economical practices in the activated sludge and sludge digestion processes

Economical Practices in the Activated Sludge and Sludge Digestion ProcessesAuthor(s): Richard H. GouldSource: Sewage and Industrial Wastes, Vol. 31, No. 4 (Apr., 1959), pp. 399-405Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25033857 .

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ECONOMICAL PRACTICES IN THE ACTIVATED SLUDGE AND SLUDGE DIGESTION

PROCESSES *

By Richard H. Gould

Associate, Greeley and Hansen, Engineers, Chicago, III.

This is one of four papers collectively forming a sewage treat

ment cost reduction symposium. Others deal with construction,

operation, and over-all regulatory considerations. The paper on con

struction eost reduction appears on page 406 of this issue. The

others will follow in early issues of This Journal.

It is believed that there is now

enough operating experience to justify the statement that considerably higher unit loads can be safely applied to two

major parts of sewage treatment

plants. These are the facilities re

quired for treatment by the activated

sludge process and those required for the digestion of sludge. For each

part, so-called conventional or stand ard unit loads have been stated in

manuals of State Health Departments and are used extensively today in sew

age treatment and sludge disposal de

sign.

In the case of activated sludge treatment, conventional design is il lustrated by a displacement period of

mixed liquor of about 6 hr, with a

mixed liquor suspended solids of about

1,500 ppm or more, and a daily load of 25 to 30 lb of applied BOD per 1,000 cu ft of aeration tank (1). By

modifying the operational design, the size of the aeration tanks may be re

duced as much as 60 per cent without

lowering treatment efficiency (2). In the case of sludge digestion, con

ventional design may be illustrated by *

Presented at the 31st Annual Meeting, Federation of Sewage and Industrial Wastes

Assns.; Detroit, Mich.; Oct. 6-9, 1958.

a unit load of 2.0 to 3.0 lb of applied solids per cubic foot of tank volume

per month. Improvements in opera tional design may permit an increase of about three times these loadings.

It is emphasized that facilities for

activated sludge and sludge digestion are but parts of a treatment plant and, as such, can be affected by imperfec tions in other portions of the plant.

For example, inability to remove

solids when required, as a result of

improper functioning of final settling tanks, may cause a good aeration sys

tem to be ineffective. Faulty screen

ing or grit removal may make digester

operation more difficult. Reliable methods of ultimate sludge disposal are essential for all elements. Sewage characteristics may vary greatly in

different locations. The effects of cli

mate, the required degree of treat

ment, and possibilities for final sludge

disposal must be considered. In sum

mary, consideration must be given to

the proper relation of the various

plant elements as well as variations in

sewage and external factors. True advances in practice are often

slow in development and result from

much experimentation and in refine ments based on actual operating ex

399



400 SEWAGE AND INDUSTRIAL WASTES April 1959

perience. Performance under full

scale operation is the ultimate test.

A treatment process must be stable,

susceptible to simple control, and pro vide easy maintenance of equipment.

Operating routines should be simpli fied to the maximum possible extent.

Stability of results and in operating procedures are necessary for any

soundly designed process.

Sludge Age

The basis for improvements in methods of treatment by activated

sludge is to be found in a better un

derstanding of the fundamental proc ess. The basic principles are quite simple. Under aerobic conditions, a

well-conditioned activated sludge will abstract the suspended, colloidal, and soluble organics from sewage in 30

min or less. The abstracted materials are attached to the mass of activated

sludge, but portions of them, particu larly those synthesized from the solu ble organics, have poor settling quali ties. To avoid excessive carry-over of

poor settling material into subsequent phases of the process, it is necessary to

subject them to a prolonged period of aerobic biological action, sufficient to alter their physical properties so that

they will settle readily. It has been found that the freshly abstracted ma

terials must be held in the aeration

system, on the average, for 3 to 4

days. This consideration led to the

"sludge age" theory (2) (3). For practical operating control, it

is necessary to relate the mass of solids in an aeration tank to the incoming load so that the sludge will maintain its ability to treat the sewage while

retaining good settling qualities in the final tanks. One method is to divide the weight of the solids in the aeration tank by the daily weight of incoming

suspended solids to give a sludge age in days. This is convenient, as the de termination can be made in a few

