activated sludge plant operation

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



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




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.



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




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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