Sludge Digestion: Operation and ControlAuthor(s): Albert B. KozmaSource: Sewage Works Journal, Vol. 16, No. 4 (Jul., 1944), pp. 700-704Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25029828 .

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700 SEWAGE WORKS JOURNAL July, 1944

SLUDGE DIGESTION?OPERATION AND CONTROL

By Albert B. Kozma

Superintendent, Butherford (N. J.) Joint Meeting

Our discussion concerns itself principally with the operation and

control of sludge digestion in separate sludge digestion tanks ; operat

ing procedures in Imhoff tanks and septic tanks will be only briefly mentioned.

The purpose of sludge digestion is to produce digested sludge by routine operation, without interruptions, in a manner reasonably free

of offensive odors. In the end product the liquor should be readily separable from the solids.

Tank Capacity

At the present stage of the art, sludge digesters are fool-proof if

properly designed, not outmoded or outgrown. The only exception is

when industrial poisons are present in the sludge which inhibit the process. For this reason and under normal circumstances, the first

step the operator should take in case of trouble, or preferably in antici

pation of trouble, is to appoint himself as a committee of one and to

investigate the design features of his tank.

The capacity of the tank is the most important item: 2 cu. ft. per capita in a plain sedimentation plant, 3 cu. ft. in a chemical treatment or trickling filter plant and 4 cu. ft. in an activated sludge plant are the minimum if the tanks are heated. For unheated tanks add at least 50 per cent. If industrial wastes are present or other unusual condi

tions exist, allow for the increased quantities of solids.

Many investigators prefer to rate digesters on sludge solids basis

and call for 2 to 3 lbs. of dry solids per cu. ft. of tank capacity per month for complete digestion and 4 to 5 lbs. for primary digestion.

Next in importance in the design of sludge digestion tanks is the provision of adequate heating facilities, proper insulation, location above ground water level, if possible, etc. In the case of hot water

heat one sq. ft. of coil area per 100 cu. ft. of tank volume is a good average figure. If special conditions prevail further investigations will have to be made such as outlined by Wittwer paper in Water Works and

Sewerage, June, 1943, or by the writer in the January, 1943 issue of the same periodical.

The depth should not be less than 20 ft. ; on the other hand, this dimension is limited by the surface area which should be sufficient to prevent the gases from causing undue agitation.

Types of Sludge Digestion Tanks

Various types are available such as those with fixed covers or float

ing covers. We distinguish single stage tanks from multiple stage tanks. Mixing and circulation of sludge can be accomplished by rotary thickeners, turbomixers or vertical propeller pumps.

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vol. 16,'no. 4 sludge collection, treatment and disposal 701

Imhoff and Septic Tanks

Imhoff tanks are designed 15 to 35 ft. deep, with 1.5 to 3 hours of

settling period and 1 to 3 cu. ft. per capita capacity in the sludge com

partments. Septic tanks are designed for 8 to 24 hrs. detention.

Operation Objectives

The 60 to 80 per cent volatile content (dry solids basis) of the raw

sludge is reduced to 40 to 60 per cent in the digested sludge, the per centage depending upon the treatment following the digestion process. If odor nuisance is a consideration higher removals are called for.

The average solids content of the raw sludge is 4 to 5 per cent, which will probably be increased to 8 to 10 per cent in the digested sludge.

The supernatant liquor should have a suspended solids content of

not over 2,000 p.p.m., less than 100 ml. per 1. of settleable solids and a

5-day B.O.D. not in excess of 1,000 p.p.m.

Backmeyer of Marion, Ohio used an ingenious scheme in his ac

tivated sludge plant to produce good supernatant ; he mixed his digested sludge in the secondary digester with cold quarry water in a 1 to 2

proportion whereupon the sludge readily separated from the liquor.

Operation Controls

Under normal conditions the pH test, the tank temperature and the volume of gas produced is all the operator wants to know to find out whether his tank is operating satisfactorily. He is also interested in the percentage of volatile matter in the digested sludge before he dis charges it.