hours. Its accuracy as a measure of

time will vary with the composition of

the sewage. Perhaps 20 to 30 per cent of the solids are destroyed by pro

longed aeration. This is counter

balanced, in part, by the conversion of soluble BOD to suspended solids that are not recorded as incoming sus

pended solids. Where the numerical values of BOD and suspended solids are about equal, the suspended solids

figure should closely approach the true sludge age. Where there are

high values of soluble BOD, it is better to base the solids level in the aeration tanks on the basis of the load of the incoming BOD rather than that of the suspended solids. Within

the range of Haseltine's recommenda tions (4) it was suggested that there should be 100 lb of aeration tank

solids for every 30 lb/day of applied BOD load. This is equivalent to a

3.33-day BOD sludge age and is basi

cally the same criterion.

Application of Sludge Age

In the applications of the principles outlined above to the conventional ac

tivated sludge flow pattern, it will be found that, with normal sewage

strengths and other usual procedures, the aeration period will approach the six hours called for in many design guides. If, however, part of the aera

tion tank is devoted to the aeration of

sewage and sludge mixtures at solids concentrations higher than that of the aeration tank effluent, the requisite

mass of solids can be kept in the sys

tem, the same effluent concentration

maintained, and satisfactory perform ance accomplished with tanks of much

smaller volume.

Step Aeration

The first application of these princi ples was in the step aeration method

(5), first used in the Tallmans Island Plant in New York City in 1939. Since then, New York has built or re

vised six other plants incorporating this principle. The usual New York

City aeration tank has four passes, or



Vol. 31, No. 4 ACTIVATED SLUDGE AND DIGESTION 401

channels. Return sludge flows through all passes in series, and sewage may be

added, in the proportions desired, at the head end of any or all of the

passes. The greatest air demand oc curs when sewage comes in contact

with the sludge; therefore, to equalize air use, sewage is applied to as many passes as practicable, consistent with the tank volumes available and the effluent concentration desired. Should the sludge index begin to deteriorate, sewage inlet gates are adjusted to shift the sewage application toward the effluent end. This reduces the ef fluent concentration so that the more

voluminous sludge occupies no more

space than the sludge of better

quality, and it is therefore not neces

sary to adjust the rate of sludge re

turn. This procedure simultaneously increases the time of sludge reaeration and usually results in a restored

sludge index. This is in marked contrast to the

early years of operation at the Wards Island Plant, under the more rigid pattern of the conventional flow sheet, when a downward spiral of deterior

ating results sometimes occurred. When deterioration occurred, sludge would occupy more space in the final tanks. Increased return rates for the

more voluminous sludge would lower

sludge concentrations and the deten tion time in the aerators, resulting in

further impairment of sludge quality. The continuation of this course would

eventually require the bypassing of

part of the load or the wasting of solids from the system prior to start

ing anew. Since the installation of

step aeration and the modification of the final tanks, average flows 25 per cent above design capacity have been treated readily, while using only one

half of the aeration tanks. Effluents

have been as good or better and of

more uniform quality than those se

cured under the original system. While step aeration is the earliest

of the complete processes that permit

a large reduction in aeration tank vol

ume, there are other methods being used in full-scale operation. These in clude the Biosorption method (6), developed at Austin, Texas, another

method discussed by Eckenfelder and

O'Connor (7), and the methods de

veloped by Kraus at Peoria (8).

Biosorption

The Austin development was an in

teresting solution to a difficult prob lem. From its performance records, the

system was using a 3.5-day BOD

sludge age and a sewage-sludge dis

placement period of 37 min. The pro portions of sludge reaeration and

sewage-sludge contact periods are al most identical to a step aeration in

stallation, where all sewage is added to the fourth or last pass. If a single point is used for sewage application, there is no way to adjust for changing loads and changing sludge condition, except by the frequently cumbersome

procedures of the conventional method. At the Hunts Point Plant in New York City, two passes are used for

sludge reaeration and two passes for

sewage-sludge contact. The quantity of air used in the latter two passes is twice that required for the sludge re

aeration passes. With a sewage-sludge contact in one pass only, there may be a problem of meeting the air re

quirements of a sewage with a high oxygen demand. Thus, there is little

margin to assure uniform results, under variable conditions.

