NATIONAL FIELD INVESTIGATIONS CENTER
CINCINNATI
OPERATIONAL CONTROL PROCEDURES
for the
ACTIVATED SLUDGE PROCESS
PART I
OBSERVATIONS
APRIL 1973
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF ENFORCEMENT AND GENERAL COUNSEL
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NATIONAL FIELD INVESTIGATIONS CENTER - CINCINNATI
OPERATIONAL CONTROL PROCEDURES
FOR THE
ACTIVATED SLUDGE PROCESS
PART I
OBSERVATIONS
by
Alfred W. West, P. E.
Chief, Waste Treatment Branch
APRIL 1973
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF ENFORCEMENT AND GENERAL COUNSEL
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FOREWORD
The Waste Treatment Branch of the National Field Investiga-
tions Center - Cincinnati is developing a series of pamphlets
describing Operational Control Procedures for the Activated Sludge
Process. This series will include Part I Observations, Part II
Control Tests, Part III Calculation Procedures, Part IV Sludge
Quality, Part V Process Control, and an Appendix. Each one of
these individual parts will be released for distribution as soon
as it is completed. The original five-part series may then be
expanded to include case histories and refined process evaluation
and control techniques.
This pamphlet has been developed as a reference for Activated
Sludge Plant Control lectures I have presented at training sessions,
symposia, and workshops. It is based on my personal conclusions
reached while directing the operation of dozens of different
activated sludge plants. This pamphlet is not necessarily an ex-
pression of Environmental Protection Agency policy or requirements.
The mention of trade names or commercial products in this
pamphlet is for illustrative purposes and does not constitute
endorsement or recommendation for use by the Environmental Pro-
tection Agency.
A. W. West
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TABLE OF CONTENTS
Page No,
OBJECTIVES 1
INTRODUCTION 1
AERATION TANKS 2
TURBULENCE 2
SURFACE FOAM AND SCUM k
FRESH CRISP WHITE FOAM k
EXCESSIVE BILLOWING WHITE FOAM 5
THICK, SCUMMY, DARK BROWN FOAM 7
SLUDGE COLOR AND ODOR 8
FINAL CLARIFIERS 8
FINAL EFFLUENT APPEARANCE 9
FINAL CLARIFIER SURFACE APPEARANCE 9
SLUDGE BULKING 9
SLUDGE SOLIDS WASHOUT 12
CLUMPING AND ASHING 12
STRAGGLER FLOG 13
PIN FLOG 11+
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OBJECTIVES
Aeration tanks and final clarifiers are studied percep-
tively for informative physical characteristics that
help identify sludge quality and process status. They are
scrutinized for clues that indicate the kind of control ad-
justments needed to achieve optimum plant performance. The
inferences of such physical findings are used to supplement the
results of other more specific control tests that dictate the
direction and magnitude of the essential control adjustments.
INTRODUCTION
Much can be learned from 'simple but perceptive sensory
observation of process features such as the type, color, and
extent of foam on the aeration tank surface and the presence •
or lack of scums and rising floe particles in the final clari-
fiers. From such observations, a skilled operator usually can
determine the basic phase his process is moving towards or is
locked into. Such observations will make him aware of more
generalized long-term requirements. They will help him reach
proper conclusions from the results of other more specific
control tests that are used to calculate process demands and to
determine the type and extent of control adjustments that are
actually needed.
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The entire series of physical observations described in
this section should be made each time the routine control tests
are performed. The appearance of the final effluent and the
aeration and clarifier tank contents should be examined at
least once during each operator's eight-hour shift.
AERATION TANKS
TURBULENCE
The operator should observe, the entire aeration tank sur-
face for turbulence. Though some of his conclusions will be
subjective and based on past experience, the extent of•surface
turbulence will indicate whether or not all sewage, return sludge,
and mixed liquor are thoroughly mixed throughout the entire aera-
tion tank. Observable surface characteristics will imply whether
or not dead spots or insufficiently mixed core areas may exist . .
within the aeration tanks. The operator should maintain, increase,
or decrease air discharge rates according to the conclusions he
reaches from the results of such observations and from supple-
mentary dissolved oxygen determinations.
He obviously should reproportion air flow through headers or
individual subheaders to correct any dead spots, unequal air dis-
tribution, or inadequately tapered aeration intensity that may
have been observed.
If serious mixing deficiencies prevail despite corrective
air distribution adjustments, he should attempt to determine which
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COMPRESSED AIR
SPIRAL FLOW
COMPRESSED AIR
CROSS ROLL
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structural, mechanical or design deficiencies may be responsible
for the difficulties. If normal air balancing procedures fail
to correct evident defects, he should be prepared to recommend
the maintenance or modification procedure that may be necessary
to eliminate the problems.
In many cases, aeration deficiencies can be corrected by
routine diffuser cleaning or by replacing existing diffusers with
more effective maintenance free units. In some cases, major
mechanical alterations may be required to relocate and increase
the number of diffusers to mix and aerate the tank contents thor-
oughly. Overall process performance has been improved at some
plants by replacing the single run of diffusers that extended
along one side wall with multiple parallel runs of diffusers ex-
tending either longitudinally or across the tank bottom.
