker, high A TC sludges.
At times the activated sludge system can become glutted with excessive sludge solids at
excessively high mixed liquor concentrations before the sludge age increases to the point
where settling rates are normally increased. Excessive mixed liquor concentration can be
reduced, settling rates can be increased and the clarifier sludge blankets lowered, in such
cases, by increasing the sludge wasting rate sharply (maybe doubling or tripling the
previously established rate) for short time periods until the clarifier sludge blanket retreats
to safe levels.
Such "blast" wasting should be discontinued as soon as possible to avoid wasting all the
way over to the sludge bulking stage.
Sludge Quality Control
The settling rates of equal concentration (ATCs) activated sludges vary according to
changes in mixed liquor sludge quality (SSCs). Young, low SSC, sludges (at equal ATC)
settle slower than old, high SSC, sludges.
On a long-termbasis, and if the plant is not consistently overleaded, bulking can usually be
eliminated by reducing the established wasting rate to increase sludge age.
But this maneuver becomes difficult at times. If wasting is reduced too sharply, or
discontinued altogether, the newly developed (slow settling) young sludge proportion of
the entire sludge mass may start to increase more rapidly than the old sludge portion. This
would reduce, rather than increase, both sludge age and sludge settling rates initially.
Multiple dilution settlometer tests will indicate whether or not an initial short spurt of
"blast" wasting is called for before the long-term reduced wasting program is initiated.
Multiple dilution tests should be run simultaneously on three portions of the mixed liquor
sample. One settlometer should contain 100 percent mixed liquor, the second one about 75
percent mixed liquor and 25 percent final effluent and the third one should contain about
50 percent mixed liquor and 50 percent final effluent.
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An initial temporary spurt of "blast" wasting will be appropriate if the diluted sludges
settle faster and concentrate to SSC values equal to or greater than the SSC of the undiluted
sample. Blasting would be discontinued and sludge wasting should be reduced as soon as
the sludge blanket recedes below the clarifier surface water level. Wasting can then be
reduced (to increase SSC and improve sludge quality) when the sludge blanket remains
safely at lower levels.
Clarifier sludge flow control adjustments must obviously be coordinated with the wasting
adjustments. Changes in sludge quality induced by the wasting program will be revealed by
the changes in the settled sludge concentration curves which in turn are used to calculate
the current clarifier sludge flow percent demand.
"STEP-FEED" OPERATION
(Discussed in Part III-B)
Aeration tanks that are arranged for step-feed loading provide excellent operational flexibility
and can be extremely effective in combating transient overloads or poor activated sludge
quality. In a four bay step-feed aeration tank, for example, all return sludge is routed to the first
bay but the wastewater can be treated in the plug-flow mode, the contact-stabilization mode or
any place in between.
Such aeration tank arrangements provide a most effective means for curing slow settling sludge
bulking by operational control adjustment. If the sludge settling rates are starting to slow down,
most of the wastewater can be routed from the first bay to the second, or to the second and third
bay to increase the sludge oxidation pressures. If serious bulking is occurring, most of the
wastewater may be sent directly to the fourth bay in contact-stabilization fashion to induce
maximum sludge oxidation. In these cases, mixed liquor concentrations leaving the fourth
aerator bay are reduced because more sludge solids are maintained in the first, second or third
bays. Return sludge flow rates can therefore also be reduced and the activated sludge will
remain in the aeration tanks for more hours per pass and for more hours per 24-hour cycle.
If the one hour SSC values for bulking sludges are extremely low (say below 6 or 7), return
sludge alone (no wastewater) should usually be routed to the head end bay, or bays. When
sludge quality improves (SSC increasing to 8 or higher levels), however, some wastewater
should again be routed to the first bay to avoid building aerator sludge concentrations up
beyond the oxidative capacity of the aeration devices. It is usually desireable to start sending
about 1/3 of the wastewater flow to bay one when SSC values start increasing significantly. The
percentage of wastewater flow routed to bay one can then be further increased while SSC values
continue rising towards optimum.
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This same procedure can be used during rainstorm induced hydraulic overloads. If the storms
do not last too long, a high percentage of the activated sludge will be stored in the first two or
three bays, instead of being flushed out of the final clarifier, if most of the wastewater is routed
toward the outlet end aeration tank bay or bays. Furthermore, when shifted to the "step" or
"contact" mode the clarifier can handle increased surface overflow rates because the lower
concentration mixed liquor (of equal quality) leaving the fourth bay of the aeration tank will
settle more rapidly than the higher concentrated mixed liquor concentration that prevailed
during plug-flow operations.
When good sludge quality is restored, the wastewater treatment pressures can be maximized by
reverting back towards the plug-flow configuration to increase the wastewater detention time in
the aeration tanks.
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Pamphlets prepared by Alfred W. West
and referred to in the
January 1978
UPDATED SUMMARY
PAMPHLETS
Part I/Part II - Observation/Control Tests
Part III A - Calculation Procedures for Plug-Flow
Part III B - Calculation Procedures for Step-Feed Process Responses
Appendix - Organization of Data, Testing Equipment and Definitions
SUPPLEMENTS
Clarifier Sludge Blanket Responses to Clarifier Sludge Flow
Percentage Control (BLT vs CSP)
Dynamic Sludge Age (DSA)
Requests for these pamphlets should be addressed to:
U. S. Environmental Protection Agency
National Training & Operational Technology Center
Cincinnati, Ohio 45268
U. S. GOVERNMENT PRINTING OFFICE: 1978-757-140/6834 Region No. 5-11
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