Kraus Method

In meeting his problems at Peoria, Kraus (9) added new concepts. In ad dition to sludge reaeration and a short

sewage-sludge contact period, he added digested sludge to the activated

sludge being reaerated. The digested material, with part of the volatiles

destroyed by anaerobic action, and

after a substantial period of aeration, is effective in the sewage purification




process. It adds weight to the acti

vated sludge and results in a good

sludge index and simplifies the prob lems of the final settling tanks. In

order to meet a problem of high unit

air use economically, this procedure supplied air to both sides of an aera

tion tank. This method needs con

firmation for application to sewages and problems less difficult than at

Peoria, but where effluent standards must be higher.

Air Considerations

The volume of air required for the

activated sludge process is related to

the amount of BOD to be removed. It also appears that relatively more air

is required to remove those parts of the BOD that are soluble, as this is

done by incorporation in biological structures. There are also practical limits to the amount of air that may be usefully applied per linear foot of

aeration tank. When unit demands are high, this may be a limitation on

the extent to which aeration tank vol ume may be reduced.

The efficiency of air use may be in

fluenced by the type and location of

diffusion media. Quite aside from

these considerations, however, there is a clear indication that, as the BOD

loading per unit volume of aeration tank is increased, there will be a cor

responding increase in the efficiency of air utilization. This is not a

straight line function, but, as sug

gested by Torpey (10), varies more

nearly as the reciprocal of the square root of the unit BOD loading. For ex

ample, if the unit BOD loading is in

creased four times, the air use per

pound of BOD may be halved. The

use of methods to accomplish the same

work in tanks of smaller size would

thus have merit, not only in the re

duction of capital costs, but also in

reduced operating costs for air com

pression.

Settling Tank Design

The effectiveness of any aeration

system, be it conventional or any of the modifications mentioned above, is

dependent on the effectiveness of the final settling tanks. The separation of the large mass of flocculent acti vated sludge carried by the mixed

liquor bears no relation to the separa tion of the more granular solids in the

primary tanks. These flocculent solids

quickly unite to form density currents that flow along the tank bottom with a

velocity of 5, or more, fpm. An ob

structing wall will deflect them up ward and cause them to return, at a

slower rate, along the surface. The

volume of the flowing sludge blanket

may be 25 per cent or more, at index

concentration, of the sewage being treated. The usual type of collector

mechanism does not have the capacity to handle this volume, therefore the removal of sludge from the tank is de

pendent on hydraulic flow. In circu lar tanks, with the sludge outlet at the

center, the time required for the re

moval of some of the sludge is unduly long. As a result of this tendency there is equipment on the market de

signed to minimize this effect. In such tanks the effluent weirs must be care

fully located to avoid the effect of the

upturn of the density currents. Simi

larly, in rectangular tanks, the point of sludge withdrawal should be placed at mid-tank, or beyond, so that the

sludge will flow to it under its own

weight, and without too much depend ence on the removal mechanism.

The burden on the final tanks is

proportional to the mass and condi

tion of the solids that pass through them. Thus, the concentration of

solids in the aeration tank effluent, the sludge index, and the rate of

sludge return all have a bearing on

the performance of any particular final settling tank. The aeration sys tem must be proportioned to hold the

requisite mass of activated sludge, but




the sludge concentration of the aera

tion effluent is limited by the form and

utility of the final tanks.

Digester Design

Advances in sludge digestion, as is true in treatment by activated sludge, have been based on a clearer under

standing of fundamental considera tions. Pilot plant work by Torpey (11), and Morgan (12), and labora

tory work by Sawyer et al. (13), among others, make it clear that the rate of activity of anaerobic biology, under controlled conditions, is rapid and quite comparable to aerobic or

ganisms. In the past, this fact has been masked by the nature of the structures provided for sludge diges tion and by the character of the ma

terials to be digested. The material

going to digestion tanks includes some

substances that will float and others that will sink. Of the floating ma

terials, including greases, only part are capable of being digested, and the remainder will collect at the surface in the form of a scum blanket. There

usually are large areas of tank bottom left essentially undisturbed by sludge removal operations. In these areas,

sludge is deposited and permitted to consolidate to form inert masses which

actually reduce tank capacity. It is

reported that the loss of effective di

gestion capacity from these two causes

often amounts to two-thirds of the

original tank volume.

