.January, 1931
IArDUSTRIAL A N D ENGINEERING CHEMISTRY
(4) Eitner, through Grasser, Handbuch fur Gerberei-Chemische Laboratorien. (5) Frey and Clarke, U. S. Dept. Agr., Tech. Bull. 169. (6) Huc, H u l k a u z cuirs S u p p l . tech., May, 1924,147. (7) Jablonski, Z . Leder. Gerberei-Chem., 1, 173 (1923). (8) Jalade, Halle auz cuirs s u p p l . tech., Feb., 1924,33; June, 1924, 161. (9) King, J. TezfileInsl., 17,T53 (1926). (10) Mosenthal, de, J . Soc. Chem. Ind., 16,443 (1907). 411) Paessler, Collegium, 1912,517.
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(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, I b i d . , 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 IV-Fresh Solids and Activated Sludge1#* Willem Rudolfs and H. Heukelekian NEWJERSEY AGRICULTURAL EXPERIMENT STATION, NEWBRKJNSWICK, N. J.
Daily additions of sewage solids, both fresh and thermophilic digestion seemed EWAGE sludge, until a activated, have been digested at thermophilic temto be around 55" C. few years ago much negperatures (50" C.) in as short a time as 2.1 days. This The effect of different temlected even in efficient digestion time can probably be reduced to 24 hours, peratures upon the rate of sewage-treatment works, has since 92.2 per cent of the total gas produced by actit h e r m o p h i l i c digestion is lately been the object of much vated sludge was obtained in 24 hours or less and shown in Figure 1. At 40laboratory and plant experi88.5 per cent of the gas evolved from the fresh solids 45" C. the time required was mentation Thermophilic in this time. The sludge produced was black, had from 42 to 43 days, a t 55" C. digestion experiments begun no odor other than the tarry odor of ripe sludge, about 8 days, and a t 70" C. over two years ago in this and had a biochemical oxygen demand corresponding 40 days, as compared with laboratory have brought reto ripe sludge. Gentle shaking, to bring the raw about 30 days for low-temsults which bid fair to usher solids in contact with the seed material, proved beneperature digestion a t 28" C. in a new era in sludge digesficial. For best results charging of raw material into Even with the best results the tion. E x p e r i m e n t s were tanks should be continuous. Preheating of the sludge time required was considered made with u n s e e d e d and is probably best and the ripe sludge can be dewatered excessive, because about half seeded solids, employing the continuously. of this time was consumed by batch process and with daily a lag period before the digesadditions of both fresh solids and activated sludge. Reaction adjustments were made with tion processes reached an accelerated rate and about threelime and other chemicals, the amounts of seed material re- fourths of the time before they reached their maximum. quired determined, and the temperature ranges established. Newer Experiments
S
.
Earlier Experiments
Digestion of fresh sewage solids a t higher temperatures (45-55 O C.) with seeding material produced under thermophilic conditions proceeded a t about twice the rate of fresh solids, seeded with ripe sludge, a t the optimum low temperature (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. Reaction adjustment of the medium to 7.8-8.2 showed that best results were obtained with these higher p H 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-temperature 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 Meeti n g of the American Chemical Society, Cincinnati, Ohio, September 8 t o
12, 1930.
* Journal Series paper of New Jersey Agricultural Experiment Station, Department of Sewage Research.
The later experiments were made in two different groupsbatch 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 determine the effect of gentle mixing a t the time when fresh solids were introduced upon the acceleration of the digestion 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 a n d Gas Production GAS PRODUCTION PER GRAM DAILYCHARGEVOLATILE MATTER ADDED INCREASE Not stirred Stirred 9.% cc. CC. % 8.7 510 710 39
16.2
372
596
60
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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The daily charges were on the basis of dry volatile matter originally in the ripe sludge. I n 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 practiced, 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
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I 40
I
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I
1
I
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50 60 70 r & M P c ' R n 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. of Increasing Daily Load upon Gas Production DAILY GASPER GRAM CHARGE DIGESTION TIME VOLATILEMATTER % Days cc. 8.7 11.5 845 16.4 6.1 875
Table 11-Effect
22.3 48.0
4.5
2.1
950 650
If a conclusion were to be based solely upon gas production, it could be assumed that a daily charge of 48 per cent volatile matter or a digestion time of 2.1 days was the maximum. However, if the percentage volatile matter decomposed is taken into consideration, it appears that additions of 16.2, 16.4, and 22.3 per cent give an average volatile-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 pertinent 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-
Vol. 23, So. 1
cates that activated sludge on a volatile-matter basis produces 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. I n a later paper the character of these substances will be discussed 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
DAILYCHARGE
Sludge Charges and Gas Production GASPER GRAM DZGKSTION TIME VOLATILZMATTKR
%
Days
cc.
15.5
6.4 4.3 2.1
430 385 405
23.2 47.2
The experimental results give no indication that a daily charge of 50 per cent volatile matter of fresh solids or activated 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 additional 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 indicated that this time was in effect shorter. The gentle shaking mentioned above was accomplished by hand once a day after the charges were introduced. Owing 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 favorable results and prevent scum formation. Charging, Heating, and Dewatering of Sludge I n order to obtain the best possible results under thermophilic 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 convenient 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 insulation of the tanks, types of heating device, and manner of heating. The amount of gas produced under thermophilic conditions is at 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 continued 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-
I X D U S T R I A L A S D ENGINEERING CHEMISTRY
January, 1931
quent digestion processes, but would probably be an advantage. This heating is an engineering problem and requires further study. The dewatering of sludge, which is preferably discharged continuously, could best be accomplished by a mechanical filter. Even if sludge is discharged several times a dav, 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.
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For larger plants fresh solids would be charged continuously, possibly preheated, kept in the tanks for probably24 hours, dewatered on mechanical filters, and carted away to dumping grounds or sold. Literature Cited Heukelekian, S e ~ , a g eWorks J., 2, 219 (1930). (2) Heukelekian, Ibid., in press. (3) Rudolfs and Heukelekian, IND. ENG. CHEM.,22, 96 (1930).
A Study of Tannery Effluent I-Effect of Various Gases upon the Nitrogen Distribution' Edwin R. Theis and Philip Kratz2 DEPARTMENT OF CHEMICAL ENGINEERIXG, LEHICHUNIVERSITY, BETHLEHEM, PA
T
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/mr
[h Hours)
'0
3
I
25
50
75
101
NS
/so
/,5
Showm Pressure o b t e , m d in fhe S y fern &,y/uent-NoNOJJunder
v'vymg p x
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 hydrogen sulfide evolved could be collected and determined quantitatively. The gases were bubbled through the effluent for 7 days, at 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
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\
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 quantities 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. I n all the cases tried soak-water effluent produced quantities of gas under acid environment but not in alkaline solution.
Experimental Procedure
It vas decided to determine just how much gas was produced when the hydrogen-ion concentration was varied. The gas produced was measured by determining the pressure in small bottles fitted with manometer tubes. I n 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 1000p. p. m., the manometers 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. I n the activated-sludge process of sewage treatment, activation is brought about through the Presented before the Division of Leather 1 Received October 27, 1930. and Gelatin Chemistry a t the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September S to 12, 1930. 2 Hunt-Rankin Leather Company research fellow, Lehigh University.
Figure 2-Apparatus for Determining Effect of Various Gases on Soak-Water
When gases were passed through the effluent, it was found that oxygen caused ammonia to be produced and evolved during i; passage through the sys(em. Upon the other hand, oxygen produced no hydrogen sulfide beyond mere traces. Hydrogen used in place Of air or oxygen caused the
