EPA-330/9-7 4-001 J|,b
NATIONAL WASTE TREATMENT CENTER
CINCINNATI
OPERATIONAL CONTROL PROCEDURES
for the
ACTIVATED SLUDGE PROCESS
PART I - OBSERVATIONS
PART II - CONTROL TESTS
MAY 1974
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WATER PROGRAM OPERATIONS
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NATIONAL WASTE TREATMENT CENTER - CINCINNATI
OPERATIONAL CONTROL PROCEDURES
FOR THE
ACTIVATED SLUDGE PROCESS
PART I - OBSERVATIONS
PART II - CONTROL TESTS
by
Alfred W. West, P.E.
Director, National Waste Treatment Center
(formerly Waste Treatment Branch, National Field
Investigations Center - Cincinnati)
MAY 1974
(Revised Nov.,1975)
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WATER PROGRAM OPERATIONS
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FOREWORD
The National Waste Treatment Center (Cincinnati) is
developing a series of pamphlets describing Operational
Control Procedures for the Activated Sludge Process. This
series, describing the "NWTC Procedures", 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 of these individual parts will be
released for distribution as soon as it is completed, though
not necessarily in numerical order. The original five-part
series raay 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 expression of
Environmental Protection Agency policy or requirements.
Parts I and II were originally printed as separate
pamphlets dated April 1973. The May 1974 printing combines
the two Parts which includes some revision concerning use of
the centrifuge and dilution settlometer tests.
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 Protection Agency.
Alfred W. West
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PART I
OBSERVATIONS
TABLE OF CONTENTS
PAGE NO.
OBJECTIVES 1
INTRODUCTION 1
AERATION TANKS 1
Turbulence 1
Surface Foam and Scum 3
Sludge Color and Odor 6
FINAL CLARIFIERS 6
Final Effluent Appearance 6
Final Clarifier Surface Appearance 7
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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
adjustments 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
clarifiers. 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.
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
surface 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 aeration tank. Observable surface
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COMPRESSED AIR - SPIRAL FLOW
Showing Diffuser Socks
COMPRESSED AIR - CROSS ROLL
Showing Diffuser Socks
COMPRESSED AIR - SPIRAL FLOW
In Use
>• .r==^a~j=~-^.~p|
~~*~~
COMPRESSED AIR - CROSS ROLL
In Use
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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
supplementary dissolved oxygen determinations.
He should reproportion air flow through headers or
individual subheaders to correct any dead spots, unequal air
distribution, 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 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 thoroughly. 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 extending
either longitudinally or across the tank bottom.
SURFACE FOAM AND SCUM
The type of foam or scum, if any, accumulated over the
aeration 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
colored, crisp appearing foam is usually evident on aeration
tank surfaces when an excellent final effluent is produced
by a properly balanced activated sludge process. Under such
circumstances the operator should continue his successful
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A
•',,•••-
BILLOWY WHITE FOAM
(Young Sludge)
THICK DARK TANK FOAM
(Old Sludge)
DARK FOAM, BAD ODOR
(Septic Sludge)
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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 proportionate 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 proportionate
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 revealed by the excessive white foam. Best results are
usually obtained by reducing the wasting rate approximately
twenty percent on each successive day until all observations
and tests reveal an improving trend. When positive
improvement is noted, the operator should maintain the
lowered wasting rate for about three more days while the
improving trends are confirmed. He should, of course,
continue to plot and review process control and response
trends which will alert him to subsequent control adjustment
policy that may become necessary. As implied previously,
wasting usually should not be discontinued completely-
Exceptionally low sludge settling rates and classic 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.
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 increased modestly, possibly twenty
percent per day, on a day-to-day basis while observing trend
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lines to determine the maximum wasting rate that should be
maintained until the difficulties are overcome and the
process is restored to proper balance.
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 increase 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 installed 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.
The operator should also observe the final effluent and
the clarifier water surface critically for additional clues
to indicate process phase and balance, and to supplement the
results of 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 procedures. Though observation of poor effluent quality
alone will not reveal specific control requirements, it
signals the need for judicious review of control and
response trends and for revised operating policies.
Specific control adjustments will be dictated by the results
of other control tests.
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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 from the trend charts (Appendix)
and by judicious use of the sludge depth blanket finder
(Part II Control Tests) many days before it actually occurs.
This type of bulking, which is practically always
associated 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 maintain aerobic conditions in the aeration tanks.