Torpey (11), Morgan (12), and

Sawyer et al. (13) have shown that

nearly 90 per cent of the total possible gas production can be expected during a 6-day digestion period. This is not

the basis for design, but is an indica

tion of the rate of activity. Actual

tank volumes required to provide a

specific displacement period depend on the concentration of the applied

sludge. Thus, in efforts toward better use of digestion tank volumes, the use

of specially designed sludge thicken

ing tanks is becoming more common.

Recommendations for Improvements

The requirements for the best utili zation of sludge digestion tank vol umes and hence a much smaller instal

lation, may be summarized as follows :

1. The maintenance of uniform tem

peratures near 90?F. 2. The maintenance of alkalinity in

excess of 2,000 ppm, which will occur

naturally with the denser raw sludges. 3. The feeding of raw sludge as

continuously as practicable, to keep the bacterial population at uniformly high levels.

4. The thickening of raw sludge to

maximum feasible concentrations up to 10-per cent solids?beyond this con

centration the sludge becomes difficult to handle.

5. The provision of means for the

prevention of dense scum blankets.

This means periodic removal of top scum, or its incorporation in the mass

of digesting sludge. 6. The provision of means for pre

venting bottom deposits, either by

multiple outlets or by induced cur

rents.

7. The circulation of the contents

within the digester to distribute fresh

solids quickly throughout the tank.

Meeting the requirements listed above will permit the safe loading of a

primary digestion tank with raw solids at a rate of over 8 lb/cu ft of tank volume per month. This is three or

more times the loading previously con

sidered suitable.

New Considerations

The essential new considerations in volved in improvements in sludge di

gestion are the methods of assuring substantial homogeneity of the tank

contents, thus making the best use of the space that is available: There are a number of available devices that




have been designed for this purpose. Some are proprietary and others are

not.

One good example, in the latter

category, is in the primary digesters of the Nut Island Plant in Boston.

Here, the two 108-ft diameter tanks are each provided with four 2-in. nozzles fixed to a tank cover above the water surface. About 1,000 gpm of

digester liquor is circulated through the four nozzles, impinging on the scum layer in such a manner as to cause a rotation of the tank surface.

As a result, the scum layer is kept in a soft and fluid condition. Periodi

cally, the undigestible scum, which has

accumulated, is drawn off through

pipes. This is possible because of the

fluid condition of the scum. Each tank is provided with four points of

sludge withdrawal, so that bottom de

posits are minimized. While these

tanks now operate at about half the safe loadings suggested above, there is every indication that they could

handle the higher loadings, particu larly if the applied raw sludge were at higher concentrations.

There are a number of proprietary devices available for better tank utili

zation, some of which have yet to be

proved by extended operating experi ence. Among these devices is a draft tube mixer.* This unit draws the scums and liquor from the top, in

large volumes, and discharges the flow near the bottom in such a way as to

keep the bottom sludge agitated and in motion. Scum layers are removed and the undigestible scum is kept

mixed with the tank contents and

passes off with the digested sludge. The mixing within the tank is

thorough. A limited range in water

level is important for the best opera tion of these units.

Another device t forces digester gas

* Manufactured by Dorr-Oliver Inc., Stam

ford, Conn.

t Manufactured by Chicago Pump Co.,

Chicago, HI.

through diffuser tubes in the center

of the tank near its bottom. The ris

ing gas creates currents vertically up ward in the center, radially outward at the top along the water surface, downward at the side walls, and across the bottom to the center. These cur

rents bring about a good mixing of the tank contents and tend to mini

mize bottom deposits. An air lift, operated by digester

gas *, is placed in the center of the

tank, and causes a similar pattern of

circulating currents. Another system? also uses com

pressed digester gas as the moving force. In this method, gas is injected at a moderate depth at several points spaced over the tank area. At each

point there is a rising current with horizontal dispersion at the top. It is reported that these currents result in adequate mixing in the upper part of the tank and reduce scum forma tion near the points of gas applica tion.

None of the devices using gas for

digester mixing make provision for the removal of that part of the scum

which will not digest. Supplementary means may be required for this opera

tion.

