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OOOR69018
CASE HISTORIES:
IMPROVED ACTIVATED SLUDGE PIANT PEKFOHMAN'JE
BT
OPERATIONS CONTROL
A. W. West - P.E.
Sanitary Engineer
Field Investigations Branch
Division of Technical Support
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
U.S. DEPARTMENT OF THE INTERIOR
CINCINNATI, OHIO
APRIL 1969
ction Agency,
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3&OTSCTIGU AGBiK
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CASE HISTORIES:
IMPROVED ACTIVATED SLUDGE PLANT PERFORMANCE
BY
OPERATIONS CONTROL
INTRODUCTION
Firsthand operation experiences during Federal Water Pollution Con-
trol Administration Technical Assistance Projects at the Sioux Falls, South
Dakota, and the St. Louis, Missouri, activated sludge treatment plants
document once again the excellent final effluent quality that can be pro-
duced "by presently available secondary treatment processes. In these two
cases, final effluent quality was greatly improved by modifying process
control procedures, and at practically no extra cost. In one case, 99 per-
cent BOD and suspended solids removals were obtained with readily avail-
able present day treatment processes and equipment.
Activated sludge plants can produce final effluents containing con-
siderably less than 10 mg/1 suspended solids and 5-day biochemical oxygen
demand (BOD). Overall reductions of 95 to 99 percent are possible. Some
of these past experiences, especially those at Sioux Falls confirm that
there is no need to approve, or accept, secondary treatment systems capable
of producing only ^5 percent reductions.
Certain essential requirements must be met, however, if consistent
effluent excellence is to be expected. The treatment plant must be proper-
ly designed with adequate built-in capacity and flexibility; plant charact-
eristics must be appropriate to the incoming load; and the process must be
skillfully controlled by conscientious qualified operators.
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2.
Performance of the Sioux Falls and St. Louis plants was measured
"by the routine BOD and suspended solids reductions determined by the plant
chemists. Though a detailed study of nutrient removals was not included,
a few scattered tests indicated only low level phosphorus removals ranging
from about 0 to 35 percent. No appreciable nitrification or denitrlfication
was observed.
Operational control adjustments were predicated upon the results of
the following tests:
1. Mixed liquor and return sludge specimens were
centrifuged to determine sludge concentration
and distribution.
2. Mixed liquor sludge settling and compaction
rates, observed in a laboratory cylinder,
revealed sludge condition.
3- The measured sludge blanket depth in the
final clarifier revealed process balance.
h. Air discharges were regulated according to
measured dissolved oxygen concentrations in
the aeration tanks.
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3-
METROPOLITAN ST. LOUIS SEWER DISTRICT - MSP
Coldwater Creek (Activated Sludge Type)
Wastewater Treatment Plant
A. PLANT DESCRIPTION
The conventional standard rate activated sludge plant had a
rated capacity of 21.0 MGD with 6 aerators, each of which is equip-
ped with spiral flow pattern air diffusers and k final clarifiers,
each equipped with plow-type sludge scrapers.
B. PLANT PERFORMANCE
The pollutional strength of the final effluent from this plant
was reduced to otic-fourth of its former strength by modified opera-
tional control. The wholehearted support and cooperation provided
by management, the recently appointed superintendent and the entire
operating group contributed greatly to the success of this project.
Previously, the average final effluent contained ho mg/1 of BOD
and 92 mg/1 of suspended solids. The process responded favorably
to modified control procedures, and during the last week of the pro-
ject, the final effluent averaged 9 mg/1 of BOD and 16 mg/1 of sus-
pended solidc.
Other "before and after" characteristics are shown in the fol-
lowing Table 1:
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TABLE NO. 1
SUSPENDED SOLIDS & BOD REMOVALS
at the
Coldwater Greek S.T.P.