Where appropriate flexibility has been designed into plants,
bulking has also been eliminated by changing the process
mode from conventional 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
clarifiers, 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 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
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CLASSIC SLUDGE BULKING
RISING CLUMPS
SOLIDS WASH OUT
FLOATING ASH
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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 revealed 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,
where excessive velocity currents are induced, every effort
should be made to balance flows into and out of the
clarifiers.
Solids washout has also been caused by hydraulic
overloading, by improper clarifier inlet port arrangements,
and by faulty final effluent weir locations. Differing from
classic sludge bulking, this type of problem is more
frequently caused by hydraulic overloads or inappropriate
final clarifier design rather than by operational control
procedures.
Clumping and Ashing
At times, large masses of sludge, possibly as large as
one foot in diameter, may be seen rising, then bursting, and
finally spreading over the clarifier surface. This has
sometimes 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 problem usually 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.
Reducing 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
diameter) may be observed rising to the clarifier surface
near the outlet 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 particularly 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
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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 have been built up over night.
Straggler floe formation can be minimized, 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 overoxidized. 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.
10
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PART II
CONTROL TESTS
TABLE OF CONTENTS
PAGE NO,
OBJECTIVES 13
INTRODUCTION 13
METER READINGS 14
DEPTH OF SLUDGE BLANKET 16
SAMPLE COLLECTION 19
SETTLOMETER TESTS 20
CENTRIFUGE TESTS 24
EFFLUENT TURBIDITY TESTS 26
DISSOLVED OXYGEN TESTS 27
11
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OBJECTIVES
Control tests, that can be run as frequently as needed
throughout each 24-hour cycle, reveal sludge quality,
process status, and final effluent quality.
Results of the settlometer, centrifuge and final
clarifier sludge blanket level tests are used to calculate
solids distribution ratios between the aeration tanks and
the final clarifiers, sludge detention time in the final
clarifiers and other factors influencing process
performance.
The coordinated results of the full test series,
including flow records, turbidity, and dissolved oxygen test
data, are ultimately used to determine the return sludge
flow, excess sludge wasting and air discharge rates needed
to maintain or restore excellent final effluent quality.
INTRODUCTION
Practically all information needed to define sludge
quality and process status and to calculate control
adjustment requirements can be determined from the results
of a few relatively simple tests. The entire series of
settlometer, centrifuge, sludge blanket depth, effluent
turbidity and dissolved oxygen tests can be completed in
about ninety minutes and can, therefore, be run as
frequently as needed throughout each 24-hour cycle. The
complete test series should be run at least once every
eight-hour operating shift. The sludge blanket and the
fifteen minute centrifuge tests should be run more
frequently whenever rapidly changing process characteristics
demand more critical scrutiny and control. Though simple,
informative, and too frequently neglected, these tests are
neither new nor difficult. They were, in fact, proposed by
E. B. Mallory more than thirty years ago. Though the
testing techniques and some of the data processing
methodology includes that proposed by Mallory, the
procedures and calculations for determining control
adjustment requirements, as discussed in these pamphlets,
were developed by me.
These discussions include only those control tests that
are used directly to identify process performance and to
13
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dictate process control adjustments. Other important
monitoring type tests, such as BOD, COD, etc., are not
included in these discussions.
METER READINGS
Each control test series should be started by reading
plant meters to record flow data that are needed to
determine process requirements.
Flow records specifically related to control of the
activated sludge process include:
AERATION TANKS
Total influent waste water flow to aeration tanks
Return sludge flow to aeration tanks
Total air to aeration tanks
FINAL CLARIFIERS
Mixed liquor flow to clarifiers
Sludge removed from clarifiers
EXCESS SLUDGE TO WASTE
Mixed liquor wasted
Return sludge wasted
The extent of additional flow data needed to evaluate
and control total plant performance will vary from plant to
plant. Other waste streams that should also be measured
could include:
Raw sewage flow
Plant drainage recycled to primaries
Primary sludge flow
Sludge thickener flows
Sludge filtrate flows
Digester supernatant flow
Sludge removed from plant
Plant drainage recycled to aeration tanks
Final effluent reuse
Final effluent flow
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- The following discussion is limited specifically to the
activated sludge portion of the treatment plant. For
multiple tank plants, having three aeration tanks and three
final clarifiers, for example, the operator should also
routinely maintain equal hydraulic loading to each of the
parallel operating units.
The following flow rates should be determined during
each test period and the 24-hour totalized value of each
should be recorded every day.
