Stabilization of Sewage Sludge Banks WILLEM RUDOLFS New Jersey Agricultural Experiment Station, New Brunswick, N. J.
had a B. 0. D. after 100 days comparable to an average domestic sewage, and after 400 days similar t o many river waters. Although old sludge has a definite influence on the water, it is small. Activated sludge followed a course similar t o raw sewage solids but shorter in duration. A two-stage B. 0. D. reduction curve is indicated, probably because of the type and quantity of organic matter present. A sludge with a high B. 0. D. apparently requires relatively less time to reach a low level than a sludge with a lower B. 0. D., but the effect on the stream will be more intense because of the high original B.0. D. and the greater rate of reduction.
Raw sewage and activated sludges were stored to determine the rate and degree of stabilization in a n effort to determine the degree and duration of the effect of sludge banks on the overlying water. The results when the raw sewage sludge is used alone or when the supernatant liquor is periodically replaced show that B. 0. D. reduction is comparatively rapid with a resulting residue requiring comparatively small quantities of oxygen and exerting little effect on the stream. Although the sludge required about 200 days to reach a level of 500 p. p. m. B. 0. D., which after 400 days had not been reduced below 400 p. p. m., the supernatant liquor
S
EWAGE solids, which settle to the bottom of a river and form deposits of varying thickness and putrescibility affect the nuisance and oxygen content of the water. Near places where raw sewage solids are discharged, these sludge banks may remain for considerable periods in place and are moved only wholly or in part during freshets. Studies of the relation between the biochemical oxygen demand and the volatile solids of sludge deposits in the Connecticut River (2) have shown that the B. 0. D. values of the sludge decreased rapidly during the first few miles below cities and more s1o~vlythereafter, indicating that the remaining residue was more resistant to decomposition. Mohlman, Herrick, and Swope (1) state that the Engineering Board of Review found that the effect of sludge deposits on the oxygen content of the Illinois River was of greater importance than the effect of the current discharge of sewage, practically all untreated. The effect is particularly great when the mass is stirred or gas-lifted. Streeter (4), discussing the effect of sewage discharged in streams, postulates that the organic matter settling a t the bottom of flowing streams “is oxidized gradually a t the usual biochemical rate, where it lies sufficiently close to the surface of the sludge deposit to utilize the dissolved oxygen content of the overlying stream water. In the lower strata of such a deposit the material does not appear to be oxidized biochemically to any large extent, although a portion of it probably decomposes anaerobically and forms soluble end products which impose a chemical demand on the overlying stream.’’ It might be expected that under the latter conditions the sludge would remain a considerable period in a highly putrescent state and be very slowly stabilized. The effect of discharge of large volumes of sludge in rivers and harbors is presumably of sufficient practical importance to be taken into consideration when sewage treatment is
considered. Schroepfer and Childs (3) calculated that the effect of sedimentation in pools created by dams accounted for a reduction in oxygen demand varying from 10 to 40 per cent during the summer months, depending upon the stream discharge. The amount of oxygen required for the oxidation of the sludge deposits in a pool of the Twin City Lock and Dam averaged 50,000 pounds per day during the summer months; during the winter the sludge apparently had only a slight effect upon the oxygen balance. The purpose of the experiments under discussion was to determine the rate of stabilization of sludge when stored as compared with similar stored sludge from which the supernatant liquor was decanted and replaced by fresh water. The results should also give an indication of the degree and duration of the effect of sludge banks on the overlying water. Frequently it is assumed that sludge banks deposited around bends of streams or behind dams exert an appreciable effect on the stream for many years and are the source of nuisances for a considerable time.
