SEWAGE TR2B&TW2IPf PLANT CEPKKABILITI
WitU Special Reference
to the
ACTIVATED SLUDGE PROCESS
by
A. W. West, P.E., Chief
Sewage Treatment Plant
Operation & Design Branch
ENVIROKMEinAL PROTECTION AGESGY
OFFICE OF ENFORCEMENT & GEITEPAL CCUIISSL
CINCIHNATI FIELD IliVES P1GATIOIK CEN2ER
IKClNylATj, OHIO
OCTOBER 1971
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TAF-L3 OF 0
PAGE NO.
I INTRODUCTION AND CONCLUSIONS 1
II DESIGN CONSIDERATIONS 1
SELECT PROPER PROCESS TYPES 2
PROVIDE GENEROUS CAPACITY 5
INCLUDE ESSENTIAL FLEXIBILITY 7
Process 7
Aeration Tanks 7
Final Clarifiers 8
Return Sludge Pumps 9
Excess Sludge Wasting 9
Emergency Chemical Treatment 10
Adjunct Facilitie s. . -. 10
MAKE PLANT TRULY CONTROLLABLE 12
Meters 12
Meter-Control Panel lA-
Automatic Controllers 1^
III OPERATIONAL CONTROL
PERSONNEL 16
DUTY DELEGATION 17
CONTROL TESTS 17
PROCESS RELIABILITY 18
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With Special .;cr^.-e ice to the
ACTIVATED SLUDGE PROCESS
INTRODUCTION & CONCLUSION
Operational experience at dozens of sewage
treatment plants has shown that, to achieve
dependable, consistently satisfactory, muni-
cipal and industrial vacte treatment plant
performance, the owners should:
Retain the best possible consultants
for design.
Hire the best trained, intelligent,
dedicated, imaginative Superintendent
and Operators available.
II DESIGN CONSIDERATIONS
Though this discussion emphasizes activated
sludge treatment, the following four general
design concepts have been found essential to
plant dependability for practically all types
of waste treatment plants.
Select the Proper Process Type.
Provide Generous Capacity.
Include Essential Flexibility.
Make Plant Truly Controllable.
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SELECT T'.W?'7"?. F^.OCFSS TIPS
A. Early in the design stage, the engineer must
exercise hie best experienced professional
judgment in selecting the Process Modification
most appropriate to the Known characteristics
of the incoming wastes, and the effluent quality
requirements.
The following illustrations, for example, are
drawn from personal plant operation experiences:
1. The "Classic Activated Sludge Process"
design - conforming generally to "10-States
Standards",usually performs satisfactorily
for "normal municipal wastes" where domestic
sewage predominates.
The "Classic Activated Sludge Process'' is
defined as the original activated sludge
system; where all return sludge and all
settled sewage enters the head end of the
aeration tanks.
2. The "Complete Mix" modification has been
found admirably suited for mixtures of dom-
estic sewage and industrial wastes with highly
variable characteristics and concentrations.
3. The "Step Aeration" modification permits an
operator to select and change his basic pro-
cess cycle to accommodate unexpected overloads,
to adjust sludge solids distribution and to
control mixed liquor sludge characteristics.
To be truly effective; the "Step" design
must permit controlled measured incremental
sewage discharge to each aerator pass or com-
partment. It must also permit discharging
100 percent of the sewage into the head end of
the aerator or to the last pass. Such an aera-
tor can then be operated in either extreme -
Classic Mode or Contact Stabilization Mode - or
any place in the middle - according to the "Step"
percentages that are selected - to meet actual
loading and system demands.
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Be mr-? to consider "T?rtiarv" or
"^Il-V"1 "lU-l::"'' J .--:2i:neht "r.yr^er:s
that luay je necessary -co meet spec-
ial water quality criteria. This
paper does not include discussion
of these important features that
are covered thoroughly by the Ad-
vanced Waste Treatment Research
Laboratory in Cincinnati.
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Whqri in ^-oivot - and vbo isn't at times? - Pilot!
The terra ''i-'ilot Studies/' having a broad, general
meaning, could include:
•'•• ftemh Scale. Waste treatability
studies can be performed effectively
aad economically at Bench Scale.
2. Pilot Scale. A specially constructed
small pilot plant (possibly 0.1 MG-D or
more capacity) may be needed if addi-
tional essential information is required.