SEWAGE FLOW INTO EACH AERATION TANK
Frequently overall treatment is impeded when aeration
tank loadings cannot be, or are not, balanced properly
between the parallel aeration tanks. The better treatment
provided by an underloaded tank unit will not compensate
fully for the poorer treatment provided by the overloaded
unit.
RETURN SLUDGE FLOW INTO EACH AERATION TANK
Maldistribution of return sludge flow, especially when
coupled with unequal sewage flow loading between parallel
aeration tanks, can further distort the proportionate
purification pressures (mixed liquor concentration and
detention times) and reduce treatment capability.
AIR DISCHARGE TO EACH AERATION TANK
Here, again, unequal distribution as well as inadequate
or excessive total air flow can degrade sludge quality and
purification. The basic indicating, recording, and
totalizing meters should be used to measure and control air
flow to each aeration tank. Simple indicating meters should
be observed to assure proper air distribution to the main
air headers feeding each bay or compartment of individual
aeration tanks.
MIXED LIQUOR FLOW INTO EACH FINAL CLARIFIER
The influent flow to each clarifier should be
determined during each test period so that the surface
overflow rates and the solids loadings can be distributed
properly to each clarifier. This is especially important
during troublesome times when rising sludge blankets warn
that classic sludge bulking may be imminent.
15
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If the clarifier flows are not balanced properly at
such times, mixed liquor sludge can be forced out over the
final weirs of the overloaded clarifier, while the sludge
blankets in the other clarifiers may remain low enough to
produce a clear effluent. Unfortunately, this may occur
when proper flow distribution could otherwise have held the
sludge blankets safely below the water surface of all
clarifiers.
SLUDGE REMOVED FROM EACH CLARIFIER
These meters, one for each tank, should also be read at
each test period and the totalized value should be recorded
each day. Here again, the need to balance sludge removal
from all clarifiers to maintain proper sludge blanket level
control is obvious.
EXCESS SLUDGE FLOW TO WASTE
This meter, or meters, should be read at every test
period and whenever the wasting rate is changed.
Procedures to use flow data and other control test
results to determine return sludge, waste sludge, and other
process requirements will be discussed in other parts of
this pamphlet series.
DEPTH OF SLUDGE BLANKET
After checking the meters, an operator following a
logical test schedule should move on to determine final
clarifier characteristics. He should determine the depth of
the sludge blanket that has accumulated within the clarifier
and observe conditions at and near the water surface.
Various types of sludge blanket depth finders are used.
The type described in the Appendix can be constructed simply
and used conveniently; and when handled properly, it becomes
a valuable, reliable operational control tool. At least two
sludge blanket finders should be provided to assure
continuity of test results, especially when one is being
repaired. One should be long enough to reach from the
bridge to the tank bottom. The other shorter one should
only be long enough to extend about half way down into the
tank.
Two other types of blanket finders are wands with
photo-electric cell actuated buzzers and a series of small
16
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SLUDGE BLANKET FINDER
LIGHT AND SIGHT GLASS
OPERATOR STARTING
DEPTH OF BLANKET TEST
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airlift pumps extending down to different measured depths
within the final clarifier. The operator should use the
type he finds most convenient, but it is suggested that the
reliable pipe and sight glass type be provided, at least as
a back-up instrument.
It takes a little practice for an operator to use a
blanket finder, or, more exactly, to know what he is looking
for, but after the first few diligent attempts he will be
surprised at how accurately and easily blanket depths can be
determined.
A cross section of most clarifiers containing a
reasonably good sludge solids distribution balance may
reveal a zone of discrete straggler floe particles settling
down to form the blanket. Somewhat farther than half-way
down into the tank, there will usually be a distinct plane
of demarcation between the individual settling sludge
particles and the relatively thin, but quite homogeneous,
upper surface of the accumulated sludge blanket. Then the
concentration of this sludge blanket will usually increase
in density down to the zone of maximum compaction at the
very bottom of the tank.
Here are two precautions - which will only be resolved
by practice. Do not stop the downward movement of the
blanket finder when the individual discrete sludge particles
are observed. Secondly, do not force the blanket finder
down to the point where the denser sludge within the blanket
actually obscures the light. Instead, move the blanket
finder down at a rather rapid uniform rate through the clear
liquor, then continue down through the zone containing
individual floe particles, and finally stop when the upper
surface of the definite homogeneous sludge mass is observed.
Take your reading at this point; you have reached the upper
surface of the sludge blanket.
