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(41) Mansfield. G. R., U. S. Geol. Survey, Professional Paper 152, 210-13 (1927). (42) Marshall, H. L., Jacob, K. D., and Reynolds, D. S., IND.ENG. CHEM..24, 86-9 (1932). (43) Matuon. G. C., U. S. Geol. Survey, Bull. 604 (1915). (44) McHargue, J . S.,J . Agr. Research, 30, 193-6 (1925). (45) Millar. C. C. H., “Florida, South Carolina and Canadian Phosphates,” p. 135, Eden Fisher 8. Co , London, 1892. (46) Moses, 0. A., U. S. Geol. Survey, Mineral Resources of the U.S., 1882, 504-21 (1883). (47) ParTIee, J. T., U. S. Geol. Survey, Bull. 640-K,195-228 (1917). (48) Pike, R. D., IND.EKG.CHEM.,22, 344-9 (1930). (49) Reynolds, D. S., and Jacob, K. D., Ibid., Anal. Ed., 3, 366-70 (1931).
(50) Reynolds. D. S., Jacob, K. D., and Hill, W. L., IXD.EKG. CHEM.,21, 1253-6 (1929).
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(51) Robertson, G. S., J . Agr. Sci., 8, 16-25 (1916). (52) Robinson, W. O., U. S. Dept. Agr., Bull. 122 (1914). (53) Rohinson, W. O., and Holmes, R . S., U. S. Dept. Agr., Bull. 1311 (1924). (54) Rogers, G. S., U. S. Geol. Survey, Bull. 580, 183-220 (1915). (55) Schreiner, O., J. Assoc. Oficial Agr. Chem., 12. 16-30 (1929). (56) Shead, A. C., Oklahoma Univ., Bull. 271, 97-102 (1923). (57) Shead, A. C., Chem. Age (New York), 31, 319-20 (1923). (58) Willard, H. H., and Greathouse, L. H., J. Am. Chem. SOC.,39, 2366-77 (1917). (59) Yoe, J. H.. “Photometric Chemical Analysis,” Vol. I, p. 182. Wiley. 1928. (60) Yoe. J. H., Ibid., p. 397. (61) Zucchri, G., Gam. chim ital., 43, 11, 398-403 (1913).
RECEIVED June 2, 1932.
Aerobic and Anaerobic Decomposition of Sewage Solids I. Changes during Decomposition Processes WILLEMRUDOLFS AND H. HEUKELEKIAN, New Jersey Agricultural Experiment Station, New Brunswick, N. J. The course of destruction of concentrated sewfitted with porous plates and HE course of destruction age solids has been &died aerobically and anconnected with an air blower. of the different complex The air was blown through in groups of volatile conaerobically. The progress of decomposition of a steady s t r e a m to k e e p t h e stituents in sewage sludge under anaerobic conditions has been vO1atile protein* fats, and B. reducmaterial in suspension. Before tion is strikingly similar under both conditions. taking a sample for a n a l y s i s , studied in considerable detail. With unseeded material the rate of destruction the loss of volume due to evapoAlthough con s i d e r a b 1 e work is greafe,yf under a;’robic conditiom, and with r a t i o n w a s m a d e up by the has beendone on the “wet comaddition of water, and a defibustion” of sewage s o l i d s in properly seeded material, greatest under anaerobic nite volume was taken out for the production of a c t i v a t e d analysis after thorough shaking. conditions* The rate sf nitrogen reduction cOrresludge, no studies have been This p r o c e d u r e was followed reported on the destruction ana sponds to fhe rate of f a t s reduction. Both under aerobic and anaerobic conditions wrotein reduca t each s a m p l i n g period, the stabilization of c o n c e n t r a t e d settled s e w a g e s l u d g e under tion takes place. Under airobic‘ conditions a volume takenfor analysis being deducted from the total volume continuous aerobic conditions. reduction in ash content is observed. before makine UD for the loss A studv of t h e c o u r s e of dr; due to evaporation. stmctio“n a n d s t a b iliza t i o n The following determinations were made: ash, pH values, of concentrated sewage sludge would conceivably coiitribute to an insight and better conception of the mechanism Kjeldahl nitrogen, ammonia nitrogen, nitrate and nitrite of the decomposition processes, and a t the same time show nitrogen, fats, and B. 0. D. Simultaneously, ripe sludge and fresh solids were also subthe fundamental differences, if any, between the anaerobic and aerobic processes. In the second place, the possibility jected to anaerobic decomposition, and similar analyses were of utilizing aerobic instead of anaerobic decomposition proc- made a t different intervals. esses is a matter of considerable practical interest. If the RESULTS OF AKALYSES few statements found in the literature, to the effect that The results of analyses made a t beginning and end of the destruction of organic matter under abrobic conditions proceeds to a greater degree and a t a faster rate, were correct, experiments are given in Table I. Volatile-matter reduction the possibility of utilizing aerobic instead of anaerobic was twice as great with continuous aeration as under anaeroprocesses would be of considerable economic and practical bic conditions. The sludge subjected to aeration was very stable after 92 days, showing a total 5-day B. 0. D. of 550 