the digestion of sewage sludge containing various concentrations of solids

The Digestion of Sewage Sludge Containing Various Concentrations of SolidsAuthor(s): C. E. KeeferSource: Sewage Works Journal, Vol. 19, No. 1 (Jan., 1947), pp. 39-42Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25030401 .

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THE DIGESTION OF SEWAGE SLUDGE CONTAINING VARIOUS CONCENTRATIONS OF SOLIDS

By C. E. Keefeb

Bureau of Sewers, Baltimore, Md.

In the operation of sludge digestion tanks attempts are often made to con

centrate the inflowing raw solids. This

practice conserves sludge gas where

tanks are heated and makes available more space for the digestion of sewage solids. It is doubtful, however, if the

average solid content in digestion tanks

exceeds 10 per cent. In view of these facts it was considered desirable to in

vestigate the extent to which partially dewatered sludge digested. If the

moisture content of raw sludge were

reduced from 95 to 70 per cent, the

dewatered material would occupy one

sixth its original volume. Should this dewatered material digest rapidly, a

considerable saving in digester volume would result.

Experimental Procedure

Raw sludge was obtained from the

mechanically cleaned primary sedimen tation tanks at the Back River sewage

works, and digested material was

drawn from a small-scale laboratory di

gester. As indicated in Table I, the raw sludge contained 85.54 per cent

volatile matter and had a pH of 5.4, and the digested material contained 57.08 per cent volatile matter and had a pH of 7.5.

The two materials were mixed in the ratio of two parts by weight of raw

volatile solids to one part by weight of

digested volatile material. The mix ture was divided into five equal por

tions, each weighing 20.42 lb. To each of four of the portions, 225 ml. of a 10 per cent ferric chloride solution was added to coagulate the solids. Three of the portions were then dewatered on a

B?chner funnel.

These sludges, Nos. 1, 2, and 3

(Table II) contained the following moistures: 67.53, 74.99, and 85.08 per

cent, respectively. The above-men

tioned volume of ferric chloride was also added to sludge No. 4, one of the con

trols, which was not dewatered. Sludge No. 5, to which ferric chloride was not added and which was not dewatered, also served as a control. Lime was added to each of the mixtures to raise the pH to 6.7. Each mixture was then

placed in a 5-gal. carboy and incubated at 37? C. The volumes and the analy ses of the gas produced were deter

mined at regular intervals. The gas was passed through calcium chloride so that any moisture entrained therein could be caught and subsequently

weighed. At the end of the experiment the volatile and the solid contents of the sludges were determined.

Discussion of Results

Sludge No. 1, the moisture content of which was 67.53 per cent at the

beginning of the experiment, digested less than any of the other materials.

During the 73-day digestion period the volatile solids on the dry basis were reduced from 72.10 to 65.82 per cent. The loss in weight of volatile solids amounted to 64.9 grams, or 29.9 per

TABLE I.?Analyses of Raw and Digested Sludges Used in Experiment

Material Quan

tity Used

(lb.)

Dry Solids

(%)

Volatile Matter,

Dry Basis

(%)

pH

Raw sludge.

Digested sludge...

12.42

8.00

4.93

5.73

85.54

57.08

5.4

7.5

39



40 SEWAGE WORKS JOURNAL January, 1947

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cent of the original raw volatile solids. The gas produced amounted to 223.9 ml. per gram of fresh volatile solids and weighed 61.1 grams.

Sludge No. 2, which had a moisture content of 74.99 per cent, digested quite

well. The volatile solids were reduced from 71.57 to 58.16 per cent, with a loss in weight of 132.2 grams. This

sludge produced 616.6 ml. of gas per gram of fresh volatile solids, which amount was greater than that from any of the other materials except sludge

No. 5.

Sludge No. 3, the moisture content of which was 85.08 per cent, lost 165.3

grams of volatile solids with a reduc tion of 72.7 per cent of the raw volatile solids. This reduction was accom

panied by an unusually low gas pro duction of 10.12 ml. per gram of fresh volatile solids. On the other hand, there was a gain of 156 grams in the moisture

content, indicating a considerable

liquefaction of solids.

Sludge No. 4 contained 94.73 per cent moisture at the beginning of the

experiment. The loss in volatile solids amounted to 162.5 grams. This loss

was accompanied by a production of 564.8 ml. of gas per gram of fresh volatile solids; this gas weighed 140.9

grams. There was also an increase of

35 grams in the moisture content, which indicated some liquefaction of solids.

Sludge No. 5, the only one to which ferric chloride was not added con tained 94.82 per cent moisture. This

sludge produced 756.9 ml. of gas per gram of fresh volatile solids. The gas

weighed 183.6 grams; the correspond ing loss in volatile matter was 176.3

grams.

The experiments indicate the possi bility of digesting raw sludge that has been dewatered and properly seeded. This method of treatment as compared with the more usual one of digesting sludge prior to dewatering presents certain disadvantages. A larger vacu

um filter installation would be required, as more material would be handled



Vol. 19, No. 1 DIGESTION OF SEWAGE SLUDGE 41

FIGURE 1.?Total gas production from sludge containing various quantities of water.

and the cake output per unit area of

filter would be less. Moreover, more

coagulant would be needed. On the

other hand, there would be a great re

duction in digestion space. Eeducing the moisture content of sludge from

95 to 75 per cent, for example, de creases it to one-fifth its original volume.

A material reduction in the amount

of gas to heat the sludge would also

result. Getting the thickened sludge into and out of digestion tanks would

present new problems. At the present time most sludge pumps handle ma

terials containing 90 per cent or more

of moisture. There are pumps, how

ever, such as those used to pump con

crete, that can handle much more concentrated materials. New ways of re

moving the thickened sludge from di

gestion tanks would have to be devised.

Digesting sludge after it had been fil

tered would in all probability solve

both the scum problem in digesters and the problem of disposal of supernatant

liquor.

It is a long step from experimenting with small quantities of sludge in

bottles and investigating the problem in a full-size plant. The results are of

sufficient interest, however, to warrant a continued study of the problem on a

larger scale.

Conclusions

The following conclusions may be drawn from the results obtained :

1. Gas production was less from

sludges to which ferric chloride was

added. 2. Reducing the moisture content of



42 SEWAGE WORKS JOURNAL January, 1947

sludge to 67.5 per cent materially re

tarded digestion. 3. There was a gradual increase in

the digestibility of the sludge as its moisture was increased from 67.5 to

85.1 per cent.

4. The sludge containing 85.1 per cent moisture produced practically no

gas. There was, however, considerable

liquefaction of the solids. There was a

loss of volatile solids of 165.3 grams and a corresponding increase in mois ture content of 156 grams.

5. The sludge containing 74.99 per cent moisture produced 8.4 per cent

more gas per gram of fresh volatile solids than the sludge containing 94.73

per cent moisture. The loss of volatile solids in the latter sludge was greater,

however, as about 10 per cent of the material liquefied.

6. The most complete digestion oc

curred in sludge No. 5.

These experiments indicate that sew

age sludge digests over a considerable

range of moistures. Although ferric chloride inhibited digestion slightly, the effect was not great. There was considerable digestion when the mois ture content was 75.0 per cent. Re

ducing the moisture to 67.5 per cent

materially retarded digestion. The Back River sewage works is

under the general supervision of Na than L. Smith, Chief Engineer of the

Department of Public Works, and

George E. Finck, Sewerage Engineer.



the digestion of sewage sludge containing various concentrations of solids

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