Concentration of Activated SludgeAuthor(s): Don E. BloodgoodSource: Sewage Works Journal, Vol. 19, No. 2 (Mar., 1947), pp. 202-206Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25030441 .

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CONCENTRATION OF ACTIVATED SLUDGE *

By Don E. Bloodgood

Associate Professor of Sanitary Engineering, Purdue University, West Lafayette, Indiana

The concentration of activated sludge has long been a problem to designers and operators of sewage treatment

plants. The designing engineers have been perplexed over sludge concentra tion when they were considering the

capacities of return sludge pumps, sec

ondary settling tanks, sludge digestion tanks and of the disposal facilities for

sludge either before or after digestion. The plant operator has been afflicted with the operation of activated sludge plants which were in some way not able to meet the requirements placed upon them.

Travaini (1) wrote in 1932 about the

problem of sludge concentration at

Phoenix, Arizona. His problem was

one of concentrating activated sludge before pumping it to digestion tanks.

Rudolfs (2, 3, 4) has written of his

experimental work in which efforts were made and logical reasons deduced

for the behavior of activated sludge. In a symposium on "Concentration of

Sewage Sludge'' by Gowdy, Keefer, Rudolfs and Donaldson (5) they dis cuss their studies and observations, and give recommendations for meeting the difficulties encountered when acti

vated sludge will not or does not con

centrate sufficiently. These authors give their readers in

formation that has been of material

help to many. There are still many difficulties experienced even today and

to say that the above workers did not

have the answers would be a gross in

justice. The information given in this

paper is offered only as an expression of opinion and a statement of experi ences and beliefs. It is hoped that in

* Presented at 20th Annual Meeting, Ohio

Conference on Sewage Treatment, Akron,

Ohio, June 21, 1946.

some small way these views may be of some assistance to those who still ex

perience difficulties.

Factors Influencing Concentration

The difficulties experienced in acti vated sludge concentration probably start with the operation of the acti vated sludge plant proper. Although the ensuing discussion may appear to relate primarily to the bulking prob lem, it is intended to develop the con

cept that the condition of the sludge is a fundamental influence on its concen

tration properties. Activated sludge has been defined

many times, yet it seems rather useless to try and describe in a few words a

substance that is made up of so many

complex substances and differs so

widely from plant to plant. The first time the writer observed the activated

sludge at Indianapolis he was amazed

because it did not have that rich brown color of the activated sludge then being handled in the Milwaukee plant.

In those days there was considerable

pickling liquor being discharged to the sewers in Milwaukee and practically none was being handled at the Indi

anapolis plant, principally because it was believed that pickling liquor has

been the cause of past difficulty in

plant operation. It was at that time

very easy to obtain an activated sludge at Milwaukee that would concentrate to

2 per cent solids in 30 minutes. Was it the iron that aided the concentration of the Milwaukee sludge ? It seems en

tirely possible that the iron was bene

ficial, but it may also be pertinent that the Milwaukee plant was not over

loaded at that time. Mathews at Gary reports that pick

ling liquor in the sewage in uniform

202

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Vol. 19, No. 2 CONCENTEATION OF ACTIVATED SLUDGE 203

concentrations seenis to improve the activated sludge rather than injure it. But before a definite conclusion is

drawn, reference to the Gary operation reports will reveal that the B.O.D. load at this plant is not excessive.

It is usual to find the sludge in the

Crawfordsville, Ind., plant with a high sludge index (200 to 500) and the

sludge in the Marion, Ind., plant with a sludge index always below 100 unless

it be during the food canning season.

The B.O.D. of the Crawfordsville sew

age is rather high (200 to 400 p.p.m.) ; at Marion it is usually below 200 and

most months it averages below 150

p.p.m. going to the aeration tanks.

The above facts assume significance to the writer in the light of his findings at Indianapolis that the degree of con

centration of activated sludge could be

varied by careful control of the amount

of B.O.D. being removed in the acti vated sludge plant. The operation records at Indianapolis indicate that the concentration of activated sludge can be controlled by adjusting the load

being placed upon it, this statement

being valid only if a sufficient amount of oxygen was being supplied at all

times.

It was not an unusual experience to have two entirely separate activated

sludge plants operating on the same

sewage and yet to find entirely differ ent sludge indexes. A similar observa tion has been made in a study of the

operating data of the Hammond, Ind.

plant where Supt. C. B. Carpenter re

ported recently that he is able to lower the sludge index by reducing the sew

age flow to one unit, and to increase the index in another by raising the

flow. He has found that the changes in flow must be made in small incre

ments and by so doing he can keep a

similar sludge index in all units of his

plant.

These statements indicate that the

character (that is, the concentration)

depends upon the degree of oxidation

being given the activated sludge. The

degree of oxidation depends upon the amount of organic matter (B.O.D.) being fed to the activated sludge.