importance. The results reported in this paper are a portion of all those p. p. m., or a reduction of 96 per cent; the same material subwhich were ohtained, and concern themselves mainly with the jected to anaerobic conditions was nearly as unstable as a t the changes observed during the decomposition processes. Re- beginning. The loss of volatile matter from the material sults on previously anaerobically stabilized material, oxygen kept under aerobic conditions was principally due to carrequirements, nitrogen and ash losses, effects of partial bonaceous substances (which constitute the larger part of the anaarobic decomposition, etc., will be presented later in more volatile matter) as indicated by the greater ether-soluble material, and also due to losses in total nitrogen. While all detail. the ammonia nitrogen originally present in the material disTESTSO N SLUDGE appeared from the aerated mixture, an increase occurred in the Definite quantities of ripe sludge and fresh solids were in- material kept under anaerobic conditions. In connection troduced into glass cylinders 2 inches (5 cm.) in diameter and with the greater destruction of volatile substances under 3 feet (0.9 meter) long. The bottoms of the cylinders were aerobic conditions, attention is drawn to the decrease in ash
T
D m
Y
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IN DUSTRIAL AN D EN GINEERIN G CHEM ISTR Y
November, 1932
content of the material. No decrease occurred under anaerobic conditions. Repetition of the experiments with different materials showed the same phenomenon. TABLEI. ANALYSESOF SLUDQES AT BEGINNINQ AND ENDOF EXPERIMENTS (Digestion period, 92 days) TILE
NHs NI-
TER
PH Q E N
VOLA-
TOTAL SOLIDSA8"I
%
%
MAT-
%
TRO-
To-
ETHER-
TAL
NITROQEN"
PROTEllb
P P. m. %
%
Fresh solids (beginning) 3 . 9 8 2 6 . 5 2 . 9 3 5 . 7 500 3 . 6 8 1 6 . 4 5 Aerobic (end) 1 . 5 8 4 0 . 5 0 . 9 4 7 . 1 0 4.38 27.4 Reduction 6 0 . 4 ... 52.8 34.0 38.5 68.0 Anaerobic (end) 2.95 34.3 1 . 9 4 7 . 4 750 5 . 0 5 1 5 . 0 Reduction 2 6 . 0 0.0 33.8 0.0 2 9 . 8 a On dry snlids basis. b Total nitrogen, minus ammonia nitrogen, times 6.25.
S0L.nMLE
5DAY
(FATs!B.O.D. 'r0 P.P. m.
...
24.72 8.47 87.0
12,600 550 96.0
... ...
14.24 57.5
10,900 13.5
For a comparison of the course of destruction of the different components of the mixtures the results obtained are shown graphically in Figure 1. Since under anaerobic conditions no loss of total nitrogen or ash occurred, these constituents could not be compared. Instead, the reduction in total nitrogen and ash under aerobic conditions is plotted to bring out the striking similarity. The difference in the rate of volatile-matter reduction under aerobic conditions, as compared with anaerobic, became increasingly greater during the course of digestion. A similar relation is observed in fats reduction, whereas the greater reduction in B. 0. D. of the aerated material began after only 22 days of incubation with thereafter rapidly increasing differences. Protein reduction progressed rapidly in the aerated material to a high peak after 35 days, followed by an identical decrease in the rate of protein destruction. These plotted results are relative, showing the percentages of protein reduced a t any time on the basis of a unit quantity of volatile solids. The rate of protein destruction increased again rapidly after about the sixty-ninth day. While the total quantity of nitrogen gradually decreased, the rate of protein destruction varied. With the increase in the rate of protein destruction, nitrites and nitrates increased, the latter reaching a maximum after the greatest rate of protein reduction had occurred. After the thirty-fifth day the ammonia-nitrogen content of the mixture decreased gradually until, after 92 days, all had disappeared. TABLE 11. CONSTITUENTS OF QLUDQE AT DIFFERENT TIMES Days
-AEROBIC-
-A4NhEROBIC-
Fats Protein Residue pH
%
%
%
33.9 31.0 20.4 30.5 31.3 31.9 25.1 12.1 12.9 13.8
20.5 21.0 22.4 24.5 21.4 25.0 27.9 45.0 48.0 45.8
45.6 48.0 48.2 45.0 47.3 43.1 47.0 42.9 39.1 40.4
5.7 6.6 7.8 8.0 7.4 7.9 8.6 s.9
S.i 7.1
F a t s Protein Residue pH
%
%
%
33.9 31.6 28.3 25.7 31.5 31.3 28.9 24.2 19.1 21.6
20.5 22.3 24.8 19.6 19.5 19.9 24.8 26.0 24.8 23.7
45.6 46.6 46.9 54.7 49.0 48.8 46.3 49.8 56.1 54.7
6.7 5.4 6.3 6.8 6.8 6.9 7.4 7.4 7.3 7.4
The mixture kept under anaerobic conditions showed similar increases and decreases in the rate of protein reduction. No nitrogen was lost, but the quantities of ammonia nitrogen produced varied with the rates of protein reduction. The rate of reduction of ether-soluble materials (fats) was persistently greater under aerobic than under anaerobic conditions. After 35 days the rate of fat destruction was 26 per cent greater under aerobic conditions and was 58 per cent higher after 56 days, remaining fairly constant thereafter, between 55 and 60 per cent. Fluctuations in B. 0. D. reduction show that during the first I1 days the biochemical oxygen demand increased in both the aerated and nonaerated mixtures, indicating the produc-
