.January, 1931 IArDUSTRIAL AND ENGINEERING CHEMISTRY 67

(4) Eitner, through Grasser, Handbuch fur Gerberei-Chemische Labora-

(5) Frey and Clarke, U. S. Dept. Agr., Tech. Bull. 169. (6) Huc, H u l k a u z cuirs Supp l . tech., May, 1924, 147. (7) Jablonski, Z. Leder. Gerberei-Chem., 1, 173 (1923). (8) Jalade, Halle auz cui rs s u p p l . tech., Feb., 1924, 33; June, 1924, 161. (9) King, J. Tezfile Insl., 17, T53 (1926).

(10) Mosenthal, de, J . Soc. Chem. Ind. , 16,443 (1907). 411) Paessler, Collegium, 1912, 517.

torien. (12) Parks and van Heuckeroth, Paint M f r s . Assocn. C . S., Tech. Circ. 'A41. (13) Porter, J . A m . Leather Chem. Assocn., 24, 36 (1929) (14) Riethof, Ib id . , 10, 456 (1915). (15) Rogers and Frey, J. IND. ENG. CHEM., 10, 554 (1918). (16) Stamm, J . Phys. Chem., 33, 398 (1929). (17) Veitch, Frey, and Clarke, U. S. Dept. Agr., Bull. 1168, 10. (18) Winandy, Halle auz cuirs suppl. tech., Oct., 1924, 314. (19) Woodroffe, J . Intern. Soc. Leather Trades Chem.. 9, 149 (1928). (20) Ziegelmann, Paint Oil Chem. Rev., 84, No. 5, 12 (1927).

Thermophilic Digestion of Sewage Solids

EWAGE sludge, until a few years ago much neg- S lected even in efficient

sewage-treatment works, has lately been the object of much laboratory and plant experi- m e n t a t i o n . Thermophilic digestion experiments begun over two years ago in this laboratory have brought re- sults which bid fair to usher in a new era in sludge diges- tion. E x p e r i m e n t s were made with unseeded and seeded solids, employing the batch process and with daily additions of both fresh solids

IV-Fresh Solids and Activated Sludge1#* Willem Rudolfs and H. Heukelekian

NEW JERSEY AGRICULTURAL EXPERIMENT STATION, NEW BRKJNSWICK, N . J .

Daily additions of sewage solids, both fresh and activated, have been digested at thermophilic tem- peratures (50" C.) in as short a time as 2.1 days. This digestion time can probably be reduced to 24 hours, since 92.2 per cent of the total gas produced by acti- vated sludge was obtained in 24 hours or less and 88.5 per cent of the gas evolved from the fresh solids in this time. The sludge produced was black, had no odor other than the tarry odor of ripe sludge, and had a biochemical oxygen demand corresponding to ripe sludge. Gentle shaking, to bring the raw solids in contact with the seed material, proved bene- ficial. For best results charging of raw material into tanks should be continuous. Preheating of the sludge is probably best and the ripe sludge can be dewatered continuously.

and activated sludge. Reaction adjustments were made with lime and other chemicals, the amounts of seed material re- quired determined, and the temperature ranges established.

Earlier Experiments

Digestion of fresh sewage solids a t higher temperatures (45-55 O C.) with seeding material produced under thermo- philic conditions proceeded a t about twice the rate of fresh solids, seeded with ripe sludge, a t the optimum low tempera- ture (28" C.) (1, 2, 3). It had been observed in both the laboratory and plant-scale experiments that the ultimate reaction of the ripe material was about pH 7.8-8.0. Reac- tion adjustment of the medium to 7.8-8.2 showed that best results were obtained with these higher pH values and that gas production was the same as from uncontrolled mixtures. Reaction control with lime and alkaline salts showed that the latter were somewhat more beneficial than lime and that the digestion time could be reduced to 10 days. It was thought possible to eliminate or reduce greatly the rather large quantities of seed material by adding alkaline salts, since one of the important functions of ripe sludge is its buffer action, but the results showed that certain quantities of fiermophilic sludge decreased the digestion time far more than the salts used as buffers.

