NATIONAL FIELD INVESTIGATIONS CENTER
CINCINNATI
RETURN SLUDGE FLOW CONTROL
BY
ALFRED W . WEST, P.E.
CHIEF-WASTE TREATMENT BRANCH
PREPARED FOR THE INTERNATIONAL WORKSHOP ON
INSTRUMENTATION CONTROL AND AUTOMATION
FOR WASTEWATER TREATMENT SYSTEMS
SEPTEMBER 1973
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF ENFORCEMENT AND GENERAL COUNSEL
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OPERATIONAL CONTROL OF THE ACTIVATED SLUDGE PROCESS
-RETURN SLUDGE FLOW CONTROL-
Alfred W. West, P.E.
Chief, Waste Treatment Branch
National Field Investigations Center - Cincinnati
Office of Enforcement and General Counsel
United States Environmental Protection Agency
The return sludge flow, or more precisely, the clarifier
sludge flow which includes both return sludge and excess
waste sludge flows, should be adjusted to meet measurable
process requirements. Attempts to maintain arbitrary return
sludge flow percentages, for example 25%, 50%, or 100%,
etc., of the wastewater flow will seldom achieve optimum
sludge quality and process balance. Fortunately, the
results of the one-hour mixed liquor settlometer test, the
15-minute mixed liquor and return sludge centrifuge test,
and the final clarifier sludge blanket test reading provide
the basic data for simple calculation of the clarifier
sludge flow rate needed to maintain or restore process
equilibrium.
Symbols Used in Formulas and Calculation Examples
ATC = Aeration Tank Concentration - The mixed liquor
concentration determined by the standard 15-minute
centrifuge test, expressed as the percent of the
centrifuge tube occupied by the compacted mixed
liquor sludge.
CFO = Clarifier Flow - Out of Clarifier (Final Effluent)
CFP = Final Clarifier Sludge Flow Percent - (CSF/CFO) From
metered" values, expressed decimally.
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CFPD = Final Clarifier Sludge Flow Percent Demand
Required sludge removal rate as a percent of the
final effluent flow (expressed decimally).
CSF = Final Clarifier SJLudge Flow - (RSF+XSF)
CSFD = Final Clarifier SJLudge Flow Demand - Required sludge
removal rate expressed in either cu m/d or mgd.
MLTSS = Mixed Liquor Total Suspended Solids Concentration
Tmg/l)~
RFP = Return Sludge Flow Percent (RSF/CFO as a decimal)
RSC = Return SJLudge Concentration - The return sludge
concentration determined by the standard 15-minute
centrifuge test, expressed as the percent of the
centrifuge tube- occupied by the compacted return
sludge.
RSF = Return SJLudge Flow (to aeration tanks)
RSTSS = Return Sludge Total Suspended Solids Concentration
Tmg/1)
SSCt = Settled SJLudge Concentration - The calculated
concentration of the mixed liquor after "t" minutes
settling in the settlometer. (SSCt = 1000 ATC/SSVt)
SSVt = Settled SJLudge Volume - The volume occupied by the
mixed liquor after "t" minutes settling in the
settlometer. (cc/1)
XSF = Excess Sludge Flow to Waste
The Clarifier Sludge Flow Demand Formula
In practice, at least once per eight-hour shift,
operators should record the actual wastewater and clarifier
sludge flow rates, determine mixed liquor settleability by
the settlometer test, check the mixed liquor and return
sludge concentrations by the centrifuge test, calculate the
clarifier sludge flow demand (CSFD) from the following
formula, and then adjust the clarifier sludge flow to
approximate the demand.
The observed rate at which settled sludge is removed
from the final clarifier can be expressed either as the.
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metered flow (CSF) or as a percentage (CSP-recorded as a
decimal fraction) of final effluent flow out of the
clarifier. The demands (CSFD or CFPD) can also be
calculated in terms of flow rates or percentages.
CSFD = CSFx(RSC-ATC)/(SSC -ATC)
CFPD = CFPx(RSC-ATC)/(SSC -ATC)
Sludge Settling (SSV) and Concentration (SSC) Test Data
Mixed liquor sludge concentration characteristics,
determined by the settlometer and centrifuge tests, define
sludge quality and are used to determine clarifier sludge
removal requirements. The settlometer test differs from the
conventional sludge settling test for SVI determination in
that:
a. A larger diameter (12.5 cm = 5 inches) shorter
length (15 cm = 6 inches) graduated cylinder is used
to minimize the settling rate distortions that occur
when slowly settling sludge is tested in the narrow
standard 1,000 cc graduated cylinder.
b. Settled sludge volume is observed and recorded at
intervals throughout one hour (or longer for special
studies) instead of the single 30-minute reading used
for SVI.
c. Sludge quality is revealed by the shape and the end-
point (1.0 hr) of the sludge concentration curve that
is calculated from the mixed liquor concentration and
the settled sludge volumes. (SSC=1,000 ATC/SSV).