Methods and Material Two types of sludges were used in the experiments-namely, raw sewage solids and sludge produced by the activated sludge process. The raw thick sewage sludge was comminuted by putting it through a household garbage grinder. The sludge pia8 then passed through a coarse screen of 0.25-inch squares and diluted with raw sewage. For the first 3 days there was a rapid evolution of gas, which caused the sludge t o rise to the surface. After 1 week this initial fermentation had decreased and the sludge was placed in carboys, each approximately two-thirds full. One portion of the sludge was stored in an open carboy, and when necessary small quantities of water were added t o compensate for evaporation. The supernatant liquor of sludge stored in another carboy was replaced at intervals with tap water amounting t o about one-third of the total volume with each displacement. 337
338
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 30, NO. 3
and less rapid with the progress of stabilization, until the B. 0. D. per gram of volatile matter became rather constant. In other words, the easily d e c o m p o s a b l e matter was destroyed, leaving a 0.6 rather resistant volatile substance behind as residue. This residue had a B. 0. D. of 0.1 gram per gram of volatile matter or roughly about one-fifth 0.5 of the original fresh solids. The fact that the original sludge had a total solids content of about 1.8 per cent (1.33 per cent volatile) may have been a factor in the relatively rapid stabilization of the ?*! 0.4 sludge. This sludge compacted in the carboys but 4 was periodically mixed with the supernatant liquor. 2” The changes in B. 0. D. of the liquor discharged I 0 0.3 are shown in Figure 2. After about 100 days this liquor had B. 0. D. comparable to an average i domestic sewage. As expected, the B. 0. D. of the 4 0 volatile matter in the liquor was about three times 0.2 m greater per unit than the B. 0. D. of the volatile matter in the sludge. Of interest is the rapid B A! change in the rate of B. 0. D. reduction per unit 0.1 of volatile matter. Gradually the B. 0. D. of the finely divided and soluble volatile matter in the liquor became the same as the B. 0. D. of the volatile matter in the sludge. When the stabilization processes were continued, the B. 0. D. of the superDays natant material became very low, about the same FIGURE1. PER CENTB. 0. D. REDUCTION AND REDUCTION PER GRAM as found in many stream waters. It is clear, thereOF VOLATILE SOLIDSOF SLUDGEOPENTO THE ATMOSPHERE AND WITH LIQUOR &PLACED fore, that the putrescible matter in the sludge disintegrated. liauefied. and Dassed into the overThe activated sludge obtained from two different plants was lying water i i proportion to the guaitity originally present. stored in open carboys under semianaerobic conditions. This means that a mass of sludge deposited in the stream h d Y s e s of the mixtures and liquid discharged were made at affectsthe B. 0.D. of the waterpassing Over it to the greatest intervals and included B. 0. D.. total solids, ash, and pH. extent during the first weeks, or, when sludge is deposited continuously, that the effect of a given quantity of sewage Fresh Solids solids is rather constant on the river. However, the rate of A part of the results obtained over a period of 400 days is decomposition is greatly influenced by the temperature of the water, and sludge deposited during cold weather would shown in Table I. The B. 0. D. of the sludge kept in an have considerably less effect on the water ,than when the open carboy was reduced from over 7000 to less than 1500 water temperature is higher. The result is that the accumup. p. m. in 130 days and to slightly over 500 p. p. m. in 200 lated solids have the greatest effect when the weather is warm days. Thereafter reduction was very slow. The material, and the stream flow low. Experience has shown this to be with the liquid periodically displaced, was reduced more rapidly during the first 100 days but did not reach an apprecithe case. In river and harbor waters, which receive sewage solid ably lower level after 400 days than the sludge which was continuously and where only small quantities of the sludge not decanted. are removed by freshets or tides, the residue exerts a continuous influence. The magnitude of this effect after one year would be only a small TABLE I. B. 0. D. REDUCTION AND B. 0. D. PER GRAM OB VOLATILE fraction of the effect of the freshly deposited MATTER PRESENT material. Even if sludge banks deposited in a -B . 0.D .-. . B , 0. D.---. o..D. of Decanted Sludge Per gram Of Liquid D$$:;gd in Open Carboy Per gram previous year were completely stirred up, the yo volatile volatile volatile whole effect could not be more than 10 to 15 per Days P. p. m. reduced matter P. P. m. reduced matter P. p. m. matter cent of the sewage solids recently deposited. In 0 7060 0.537 7000 0.526 ..., cases where the sewage is removed from the 10 6180 1215 0.470 5550 20:B 0.418 1700 1:51 18 7480 4-5.0 0.445 5860 16.2 0.420 1775 1.28 1.52 water course, it is clear that the river should 38 6200 12.5 0.382 5720 18.5 0 426 730 111 3860 45.2 0.280 2400 65.3 0 240 180 0.51 rapidly improve, but the old sludge will exert a 129 1475 79.0 0.251 1295 81.5 0.210 115 0.27 small but definite influence on the condition of 1075 84.8 0 237 800 86.0 0 187 60 0.23 159 175 955 86.5 0.217 655 89.3 0 180 52 o.17 0.22 the water. This conclusion is in accord with 46 0.180 515 92.6 0 163 520 93.4 209 344 425 94.0 0.110 430 93.4 0 104 10 0.08 observations made in rivers from which sewage 400 3.5 0.04 93.8 0.100 94.6 0 104 40 1 440 is removed or greatly reduced by proper treatment. The relation between B. 0. D. reduction and stabilization of the sludge can be shown by the actual B. 0. D. of The plotting of the percentage B. 0. D. reduction of the the material and the change in pH values during storage (Figtwo sludges (Figure 1) shows that the decreases were parallel. ure 3). During the first few weeks the pH values continued to Early in the decomposition a rapid drop followed by a dedecrease while the B. 0. D. of the material fluctuated greatly, cided increase was noted in the B. 0. D. of the sludge, probbut when a gradual decrease in B. 0. D. took place the pH ably caused by the rapid liquefaction. Calculation of the B. 0. D. per gram of volatile matter present (Figure 1) values remained constant. After the B. 0. D. began to deshowed a rapid decrease a t the beginning, which became less crease rapidly, the p H values of the sludge increased greatly. 9)
i
.