Before After
Suspended Solids
Saw fao/l) 173 198
Primary Effluent (mg/l) 155 1^2
Final Effluent (mg/l) 92 l6
Activated Sludge deduction UO^ 8956
Total Plant Reduction U6> 92$
BOD
Raw ( rag/I ) 150 l62
Primary Effluent (rng/l) 152 130
Final Effluent (mg/l) ho _ g^
Activated S.ludge Reduction l^k 93/J
Total Plant Reduction 73% 9^%
IMPROVED OPERATIONAL CONTROL
1. Number of Aerators and Glarifiers in Service
Before changes were made, the activated sludge looked
old arid septic, smelled "bad, and was settling too
rapidly. Too many clarifiers, and too few aerators
were in service. One of the four clarificrs was taken
out of service and one additional aerator was placed
in service to augment the three operating units. The
computed characteristics of the 3 aerator - k clarifier
vs. the Ij- aerator - 3 clarifier activated sludge systems
were changed as shown in the following Table 2:
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TABLE NO. 2
COMPUTED PLANT CHARACTERISTICS
Coldwater Creek S.T.P.
Fo_rrne_r Changed To
Aerator-Claririer Combination 3A/'4C ':A/3C
Mixed Liquor Suspended Solids 7,200 3,]i-00
Return Sludge Flow (/0 of Sew-
age Flow) S3 30
Flow Capacity (MGD) 13.
Aerator Detention (HRS) 3.0 5.;4
Claririer Dcte^lioi (ESS) 2.0 2.0
The h aerator - 3 clarifier combination provided a more
practical theoretical mixed liquor suspended solids con-
centration demand of approximately 3>'-i-00 rng/1 as opposed
to the impractical 7>200 mg/1 demand of the former 3
aerator - h clarifier combination. Similarly, calcu-
lations indicated that the revised plant combination could
"be operated at 30 percent return sludge, whereas the old
combination required in excess of 90 percent return sludge.
2. Control Tests
The standard 1-liter graduated cylinders were replaced by
the larger diameter (5" dia. x 6" deep) 2- liter Mallory
Direct-reading Settleometers for mixed liquor sludge
settling rate determinations. Each mixed liquor specimen
was settled for one hour, and the volume of the cylinder
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6.
occupied by the settled sludge was recorded at every
5-minute interval during the first half-hour aixi at
every 10-minute interval during the second half-hour.
Mixed liquor and return sludge specimens were centri-
fuged for 15-rainute periods to determine their rela-
tive densities and the solids distribution ratios.
Sludge concentration characteristics were computed from
the settling and ccntrifuged tests.
The thickness of the settled sludge "blanket in the
fi.iul clarificra was Pleasured periodically with a
sludge blanket finder.
The dissolved oxygen concentration in the aeration tanks
was measured to indicate air discharge requirements.
Relative final effluent quality trends were determined
rapidly with a turbidimeter.
Control Ad,''ustr.ients
In this case operations and effluent quality were im-
proved "by:
Increasing the excess sludge wasting rate to
reduce sludge age.
Reducing the return sludge pumping rate to con-
form to the requirements of lowered mixed liquor sludge
concentrations.
Increasing the air supply and then adjusting
it to try to maintain "between 1.0 and 3-° mg/1 of dis-
solved oxygen in the aeration tanks.
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7.
D. FAVOiUBLE FEATURES AT EXISTING PLANT
The extensive improvements would not have been possible without
the dedicated and effective cooperation provided by the Superintendent
and operators.
Adequate tank capacity in multiple aerator and clarifier units
were available to permit changing the process characteristics and to
accommodate present dry-weather flows.
E. PLANT DEFICIENCIES
1. Plow-type sludge scrapers increased the settled sludge
detention time in the anaerobic environment of the final
clarifiers.
2. "Spiral flow" air diffuser placement limited mixing and
oxygen transfer rates in the aerators.
3. Hydraulic short circuiting plus strong velocity currents
in final clarifiers were at times directly responsible
for carrying increased amounts of suspended solids to the
final effluent.
k. No scum removal devices were provided for the final clari-
fiers.
5. Return sludge capacity was limited.
6. Meter problems, and lack of remotely controlled mechani-
cal valve actuators or automatic sensor-controllers lim-
ited process controllability.
F. FUTURE POTENTIAL
The problems encountered at this plant indicate that the pollu-
tional load in the effluent can be cut in half once again when the
more significant constricting plant deficiencies are corrected.
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3.
SIOUX FALLS, SOUTH IAKOTA
A. PIANT DESCRIPTION
Approximately 3-5 mgd of strong 2,500 rag/1 BOD meat-packing
waste plus 6.0 mgd of normal domestic sewage were treated at Sioux
Falls. The packing plant waste was settled, pretreated in high-
rate trickling filters, and then combined with raw domestic sev-
age for discharge to primary clarifiers. The activated sludge
system treated the settled domestic sewage and polished the pre-
treated industrial wastes.