The blanket reading station should be located at a
point where the single measurement will approximate the
average depth of the entire sludge blanket. A station on
the final clarifier bridge, about one-third of the tank
radius in from the outer tank wall, will usually satisfy
this requirement. The selected station should be marked so
that blanket depths are always measured at the same
location.
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SAMPLE COLLECTION
Samples for the control tests and other laboratory work
must be collected on time, from appropriate locations, and
according to approved procedures; and they certainly must be
representative. These elementary principles can not be
violated without cost.
CENTRIFUGE TEST SAMPLES
After observing the clarifiers, return sludge samples
should be collected for the centrifuge test. Selection of
the sampling station for collecting samples that truly
represent the actual quality of the entire return and waste
sludge flow is most important. It is best to collect the
sample from the point where the thoroughly mixed return
sludge from all clarifiers enters the aeration tanks. If
such location is inaccessible, the sample should be taken
from a tap off the return sludge pump discharge header. The
sample pipe should obviously be flushed out thoroughly
before each sample is collected. Since mixing may be
inadequate, sampling from deep wet wells should be avoided
if at all possible.
In addition to the sample collected for basic plant
control, individual return sludge samples should be
collected from each final clarifier to check or to balance
multi-tank performance.
At times, samples should be collected from each of the
individual sludge draw-off tubes in each final clarifier.
This series of samples need be collected only as frequently
as required to assure uniform sludge withdrawal from the
entire clarifier floor area. Ordinarily, such "trim spin"
samples are collected about once each week.
The samples collected for the settlometer test, as
described in a following section, will also be used to
determine the centrifuged concentration of the mixed liquor
solids.
TURBIDITY TEST SAMPLES
Special samples should also be collected from the final
clarifier to determine the turbidity of the treated waste.
These samples for turbidity should be collected deliberately
from the clearest area of the final clarifier water surface
19
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that contains the least surface sludge or scum that may be
present. When collected in this manner, these samples will
indicate specific process performance, undistorted by
clarifier or other equipment defects that may need
correction. Other 24-hour composite samples collected for
plant monitoring (BOD, COD, TSS, etc.) will be used to
determine net process performance.
SETTLOMETER TEST SAMPLES
The mixed liquor sample for the settlometer test should
be collected last of all. This sample, which should
represent the average quality of all mixed liquor flowing
out of all aeration tanks, will also be used to determine
solids concentration both by centrifuge and by weight.
Preferably, this sample should be collected from the common
discharge flume that contains all mixed liquor flowing from
the outlet of all active aeration tanks. If the common
discharge header is inaccessible, equal volumes of mixed
liquor should be collected from the outlet end of each
aeration tank and composited into a single representative
sample for the settleability and solids concentration tests.
The necessity for collecting mixed liquor samples in
wide-mouthed containers, rushing the samples to the
laboratory, and starting the settlometer test immediately,
can not be over emphasized. After collection, the samples
should be subjected to the absolute minimum amount of
agitation and aeration. Improper sample handling, such as
violent shaking or splashing, or unwarranted delay between
the time that the sample is collected and the time it is
poured into the settlometer, can alter sludge settling
characteristics and induce erroneous settlometer test
results.
SETTLOMETER TESTS
Preferably, mixed liquor sludge settleability tests
should be determined in a clear glass cylinder with a larger
diameter and a lesser depth than the standard 1,000 cc
graduated cylinder that is frequently used for this test. A
glass cylinder shaped similar to the standard two-liter
beaker, but with better graduations and without the rounded
bottom edge, would be satisfactory. Many operators now
prefer the five inch diameter, six inch graduated depth,
two-liter Mallory settlometer that is graduated in tenths
and hundredths of the settlometer volume. Satisfactory
settlometers can also be fabricated from five inch diameter
clear plexiglass cylinders.
20
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•MM
TEST SET-UP
5 MINUTE READING
1 HOUR APPEARANCE 1 HOUR READING
SETTLOMETER TEST
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The mixed liquor sludge sample should be stirred and
then poured into the settlometer carefully and rapidly and
with the least possible amount of additional aeration or
disturbance. The settlometer contents should then be
stirred gently to assure thorough mixing; then all swirling
should be dampened immediately with a wide paddle (four inch
sheet aluminum works well) before the timer is started.
The volume of the settlometer occupied by settled
sludge should be read and recorded at every five-minute
interval for the first thirty minutes and then every ten-
minute interval for the second thirty minutes of the one
hour settlometer test. This is the standard test duration.
