a discussion of the symposium on sludge

A Discussion of the Symposium on SludgeAuthor(s): H. HeukelekianSource: Sewage Works Journal, Vol. 16, No. 4 (Jul., 1944), pp. 715-719Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25029831 .

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Vol. 16, No. 4 SLUDGE COLLECTION, TREATMENT AND DISPOSAL 715

in supernatant chlorination in the plants at Morristown, Little Falls, Ridgewood, and elsewhere. As far as we are concerned, the advantage of the treatment lies in the modification occurring in the carbonate

equilibrium which, combined with the reduction in B.O.D., substantially reduces the shock load on the plant and permits return of the super natant to the sewage without causing a reduction in overall plant

efficiency. In experiments at Ridgewood in August, 1943, 6,000 g.p.d. of super

natant was chlorinated to a residual of 20 p.p.m. The treatment re

duced the alkalinity from 3,230 p.p.m. to 1,210 p.p.m. and lowered the

pH from 7.1 to 6.0.

This step combined with a proper technique in spreading the return over the period of highest plant inflow minimizes the load on the plant and makes it possible for the plant to function satisfactorily when other

wise it might not. At the Ridgewood plant, the supernatant return is distributed over the entire 24 hours, thus attaining a maximum of dilu tion. This precaution alone has sufficed to prevent upset of the other

biological plant processes.

A DISCUSSION OF THE SYMPOSIUM ON SLUDGE *

By H. Heukelekian

Assoc, Dept. Water and Sewage Besearch

These papers deal with four important operational phases of the sludge problem; namely, (1) collection and pumping, (2) digestion, (3) dewatering and drying, (4) the disposal of supernatant liquor.

Because of the time limitation, a comprehensive discussion of all the different phases cannot be attempted. Certain specific problems, how

ever, will be discussed. In order to limit the scope, the discussion will deal mainly with primary sludge.

The frequent removal of sludge from sedimentation tanks is essen

tial and is now generally recognized as good practice. The disadvan

tages arising from infrequent removal of sludge are : (a) deterioration of the effluent, (6) scum formation in sedimentation tanks, and (c) pos sible retardation of digestion. In addition to frequent removals of

sludge, thoroughness of removal should also be emphasized. If sludge is not completely removed from corners and side walls of the tanks, it seeds the freshly-accumulated sludge and speeds up septic action, espe

cially during the summer months.

The frequent removal of sludge, on the other hand, is usually ac

companied by a sludge of low solids concentration, entailing the addi tion of large volumes to the digester and resulting in cooling the tank contents and reduction in the capacity of the tank. Slow rate of re

moval of the sludge from the clarifier sump prevents cone formation *

Journal Series Paper of the New Jersey Agricultural Experiment Station, Rutgers Uni

versity, New Brunswick, New Jersey.



716 SEWAGE WORKS JOURNAL July, 1944

and the transfer of thin sludge. If maximum concentration of sludge is desired, thickening in separate or especially designed hopper tanks before addition to the digester is called for. The advantages of such a scheme would be a greater separation of liquor than is feasible in the

digester, where evolution of gas prevents quiescent compacting, except in two-stage digesters especially designed for this purpose. Fischer

has shown that concentration of fresh solids retards digestion but that retardation is very slight and is more pronounced with concentrations above 12 per cent.

Studies show that compacting of fresh solids is affected by time, temperature, and initial concentration of solids. The rate of compact

ing decreases with increasing concentration. The rate of concentration is greatest during the first 12 to 24 hours and decreases gradually there after. It is possible and may prove desirable to obtain solids concen

tration of 10-15 per cent with storage under quiescent conditions.

Temperature has an important bearing on the rate of compacting of

sludge, greater concentration being obtained during the summer than in winter with equal periods of quiescent compacting. Pumping diffi culties, due to greater loss of head, will increase with greater solids concentration and with lower temperatures.