Such units can validate process suit-
ability, relative tank sizes, and system
dynamics for the proposed full size plant.
3. Demonstration Scale. In some cases,
full sized plant units must be utilized to
demonstrate the relative effectiveness of
various facility arrangements. At times
certain units in existing plants can be
modified for such study before the design
of plant additions is initiated. For ex-
ample, it is impractical, if not impossible,
to prove the suitability of various over-
flow weir arrangements for 150 ft. diameter
final clarifiern from studies on 10-ft. dia-
meter pilot scale models.
The type and extent of Pilot Studies will obviously de-
pend upon effluent requirements; the specific informa-
tion needed; and the size, complexity and cost of the
treatment facilities to be constructed.
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Benign criteria such as, "10-Stateo Standards" and others,
should be interpreted ski llfivlly, aid used as intended. In other
words, moot suggestions in such Manuals should be considered as
minimum, and not maxima, requirement" to achieve plant depend-
ability and to provide es',entic,l factors of safety. Generous
capacity is always welcomed by plant operators and by plant managers
facing the necessity for day-in, day-out, dependable and acceptable
plant performance and effluent, quality. Surplu^ capacity, if any,
that might be provided will in most cases be used up rapidly as the
communities face unexpected rapid population growth and unpredictable
development of new waste-producing industries within the area.
A. Some design criteria, based on 2k-hour average flow
and load, include appropriate diversity factors to
accommodate the normal cyclic peaks that occur dur-
ing each 24-hour period. Such 24-hour design cap-
acities should be based on the averages anticipated
during the maximum flow and loading producing week
of the year. Obviously, any abnormally high short-
term peak load:1, that can be anticipated, must also
be included in the design loading.
B. Accommodate full load with either one aerator
or one clarifier out of service for maintenance.
You all know that at times equipment units must
be taken out of service for maintenance or repair.
Out of dozens of activated sludge plants, I can
only recall one or two that did not have a final
clarifier down for maintenance at some time dur-
ing my stay at the plant. In a four-tank design,
for example, this means that clarifiers must be
designed so that three of the four can successful-
ly handle the estimated total design flow. Simil-
arly, aeration capacity must be large enough so
that three of the four aerators can handle the
total estimated design load.
G. Provide multiple aerators and clarifiers; prefer-
ably four each for medium to large plants.
This is, of course, a corollary to the previously
mentioned requirement.
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Wi-.en aay ono
unit is taken out of service. Though discussions
of relative aerator to clarifier volumes usually
becomes controversial, my experience indicates
that properly "balanced operation can "be obtained
vhen the total i^rator volume approximates twice
the total clariii^r volume. ,-/ixn this relation-
ship, for er.a:;>ole_, full plant operation of a four-
aerator/fouv-.'j.lc.rifier plaat might require approxi-
mately fifty rercont return -:Iudge pumping. With
one aerator out o/_' -ervice, the return sludge pump-
ing demand rni^hb increase to ICO percent; and con-
versely, with oue clarifier out of service, some-
thing in the neighborhood of thirty percent return
sludge pumping mi ght be required. Other process
requirements, that will change when individual units
are taken out of service, can usually be accommodated
effectively under such circumstances.
D. Include the estimated plant recycle (thickener over-
flow, filter underflow, dilution flows, etc.) in the
design load. In one extreme case, the additional
flows imposed by effluent dilution of sludge thick-
ener influent, scrubbing water for the furnaces, etc.
approximated fifty percent of the incoming sewage flow
volume. In most cases, such recycle flows can easily
exceed ten percent and possibly approach twenty percent
of design flows. These additional flows will influence
aeration tank detention times and final clarifier over-
flow rates and must be considered in the design of ade-
quately sized units.
E. Sludge handling facilities must be designed to accom-
modate the maximum (not the average) anticipated sludge
quantities; and with units down for maintenance. Remem-
ber, sludge wasting requirements at activated sludge
plants can vary greatly from day-to-day in response to
sewage loads, process equilibrium, and mixed liquor
sludge quality. At times, extremely high wasting rates,
greatly in excess of the anticipated average, will be
needed to restore process equilibrium when a system is
sliding out of balance.
Generous sludge holding-equalizing tank capacity is
essential and can reduce substantially the danger of
periodic overloading of sludge processing equipment.