When sludge settles extremely slow, or when bulking
actually occurs, the settlometer test period should be
extended beyond the one hour reading. By returning to
observe the settled sludge volume after 2, 3, 4, etc. hours,
the operator will be able to determine ultimate sludge
compaction for a more thorough process evaluation.
Running simultaneous multiple settlometer tests on
diluted specimens of slowly settling mixed liquor can help
the operator decide whether to increase or decrease the
sludge wasting rate. At least two dilutions should be made.
One settlometer should contain undiluted mixed liquor, one
should contain about 75% mixed liquor diluted with 25% final
effluent, and a third settlometer should contain half mixed
liquor and half final effluent.
Though this expanded multiple dilution test need be run
only once per day while the slow settling problem persists,
it should follow the previously stated standard procedures.
If confirmed by other control test results, these multiple
settlometer test results can reveal the following control
adjustment needs.
If the diluted samples settle much more rapidly than
the undiluted sample (especially during the first 10
minutes), the system contains too many fair to good quality
mixed liquor solids and sludge wasting should most probably
be increased.
If the diluted samples settle at the same rate, or only
slightly faster than the undiluted mixed liquor sample
(especially during the first 10 minutes), the mixed liquor
sludge is truly bulky. This has been caused frequently by
excessively heavy wasting which reduced sludge age way below
the optimum. Sludge wasting should usually be reduced (but
22
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not down to zero) modestly, day by day, to develop a sludge
that can concentrate more.
Operators who set up a test cylinder, walk away, and
then return for only one single observation after thirty
minutes, miss most of the information that actually defines
sludge quality. A single thirty-minute test, to determine
SVI, is somewhat like a five-day BOD, in that it reveals
only one point in a progressive reaction and it will not
help an evaluator who is really interested in reaction rates
and the full impact of ultimate demands.
The first five-minute reading is one of the two most
important observations for this test. During this first
five minutes the conscientious operator will critically
observe how the sludge particles agglomerate while forming
the blanket. He will see whether the sludge compacts slowly
and uniformly while squeezing clear liquid from the sludge
mass or whether tightly knotted sludge particles are simply
falling down through a turbid effluent. He will also
observe how much and what type of straggler floe, if any,
remains in the supernatant liquor above the main sludge
mass.
The importance of conscientious, perceptive observation
during the first five minutes can not be over emphasized.
During these first five minutes the operator will acquire
additional insight into sludge character and quality, and
will be in a much better position to evaluate what the
settlometer test reveals.
The sixty-minute reading is the second most important
observation. It provides a check on final clarifier sludge
blanket characteristics and is used to compute process
equilibrium indices and operational control adjustment
requirements.
After the tests are completed, the settlometers should
remain undisturbed for at least four hours. The time at
which previously compacted sludge starts to swell and rise
to the surface is another important indicator of sludge
quality- Settled sludge should not remain down forever, any
more than it should gasify and pop to the surface during the
sixty-minute test cycle. Well-oxidized sludge will
frequently begin to swell somewhat after ninety minutes and
will usually float to the surface within two to four hours
after the test was started.
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A portion of the sample collected for the settlometer
test should be saved for solids determinations.
CENTRIFUGE TESTS
The centrifuge permits rapid determination of mixed
liquor and return sludge solids concentrations for immediate
use in calculating solids distribution ratios, effective
return sludge percentages, solids concentration rates,
clarifier sludge detention times, and a host of other
process relationships used to determine operational control
demands.
The use of 12.5 ml API (American Petroleum Institute)
centrifuge tubes in a clinical centrifuge, with a six place
horizontal head, revolving at full speed (position 7 on the
International No. 428, for example) has been found
convenient and appropriate for sewage plant control testing.
The mixed liquor and return sludge specimens should be
centrifuged for a standardized fifteen minute interval to
assure consistent compaction and to avoid discrepancies that
can be introduced by slight variations in testing time when
shorter intervals (say from one to three minutes) are used.
Shorter time intervals may be used for "trim spins" and
other special tests when many specimens are centrifuged to
determine the solids balance in multiple aeration tanks or
clarifiers.
The centrifuge is an exceedingly useful and convenient
tool for searching out and correcting unbalanced sewage or
return sludge flow distribution as well as unequal mixed
liquor and return sludge solids concentrations in multiple
tank plants. It has also been used effectively to determine
return sludge percentages in the all too many cases where
flow meters have been missing or were in error. Mixed
liquor and return sludge concentration data frequently are
plugged into the mixing formulas, described in Part IIIA.