The question that might next be asked is, if the sludge index is af fected by loading, how does it affect the return sludge concentration? In a

previously published paper by the au

thor (8), a settling rate graph is shown

for a good activated sludge and for a

poor one. The good activated sludge, when settled in a cylinder, soon

reaches a concentration that does not increase materially with a longer set

tling period. From a typical curve it is apparent that there is not a marked decrease in volume in the sludge after 30 minutes of settling. This seems to indicate that a particular sludge from a particular plant will settle to a given concentration, but can not be concen

trated further unless the sludge is

physically changed in some manner.

There are several methods by which the

sludge can be changed so that it will concentrate further; two of them will be discussed here.

The first method has to do with the further oxidation of activated sludge by more aeration. Just what further oxidation does to sludge to make it set tle no one can be quite sure. It seems

quite logical that the swelling of the

sludge might be caused by increased numbers of organisms which have de

veloped in their effort to utilize all of the bacterial food present. On the other hand, it seems almost as reason

able to believe that the swelling of the

sludge may be due to the absorption of bacterial food into the gelatinous

masses surrounding the bacteria. It does not matter, however, which

of these two theories are accepted, be cause there appears to be a logical ex

planation of the improved concentra tion that results from continued aera

tion. If the sludge expands because of increased numbers of organisms, con

tinued aeration without the further ad dition of food will eventually result in a shortage of food and a consequent

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204 SEWAGE WORKS JOURNAL March, 1947

death of organisms. If there is a swell

ing of the gelatinous masses because of the great quantity of food absorbed by them, continued aeration without the addition of food will result in a deple tion of the food and a possible shrink

ing of the gelatinous masses.

The reports of Tapleshay (6) and

Smith (7) on the chlorination of acti vated sludge seem to indicate that the addition of a small amount of chlorine to the return sludge may keep the bac terial counts within the limits neces

sary to produce a sludge which will concentrate satisfactorily. This is a

second method by which the sludge is

changed in physical character.

Concentration a Function of Sludge Index

So much for the theories. The plant operator is more concerned with what can be done in a practical way to solve the problem of sludge concentration in an activated sludge plant by control of

the degree of oxidation. If an acti vated sludge is being given all of the

oxygen possible it can handle only so

much B.O.D. To go beyond the limit of B.O.D. loading will result in a break

down of the process and in the produc tion of a sludge which can not be con

centrated for further disposal in sludge

digestion tanks. The concentration of sludge for dis

posal (waste sludge) can be controlled

by adjustments in the activated sludge

plant. Many times this is not a feasi

ble method however, because the ca

pacities of the activated sludge units

may be inadequate to give an oxidized

sludge that will settle to the desired

concentration necessary for disposal. In the Milwaukee plant a separate unit

is used to condition the waste sludge for the vacuum filters. In 1938 the

average solids content of the sludge fil

tered was 1.50 per cent, in 1936 it was

1.16 per cent and in 1937 it was 1.26

per cent. The maximum concentration

for any month during these three years was 1.65 per cent. These rather high

concentrations are, it is believed, ob tained by a more complete oxidation of the sludge in the conditioning units, and are certainly not obtained by

merely holding the sludge in the sedi mentation tanks.

The writer is quite certain that if the

average sludge index for the years above were known, that the reciprocal of the sludge index times 100 would

give percentage sludge concentrations

approximating those given above. By this it is meant that the probable sludge index for the above years would have been approximately as follows :

Waste Sludge Probable Year Solids (%) Index

1936 1.16 86 1937 1.26 79 1938 1.50 66

The writer has seen waste sludge pro duced in the Milwaukee plant that

would settle to 2 per cent solids. This is perhaps an indication of what can be done within an activated sludge plant to concentrate the waste sludge solids.

Sludge Concentration Tanks

In most cases, plant facilities are

such that it is impossible to concentrate the waste sludge within the treatment

works and many efforts have been made to use resettling tanks for the concen

tration of the waste activated sludge. It has been the writer's experience

that it is quite impractical to concen

trate waste activated sludge which has a high initial sludge index unless the

sludge can be held sufficiently long for

aerobic decomposition to take place and thereby change the physical com

position of the sludge. Travaini re

ported that he obtained high concen

trations but was troubled by odors; this certainly indicates that decomposi tion of the sludge had started.

The use of chlorine in concentration tanks may serve two purposes: (1) The chlorine may be of assistance in

keeping down troublesome odors; (2) Chlorine may so act upon the sludge

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Vol. 19, No. 2 CONCENTRATION OF ACTIVATED SLUDGE

TABLE 1.?Operation Data on Sludge Concentration Tank at Indianapolis

205

Year Days Oper

ated

Activated Sludge (m.g.)

Concen trated

Sludge (m.g.)

Waste Activated

Sludge Solids

(%)

Concen trated

Sludge Solids

(%)

Solids Treated (1,000 lb.)