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INCLUDE ESSENTIAL FLEXIBILITY
Design considerations that effect, and permit operating
flexibility are presented in brief outline form. Though readily
understood, and almost universally accepted; omission of one or
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INCLUDE ESSE3STIAL FLEXIBILITY
Design considerations that effect, and permit operating
flexibility are presented in brief outline form. Though readily
understood, and almost universally accepted, omission of one or
more of these elements has been observed in all too many plants.
Lack of essential flexibility frustrates operators and degrades
effluent quality. Give the operator the tools he needs to modify
anticipated control schedules in order to accommodate some of the
unforeseen difficulties he will almost certainly face at times.
A. Process
1. If the activated sludge process
is selected for secondary treat-
ment; evaluate the relative ad-
vantages of both the "Classic"
process and the "Complete Mix"
modification with reference to
the type and characteristics of
the incoming raw wastes, and the
plant performance requirements
needed to meet final effluent and
receiving water quality objectives.
2. Provide for "Step" operation of any
"Classic" or conventional activated
sludge system.
3. Be sure any "Step" design can be
operated in all modes; from Classic,
through variable step proportions,
and to "Contact Stabilization".
k. Where practical, provide each aerator-
clarifier combination (or each group
of aerators and clarifiers) with its
own separate return and waste sludge
pumping.facility.
B. Aeration Tanks
1. Provide truly effective mixing and oxy-
genation.
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2. Assure arainrt "coreinq;" and
compromise by ineffective modifi-
cations of inappropriate basic
aeration devices.
3. Provide truly controllable, measur-
able, variable outputs for mechani-
cal aerators pr air blowers.
k. Provide separate, controllable,
metered air headers for each pass,
or compartmented zone of the aera-
tion tank. For example, don't con-
nect the "A" pass (with its relative-
ly high air demand) of one aerator
and the "C" pass (with its relative-
ly low air demand) of an adjacent
aerator to a common air header.
5. Consider provision of recording D.O.
meters. (Preferably actuating air
blower controllers.)
6. Consider provision of mixed liquor
solids concentration sensor and re-
corder. (Also similar for return
sludge solids concentration.)
C. Final Clarifiers
1. Obviously - minimize excessive veloc-
ity currents and short-circuiting.
2. Don't skimp on surface area. Contem-
porary high capacity designs appear to
require considerably less than the con-
ventional 800 Gals/Day/Sq. Ft. overflow
rate.
3. For large tanks (certainly for 100 feet
in diameter and greater) provide appro-
priately located and properly spaced
multiple effluent weir launders.
k. Provide effective surface scum collec-
tion and removal devices.
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5. Consider 12 feet as a minimum
practical side wall depth.
(Despite calculable theory -
shallow tanks aggrevate floe
carry-over characteristics.)
6. Provide combination suction-
sciraper sludge collectors to
minimize clarifier sludge de-
tention time. (Be sure that
the actual sludge withdrawal
capacity equals at least 100
percent of design waste flow
plus recycle.)
D. Return Sludge Pumping Facilities
Provide multiple, remotely ad-
justable, return sludge pumps
capable of returning at least
100 percent of design waste
plus recycle flow with one pump
out of service for maintenance.
^' Excess Sludge Wasting
1. Provide metered, controllable,
waste sludge pumps - separate
from the return sludge pumps.
2. Be sure they can be operated at
the low wasting rates required
at times. (A valved interconnec-
tion to the return sludge pumping
system can accommodate the excep-
tionally high wasting demands that
occur occasionally.)
3. Provide a suitable valved inter-
connection to permit wasting either
from the aerator outlet (mixed liq-
uor) or from the clarifier sludge
withdrawal system (return sludge).
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F. Emergency Chemical Treatment
1. Provide feeders and piping to per-
mit emergency application of chem-
icals to aerators or clarifiers (
and primary tanks).
2. For example - application of poly-
mers and ferric chloride to the
clarifier inlet has solved serious
classic bulking; and apparently
without destroying other desirable
sludge characteristics.
G. Adjunct Facilities
1. General - This Section will only
highlight general concepts concern-
ing certain related plant facilities.
2. Primary Clarifiers - Don't skimp on
size and surface overflow rates, es-
pecially if excess activated sludge
is to be wasted to the primary clari-
fiers.