It is then possible to determine either the primary effluent
flow rate into the aeration tanks or the sludge withdrawal
rate from the final clarifiers when either of the meters for
these two locations is missing or inoperative.
Some operators question the use of the centrifuge for
sludge concentration determination because such tests
results can not always be correlated with suspended solids
determinations by the laboratory balance. The variation in
the ratio of mixed liquor suspended solids concentration
determined by the laboratory balance to that determined by
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PROPER SAMPLE HANDLING
FOR TURBIDITY TEST
CENTRIFUGED
SLUDGE SAMPLES
TURBIDITY READING
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the centrifuge is affected by sludge age and oxidation and
is, therefore, one of the virtues of the centrifuge test;
not a defect. Furthermore, the centrifuge test result,
because it is influenced by the specific gravity and the
surface area of the sludge mass, provides a realistic
measure of effective sludge concentration.
EFFLUENT TURBIDITY TESTS
Two turbidity determinations should be made on the
sample collected from the final clarifier. A photo-electric
activated readout type turbidimeter with results expressed
in Jackson Turbidity Units (JTU) or Formazin Turbidity Units
(FTU) is recommended. The turbidity of excellent final
effluents usually ranges between 1.0 and 3.0 JTU. Final
effluent turbidities greater than 3.0, and especially those
exceeding 10.0 JTU, indicate that the full purification
capability of the activated sludge process has not been
achieved.
The first turbidity determination should be made
shortly after the settlometer and centrifuge tests have been
started. The sample should be stirred (only for the first
test) before it is poured into the turbidimeter test vial.
The vial exterior should always be wiped clean and dry
before insertion into the turbidimeter. This precaution
will protect the internal components and enhance accuracy.
The final effluent sample should then be put aside for
a one-hour settling period to permit gravity separation of
extraneous floatable scums and settleable solids that may be
attributed to flow overloads or inappropriate final
clarifier features. At the end of this time interval, the
turbidity of the subnatant liquor in the settled final
effluent sample should be determined.
Special additional precautions should be observed while
pouring the "settled" specimens into the turbidimeter test
vial. The specimen container should be carefully lifted
from the work bench and any accumulated surface scum should
be poured to waste before the settled liquid (free from any
sludge that may have settled to the bottom of the container)
is poured into the test vial. The pouring operation must be
gradual and continuous to avoid creating air bubbles and to
eliminate surface scums or settled solids that might
otherwise cause erroneous interpretation of the test
results.
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When performed properly, the turbidity test will
reflect process performance. If the final effluent contains
excessive suspended solids, for example, differences in the
turbidity test results on settled versus unsettled effluent
specimens will indicate whether the problem was created by
improper process control, hydraulic overloads or by
defective final clarifiers. In other words, if the final
clarifiers lack scum baffles and skimmers or have
inappropriately placed final effluent launders, or if the
tank geometry induces unnecessary velocity currents, the
turbidity of the settled specimen will reveal the true
effect of control adjustments on sludge characteristics and
process equilibrium. Many times the final effluent produced
by good sludge at proper process equilibrium contains
unnecessarily high concentrations of solids that are
needlessly forced out of the clarifiers by inappropriate or
malfunctioning equipment. The object of this turbidity test
is to define process response and control requirements
rather than to document built-in, and usually not
immediately correctable, plant defects.
Turbidimeter calibration and use should follow
manufacturer's instructions. Where multiple turbidity
standards are supplied with the equipment, the turbidimeter
should be calibrated with the standard closest to the
anticipated effluent turbidity. If, for example, 1.0, 10.0,
and 100.0 FTU standards were provided, and the equipment had
been calibrated with the 100 FTU standard, but the effluent
turbidity read out at less than 10 FTU; the meter should be
recalibrated with the 10 FTU standard and reread.
DISSOLVED OXYGEN TESTS
At least once per day, and preferably once during each
eight hour shift, dissolved oxygen concentrations should be
determined at the inlet and at the outlet ends of each
"pass" or compartmented area of the aeration tanks.
Additional dissolved oxygen determinations should be made
more frequently at "one" or "a single" aeration tank
sampling station to reveal dissolved oxygen variations
throughout each 24-hour cycle. These additional tests
usually are run once every four hours and should be
scheduled to include the intervals when influent loads
increase in the early afternoon and decrease in the early
morning hours. A battery-operated dissolved oxygen field
probe is much more convenient to use and provides more
accurate mixed liquor DO values than the Winkler Method.
S. GOVERNMENT PRINTING OFFICE: 1975/657-695/5351 Region No. 5-I I 27
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