The experiments on the effect of temperature upon sludge digestion were conducted a t temperatures between 5" and 70" C. (40" and 160' F.). The optimum for low-tempera- ture digestion was about 28" C., whereas the optimum for

1 Received September 27, 1930. Presented by Willem Rudolfs under the title "Further Experiments on Thermophilic Sludge Digestion" belore the Division of Water, Sewage, and Sanitation Chemistry at the 80th Meet- i n g of the American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930.

* Journal Series paper of New Jersey Agricultural Experiment Station, Department of Sewage Research.

thermophilic digestion seemed to be around 55" C.

The effect of different tem- peratures upon the rate of t h e r m o p h i l i c digestion is shown in Figure 1. At 40- 45" C. the time required was from 42 to 43 days, a t 55" C. about 8 days, and a t 70" C. 40 days, as compared with about 30 days for low-tem- perature digestion a t 28" C. Even with the best results the time required was considered excessive, because about half of this time was consumed by a lag period before the diges-

tion processes reached an accelerated rate and about three- fourths of the time before they reached their maximum.

Newer Experiments

The later experiments were made in two different groups- batch process and daily additions. I n both groups fresh solids used were obtained from primary settling tanks a t Plainfield, N. J., and activated sludge from the returned sludge pipe a t Tenafly, N. J.

Carboys fitted with extra holes for solids additions, sludge removal, and gas collection were inverted so that the necks of the bottles served as sludge hoppers. Provision was made to draw either sludge or supernatant liquid. Only a few of the results obtained with daily solids additions and daily sludge withdrawals are here reported. The activated sludge was added after settling for about 1 hour. All experiments reported in this paper were made a t 50" C.

Results with Fresh Solids

With increasing quantities of fresh solids added daily, it would be logical to assume that there would be less intimate contact between the ripe sludge and fresh solids. To de- termine the effect of gentle mixing a t the time when fresh solids were introduced upon the acceleration of the diges- tion processes, several series of parallel experiments were conducted. The results for two series with different daily additions obtained a t the end of the charging periods are given in Table I.

Table I-Fresh Solids Charges and Gas Production GAS PRODUCTION PER GRAM

DAILY CHARGE VOLATILE MATTER ADDED INCREASE Not stirred Stirred

9% c c . CC. % .- 8.7 510 710 39 16.2 372 596 60

68 INDUSTRIAL AND ENGINEERING CHEMISTRY Vol. 23, So. 1

The daily charges were on the basis of dry volatile matter originally in the ripe sludge. In other words, if 100 grams of dry volatile matter were present, the daily additions were 16.2 grams of fresh dry volatile matter. From the results i t is evident that, with greater charges than ordinarily prac- ticed, gentle stirring in order to produce more intimate contact is beneficial. Moreover, with increasing charges the percentage increase in gas production becomes greater. This does not mean that the ultimate amount of gas produced would be greater from the mixed materials, but that the mixing brought the fresh solids into close contact with the ripe sludge quicker and thereby accelerated the biological activities. Releases of gas caused by stirring might be of additional advgntage to obtain an even flow of gas.

I I I I I 1

I I I I I I 40 50 6 0 70

r&MPc'Rn rue€, 0 C . Figure I-Effect of Temperature upon Rate of

Thermophilic Digestion

Results on decreasing the digestion time by increasing the daily charges and withdrawing the calculated amountsiof ripe sludge produced daily are given in Table 11.

Table 11-Effect of Increasing Daily Load upon Gas Production

% Days cc. DAILY GAS PER GRAM

CHARGE DIGESTION TIME VOLATILE MATTER

8 . 7 16 .4 2 2 . 3 48 .0

11.5 6.1 4 . 5 2.1

845 875 950 650

If a conclusion were to be based solely upon gas produc- tion, it could be assumed that a daily charge of 48 per cent volatile matter or a digestion time of 2.1 days was the maxi- mum. However, if the percentage volatile matter decom- posed is taken into consideration, it appears that addi- tions of 16.2, 16.4, and 22.3 per cent give an average vola- tile-matter reduction of 72.7 per cent, whereas the 48 per cent volatile-matter addition caused a reduction of 73.3 per cent. Moreover, the sludge withdrawn had a 24-hour B. 0. D. of 1470 p. p. m. for each per cent of volatile matter, which indicates a good digested sludge.