3« Sludge Handling Facilities
a) Must be designed to handle
maximum (not average) anti-
cipated loads,, with units
down for maintenance.
b) Must be provided with gener-
ous storage, or equalizing,
tanks to accommodate periodic
peak requirements that will
at times exceed even estimated
maximum loads.
c) Then provide means for dis-
posing of partially processed
sludge during breakdowns.
1) Take a sludge thickening -
filtering - burning process,
for example:
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Be sure filtered, sludge can
be collected, conveyed out of
the building and hauled away
in case of serious furnace
breakdown.
2) Consider, for example, sludge
digestion: -
Provide facilities to collect,
pump, and haul supernatant and
partially digested sludge to
prescribed land disposal if di-
gestion or drying facilities are
seriously overloaded.
3) Plan an "out" so that the secon-
dary process and effluent quality
need not be degraded by break-
down of other plant facilities.
k. Equalizing Tanks - In special situations
"(separate interceptors collecting slug-flow
strong wastes) provide adequate raw waste
holding tanks to permit uniform process
loading throughout the 24-hour cycle.
5- Holding Ponds - Consider holding ponds for
effluent polishing, or for storage and re-
cycle of primary effluent, during periodic
severe plant overloading.
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A. Meters
svrtern is a controllable
process that, rnur.t include appropriate meters
and accurately controllable equipment,gates,
valves, pumns and blowers for optimum perform-
ance. More importantly; it should be developed
and run by intelligent competent designers and
operators.
1. Obviously - the most reliable, proper
type, meters should be specified.
2. Throughout the plant, meters can range
from the most simple elementary type to
the highly sophisticated system; depend-
ing upon the specific output needed.
3. A single subcontractor should supply, and
and be fully responsible for satisfactory
performance of, the entire meter-controller
package.
k, A qualified instrument technician should be
included on the staff of all large plants.
A capable technician, from within the com-
munity if possible, should be retained for
periodic meter maintenance and emergency
repair at smaller plants.
5. Separate independent meters are needed at
each plant unit requiring control adjust-
ment.
A summator, in addition to read-out from
individual meters, is helpful for multi-
unit plants. But beware of a subtracter
as the sole means of obtaining an essential
third flow measurement from two other inde-
pendent meters.
6. Beware of relying too greatly on so-called
hydraulic similitude for balancing flows
between multiple units. Individual meters
and control gates are usually needed in
critical areas.
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7. When der.i^n contemplates a phased series
enougn) to permit accurate measurement of
the relatively low initial flow rates.
When pipc:3 are sized for future additions,
it Tr.ay be necessary to install replaceable
meters in temporarily reduced pipe sections.
8. Be certain that metering is adequate to per-
mit accurate control adjustments, maintain
essential balance in multiple parallel plant
units, to document plant performance, and to
evaluate process and effluent quality require-
ments.
a) Measure either plant influent or
effluent separately. A back-up
sensor and indicator on the other
will be useful. Don't depend on
mechanical addition of other inter-
nal meters for this value.
b) Be sure that waters recycled within
the plant are metered and can be
accounted for. (Thickener influent
and effluent, dilution water, fur-
nace condenser spray, etc.)
c) Provide individual meters for each
of the following similar parallel
plant features. (i.e. Four meters
for influent flow to four parallel
aerator s.)
l) Return Sludge Flow -
To each Aerator.
From each Glarifier.
2) Waste Water Flow -
To each Aerator.
To each pass in "Step"(*).
From each Clarifier(**).
3) Mixed Liquor Flow
To each Clarifier.
U) Air Discharge -
To each Aerator.
To each "pass" of each Aerator(*).
5) Waste Sludge - One meter for each
individual aerator-clarifier battery
(*) Can be calibrated manometers or simple indicating meters,
(**) Desirable, but can be eliminated if all other meters are provided.
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1. General
In large multi-unit plants it is
utterly impractical to adjust or bal-
ance flown manually at valve or pump
locations that are almost always far
removed fron the meter panels.
A centrally located meter-control
panel, wired to mechanical valve and
puno actuc.to.-s, permit:; accurate ad-
justment of critical flows while ob-
serving; the restored response. This
applies in principle, though not in
degree, to small as well as large
plants.
* 2. Remotely actuated controllers should
be provided for:
a) Return and waste sludge pumps.
b) Proportioning waste water and
return sludge flows to individ-
ual aerators.
c) Proportioning mixed liquor flow
to, and return sludge withdrawal
from, individual clarifiers.