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tion of less stable substances. Thereafter the stabilization of the aerated solids increased, with minor fluctuations. The rate of stabilization of the solids receiving no air was slower and fluctuated in a more pronounced way.
DISCUSSION OF RESULTS Examination of the curves in Figure 1 brings out the striking similarity in the progress of volatile matter, protein, fats, and B. 0. D. reduction under both anaerobic and aerobic conditions. Although the organisms active in the oxidation and reduction processes are different, producing different intermediate and end products, the cycles or succession of the
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FIGURE1. COURSEOF DESTRUCTION OF COMPONENTS OF SLUDGE groups of substances destroyed are the same. There was, however, a difference in the rate of destruction and stabilization. The two distinct successive stages in the rate of volatile-matter destruction are more clearly marked under aerobic conditions. A comparison of the rates of destruction of proteins and fats both under aerobic and anaerobic conditions show that, when protein reduction was greatest, fats reduction was lowest and vice versa. This is more clearly shown when the percentages of sludge constituents present in the mixtures a t any time are compared (Figure 2 ) . If the percentages of protein or fat decrease, the rates of destruction should increase; when the percentages of protein and fat increase, the rates of destruction should decrease; a straight line would indicate that the different components are destroyed a t an even rate. The curves for volatile matter, fats, nitrogen, and ash reduction show increased and constant rates; the protein reduction curves show also decreased rates of destruction. There are stages which can be clearly distinguished. The stages are indicated by increased, constant, and decreased rates following each other. These stages of digestion in relation to time correspond for the different constituents. During the first part of anaerobic digestion the rate of fat destruction was greater than the rate of protein destruction; toward the end of this stage (from the twenty-eighth to
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the fifty-eighth day) it was reversed. This repeated itself during the second stage of digestion. Changes in pH values (Table 11) correspond to these changes and stages. Carbonaceous materials in the volatile matter, other than fats and proteins (designated as residue in the table), were destroyed a t a fairly even rate with fluctuations but with gradually and slightlydecreaiing rate. T h e comparatively even rate of fat and protein destiuction during the first 40 days of aeration is of considerable interest since it i n d i c a t e s t h a t during the first stage of stabilization (the so-called c a r b o n a ceous stage) the rate of p r o t e i n reductions was nearly the same as the rate of fat reduction, whereas the rate of destruction of t h e remaining material was nearly constant. During that part of the s e c o n d s t a g e when fat reduction was rapid, the rate of protein destruction decreased DAY3 g r e a t l y a n d inFIGURE2. PERCENTAGES OF SLUDGE c r e a s e d only after CONSTITUEKTS PRESENT AT ANYTIME fat destruction was nearly c o m p l e t e . It should be remembered that this rapid fat deqtruction occurred during the most rapid nitrification processes. Since microbrganisms can obtain the required nitrogen for protein assimilation both from ammonia nitrogen and nitrate nitrogen, it would appear that they obtain it first from the ammonia nitrogen present and during the later stages from the nitrates. The succession of processes occurring in the sludge during aeration may conceivably take place in sludge banks deposited in rivers. Calculations made from sludge banks (6) tended to show that the rates of destruction of nitrogenous and carbonaceous materials were the same under these conditions. The laboratory results indicate that stabilization of nitrogenous and carbonaceous substances in sludge deFosited on the bottom of rivers, being in contact with oxygen from the overlying water, may occur a t the same rate until actual nitrification takes place, while thereafter carbonaceous substances (fats and others) are stabilized a t a greater rate than nitrogenous substances. The two successive stages in the stabilization processes are reflected in the B. 0. I). curves. As might be expected, the 13.0. D. increases when the rate of carbonaceous destruction increases. Since carbonaceous material is the most abupdant ingredient the fluctuations in B. 0. D. reduction follow more closely the changes in the rate of destruction of carbonaceous materials. The sludge subjected to anaerobic conditions was not well digested a t the end of the period given. It requires considerable time before a balanced flora is established when frgsh solids are incubated under anaerobic conditions. When the material is properly inoculated, the digestion time is reduced to one-sixth or less. The reduction in total ash content of the aerobic cultures was unexpected. Repetition of the experiments with sludge