Dr. Rudolfs found in his classical investigations that digestion passes through three stages, namely, (1) acidification (intensive acid production), (2) liquefaction (acid digestion), and (3) gasification (intensive digestion and stabilization). In normal operation the third stage predominates ; hence the pH is at or near to 7. The temperature determines the rate of digestion; therefore, 4 to 5 per cent solids can be added to the digesting sludge at 80? F. and less at lower tempera tures. Temperatures of 80 to 85? F. are considered optimum, both from the standpoint of the rate of the process and due to certain re

strictions imposed by the hot water heating facilities (140? F. being the maximum safe temperature of the feed water). The volume of gas

produced is usually l.U cu. ft. per capita or 10 to 15 cu. ft. per lb. of volatile matter digested.

When the tank is being started into operation or other unusual con

ditions prevail, further control tests will be necessary. The carbon dioxide content of the sludge gas, if in excess of 35 to

40 per cent, predicts trouble one to two weeks ahead of time and it is an important test if acid conditions are anticipated. Its determination is very simple by means of an inexpensive gas analyzer, by bubbling the

gas through potassium hydroxide solution.

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702 SEWAGE WORKS JOURNAL July, 1944

If a more sensitive index is desired than the pH test, the total alka

linity test is suitable. It should not be less than 1,000 p.p.m. In case of industrial wastes, it is advisable to control the process by

means of the volatile acids which are determined as follows :

200 ml. of the sample is measured in a distilling flask ; 5 ml. of con centrated sulfuric acid is added and 150 ml. distilled into a receiver. The distillate is titrated with 1/10 N sodium hydroxide and results are calculated as acetic acid (10 p.p.m. per ml. of sodium hydroxide) and

reported in p.p.m.

A recently proposed test by the engineers of the Dorr Company calls for the determination of the ratio of free carbon dioxide to the alka

linity. To avoid the direct determination of the free carbon dioxide, which is a rather cumbersome process, they propose to follow the pro cedure outlined by Prof. Hoover in his book entitled Water Treatment, page 161, consisting of the determination of the pH and alkalinity di

rectly and in the computation of the free carbon dioxide from the fol

lowing equation:

pH ? 6.3075 + log C02 = log alkalinity

where both the C02 and alkalinity are expressed in p.p.m., the latter in terms of CaC03. A graphical chart is available to determine the free carbon dioxide from the other two values. This method appears to be

much simpler than the volatile acids determination and has interesting possibilities.

Control or Acid Conditions, Scum and Temperature

The control of acid conditions, scum and temperature is somewhat interrelated. The most common method used to rectify acid condi tions is by addition of lime, although ammonium sulfate treatment was used in places. The latter has found considerable application in the control of disturbances caused by industrial wastes, also in the control of scum.

For controlling scum, high temperatures are recommended, attain able by insulating the top of the tanks or, as Mr. Taylor, Consulting Engineer of New York City, advocates, by using jetting pipes to sprin kle supernatant on the top of the scum. The latter method breaks up the scum besides heating it. Hood of Ridgewood, N. J., has been suc cessful in controlling scum with activated carbon.

For maintaining uniform temperatures and increasing gas produc tion, the recirculation or mixing of the sludge is found to be useful.

Sludge Input and Withdrawal Technique

If means are available to dispose of the sludge at regular intervals all year round, such as by covered sludge drying beds or vacuum filters, the amount of ash in the digested sludge withdrawn should be the same as in the raw sludge added. If, however, sludge drying facilities are

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Vol. 16, No. 4 SLUDGE COLLECTION, TREATMENT AND DISPOSAL 703

limited at the beginning of the poor drying season, the digester has to be sufficiently emptied to make room for the raw sludge received during the poor drying period. We have not time to fully outline the arith

metic of this problem but merely point out that sufficient seeding sludge should be left in the tank so that if an average tank temperature of 60?

F. is anticipated, the daily amount of fresh solids added should not exceed 2 per cent by weight, and at 80? F. the daily charge should not exceed 4.5 per cent.