C. Automatic Controllers
1. General
Density sensors coupled to automatic
controllers should be provided as part of
the adjunct sludge handling and disposal
facilities. Such control, according to the
operator's "set point" requirements greatly
improves sludge thickener, digester, etc.
performance and minimises the supplementary
recycle load on the secondary treatment
facilities.
2. Other automatic controllers that can im-
prove plant performance are:
a) Blower control by D.O. sensors.
b) Meter actuated controllers to pro-
portion return sludge punpage
according to the cyclic incoming
waste water flow rates.
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y.-rcd ] iruo.* and return cruxlge
concentration censors are being
developed to control return
sludge pumping and solids distri-
bution.
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III OFT'T'ATJCTAL "CXriOL
operator or :ie tidier qualiiieatioas would be
academic at "bent. It is unquestionably true
that oualified operators are required to
achieve the high quality effluent that can be
produced by properly designed waste treatment
plants. Cf even greater importance; dedicated,
experienced,•operator ingenuity is needed to
get the best out of plant3 that may suffer
from certain design defects. Though some of
the requirements for proper operational control
discu'red here may reem repetitious to a few,
all of the EC elements have been observed again
and attain where coafomance to such principles
has enhanced pollution abatement immeasurably
or, conversely, where neglect has degraded final
effluent quality.
FERSONTIEL
1. Hire the best qualified people available.
2. Exert your greatest, and most effective,
support to Operator Certification programs.
3. Send selected personnel to training courses
and to similar treatment plants to upgrade
their knowledge.
k. Conduct continuing in-house training.
5. Inspire all operators to recognize that
consistent production of the best possible
final effluent quality is their foremost
job responsibility.
6. In medium to large activated sludge plants;
be sure there is a staff position - somewhere
between the Superintendent and crew chief
positions - with the specific responsibility
to evaluate plant performance, cause and effect
relationships, and to direct process control
operations.
7. Obviously; provide tools and facilities the
staff needs to accomplish their objective.
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1. Practically every one is, or should be,
aware of the need for properly planned
safety, preventive maintenance, and em-
ergency repair duty delegation.
2. Conduct critical reviews of routine opera-
ting procedures. Minimize less important
activities to provide additional time to
beef-up the more essential and productive
operational tasks.
3. Be sure crew chiefs are fully aware of
their specific responsibility and author-
ity. Excess sludge wasting schedules, for
example, are dictated by process and efflu-
ent quality requirements. Wasting adjust-
ments should, therefore, be directed by the
secondary process crew chief; not by (ex-
cept in extreme emergency) the crew chief
in charge of sludge handling.
k. Written Standing Orders and Special Instruc-
tions should be posted in the control office
to avoid confusion among shift operators
coming on duty aro\md-the clock.
CONTROL TESTS
1. Again; all of us are aware of the need for
conscientious, timely and proper sample
collection.
2. Essential control tests should be run at
least once every 8-hour shift; and more fre-
quently when needed during troublesome times.
3. The control test series for activated sludge
should include the all too frequently neg-
lected following routines:
Aeration Tank D.O.
Clarifier sludge blanket depth.
Final effluent turbidity.
Mixed liquor sludge settlometer test.
Sludge concentration by centrifuge tests.
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d;.r.:and.~ from
c,er,T; series 'trill c'e scribe
process status and dictate the type and magni-
tude of control adjustments needed to maintain,
or restore, proper process performance.
PROCESS RELIABILITY
1. The senior staff member directing process
control operations snould:
a. Summarize and evaluate all essential
control terrc. data, results of demand
calculations, and extent of control
adjustments daily.
b. Develop (and keep up-to-date) running
trend charts illustrating significant
features of plant performance. For
example:
Plant Loading
Sludge Settling Characteristics
Sludge Concentration Characteristics
Sludge Blanket Depth
Final Effluent Turbidity, etc.
c. Study trend charts and determine cause-
effect relationships between process con-
trol and plant performance to:
l) Document loadings or control pro-
cedures that have caused trouble
and must be avoided in the future.
2) Identify procedures that have proved
successful and should be continued.
2. By conscientious, intelligent application of the
basic operational requirements, discussed prev-
iously, the Director of a properly designed treat-
ment plant will achieve consistently reliable plant
performance and excellent final effluent quality.
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