The aeration taaks, which operate as "complete-mix" u:iits,
ai-v provided with effective "turbine" type aerators. The "rim-
flow" final clarifiers are equipped with appropriate suction type
sludge removal mechanisms.
B. PIAHT PERFORMANCE (Fall of 1968)
During the October and November 1968 technical assistance
project, when the plant is severely overloaded, the pollutlonal
strength of the final effluent was reduced to about one-half of
its forncr strength by modified operational control.
During the summer months, when plant loadings are within
design capacity, the superintendent achieves remarkable reduc-
tions through the plant.
During late fall and early winter, however, industrial waste
loads increase with the increased kill at the meat-packing plant,
trickling filters freeze, the activated sludge unit is greatly
overloaded, and effluent quality suffers.
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1. ijiqnii'ioc. it Loading Character!sti cs
The more significant Gctobcr-JIoveriber 1968 loading
characteristics that influence performs.ice of this
plant veri shown on Table 3. Aerator loadings; \rhich
vere high at all times, averaged 11? pou.ids of BOD per
1,000 cubic foot of aerator. The vacte load to acti-
vated sludge solids ratio usually exceeded 1.0 pound
of BOD per povid of mixed liquor solids. (lTon,
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10.
2. Control Tests
Here again settlometers, centrifuges, sludge blanket
finders and turbidimeters were introduced to detect
sludge conditions, process balance and operational
control demands.
3. Control Adjustments
The effluent quality was improved by increasing the
return sludge percentages from about 30 percent to
as high as 200 per cent and by "tight-rope" sludge
wasting control to increase mixed liquor sludge con-
centrations from about 1,000 to 3>000 mg/1 without
upsetting the aerators and clarifiers. The plant
was greatly overloaded and more aerators and clari-
fiers, especially aerators, arc needed to handle the
organic overload to the activated sludge system.
H. Resultjs
Final effluent quality produced in 1968 is compared
with that obtained during the fall of 1967 when in-
fluent loadings were similar (Figures 1 and 2).
The average BOD had been reduced from 30 to 20 mg/1
and the average suspended solids conce nitration from
35 to Ik mg/1. The pollutional loading to the Big
Sioux River had been reduced almost one-half by the
modified control procedures.
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10 SO 90
% OF TIME EQUAL TO OR LESS THAN
FIGURE 1— SIOUX FALLS. S.D. FINAL EFFLUENT 5 DAY B.O.D. MONTHS
OF OCT. a NOV. 1967 VS. 1968.
10 90 90
* OF TIME EQUAL TO OR LESS THAN
99
FIGURE 2 — SIOUX FALLS, S.D. FINAL EFFLUENT TOTAL SUSPENDED
SOLIDS MONTHS OF OCT. ft NOV. 1967 VS. 1966.
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100
10
% OF
50 90 99
EQUAL TO OR LESS THAN
TIME
FIGURE 3- SIOUX FALLS. SD 5 DAY BOD REDUCTIONS
PLANT (AUG. 8 DEC.,1967).
THROUGH ENTIRE
100
10 50 90
% OF TIME EQUAL TO OR LESS THAN
FIGURE 4 — SIOUX FALLS ,S.D. SUSPENDED SOLIDS REDUCTIONS
ENTIRE PLANT (AUG. 8 DEC., 1967).
THROUGH
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90
8O
70
v
e eo
d
CD
z
Lu
50
>- 40
Ul
t 30
LJ
20
10
73
15
I 10 5O 90 99
% OF TIME EQUAL TO OR LESS THAN
FIGURE 5 — SIOUX FALLS , S.D. FINAL EFFLUENT 5 DAY BOD MONTHS
OF AUG. 8 DEC.. 1967.
I 10 50 90
% OF TIME EQUAL TO OR LESS THAN
99
FIGURE 6 — SIOUX FALLS, S.D. FINAL EFFLUENT TOTAL SUSPENDED SOLIDS
MONTHS OF AUG. 8 DEC.,1967.
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11.
IAIIT I-KRFO.MMCE (Su>nraor I(}6j)
The feature story at this plant relates to the Superintendent's
ability to obtain 99 percent BOD and suspended solids reductions
during summertime when the plant loadings remain witMn desired
capacity.