The septicity produced during the compacting period will have little if any effect on the rate of digestion, but too high a concentration of solids in the digesting mixture may retard the digestion, because of high concentration of intermediate and end products such as volatile acids

accumulating in the smaller volume of liquor. From the standpoint of digester operation, the uniform loading of

the raw solids, whether compacted or not, makes for more successful results than does intermittent application of large batches. A more

uniform gas evolution can be expected from such a procedure. At present, the digestion of raw sludge of domestic origin in tanks

of adequate capacity and with heating facilities offers few major prob lems. The gross changes taking place during digestion in respect to the quantity and quality of gas, volatile matter reduction, and the im

portance of optimum pH value are well recognized and need little em

phasis. What is urgently needed for further progress in this direction is a more thorough and intimate understanding of the intermediate re

actions which result in these gross changes. It is quite possible that the same end results may be obtained under slightly different condi tions through different channels and side reactions. As the complete chain of events during the complicated process of digestion is better understood, it may be possible to direct the course in certain directions in order to obtain certain desirable products or to accomplish the stabi lization of sludge in shorter time. In order to decrease the digestion time from 35-40 days we must find out what the slowest reaction is, in the whole chain of events. We recognize now that digestion entails two

main and entirely different processes ; the first liquefaction or hydroly sis, which is the preliminary stage in rendering the complex and large size materials into forms more readily available for the methane fer




menting organisms which take part in the second stage and convert the

liquefied and hydrolyzed materials into methane and carbon dioxide. This is the gross picture. There are, however, wheels within wheels

in each stage and the above generalization does not adequately convey a picture of the complex and manifold reactions within each stage. It

might be surmised that liquefaction is the slower of the two stages since, if material in a soluble form such as sugars is used instead of complex and large organic particles as in raw sludge, the speed of digestion and

gasification is greatly accelerated and greater loadings per unit tank

capacity can be made. Attempts to accelerate liquefaction by the addi

tion of external sources of enzymes have not been fruitful. Preliminary

septic digestion of fresh solids before their subjection to methane fer mentation with ripe sludge has not resulted in curtailing the digestion

time. On the other hand, we also know that the accumulation of too

great a concentration of liquefied products such as volatile acids is not

healthy for methane fermentation. In other words, if the rate of lique faction exceeds greatly the rate of gasification, because of either a de

pression of the activities of gasifying organisms or an acceleration of

the activities of liquefying organisms, the whole process of digestion is

upset. It appears from the foregoing that both stages must be accel

erated to the same extent if further increase in the efficiency of digestion is to be expected.

It appears that the addition of inorganic nitrogenous compounds such as ammonium sulfate as suggested recently by Schlenz is not the

answer to this question. It has been shown that the ammonia nitrogen content increases from 100 p.p.m. in raw sludge to 1,000 p.p.m. in di

gested sludge, thus indicating that nitrogen for the growth of micro

organisms is present in the sludge in quantities greater than required.

Sludge digestion, in addition to stabilizing and reducing the volume of the sludge, is a preparation to increase the rate of dewatering of

the sludge. Emphasis will be placed here on the dewatering of the

sludge on sand drying beds, since this method is more simple and eco

nomical for small installations, although some of the generalizations

apply to vacuum filtration as well. There are two forces which operate in dewatering of the sludge in sand drying beds: first, drainage, and

second, evaporation. Drainage is more pronounced during the first

12-18 hours. The flow by drainage is uniform during this initial pe riod and is negligible thereafter. If the sludge is well digested, release from the tank pressure will liberate sufficient gas to float the solids and permit the drainage of the liquor through the bed rapidly. To assist this, natural process acidification of the sludge or alum application will

liberate more gas from carbonates and induce flotation of the solids.