Reduction in Volume

(%)

Loading (gal. per sq. ft.

per day)

Susp. Solids

Decanted Eff. (%)

1931 1932 1933 1934 1935 1936 1937

255 288 47

130 245 138 187

241.65

310.87

32.13

159.66 44.81

23.27

90.74

83.22

64.10

11.62

57.34

7.16

4.78

56.29

0.50

0.32

0.37

0.95

0.31

0.69' 1.04

.1.53

1.54

1.09

2.66

1.96

2.78

1.69

10164.15 8192.68

1040.94 1264.89 1166.62

1382.46 7619

64.3

79.4

63.8

64.1

84.0

56.0

38.0

272 328 208 373

56 58

145

45 43 93

134 61 38 74

that it changes the original physical characteristics.

Indianapolis Experience

The waste sludge, concentration tank at Indianapolis has 3,292 sq. ft. of sur

face area. In the years past it has been

operated in many different ways. One of these was to pump to the tank all of

the waste sludge and take the resulting sludge concentration. One other way

was to pump to the concentration tank a smaller volume of sludge and thereby allow for a longer retention period. It is most difficult to analyze the data

(Table 1) to determine which was the most satisfactory method because the

sludges going to the concentration

tank, during the years for which infor

mation is available, were all quite different.

The sludge removed from the concen

tration tank had lost all of those char

acteristics identifying it as activated

sludge. The odor of the decanted ef fluent was always bad unless activated

sludge plant effluent was being pumped to the surface of the concentration

tank, thereby washing away or dilut

ing the less desirable liquor of the con centrated sludge.

There is no question but that the

Indianapolis concentration tank was of

great help in reducing the volume of

sludge that required disposal. The lo cation of this unit at a prominent point near the activated sludge plant was

far from being an asset from the stand

point of appearance and fragrance. It

is certainly suggested that all tanks

constructed for this purpose be located as far from the public eye and nose as

possible, as the beneficial service ren

dered by such tanks is certainly not

apparent to the casual observer.

Conclusions

The following conclusions are offered on the basis of the author's experience and observations concerning activated

sludge concentration:

1. The concentration of activated

sludge can be controlled by changes in the method of plant operation.

2. Concentrations of activated sludge suitable for immediate discharge to

digestion tanks can be obtained in the activated sludge plant.

3. The reciprocal of the sludge index times 100 is the maximum sludge con centration that can be expected from control of the activated sludge plant.

4. Sludge concentration tanks will

give satisfactory results if the sludge can be held in them a sufficient length of time to bring about changes in the

physical characteristics of the sludge. 5. It is difficult to operate concentra

tion tanks without odor nuisance. Con

sequently, they should be designed and located so that there will be no objec tionable conditions.

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206 SEWAGE WOEKS JOUENAL March, 1947

References

1. Travaini, Dario, "

Sludge Thickening at

Phoenix, Arizona.'' Water Works and

Sewerage, April, 1934.

2. Eudolfs, Willem, and Logan, Eobert P., "

Sludge Compacting. ' '

Sewage Works

Engineering and Municipal Sanitation,

14, 10 (Jan., 1943). 3. Eudolfs, Willem,

" Effect of Temperature

on Sludge Concentration. 'y

This

Journal, 15, 894 (1943). 4. Ibid., "Concentration of Activated Sludge

by Compacting and Flotation." This

Journal, 15, 642 (1943). 5. Gowdy, R. F., Keefer, C. E., Rudolfs, Wil

lern, and Donaldson, Wellington, "Con

centration of Sewage Sludge?A Sym

posium. "

Proceedings, American So

ciety of Civil Engineers, 70, 1276 (Oc

tober, 1944). 6. Tapleshay, John A., "Sludge Density

Control. "

Water and Sewage Works,

March, 1946.

7. Smith, E. E., "Control of Activated Sludge

Bulking." Water Works and Sewer

age, June, 1941 and October, 1935.

8. Bloodgood, Don E., "Application of the

Sludge Index Test to Plant Operation.n Water Works and Sewerage, June, 1944.

A CORRECTION IN THE FERTILIZER MANUAL

All holders of copies of FSWA Manual of Sewage Works Practice No. 2, Utilization of Sewage Sludge as Fertilizer, are asked to note an error on page 84, in the fourteenth line from the bottom. The word "not" should be deleted in this line so that the statement shall read "Chemical precipitation or intermittent sand filtration did remove

amoeba cysts."

Attention was called to this error by Bloise B. Cram, author of the

paper "The Effect of Various Treatment Processes on the Survival of Helminth Ova and Protozoan Cysts in Sewage" (This Journal, 15, 1119; November, 1943), which is credited as the authority for the above statement. Our apologies go forth to Miss Cram for the unfortunate

oversight that resulted in the mistake.

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