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Activated Sludge Plant OperationAuthor(s): T. R. HaseltineSource: Sewage and Industrial Wastes, Vol. 24, No. 12 (Dec., 1952), pp. 1533-1537Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25032060 .
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Vol. 24, No. 12 ACTIVATED SLUDGE OPERATION 1533
ACTIVATED SLUDGE PLANT OPERATION *
By T. R. Haseltine
Partner, The Chester Engineers, Pittsburgh, Pa.
Nature of Activated Sludge
The impurities in sewage are pres ent in three forms: suspended, col
loidal, and dissolved. Only the heavier
suspended matter can be removed by plain sedimentation. The cloudiness of sewage is due to the colloids and
finely divided suspensoids. When set tled sewage is aerated for protracted periods, those particles will coalesce to form light feathery floe, which settles
slowly upon quiescent standing. The
supernatant liquor will be relatively clear. If that liquor is discarded and the thin sludge is reaerated in a sec ond batch of settled sewage, a shorter
period will suffice to produce clarifica tion and more sludge will result. If this cycle is repeated several times, it
will be found, at first, that as the amount of sludge increases, the clarifi cation time decreases, but eventually a
point will be reached beyond which further increments of sludge are of little or no value in reducing clarifica tion time. At that point clarification
may occur in 10 to 30 min.
During the process of flocculation and clarification many of the bacteria and protozoa in the sewage are ad sorbed on or enmeshed in the floe. In the presence of abundant dissolved
oxygen some forms die off, or remain
dormant, while others multiply rap
idly. By the action of their growth these organisms convert the pollutants adsorbed on the sludge or dissolved in
the sewage to carbon dioxide, sulfates, nitrates, and the living protoplasm of their own bodies. This process is
termed biological oxidation. During its progress bacterial enzymes are se
creted which hasten clarification. *
Presented at 1952 Annual Meeting, Penn
sylvania Sewage and Industrial Wastes Assn. ; State College, Pa.; August 27-29, 1952.
It is important to remember that floc
culation, clarification, and oxidation, as encountered in the activated sludge process, are dependent upon biological activity, which in turn is dependent upon oxygen. Without that biological
activity flocculation would be slower, clarification would be less complete, and oxidation might not occur at all.
Both oxidation and clarification take
place most rapidly when the sludge and
sewage are first brought together, pro
viding there is sufficient oxygen available. But, unlike clarification, oxi dation cannot be completed in a few
minutes?several hours, or even days,
are required for that. Unless sufficient time is allowed between cycles for oxi dation to catch up with adsorption, the
settleability of the sludge deteriorates
and, if oxidation continues to lag, the
clarifying power of the sludge is also
reduced.
Classification of Activated Sludge Processes
There are several different modes of
operation or degrees of treatment that
conform to the accepted definition (1) of the activated sludge process.
In the early days all activated sludge
plants were expected to produce a
clear, highly nitrified effluent contain
ing less than 10, and frequently less
than 5, p.p.m. of 5-day B.O.D. or sus
pended solids. In such a plant a non
nitrifying sludge was obviously un
satisfactory. Later many authorities concluded that nitrification was un
necessary. It was found that at most
plants considerable reduction in air, and hence power consumption, could be
made if nitrification was avoided. The
effluents from these plants still had
about the same suspended solids and
B.O.D. as those from the nitrifying
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1534 SEWAGE AND INDUSTRIAL WASTES December, 1952
process. Both of these processes are
termed "conventional" process and
both yield about 90 per cent reductions in applied B.O.D. when treating a pre settled typical municipal sewage.