Experiments have shown that drainability of the sludge increases with increased gas production and that optimum drainability of sludge co

incides with the period shortly after the peak of gas formation. Stor

age of digested sludge decreased the drainability. Of the two forces, drainage removes a greater percentage of mois

ture (60 per cent), at prevailing bed temperatures up to 130? F., than



718 SEWAGE WORKS JOURNAL July, 1944

evaporation, which is the slower of the two forces. Naturally, drainage is not affected so much by an increase in temperature as is evaporation. Since circulation of air increases the rate of evaporation, the surround

ing walls in an open drying bed should not be built too high. South African experiments have shown that the effect of rain is not so con

siderable as is generally supposed. Its effect varies according to the

length of time the sludge has been on the bed before the rain. When rain falls before drainage has stopped, all the rain passes right through the sludge and the bed, just as though there had been no sludge. There is no tendency to absorb the rain. When rain falls after drainage is

complete and while evaporation is a major factor, the rain does not

drain through the bed but is absorbed by the sludge. The cracks are re duced in size and the drying time is increased. When rain falls after the sludge is thoroughly dried out and the cracks are large, extending down to the surface sand, it is not absorbed to any large extent by the

sludge but flows rapidly through the bed. In discussing the supernatant liquor problem it might be well to

define what constitutes an ideal or good supernatant liquor. Under

ideal situations the supernatant liquor should have only a small quan

tity of settleable solids, should be low in suspended solids and B.O.D., and should have a brownish color. There are no accepted standards

for the quality of supernatant liquor, but the following are attainable values: total solids 3,000-4,000 p.p.m., suspended solids 1,000-2,000 p.p.m., B.O.D. 1,000-2,000 p.p.m., volatile acids 2,000-3,000 p.p.m.

Intermediate and end products of liquefaction and gasification of

the digesting sludge find their way into the supernatant liquor. These are either in soluble or colloidal forms. During the period in which

liquefaction predominates, the quantity of the intermediate products such as volatile acids and colloidal material in the supernatant liquor is high. As the digestion nears completion, most of these products are destroyed, leaving only the relatively resistant materials in the

liquor. The ideal supernatant liquor as described above is not always at

tained; complete separation of the liquid from the sludge does not al ways take place in the digestion tanks. Under very aggravated con

ditions there is little difference in the percentage of solids vertically in the tank. Under less aggravated conditions the liquor may repre sent a thin sludge with 1 or 2 per cent solids. Naturally, the type of disposal of the supernatant liquor will depend on the nature of the

liquor. When the supernatant liquor approaches the values given above,

returning it to the influent of the plant will have no appreciable effect on the subsequent treatment processes, even with the activated sludge process, especially if the return of the overflow is spread over the en

tire day. Supernatant liquor which is more like a thin sludge should not be returned to the influent of the plant, but disposed of in lagoons on sludge drying beds or chemically treated.

The special treatment of supernatant liquor by atomizing aeration




and subsequent settling has been reported to reduce the original B.O.D.

from 1,500 p.p.m. and the suspended solids from 2,200 p.p.m. down to

values found in normal sewage. But more results with supernatant

liquor with still higher values would be required to establish the value

of this treatment.

Wide fluctuations occur in the nature of the supernatant liquor at

the same plant at different times, as well as at different plants. One of

the factors is whether secondary sludge, especially activated sludge and

to some extent humus sludge, is digested together with primary sludge in the digestion tanks. It seems that the gelatinous nature of activated

sludge is not readily destroyed by digestion, hence making it difficult for

liquid to separate from the sludge. The concentration of liquefied col

loidal and soluble materials in the liquor will depend, among other

things, on the concentration of the digesting solids. It is reasonable to

assume that a thicker sludge will throw into the liquor more products than a thinner sludge, if everything else is equal. When the loading rate into the digester is high, it is to be expected that the greater gas produced per unit volume of tank capacity will stir up the tank contents and prevent the separation of the liquor. Pumping sludge with high

moisture content into the digester will reduce the detention time for supernatant liquor, giving rise to poor quality overflow. Increased

temperature from 70? to 95? F. has no appreciable effect on the quality of liquor. In unheated tanks, however, when the temperature rises in

the summer, effects similar to overloading are obtained due to violent

gas evolution from the partially digested material accumulated during the winter months. Two-stage digesters are designed to segregate the

active digestion period, with its concomitant high rate of gas evolution and stirring up of the sludge, from the more quiescent storage con

ducive to compacting of the sludge and separation of the liquor.



a discussion of the symposium on sludge

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