SURFACE FOAM AND SCUM
The type of foam or scum, if any, accumulated over the aera-
tion tank surface, and to a lesser extent, the color of the mixed
liquor sludge reveal process status and indicate generalized long-
term sludge wasting requirements.
Fresh Crisp White Foam
Only a modest accumulation of white, or at least light col-
ored, crisp appearing foam is usually evident on aeration tank
surfaces when an excellent final effluent is produced by a prop-
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erly balanced activated sludge process. Under such circumstances
the operator should continue his successful control policies until
the physical characteristics or the results of other control tests
diverge from optimum.
Excessive Billowing White Foam
If the aeration tanks are covered by thick voluminous billows
of white sudsy foam, the operator can be quite certain that the
sludge is too young and that sludge age should be increased by
reducing the sludge wasting rate.
Sludge age, which is controlled by the sludge wasting rate,
indicates the approximate number of days that the activated sludge
remains in the system before being discarded. Prolonged excessive
sludge wasting will reduce sludge age by increasing the propor-
tionate amount of newly developed floe in the system. Conversely,
unduly low wasting rates will increase the number of days the
sludge is retained^ in the system and will increase the propor-
tionate amount of older sludge.
Sludge wasting rates should be decreased only gradually on a
day-to-day basis to correct the process imbalance that was re-
vealed by the excessive white foam. Best results are usually
obtained by reducing the wasting rate approximately twenty per-
cent on each successive day until all observations and tests
reveal improving trend lines. When positive improvement is
noted, the operator should maintain the lowered wasting rate for
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BILLOWY WHITE FOAM
(YOUNG SLUDGE)
IP
. ' « ^ 1 ,' 1
>;d
THICK DARK TANK FOAM
(OLD SLUDGE)
DARK FOAM, BAD ODOR
(SEPTIC SLUDGE)
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about three more days while the improving trends are confirmed.
He should, of course, continue to plot and review process con-
trol and response trend lines which will alert him to subse-
quent control adjustment policy that may become necessary. As
implied previously, wasting usually should not be discontinued
completely. Exceptionally low sludge settling rates and
classic sludge bulking that can accompany this type of foam
generation may be corrected by reducing air discharge rates
to lower the mixed liquor dissolved oxygen concentration to the
0.5 to 1.0 mg/1 range.
Operators who have actually gone through this white foam
cycle realize that not all foam is caused by detergents.
Thick, Scummy, Dark Tan Foam
At the other extreme, the operator may observe a more
dense and somewhat greasy scummy layer of deep tan to brown
foam covering the entire aeration tank surface. Such a foam
almost always indicates that the sludge is too old and possibly
over oxidized. The obvious answer is to increase sludge wasting
rates. Here again, the sludge wasting rate should usually be in-
creased modestly, possibly twenty percent per day, on a day-to-day
basis while observing trend lines to determine the maximum wasting
rate that should be maintained until the difficulties are overcome
and the process is restored to proper balance.
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Referring to the dark tan foam cover, many operators have
recently voiced concern about the effects of massive Nocardia
concentrations in aeration tank foams. Some of these scummy foams
are, in fact, loaded with Nocardia, but I know of no case in which
Nocardia caused the foam or the associated difficulties; they were
there because they enjoyed the environment. Increased sludge
wasting has eliminated both scums and Nocardia.
SLUDGE COLOR AND ODOR
At times a poor quality extremely dark brown-colored sludge,
sometimes almost black, releasing hydrogen sulfide odors, may be
observed in the aeration tanks. It does not take much experience
to recognize this problem. Most operators would logically in-
crease air discharge rates immediately to provide 2-3 mg/1
DO throughout the tank contents. In severe cases, when such
color and odor persists, despite proper control measures, they
should question the adequacy of the aeration devices in-
stalled at their plants. Under such circumstances, the operator
should clean or replace the existing diffusers and recommend
appropriate mechanical modifications as discussed in the section on
turbulence and mixing.
FINAL CLARIFIERS
The operator should also observe the final effluent and the
clarifier water surface critically for additional clues to indi-
cate process phase and balance, and to supplement the results of
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other control tests to determine sludge wasting and air control
requirements.
FINAL EFFLUENT APPEARANCE
If the final effluent appears clear and attractive, or is
improving day by day, obviously the operator should continue his
present control policy if all control measurements are in the
proper range.
Conversely, if it appears turbid or contains noticeable
solids, he should modify his operational control policies and pro-
cedures. Though observation of poor effluent quality alone will
not reveal specific control requirements, it signals the need for
judicious review of control and response trend lines and for
revised operating policies. Specific control adjustments
will be dictated by the results of other control tests.