A later development has been the
so-called high-rate activated sludge process, which does not attempt to pro duce a high quality effluent. When
treating a typical municipal sewage it
yields an over-all B.O.D. reduction of
about 75 per cent, which is equivalent to about 60 per cent on the basis of
settled sewage. Still farther down the ladder of
B.O.D. removal, are straight aeration
plants with relatively long aeration pe riods. Although these plants return no sludge to the aerators, their aeration
periods are so long that the floe formed
in the aerators may have appreciable biological activity. These plants may
yield over-all B.O.D. reductions of 60
per cent, or about 40 per cent on the
basis of the applied settled sewage
(2) (3). Recently the so-called "Biosorption
Process "
has been announced. Like some of the earlier activated sludge
plants, this process omits any primary treatment other than screening and grit removal. According to the flow sheet for this process, equal volumes of
normal crude sewage and activated
sludge are intimately mixed for 35
min., after which the mixed liquor en
ters a conventional settling basin. Set tled effluent is discharged to the stream.
Sludge withdrawn from the settling tanks is passed through a "sludge stabilizer" or "aerobic digester" hav
ing a 21/2-hr. aeration period before it
is again mixed with crude sewage. The
"sludge stabilization" or "aerobic di
gestion" is simply sludge reaeration under a new name. Sludge reaeration
has been used in either nitrifying or
non-nitrifying conventional plants and also in some high-rate plants. In the
author's opinion, the newest things about the Biosorption Process are the names used and the short contact pe riod between sludge and sewage.
Regardless of the particular type of
activated sludge process under consid
eration, much the same control tests are
used. However, their significance and the interpretation of their results may
differ, depending upon the degree of treatment being sought. The various control tests most commonly used are
(a) those to regulate aeration, (b) those to control the amount of sludge, and (c) those to measure the condition of the sludge.
Dissolved Oxygen Tests
Dissolved oxygen tests are by far the best and perhaps the only logical
method of controlling aeration. In any aeration tank there are two opposing forces?the aerator device injecting air (oxygen) into the sewage-sludge
mixture, and the biological organisms using oxygen in their life processes. So long as the first force exceeds the
second, there will be some dissolved
oxygen in the mixed liquor and so the
operator can be reasonably certain that aeration is adequate. If there is no
dissolved oxygen present then the re verse is true, no matter how much air is being applied.
The oxygen requirements depend primarily upon the amount and condi tion of the sludge present, and are al
ways greatest at the point where the
sludge and sewage first come in contact with one another. When considerable
sludge is being carried, the variations in volume and strength of the sewage have little, if any, influence on the air
requirements and sa aeration may con
tinue at a practically constant rate. When lesser amounts of sludge are
carried, there will be some hourly vari
ation in oxygen requirements. Under those conditions, and in larger plants, the saving in power resulting from in
telligent variation in air supply may be sufficient to warrant making dis
solved oxygen determinations several
times a day. However, in small plants, or those carrying a high concentration
of sludge, one set of determinations
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Vol. 24, No. 12 ACTIVATED SLUDGE OPERATION 1535
per day, preferably about 6 hr. after
the peak sewage load reached the aera
tors, should suffice. The dissolved oxygen determinations
should be made at various points from
inlet to outlet of the aerators. At least
in conventional plants, it is desirable to always have at least a trace of oxy
gen at the inlet end and gradually in
creasing amounts at succeeding points along the aerator. If a highly nitrified effluent is desired, it may be desirable to maintain as much as 3 to 6 p.p.m. of oxygen at the aerator outlets, but where nitrification is not desired 1 or
2 p.p.m. is probably adequate. Because oxygen requirements are
greatest when sewage and sludge first come together, it is frequently neces
sary to apply more air at the inlet ends of the aerators and gradually de crease aeration toward their outlets. This is termed "tapered aeration." Another solution to the problem is
"step aeration," where either or both
the return sludge and the sewage are
introduced at several different points along the aerators (4) (5). Most plants have not been designed to permit use
of step aeration.