Vol. 24, No. 11
digested previously under anaerobic conditions showed the same relative results. Discussion of these results will be offered subsequently. It is of interest to note that the reduction in ash corresponded to the degree of destruction of fats. When the rate of destruction of fats was slow, the rate of ash reduction decreased; when no reduction in fats took place, no reduction in ash occurred. When the rate of protein destruction increased, the ash content remained constant. Although the percentage of ash in the aerated material increased in relation to the volatile matter from 26.5 to 40.5 per cent, the actual decrease in ash per given quantity of solids was 38.5 per cent. In comparison, the volatile-matter reduction amounted to 68.0 per cent, and the principal constituent, fat (which was originally 34 per cent of the volatile matt.er), was reduced by 87 per cent. The loss in ash was only 20 per cent as compared with the loss of total volatile solids, and 47 per cent as compared with the loss in fats. The similarity of the rates of destruction of carbonaceous and nitrogenous matter and the apparent interrelation of the differentphenomena observed, both under aerobic and anaerobic conditions, appears to be of distinct and of general interest in connection with the manner or mechanism of volatile matter destruction under conditions which are assumed to be of opposite character. It is assumed that the nat,ure or character of the volatile matter determines the degree of destruction, whereas the types of organisms determine the mode of destruction, and environmental factors, such as air, temperature, reactions, etc., determine the rate of destruction. The environmental factors, in turn, determine the type of organisms so that these factors also affect indirectly the mode of destruction. If the manner of destruction of both aerobic and anaerobic organisms is similar, the only difference would be the production of different intermediate and end products. Since the degree of destruction is determined by the character of the volatile matter, and the rate of destruction becomes exceedingly slow after the more easily decomposaLle materials have been destroyed, it may be possible that rearrangement of the molecular complexes in the resistant material occurs more rapidly under aerobic than under anaerobic conditions. Tenney and Waksman (5) studying decomposition of four plant materials-namely, corn stalks, rye straw, oak leaves, and alfalfa plants-conclude from their work that the rapidity (rate) of decomposition under aerobic conditions is greater than under anaerobic conditions. These conclusions seem to check the present work, provided the materials incubated under anaerobic conditions are not properly seeded. To destroy 68 per cent volatile matter under aerobic conditions required 92 days, whereas a t that time only 33.8 per cent volatile matter was destroyed of the unseeded material kept under anaerobic conditions. Experiments conducted with properly seeded materials, using large quantities of inoculum, showed the reverse. These results agreed with other laboratory and plant results which show that, under proper conditions, from 60 to 70 per cent volatile matter is destroyed in 30 to 60 days. Tenney and Waksman conclude further that “although certain initial processes of decomposition may well go on under. anaerobic conditions, as in sewage tanks, for the complete disintegration of organic residues, proper aeration is required.” The present authors have been unable to find in the literature evidence which shows how much time is required for “complete,’ diyititegration of organic residues under aProbic conditions. From work carried on here it is known that organic residues kept under anaerobic conditions are not completely destroyed after several years of incubation. Thus far it has been found that those plant residues which are capable. of being destroyed within a comparatively short time are, with proper seeding, more rapidly destroyed under anaerobic conditione than when the same material, properly seeded.
IImx :IMI fniinrt olihiiiicd iiiidcr zu+obic widition*, i s to aerol.i