-
MARCH, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
2000
P
0
a 1600
*6
a
b
1200
9 % m
g
800
400
&YE
FIGURE2. B. 0. D. REDUCTION OF SUPERNATANT LIQUOR
339
banks caused by raw sewage solids. In order to determine the rate of B. 0. D. reduction of activated sludge when stored under semianaerobic conditions, two sludges obtained from different plants were placed in open carboys. The B. 0. D. results for a period of 180 days are graphically shown in Figure 4. The rate of B. 0. D. reduction was very rapid during the first 2 to 4 weeks; then followed a brief interval of little or no reduction and again a period of rather rapid reduction which gradually decreased. The plotted results indicate a twostage type of curve similar to those reported for stabilization of sewage in a stream. This two-stage curve corresponds to those presented for fresh sewage solids, except that the extent of the hump or break in the curve is less. It is questionable whether these two-stage curves can be explained in terms similar to those frequently used for B. 0. D. curves of sewage in water. There seems to be no doubt that in the first part of the curves liquefaction greatly overbalances the degree of oxidation, whereas in the second part liquefaction and oxidation become more equal. In reality these two-stage curves
In sludge digestion the pHvalues are indicative of the progress of digestion. I n the case of sludge banks the pH value would indicate the relative stabilization of the material. The curveR presented are for the sludge kept in an open carboy without replacement of supernatant liquor. The results for the decanted material are essentially the same, except that the highest pH value reached was 7.4. The effect of decanting was perhaps greater than is indicated by the curves and figures presented. After 200 days the supernatant liquor of the decanted sludge had become transparent, whereas the liquor of the open bottle was still turbid with a brownish color. The degree of stabilization is further indicated by the results obtained on the supernatant liquor after 400 days: Open Dissolved oxygen, p. p. m. 0 Ammonia, nitrogen, p . p. m. 260 1.6 Nitrate nitrogen, P. P. m. Color Opaque brown ~~
Deoanted 1.a
0.7
0.8
Clear
In the decanted material dissolved oxygen was present, and only small quantities of ammonia remained; in the open carboy no dissolved oxygen was found with a large amount of ammonia. The decanting had apparently removed the soluble nitrogenous substances to such an extent that the amount of nitrates was less than in the material stored in the open bottle.
Days
FIGURE3. RELATION BETWEEN B. 0. D. REDUCTION OF SLUDGE AND PH
Activated Sludge In the activated sludge treatment process most of the putrescible material present in the sewage is removed and partly stabilized. A portion is assimilated by the microbial growth. Under anaerobic conditions the floc decomposes in a manner similar to that of settled sewage solids. It can be expected that activated sludge discharged into a water course and forming sludge banks will behave similarly to the sludge
VALUF,S
indicate that the rate of B. 0. D. reduction varies more in the first part than the second, on account of type and quantity of organic matter present. More information is needed before the results can be expressed by a mathematical equation, The two different sludges, which originally had B. 0. D. values of 4350 and 2860 p. p. m., respectively, followed the same course, and in about 80 days more than two-thirds of the B. 0. D. had been reduced. After 170 days the B. 0. D. of the Tenafly sludge was reduced to 450 p.. p. m., and the Madison-Chatham sludge to 283 p. p. m.; then the
340
INDUSTRIAL AND ENGINEERING CHEMISTRY
B. 0. D. reduction became slow, so that after 390 days the values were 213 and 218, respectively. At this time the liquids in the carboys were a distinctly greenish color, caused by microorganisms.