The settlometer test results displayed in Figure 1 show
that the Settled Sludge Volume (SSV) reached 235 cc per
liter at 40 minutes, 215 at 50 minutes, and 200 at 60
minutes.
The Settled Sludge Concentration (SSC) curve, revealed
that the mixed liquor sludge, with an initial centrifuged
concentration of ATC=3.0% at time zero, compacted to a
calculated concentration of 12.8% in 40 minutes, 14.0% in 50
minutes, and 15.0% in 60 minutes.
Using these sludge concentration test data and the waste
water and the clarifier sludge flow meter readings, the
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FIG. 1
Sludge Settling and Concentration Characteristics
1000
ssv a ssc
(SSC = 1000 ATC/SSV)
ADJUST RSC
TOWARDS SSC
ACCEPTABLE RSC
RANGE- SSC40_60
0
10 20 30 40 50
SLUDGE SETTLING TIME - SST, min.
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operator can calculate the clarifier sludge withdrawal rate
that will provide approximately one hour sludge compaction
time in the clarifier.
Ideally, removing sludge from the final clarifier at a
concentration approximating the 60-minute SSC value usually
minimizes the sludge detention time in the anaerobic final
clarifier environment and reduces the amount of water that
is pumped back with the sludge solids to practical minimums.
In practice, operators usually use the 50-minute SSC as
their target value to minimize the chance of increasing RSC
above the 60-minute SSC value and thereby lengthening the
clarifier sludge detention time unduly during periods of
rapidly rising waste water flows. SSC targets greater or
less than the 50 minute value are used only in exceptional
cases when other control procedures are also modified to
overcome plant or process balance abnormalities.
It may be impractical to try to maintain RSC at the
precise target SSC value at all times. The operators,
therefore, usually select a range of RSC's, between SSC 40
and SSC 60, within which the clarifier sludge flow is not
changed even though RSC does not equal the SSC target value.
Therefore, for the sludge concentration characteristics
shown in Fig. 1, the operator should change the clarifier
sludge flow rate only if the observed return sludge
concentration (RSC) value is less than 13% or greater than
15%. If it is, he should insert ATC=3.0 and SSC =14.0 into
the clarifier sludge flow demand formula and then adjust the
clarifier sludge flow to meet the current demand.
Clarifier Sludge Flow Demand Examples
The following examples illustrate the step-by-step
calculation procedures used to determine the clarifier
sludge flow needed to achieve process balance where mixed
liquor sludge characteristics equalled those in Fig. 1 and
the observed return sludge concentration (RSC) equalled
16.0%. Observed flow meter readings are posted in the
examples.
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Examples
Metric Units
Observed:
ATC
RSC
SSC
CFO
RSF
XSF
English Units
Observed:
3.0%
16.0%
14.0%
37,850 cu m/d
18,170 cu m/d
760 cu m/d
ATC
RSC
SSC
CFO
RSF
XSF
3.0%
16.0%
14.0%
10.0 mgd
4.8 mgd
0.2 mgd
Wanted: CSFD and CFPD required to reduce RSC from 16%
to 14%
Therefore:
CSF = RSF+XSF
= 18,170 + 760
= 18,930 cu m/d
CSFD = CSFx(RSC-ATC)
/(SSC50-ATC)
= 18,930x(16.0-3.0)
/(14.0-3.0)
= 18,930x1.18
= 22,340 cu m/d
CFP = CSF/CFO
= 18,930/37,850
= 0.50
CFPD = CFPx(RSC-ATC)
7(SSC50-ATC)
= 0.50x1.18
= 0.59
= 59%
Therefore i
CSF =
RSF+XSF
4.8 + 0.2
5.0 mgd
CSFD = CSFx(RSC-ATC)
/(SSC50-ATC)
= 5.0x(l6.0-3.0)
/(14.0-3.0)
= 5.0x1.18
= 5.91 mgd
CFP =
CSF/CFO
5.*0/10.0
0.50
CFPD = CFPx(RSC-ATC)
/(SSC50-ATC)
= 0.50x1.18
= 0.59
= 59%
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The operator should now increase the clarifier sludge
removal pumping rate from 18,930 cu m/day (5.0 mgd) to
22,340 cu m/day (5.9 mgd). In actual practice, operators
usually select a maximum allowable sludge flow increase, or
decrease, that they will not exceed at any single test and
control period. In this case of a 37,850 cu m/d (10.0 mgd)
plant, for example, the maximum clarifier sludge flow
adjustment permitted after each test period might have been
limited to 3,785 cu m/day (1.0 mgd). This is a precaution
against over-control that could otherwise occur if very
large flow changes are called for.