Results for Activated Sludge Daily additions of activated sludge were made in a similar

way as indicated for fresh solids. Some of the more perti- nent results are given in Table 111.

St is now well known that activated sludge produces much less gas than fresh solids. Comparison of the averages for Similar charges of fresh solids and activated sludge indi-

cates that activated sludge on a volatile-matter basis pro- duces only 50 per cent of the gas produced by fresh solids. It is evident, therefore, that during the activation processes large quantities of gas-yielding substances are destroyed. In a later paper the character of these substances will be dis- cussed in more detail. The results given in Table 111 show clearly that a digestion time of 2.1 days is sufficient under thermophilic conditions.

Table 111-Activated Sludge Charges and Gas Production

DAILY CHARGE DZGKSTION TIME VOLATILZ MATTKR % Days cc.

GAS PER GRAM

15.5 23.2 47.2

6 . 4 4 . 3 2 . 1

430 385 405

The experimental results give no indication that a daily charge of 50 per cent volatile matter of fresh solids or acti- vated sludge is the upper limit. The large volumes of gas produced and the quantities of sludge to be added became cumbersome for further laboratory experimentation and addi- tional work is to be conducted on a semi-plant scale. One of the reasons why it is believed that the upper limit is not reached with a 50 per cent charge is that 92.2 and 88.5 per cent of the total gas produced by the activated sludge and fresh solids, respectively, evolved within 24 hours after charging. Hourly measurements of gas production indi- cated that this time was in effect shorter.

The gentle shaking mentioned above was accomplished by hand once a day after the charges were introduced. Ow- ing to the high temperature a considerable portion of the sludge introduced had a tendency to rise to the surface and a stirring device that would bring the raw material into contact with the ripe sludge should produce even more favor- able results and prevent scum formation.

Charging, Heating, and Dewatering of Sludge

I n order to obtain the best possible results under thermo- philic conditions, the charging of the sludge into the tank should be continuous. This might be possible only a t the larger plants. Smaller plants would probably resort to two or three charges a day or as many more as would be con- venient and consistent with the amounts of sludge collected in the settling basins.

Heating the sludge to these higher temperatures brings up the question of amount of gas produced, proper insula- tion of the tanks, types of heating device, and manner of heating.

The amount of gas produced under thermophilic condi- tions is a t least the same as that produced under optimum lower temperature conditions. The gas produced will heat the tanks in winter to 20" C. and in summer to 28-30' C. This procedure requires from 25 to 30 days a t the higher temperature and about 40 days at the lower. Without lengthy calculations it is evident that evolution of the same quantities of gas in 24 or 48 hours as are produced in 30 days would offset the increased heat losses and greater amounts of heat required to keep the tanks at the higher temperatures.

Experience over several months has already shown that heating with hot water circulated through iron pipes does not result in a heavy coating on the outside of the pipes, as was expected. But it is possible that such heating con- tinued for several years might cause a sufficiently thick cake of sludge on the pipes to interfere seriously with the heat transmission. Perhaps preheating of the fresh sludge with boiling water or live steam would be better. Complications in this respect are less to be feared because even boiling or sterilizing of the sludge would not interfere with the subse-

January, 1931 IXDUSTRIAL A S D ENGINEERING CHEMISTRY 69

quent digestion processes, but would probably be an advan- For larger plants fresh solids would be charged continuously, tage. This heating is an engineering problem and requires possibly preheated, kept in the tanks for probably24 hours, de- further study. watered on mechanical filters, and carted away to dumping

The dewatering of sludge, which is preferably discharged grounds or sold. continuously, could best be accomplished by a mechanical filter. Even if sludge is discharged several times a dav, Literature Cited

\

a mechanical filter would be better than sand filters, and probably more economical. A thermophilic digestion unit would therefore make it possible to do away with sand beds.