Where sludge reaeration tanks are
used, the air supply to them should be so adjusted as to maintain about the same dissolved oxygen as recommended for conventional mixed liquor aerators. One reason sludge reaeration tanks
have come into ill repute is the fact that the solids concentration in them, and hence their oxygen requirements, are so high that it is virtually impos sible to maintain any dissolved oxygen in them. Under such conditions reten
tion of sludge in those tanks did more
harm than good. Retention of sludge in the bottom of
final sedimentation basins is even more
harmful. Some operators make routine
measurements of sludge depths in final
tanks and vary the rate of sludge withdrawal so that those depths never
exceed 2 ft. Even lesser depths may be desirable.
By using sludge return rates as high as 50 per cent, or even 100 per cent, it is possible not only to avoid sludge
build-up in final tanks, but also to
dilute the sewage-sludge mixture at the
aerator inlets (or the sludge in re
aeration tanks) so that oxygen require ments will not be so great at those
points. This is an alternative to either
tapered or step aeration that is par
ticularly applicable to small plants.
Amount of Sludge
The amount of sludge present in the
mixed liquor is most accurately meas
ured gravimetrically by making sus
pended solids determinations. Origi
nally it was usually measured volu
metrically by observing the percentage of sludge in a graduate of mixed liquor after a definite standing time. The
objection to this method of measure
ment is that the volumetric reading is
affected almost as much by settling characteristics of the floe as it is by their weight. If both methods of meas
urement are used, it is possible to calcu
late the "sludge index," which is a
useful indicator of sludge condition.
Centrifuging is a third method of
sludge measurement. It is more rapid than the other methods, but less ac
curate than the suspended solids test.
All three methods are more fully dis
cussed elsewhere (6). The proper amount of sludge to
carry depends upon the degree of
treatment desired and the capacity of
the aeration equipment. Generally
speaking, diffused air plants can carry more sludge than those depending upon mechanical aerators. Typical mixed
liquor solids concentrations for normal
sewage are probably about as follows:
Conventional nitrifying plants, 3,000
p.p.m. or more.
Conventional non-nitrifying diffused
air plants, 1,000 to 2,500 p.p.m. Conventional non-nitrifying mechani
cal aeration plants, 600 to 1,500
p.p.m.
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1536 SEWAGE AND INDUSTRIAL WASTES December, 1952
High-rate plants, never over 600 to
800 p.p.m., and frequently as little as 200 to 300 p.p.m.
Straight aeration plants usually have a higher solids concentration in the
mixed liquor than in the settled sew
age, despite the fact that they have no sludge return ; perhaps 100 p.p.m. would be typical.
At present, frothing in aerators is
generally attributed to detergents in the sewage. However, low solids con
centrations at least aggravate the prob lem. The author encountered foaming at Salinas, Calif., whenever the solids
were low, back in 1930 before the use
of detergents was common.
Measures of Sludge Condition
Sludge Index
By far the most common measure of
sludge condition is the sludge index, which is the ratio of the percentage of
sludge by volume after standing 30 min. to the percentage of sludge by weight as measured by the suspended solids determination. It is a measure of the settling characteristics of the
sludge; a high index indicates poor
settleability, and vice versa. A bulked sludge is simply a sludge
that, after a given settling period, oc
cupies a greater volume than would a
normal sludge containing the same
weight of suspended solids. Bulking is best measured by the sludge index.
Whether or not a bulked sludge will
pour over the weirs of final settling tanks depends upon the design of those tanks and the amount of sludge being carried. The more generous the
design of the tanks, the higher the
sludge index that can be tolerated in a given mixed liquor solids concentra tion (or, for a given index, the higher
may be the solids concentration) be fore the tank effluent becomes fouled
with sludge. In conventional plants, either ni
trifying or non-nitrifying, the primary cause of bulking is the failure of bio
logical oxidation to keep pace with
adsorption. The reason for that lag may be too short an aeration period,
inadequate air supply, increased pol lution load on the plant, insufficient
sludge, septic sewage or return sludge, germicidal waste, excessive oil and
grease, etc. Unless the cause is found
and corrected, the sludge index in
creases, bulking occurs and, if it con
tinues, the final effluent will eventually become fouled with sludge. In that
event, the operator must reduce the amount of sludge he is carrying, even
though the initial cause of the lag in
oxidation may have been insufficient
sludge. The net result of operation with a reduced amount of sludge will be a lower degree of treatment than had been hoped for.