Percent .reductions during the summer (August) and the winter
(December) are illustrated on Figures 3 and k, and the effluent
quality (in tevtns of BOD and suspended solids concentrations) is
shown on Figures 5 and 6.
During August 19&7 the combined BOD of the; combined industrial
a-id domestic wastes averaged 900 mg/1 and was reduced to an aver-
age of 9 mg/1 iu the final effluent. Similarly, total suspended
solids were reduced from 6^0 to 5 rog/1. These are remarkable
reductions: especially in view of the high BOD loadings shown in
Table !|.
TABLE NO. 4
SIGNIFICANT LOADING CHARACTERISTICS
(Sioux Falls, South Dakota, Activated Sludge Plant)
Summer vs. Fall 1967
SUMMER FALL
(Aug. 19C7)(Dec. 1967)
BOD_ JLoad to Aerators
Pounds per day 18,000 37,tl-00
Pounds per 1,000 cu.ft. Aerator 103 160
Pounds per pound mixed liquor solids 1.3 1.2
Clarifier Surface Loading Rate
Gals./sci. ft./day 720 6UO
EOP Reductions
Total - Raw to Final 99 97
Activated Sludge Alone 95 90
TSS Reductions
Total - Raw to Final 99 96
Activated Sludge Alone 96 83
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D. FAV03A3LE FEATURES AT THE EXISTING PIAHT
1. Dedicated, skilled supervision and operation, arc primarily
responsible for the unusually high purification efficien-
cies developed at this plant.
2. The aeration tanks were equipped with effective turbine-
type aerators, rather than the less efficient conventional
spiral flow types, (Compressed air was discharged through
sparger rings located at the bottom of the aerators below
nlxi i ig paddle s.)
3. The aerators operated as "complete mix" (rather than "plug
flow") systems.
h. The final clarifiers were equipped with suction-type (rather
than scraper type) sludge removal mechanisms for rapid re-
turn of settled sludge to the aeration tanks.
5. Adequate return sludge pumping capacity, exceeding 150 per-
cent of average raw waste flows, permitted over compensa-
tion when necessary.
E. PIAIW DEFICIENCIES
1. Activated sludge plant was overloaded. It needed additional
aerators.
2. Industrial waste trickling filters froze up in winter, and
imposed greater loads upon the activated sludge system.
3. There was insufficient air to maintain 1.0 to 2.0 mg/1 D.O.
in aerators at all times.
'(-. Scum removers had not been provided for the final clarifiers.
5. Lac'c of sufficient meters, remotely controlled mechanical
valve actuators, and automatic sensor-controllers limited
process controllability.
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CONCLUolUNS
Experiences at Sioux Falls and 3t. Louis provide confirmation, that
the- activated sludge process can be operated at exceptionally high purifica-
tion efficiencies to produce attractive clear final effluents.
They also highlijhted the following design and operation requirements:
A. DESIGN
1. Frovide adequate plant capacity for growth and
for repairs during equipment outages.
2. Flexibility. (Give* tlie operators a chance.)
Provide ability7 to increase or decrease the
number of aerators or clarifiers in service
a-it] to convert to modified activated sludge
schemer, suc]^ as "step aeration" if required.
3. Avoid "spiral flow" aeration.
Ucc suction devices in final clarifiers fox1
vapid removal of fresh sludge. Also provide
surface scvin removers.
r, . i'lalic air supply, sludge return and sludge
wc.3t.iAg equipment truly variable and con-
veniently controllable.
o. Irovide essential meters and sensors, remote
valve actuators and automatic ratio control-
lers where required.
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1't.
B. OIERATION
1. Recruit and retain conscientious, intelligent,
trained, and certified plant operators.
2. i-rovidc practical "on-the-job" work experience
type trairin~.
3. Provide 2'i-hour operation. Test, evaluate and
adjust process at least once every 8-hoxor sMft.
'-. 1'lake best use of existing facilities and advise
consulting eu^ineers of operational reqiiirc'..ients
for pla-it additions and modifications.
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Environmental! Protect ion Agency
Library, IK,-j-ion V
1 North to'acker Drive
Chicago, Illinois 60606
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II if 111 r II1
: 111
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