During the next 8-hour shift, or more frequently if
necessary, the operator would make similar observations and
calculations and then readjust, if necessary, the clarifier
sludge flow to meet the new demand.
During each testing period the operator should also
calculate the sludge detention time in the final clarifier
and attempt to maintain this value between 30 and 90
minutes. If the process is so badly unbalanced that the
clarifier sludge detention time equalled or exceeded two
hours, he should increase the clarifier sludge flow rate
more aggressively and probably also step up the sludge
wasting rate. The clarifier sludge flow demand, discussed
in this paper, must be coordinated with other measurable
process requirements that will be described in a forthcoming
PROCESS CONTROL pamphlet being developed by the National
Field Investigations Center.
Control adjustments could be accomplished more
conveniently and accurately if the treatment plant were
equipped with an automatic clarifier sludge flow rate
controller that continuously adjusted the metered clarifier
sludge flow to equal a preset percentage of the measured
waste water flow. In such case, the operator would move the
controller set-point from 50% to 59% and the increased
sludge flow rate would continue to respond to waste water
flow rate changes.
Looking to the future, it is easy to visualize how this
procedure may be further automated when reliable continuous
activated sludge concentration sensors become available.
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Response to_ Return Sludge Flow Adjustments
The return sludge concentration and the mixed liquor
sludge concentration responses to changed return sludge flow
percentages are illustrated in Figures 2 and 3, which
contain two weeks of data from the Merrimack, New Hampshire,
Waste Treatment Plant operating log. During the period, the
return sludge percentage was increased from 15 to 127, and
then reduced to less than 26.
Return Sludge Concentration (RSTSS in mg/1)
Return sludge concentrations respond rapidly and
inversely to return sludge flow adjustments. This normal
relationship is evident in Figure 2 where the return sludge
concentration fell from 5640 to 2280 mg/1 while the return
sludge percentage increased. Return sludge concentration
then increased to 7110 mg/1 while the return sludge
percentage was reduced.
Mixed Liquor Concentration (MLTSS in mg/1)
Mixed liquor concentrations, and the associated sludge-
age and food-to-microorganism ratios, respond strongly to
sludge wasting adjustments but they are not normally
affected greatly by return sludge flow adjustments unless
the process is badly out of balance. In fact, a direct
response, where mixed liquor concentrations increase with
higher return sludge flows, usually occurs only after the
process balance has been upset by excessive accumulations of
sludge solids in the final clarifier.
As shown in Figure 3 the mixed liquor concentration
increased only modestly from 1230 to 1470 mg/1 while the
return sludge flow percentage rose drastically from 15 to
127. Then, the mixed liquor concentration continued to
increase to 2400 mg/1 even though the return sludge
percentage adjustment was reduced from 127 to 37.
Sludge Concentrations vs. Sludge Wasting
Process response to variations in sludge wasting is
discussed only briefly since Sludge Wasting Control is
beyond the scope of this paper.
Comparison of the mixed liquor and the return sludge
concentration curves with the pounds of sludge wasted curve
(Figure 4) reveals that the effects of wasting were
overwhelmed by other control adjustments from July 17 to
8
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FIG. 2
Return Sludge Concentration Response
to Return Sludge Flow Percentage Adjustments
N.
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WITH INCREASED RSP
WITH DECREASED RSP
F S S M T W T
20 21 22 23 24 25 26
JULY 1973
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FIG. 3
Mixed Liquor Concentration Response
to Return Sludge Flow Percentage Adjustments
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MLTSS CONTINUED TO
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16 17 18 19 20 21 22 23 24 25 26 27 28 29
JULY 1973
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FIG. 4
20000
Mixed Liquor and Return Sludge Concentrations vs.
Kilograms (Pounds) of Sludge Wasted
200
9000
8000
7000
6000
5000
4000
- 3000
- 2000
X
o>
a
LU
-1000
900
800
700
600
- 500
- 400
- 300
- 200
§
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MTWTFSSMTWTFSS
16 17 18 19 20 21 22 23 24 25 26 27 28 29
JULY 1973
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July 26. The sludge concentrations did not increase with
the decreased wasting rates, nor did they decrease with the
increased wasting rates to the extent they should have if
sludge wasting had exerted the principal control pressure.
As evidenced in Figure 2, the drastically increased and then
decreased return sludge flow percentage, that reduced and
then increased the return sludge concentration, exerted the
predominant control pressure during this ten-day interval.