4000
-Open
Carboy-
-Decanted
Sludge-
PH
volatile matter reduced
%
volatile matter reduced
Days
%
-
I
Tenafly
5 TABLE11. VOLATILE MATTERREDUCTION A N D PH VALUES
I
VOL. 30, NO. 3
_____ Madison-Chatham
1
3000
r3l
E r4
%
2000
Y k
%
.
d c
9 0. m a
1000
Original concentration. I
I
40
I
I
80
120
I
1
160
Days
From the results with activated sludge it appears that the time required to reach a certain level of B. 0. D. is considerably shorter than that for the raw sewage solids. However, this is only apparent, because the original B. 0. D. of the activated sludge was lower. Calculations show that on a volatile matter basis there is a fairly direct relation between the B. 0. D. of the different sludges and the time required for stabilization. The relation does not appear to be directly proportional. For instance, if 120 days were required for a sludge with an original B. 0. D. of 3000 p. p. m. to reach a level of 450 p. p. m., 160 days would be required for a sludge of 4000 p. p. m., and 310 days when the original B. 0. D. amounted to 7000 p.p.m. If this assumption is correct, it indicates that a sludge with a high B. 0. D. will require relatively less time to reach a low level than a sludge with a lower B. 0. D., but that the effect on the stream will be more
FIGURE4. EFFECTOF STORAGE ON B. 0. D. ACTIVATEDSLUDGE
OF
intense on account of the higher original B. 0. D. and the greater rate of reduction.
Literature Cited (1) M o h l m a n , Herrick, and Swope, IND. ENG.CHEM.,23, 209 (1931). (2) Rudolfs, W., Sewage Worka S.,4, 315 (1932). (3) Schroepfer and Childs, Ibid., 3, 693 (1931). (4) Streeter, H., Ibid.,3, 713 (1931). RECEIVED September 15, 1937. Presented before the Division of Water, Sewage, and Sanitation Chemistry a t the 94th Meeting of the American Chemical Society, Rochester, N. Y., September 6 to 10, 1937. Journal Series Paper, Division of Water and Sewage Research, N. J. Agricultural Experiment Station.
Sugar Changes in the Banana during Ripening /y OMPLETE quantitative data on the interrelation of b the sugars the banana ''( sapienturn L* Gros Of
Michel) at various Of ripeness are meager and contradictory. Accurate information of this type is of value in studying commercial ripening practices and in formulating dietaries in which specific and total carbohydrate intake must be controlled. The available data on this subject are summarked in Table I. It would be to accept as representative any sing1e Or average from I for reasons: in (a) The is unknown several variety of the fruit used for analysis cases: ( b ) the methods of analysis used and the expression of results are not comparable; (c) the stage of ripeness of the fruit a t time of analysis is not known with a great enough degree of exactness in most cases; and (d) the method of ripening in each case is probably different. The purpose of the present study was to make an investigation into the quantitativerelations of the sugars of the G~~~ Michel banana, the variety of commerce in the United States, at various stages of ripeness. The ripening treatment which
G. L. POLAND, J. T. MANION, M. W. BRENNER, AND P. L. HARRIS United Fruit Company, New York, N. Y.
the fruit received was standard commercial practice (7'). Previous work (11)had shown that glucose, fructose, sucrose, and maltose were present in ripe banana pulp but that maltose occurred only in small amounts. Furthermore, Widdowson and McCance ( I s ) proved that the amount of maltose in bananas must be insignificant since glucose and fructose determined separately accounted for all of the reducing sugars present.Consequently, for purposes of this study, maltose was disregarded and only glucose, fructose, and sucrose were determined,
Methods PREPARATION OF SAMPLE.Four to six bananas, selected as representative of the required degree of ripeness, were peeled and pulped. Ten-gram samples were weighed into 250-ml. VOlUmetric flasks, 125 ml. of neutral 50 per cent alcohol and 2 grams of calcium carbonate were added, and the mixture was refluxed for one hour on a steam bath. Subsequently the flasks were allowed to stand at room temperature overnight, made up to vol-