Heukelekian, S e ~ , a g e Works J., 2, 219 (1930). (2) Heukelekian, Ibid. , in press. (3) Rudolfs and Heukelekian, IND. ENG. CHEM., 22, 96 (1930).

I

A Study of Tannery Effluent I-Effect of Various Gases upon the Nitrogen Distribution'

Edwin R. Theis and Philip Kratz2

DEPARTMENT OF CHEMICAL ENGINEERIXG, LEHICH UNIVERSITY, BETHLEHEM, PA

HEIS and Lutz (1) made a preliminary study of the effect of nitrate oxygen upon tannery soak water and showed that treating such water with 1000 p. p. m.

of sodium nitrate gave a rapid evolution of gas which was largely nitrogen, a rapid reduction of the added nitrate to nitrite, free ammonia, and free nitrogen. They also showed that the sulfur compounds were acted upon to produce hydrogen sulfide.

During the past year this work has been expanded to cover the effect of varying pH of the soak-water effluent and the relation of this effluent to pH under the influence of nitrate oxygen.

T

T/mr [h Hours) '0

25 50 7 5 101 NS /so /,5

Showm Pressure o b t e , m d in fhe S y fern &,y/uent-NoNOJJunder v'vymg px

I 3

Experimental Procedure

It vas decided to determine just how much gas was pro- duced when the hydrogen-ion concentration was varied. The gas produced was measured by determining the pressure in small bottles fitted with manometer tubes. In the bottles were placed 100 ml. of the effluent, the pH of which had been adjusted, the system was brought to 37' C., enough sodium nitrate added to give a solution of 1000 p. p. m., the manome- ters attached, and readings taken a t given intervals. The pH of the samples was varied from 1 to 10. Figure 1 shows the results obtained.

It was further desired to find the effect of various gases upon the effluent in question. In the activated-sludge process of sewage treatment, activation is brought about through the

medium of air; therefore it was desirable to know just what effect would be produced when hydrogen or oxygen was used in place of air.

A definite amount of effluent was placed in an apparatus, as shown in Figure 2, so arranged that any ammonia or hydro- gen sulfide evolved could be collected and determined quan- titatively. The gases were bubbled through the effluent for 7 days, a t the end of which period more of the same effluent was placed in the apparatus and the gases again passed through the system for 7 days. This passage of gas was continued for about 4 weeks. At the end of each 7 days the amounts of ammonia and hydrogen sulfide evolved were determined. In the residual effluent nitrogen-distribution determinations were made consisting of the determination of total nitrogen, protein nitrogen, free ammonia nitrogen, formaldehyde titration, and amount of volatile fatty acid formed (these acids would be formed through the deaminization of various amino acids). Figures 3 to 7 show the results obtained from such experiments.

Discussion

When the pH of the sewage-nitrate system was varied, it was found that during the first 24 hours of incubation (Figure 1) the solutions having a pH of 1 to 5 produced goodly quan- tities of gas, while those solutions of higher pH not only did not produce gas but actually adsorbed gas; in other words, from pH 1 to 5 the pressure was positive but above 5 it was negative. In all the cases tried soak-water effluent produced quantities of gas under acid environment but not in alkaline solution.

Figure 2-Apparatus for Determining Various Gases on Soak-Water

Effect of

When gases were passed through the effluent, it was found that oxygen caused ammonia to be produced and evolved

1 Received October 27, 1930. Presented before the Division of Leather and Gelatin Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September S to 12, 1930.

during i; passage through the sys(em.

traces.

Upon the other

Hydrogen used in place Of air or oxygen caused the hand, oxygen produced no hydrogen sulfide beyond mere

2 Hunt-Rankin Leather Company research fellow, Lehigh University.