High-rate activated sludge plants generally have a higher sludge index than do the conventional plants.
However, when oxidation lags suffi
ciently behind adsorption for a con
siderable period of time, clarification itself is reduced. In such cases, the
sludge index is lower than it is when there is less lag. Since such is the
case, an increase in sludge index at a
high-rate activated sludge plant may indicate the need for reducing, rather than increasing, the aeration period or
the air consumption. A slight excess of oxidation over ad
sorption favors nitrification and results in a low sludge index. However, too
great an excess produces a dense ' '
pin point" sludge that does not yield as
clear an effluent as does the more floc culent sludge with a higher index. This
may be another objection to sludge reaeration in conventional non-nitrify
ing plants. However, it is believed
that in any properly designed and op erated conventional plant sludge re
aeration can be used to advantage.
Sludge Age A less common measure of sludge
condition is sludge age. As usually calculated, the sludge age is the ratio
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Vol. 24, No. 12 ACTIVATED SLUDGE OPERATION 1537
of the total weight af activated sludge solids in the plant to the weight of
suspended solids removed from the
daily flow of settled sewage. Theoreti
cally, at least, it might be more ac^ curate to use the weight of total rather than suspended solids removed from the settled sewage. There may be other
objections to the term, also. However, it has proved a useful tool in control
ling some plants. Generally speaking, the older the sludge, the lower will be its sludge index and the higher will be its ash content. Only old sludges can
be nitrifying sludges, because nitrifica tion does not take place until after
much of the carbonaceous material has been oxidized.
Volatile and Ash Content
The routine determination of ash or
volatile matter in the return sludge is also a useful indicator of when to ex
pect a change in sludge index. There does not seem to be any universal
optimum volatile content for all plants. But, in most conventional plants, a
rise in volatile content of the return
sludge precedes a rise in sludge index and vice versa. Apparently this rela
tionship does not necessarily hold for
high-rate and/or straight aeration
plants.
Microscopic Examination
Microscopic examinations of sludge are of value in conventional plants? particularly observations of the num
ber and type of protozoa. Unless there is a preponderance of the larger ciliate
protozoa over the smaller flagellates the operator can be certain that oxida tion is lagging behind adsorption.
Miscellaneous
Other tests that are sometimes used
to measure the condition of the sludge include measurements of its short-time
oxygen requirements, comparisons be tween the dissolved solids concentra
tions in the settled sewage and in the
plant effluent, grease determinations, etc. Oxidation potential may also be a
useful tool, not only to measure sludge
condition, but also to control aeration.
References
1. "Glossary, Water and Sewage Control En
gineering." A.P.H.A., New York, N.
Y. (1949). 2. Hatfield, W. D., "Operation of Pre-Aera
tion Plant at Decatur, Illinois." Sew
age Works Jour., 3, 4, 621 (Oct.,
1931). 3. "The Operation and Control of Activated
Sludge Sewage Treatment Works."
A.P.H.A. Committee Report, Sewage Works Jour., 14, 1, 19 (Jan., 1942).
4. Gould, R. H., "Operating Experiences in
New York City. ' '
Sewage Works Jour.,
14, 1, 70 (Jan., 1942). 5. McKee, J. E., and Fair, G. M., "Load
Distribution in the Activated Sludge Process." Sewage Works Jour., 14, 1, 121 (Jan., 1942).
6. Haseltine, T. R., ' '
Operating Fundamentals
of the Activated Sludge Process."
Water and Sew Works, 98, 4, R-144,
(1951).
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