Conversely, the relative effects of the wasting and the
return controls reversed after July 26, when the response to
the exceptionally high sludge wasting rate overpowered the
lesser response to the more moderate changes in the return
sludge flow percentage. Sludge wasting on July 26 and 27
had been increased to more than three times the previous
ten-day average. Both the mixed liquor and the return
sludge concentration trends reversed and then decreased in
logical response to the predominating sludge wasting
control.
Practical Precautions
This clarifier sludge flow adjustment procedure, when
coordinated with proper aeration and sludge wasting
techniques, has improved performance at practically all
plants at which Waste Treatment Branch personnel
demonstrated control procedures. Although such procedures
can be helpful, they may not solve problems that are imposed
by gross overloads or by improper plant design.
Furthermore, certain hydraulic limitations must be taken
into consideration at most plants. There is usually a limit
below which the return sludge flow cannot be reduced without
impeding proper sludge collection or plugging return sludge
piping. Similarly, there is usually a limit beyond which
the return sludge flow cannot be increased without creating
excessive turbulence and scouring flow velocities within the
final clarifier. Most operators identify these limits from
practical experience and follow the demands throughout the
acceptable return sludge flow range. In general, reducing
the return sludge flow percent below 15 or increasing it
above 150 may induce more problems than those that are
solved.
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Actual Results
Two examples of final effluent quality achieved while
following these control procedures are the most recent 30-
day average data summaries from the Merrimack, New
Hampshire, and the Albany, Oregon, waste water treatment
plants. The National Field Investigation Center's technical
support projects were completed on September 30, 1973, at
Merrimack and on August 9, 1973, at Albany.
The Merrimack plant treats 8,330 cu m/day (2.2 mgd) of
wastes. About 95% of the flow is brewery waste and the
remainder comes from a paper coating factory. Wastes are
pretreated by a trickling filter before entering the
complete-mix type, activated sludge plant equipped with
surface-mechanical aerators.
The Albany plant treats 20,820 cu m/day (5.5 mgd) of
domestic sewage. Local industries contributed less than 5%
of the waste flow during the project. The plant is a
conventional complete-mix activated sludge plant utilizing
compressed air.
During the first two months of each project, NFIC-C
personnel trained plant personnel to use the coordinated
control procedures that have been developed by the Waste
Treatment Branch. Consultation was provided for the next
month or two and the final month's performance, summarized
in the Table 1, was achieved by plant personnel without
further assistance from NFIC.
TABLE 1 Activated Sludge Plant Performance
Merrimack Albany
BOD SS BOD SS
Concentrations (mg/1)
Raw Waste
Aerator Influent
Final Effluent
Reductions (%)
Preliminary Treat.
Act. Sludge Process
Total Plant
1168 524
340 325
6.9 6.4
70.9 38.0
98.0 98.0
99.4 98.8
191
141
5.4
26.2
96.2
97.2
254
172
6.0
32.3
96.5
97.6
The turbidity of settled final effluent averaged 1.8
JTU at Merrimack and 2.1 JTU at Albany.
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Summary
The real value of this control procedure is that it
responds to practically all loading, process balance and
sludge quality characteristics to reveal the clarifier
sludge flow rate that will best satisfy the net requirement
of all these interacting variables. The calculated demand
satisfies the coordinated requirements imposed by changing
mixed liquor sludge concentrations and quality, sludge
solids distribution between the aeration tanks and the final
clarifiers, and the waste water flow rates. On a
progressive long-term basis it also responds to changes in
organic loadings and the interrelated sludge-wasting rates.
This control procedure satisfies the dynamic requirements of
the total process.
Author's Note
A series of pamphlets describing Operational Control
Procedures for the Activated Sludge Process is being
developed by the Waste Treatment Branch of the National
Field Investigation Center - Cincinnati. Part I
OBSERVATIONS, Part II - CONTROL TESTS, and Part III-A
CALCULATION PROCEDURES are available for distribution. Part
IV - SLUDGE QUALITY, Part V - PROCESS CONTROL,, APPENDIX, and
a series of CASE HISTORIES will come later. This paper is
essentially a preview of a section of proposed Part V
PROCESS CONTROL. Though this paper is limited to a
discussion of the clarifier sludge flow control procedures
that were evolved by me and are demonstrated by the Waste
Treatment Branch, it should be recognized that coordinated
control of aeration tank mixing and dissolved oxygen
concentrations along with excess sludge wasting procedures
is also necessary to obtain best plant performance and
effluent quality from the activated sludge process.
Alfred W. West
U.S. GOVERNMENT PRINTING OFFICE 1973- 758-488/1048 14
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