FINAL CLARIFIER SURFACE APPEARANCE
Sludge Bulking
Operators who have experienced true classic sludge
bulking find it all too easy to remember and identify. Such
conditions are evidenced by a homogeneous appearing sludge
blanket that extends throughout the entire clarifier, and can be
observed at the water surface while the mixed liquor solids pour
out over the final effluent weirs. Though at times induced
by shock loadings, and aided and abetted by ineffective aeration
devices, classic sludge bulking usually is caused by improper
operational control rather than by inadequate plant capacity.
Furthermore, impending bulking usually can be recognized many
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CLASSIC SLUDGE BULKING
RISING CLUMPS
SOLIDS WASH OUT
FLOATING ASH
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11
days before it actually occurs by judicious use of the sludge
depth blanket finder described in Part II Control Tests.
This type of bulking, which is practically always associ-
ated with young sludge, usually can be eliminated by reducing
sludge wasting rates, increasing return sludge flow rates, and
reducing air discharge down to the minimum rates that will still
maintain aerobic conditions in the aeration tanks. Where appro-
priate flexibility has been designed into plants, bulking has
also been eliminated by changing the process mode from conven-
tional plug flow to step flow by introducing the primary effluent
into the second or third bay of the aeration tank.
In some cases where such control adjustments have failed,
emergency chemical treatment has cured classic sludge bulking.
Some operators have successfully applied polymers and ferric
chloride or alum to the mixed liquor entering the final clarifier
without destroying desirable sludge characteristics. Laboratory
jar tests should be performed to indicate the type of chemical, the
dosage rate, and the pH range that will be most effective. If the
chemical additives do not cure actual bulking in the final clari-
fier s, even though the sludge samples settled and compacted in
the laboratory jar tests, the chemicals should be added at
different points between the aeration tanks and the final
clarifiers until best results are obtained. It is usually best
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to apply chemicals to the wet well preceding, or the pipe
line leading to, the final clarifier.
Sludge Solids Washout
Excessive sludge washout over the final effluent weirs,
when the upper surface of the sludge blanket is more than three
feet below the clarifier water surface and when sludge settles
properly in the laboratory, should not be confused with
classic sludge bulking. At times this type of severe effluent
degradation has been observed while the settlometer test re-
vealed excellent sludge quality. In many multiple clarifier
plants this has been caused by unequal mixed liquor flow into,
or by unequal return sludge removal from, individual final
clarifiers. Under such circumstances, every effort should be
made to balance flows into and out of the clarifiers.
Solids washout has also been caused by hydraulic overloading
and by improper clarifier inlet port arrangements and faulty final
effluent weir locations. Differing from classic sludge bulking,
this type of problem is more frequently caused by hydraulic over-
loads or inappropriate final clarifier design rather than by op-
erational control procedures.
Clumping and Ashing
At times, large masses of sludge, possibly one foot
in diameter, may be seen rising, then bursting, and finally
spreading over the clarifier surface. This has sometimes
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13
been called "clumping." At other times, smaller sludge particles
usually deep brown to gray in color, may rise and then spread over
the tank surface. Some operators call this "ashing." This prob-
lem occurs when sludge age has been permitted to increase beyond
the optimum equilibrium requirement of the process cycle and it
can usually be eliminated by increasing sludge wasting rates. Re-
ducing air discharge rates to the minimum levels that will still
maintain aerobic conditions in the aeration tanks has also been
helpful.
Straggler Floe
At times, small, almost transparent, very light fluffy,
buoyant sludge particles (one-eighth to one-quarter inch in diam-
eter) may be observed rising to the clarifier surface near the out-
let weirs. This condition is usually intensified in a shallow
clarifier and may be especially noticeable at high return sludge
flow rates. When this type of straggler floe is observed while
the final effluent is otherwise exceptionally clear, and partic-
ularly if it prevailed even during relatively low surface overflow
rates, it implies that sludge age should be increased moderately
towards optimum. Since this type of straggler floe usually occurs
at relatively low mixed liquor solids concentrations and is
usually intensified during the early morning hours, it is believed
that these particles are fresh, low density portions of new sludge
that has been built up over night. Straggler floe formation can
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be minimized, and usually eliminated, by reducing sludge
wasting rates moderately to increase sludge age while return
sludge and air discharge rates are controlled to meet process
demands that are calculated from other control tests.
Pin Floe
At other times, very small compact pin floe, usually
less than one-thirty-second of an inch in diameter, may be
observed suspended throughout moderately turbid final clarifier
tank contents. This is a strong indication that sludge age
has been increased unduly, and the sludge has become over-
oxidized. This will be confirmed by the settlometer test if
rapidly settling discrete sludge particles appear granular
rather than flocculant, and accumulate rather than compact while
forming a settlometer sludge blanket. In essence, granular
sludge particles were falling through a turbid liquor rather
than compacting and squeezing out a clear final effluent.
When these final clarifier characteristics are confirmed
by the settlometer test, the sludge wasting rate should be
increased while return sludge flow is adjusted to meet other
control test demands.
T> US GOVERNI/ENT PRINTING OFFICE 1973—758-487/1025
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