Gas Production

The digester gas is a valuable by-product of the digestion process and careful attention should be given to its full utilization. Its average

methane content is 70 per cent and its heat content 600 B.T.U. In 1-5

to 1-12 mixtures with air the gas is highly explosive and for this rea

son and that it is an asphyxiating gas, we cannot place enough emphasis on the need that safety precautions be strictly followed. The mainte

nance of safety devices such as flame traps, vacuum relief valves,

grounding of motors, explosion-proof lighting fixtures, etc., is a must in

any plant. Prohibition of smoking, elimination of water seals, closing of petcocks after manometer readings, closing of automatic condensate

drip traps, and checking of automatic gas pilot valves are equally im

portant precautions. The principal use of this gas is to heat the tank, and to accomplish

successfully this purpose it is very important to keep the boiler clean, not to operate it under 180? F. and to use hot water mixing valves to

cut this down to 140? F., the maximum permissible temperature in the

coils. Care should be taken to maintain proper gas pressure (usually 2 inches) at the gas burners.

If gas is used for power generation further steps must be taken to

suit it for the purpose. The maintenance of the machinery presents some problems but the scope of this paper does not permit us to go further into this subject.

Starting the Digester

The starting of the digester is a most difficult manipulation if no seeded sludge is available. There are two schools of thought as to the

procedure. The first recommends filling of the tank with sewage to avoid dangerous gas-air mixtures, then slow introduction of heavy

sludge with heat from an independent source of fuel supply. Tempera ture and pH control, addition of lime and recirculation will eventually help to attain the desired end. On the other hand, the Dorr Company does not recommend the use of sewage but sludge alone. Sludge should

be pumped as thick as possible and it should be heated as soon as it reaches the heating coils ; then proceed as outlined above. The roof

hatches will not be closed until actual gas production starts, to be sure

that all air is expelled.

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704 SEWAGE works journal July, 1944

Cost of Operation

At the Joint Meeting at Rutherford, the 1942 fixed charges amounted to $2.35 and the operating charges to $3.00 per dry ton. Average fig ures are $3 to $4 for fixed charges and $2 to $3 for operating charges.

Garbage Digestion

At Cornell University, experiments were conducted by Malcolm and

Straub on the digestion of garbage and pure vegetables mixed with

sewage sludge. They found that the pH and volatile solids were

slightly higher than in digested sludge alone. About 13 cu. ft. of gas was produced per lb. of volatile matter, which gas contained 40 per cent

carbon dioxide, therefore, having a lower B.T.U. value than in the case

of ordinary sludge digestion. The digestion of garbage offers interesting possibilities for dispos

ing of this troublesome material and for power production, after some

of the inherent difficulties are ironed out. The little information avail able indicates that combined sludge and garbage requires twice as much

digestion tank space per capita as sludge alone.

SLUDGE DEWATERING

By J. K. Adams

Superintendent, Sewage Treatment Plant, Tenafiy, N. J.

This title could include a dozen or more subdivisions, each of which would supply material for an entire paper, so I can only touch the high spots. Sludge dewatering is still the principal problem at most plants and it is accomplished in various ways.

It is worth noting that, although widely practiced, dewatering is not always necessary. Wet sludge may be disposed of by dilution in large bodies of water as is done by several cities located near the ocean. It

may also be disposed of on land by discharge to low ground or into trenches or lagoons. Where this is done in the United States it is usu

ally with apologies and an explanation that it is an emergency measure due to lack of sludge bed drying area or for temporary use during win ter months. The objections to it are obvious, even for digested sludge,

while raw primary or raw activated sludge, so handled, creates a serious nuisance due to odors and flies.

Eeturning to the dewatering problem, the principal reason for taking this step is to reduce the volume of material to be handled, whether it be for ultimate disposal on land as fill or for fertilizer or by incineration.

Even in this mechanical age sand beds are the most widely used means of dewatering sludge.

Mr. Langdon Pearse's report to the A. S. C. E. in January 1944 (4) states that in 1941 there were 3,909 places using sludge drying beds of

which 348 were covered. By far the larger part of these installations

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