When primary digestion tanks are op erated with concentrated raw sludges, the effectiveness of secondary diges tion is greatly diminished. If the

maximum of liquor is extracted from the raw sludge, by sludge thickening

methods, it is found that very little more can be decanted from the sec

ondary digestion tanks. This is par ticularly true if solids from secondary treatment are present. Secondary di

gesters lose effective capacity from the

accumulation of undigestible scums

and bottom deposits. Agitation to

correct these conditions will preclude any possibility of further liquid sepa

X Manufactured by Walker Process Equip ment, Inc., Aurora, 111.

$ Manufactured by Pacific Flush Tank Co.,

Chicago, 111.




ration. As a possible alternative, smaller storage tanks may be substi

tuted for secondary digestion tanks, depending on operating circumstances and on the method of final sludge dis

posal.

Summary

Current practice in sewage disposal, as acceptable to control agencies such as state health departments, is out lined in standards established by many such departments. As brought out in this discussion, many of these stand ards recommend the sizes for aeration tanks to be more than twice as large and digestion tanks to be more than three times as large as are necessary

under today's well established meth ods and principles of design and op eration. While existing standards serve a useful purpose, there is always the danger of their tendency to limit

imagination and initiative in design practice. However, regulatory agen

cies have generally given sympathetic consideration to the use of advanced

practices.

References

1. "Standards for Sewage Works, Illinois,

Indiana, Iowa, Michigan, Minnesota,

Missouri, New York, Ohio, Pennsyl vania and Wisconsin.'' Upper Mis

sissippi River Board of Public Health

Engineers and Great Lakes Board of

Public Health Engineers, May 1952

(partially revised July 1954).

2. Torpey, W. N., "Practical Results of

Step Aeration. ' '

Sewage Works Jour.,

20, 5, 781 (Sept. 1948). 3. Gould, R. H., "New Horizons for Acti

vated Sludge.'' Eng. News-Mec, 143,

p. 180 (Sep. 1, 1949). 4. Haseltine, T. R., "A Rational Approach

to the Design of Activated Sludge Plants.'' Water and Sewage Works,

102, 487 (Nov. 1955). 5. Gould, R. H., "Tallmans Island Works

Opens for World's Fair." Munie.

San., 10, 183 (Apr. 1939). 6. Ullrich, A. H., and Smith, M. W., "The

Biosorption Process of Sewage and

Waste Treatment." This Journal,

23, 10, 1248 (Oct. 1951). 7. Eckenfelder, W. W., and O'Connor, D. J.,

"The Aerobic Biological Treatment

of Organic Wastes." Troc. 9th Ind.

Waste Conf., Purdue Univ., 87, 512

(1955). 8. Kraus, L. S., "Dual Aeration as a Rugged

Activated Sludge Process." Sewage Works Jour., 27, 12, 1347 (Dec. 1955).

9. Kraus, L. S., "The Use of Digested

Sludge and Digester Overflow to Con

trol Bulking Activated Sludge. ' ' Sew

age Works Jour., 17, 6, 1177 (Nov.

1945). 10. Torpey, W. N., Personal Communication.

11. Torpey, W. N., "Loading to Failure of

a Pilot High-Rate Digester." This

Journal, 27, 2, 121 (Feb. 1955). 12. Morgan, P. F., "Studies of Accelerated

Digestion of Sewage Sludges." This

Journal, 26, 4, 462 (Apr. 1954). 13. Saywer, C. N., Howard, F. S., and

Pershe, E. R., "Scientific Basis for

Liming of Digesters." This Jour

nal, 26, 8, 935 (Aug. 1954).

GRADUATE TRAINING

The Department of Civil Engineering of Syracuse University is of

fering a new curriculum available to persons holding B.S. degrees in

Engineering, Chemistry, or Bacteriology leading to the degree of Master of Science in Sanitary Engineering.

Research Assistantships, available in this program commencing June

1,1959, have a remuneration of $3,000 per year, plus remission of tuition

and fees. Address inquiries to Chairman, Civil Engineering Department, Syra

cuse University, Syracuse 10, N. Y.



economical practices in the activated sludge and sludge digestion processes

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