INDUSTRIAL AND ENGINEERING CHEMISTRY
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of S o l u b l e Arsenic Formed f r o m PbHAaOr C. P. ~ M ~ C O S . M O ( O H ) ~ . ~ a fHt e~rO4-Day Digestion
Table X-Amount
BASICCARBONATE/2 GRAYSPhHAsO&/LITER
Mg. Blank (no carbonate) 21.3 42.2 84.4 168.8 337.6
WATER-SOLUBLE ARSENIC/LITER MK. 1.35 12.60 23.00 41.30 6 5 , 50 63.00
Literature Cited (1) Assocn., Official Agr. Chem., Mrthods, p. 50 (1925). (2) Ginsburg, J. M., J . Econ. Enlomol., I O , 625 (1927).
Vol. 23, No. 10
(3) Goodwin, w,,and Martin, H., J . Agr. Sci., 18,460 (1928). (4) Haywood, J. K.,and McDonnelI, C. C., U.S. Dept. Agr., Bur. Chem., Bull. i s 1 (1910). (6) Meulen, P. A. van der, and Leeuwen, E. R. van, J. Agr. Research. SO, 313-21 (1927). ( 6 ) Mogendorff, N., N. J. Agr. Expt. Sta., Bull. 419 (1925). (7) Patten, A. J., and O'Meara. P., Mich. Agr. Expt. Sta., Quarterly Bull. I, No. 2, 83 (1919). (8) Robinson, R. H., J . Econ. Enlonol., 12, 429 (1919). (9) Spoehr, H. A., "Photosynthesis," p. 41, Am. Chem. SOC.Monograph, Chemical Catalog. 1926. (10) Sweeter, L. R . , and Kokoski, F.J., unpublished data. (11) Swingle, H. S., J. Agr. Research, 89, 393 (1929).
Acid Sludge Digestion" E. L. Pearson and A. M. Buswell ILLINOIS STATEWATERSURVEY, U R B A N AILL. ,
CID conditions have frequently been reported as one of the difficulties in the operation of plants designed for the stabilization of sewage sludge by ana6robic digestion. The precise cause of this anomalous condition is not completely understood and previous observations have been made under conditions where quantitative data on the course of the reaction could not be obtained. In a series of experiments carried out in this laboratory on sludge digestion, one case of acid digestion was encountered under conditions such that quantitative data could be obtained. The data are reported herewith. Since the preceding and succeeding experiments in this series followed the normal course of digestion in the same apparatus and under similar chemical conditions, the authors do not believe that the acid condition reported here was due to chemical poisons, hut rather to the chance development of the wrong bacterial flora. Two methods of measuring the extent of sludge digestion are commonly used, and both have been shown to be subject to appreciable error. One, considering the gas produced as a direct measure of the volatile matter destroyed, has been discredited by the work of Buswell and h'eave ( 2 ) who have shown that some constituents of sewage grease, a major source of gas, can produce gas equivalent in weight to as high as 150 per cent of the weight of the material digested. The other, the direct determination of the volatile matter entering and leaving the digestion system, is subject to analytical and sampling errors. The chief analytical error is the loss of substances (ammonium compounds, volatile acids, and carbon dioxide) volatile a t the temperature of the determination of total solids. This error can be overcome, in part a t least, by separate determination of the volatile compounds, which may then be added to the results of the determined solids. The ignition temperature must also be taken into consideration in interpreting the results obtained. The writers (5) in a previous study found that some grit and solids, such as fruit and vegetable seeds, which might pass through a grit chamber would be held in suspension by the more gelatinous fresh sludge and, therefore, included in samples of the material fed, but would settle so rapidly frcm digested sludge that representative samples could not be obtained from large volumes by ordinary sampling methods. The errors in both methods are in a direction which would indicate a greater apparent reduction of volatile matter than would actually be obtained. The second method is the more direct and the errors in it would seem to be more easy to correct.
A
Presented before the Division of Water, 1 Received May 11, 1931. Sewage, and Sanitation Chemistry at the S l s t Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931. t This work was carried out with the assistance of funds furnished by the Chemical Foundation.
Experiment on Acid Sludge Digestion
The present experiment was started in an effort to minimize sampling errors in handling small amounts of material and still retain some semblance of operating conditions. A 28-liter galvanized-iron tank, equipped for gas collection and liquor circulation over the scum and through the sludge a t the bottom, was used for a digestion tank. This was not seeded, but was charged with 700 cc. of fresh solids from a nidus tank, and raw sewage was added to fill the remainder of the tank volume. Daily additions of 700 cc. of fresh solids were made thereafter, and the displaced liquor saved for analysis. Solids, alkalinity, ammonia nitrogen, and pH were determined daily on both the fresh solids and the overflow liquor. Volatile acids were determined from time to time and ash, grease, total organic nitrogen, and crude fiber were determined on composited samples. The temperature of the vessel was maintained a t 27" C. by thermostatic control. Later on when foaming became persistent, a float in the feeding pipe was connected to a relay to start the liquor circulator automatically when the foam reached a certain level. In previous unseeded experiments (4) involving a similar procedure, normal digestion developed in about 25 days. I n this particular experiment normal digestion failed to develop a t all. The pH had gradually fallen from 7.0 to 6.7 and continued t o decrease. Foaming had become persistent, although it was not accompanied by increased gas production. The volatile acids had increased to 800 p. p. m. At the end of 60 days the pH had reached 6.4 and the volatile acids 2000 p. p. m. Gas production up t o this time had averaged less than a liter per day. Lime was added a t this time t o bring the pH to 7.6 and the contents circulated through both the top and the bottom connections to obtain proper mixing. NO change could be detected up to the seventieth day when, to insure the presence of a proper inoculum, if conditions had become favorable to the develobment of a normal flora, 500 CC. of actively digesting sludge were added. After 80 days gas production had not increased, the pH had decreased to 7.4 and the volatile acids had increased to 2500 p. p. m. The contents of the tank were then sampled and analyzed. The material had a decidedly offensive odor and was very colloidal. The sludge was uniformly distributed throughout the tank. Filtration was so difficult that separate analysis of liquid and solid fractions was not attempted. The experiment was continued after sampling, and on the eighty-seventh day one liter of actively digesting sludge was added. After a total of 130 days, the pH hadreached 6.1 and thevolatile acids 3800p. p. m., and gas production had not increased. A microscopic examination of the material revealed a heterogeneous flora quite similar to that of a normqlly digesting mixture except that micrococci were not nearly as abundant in the acid mixture.
October, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
The cause of the failure of normal digestion is not known. Seeding or liming earlier in the experiment might have been of more benefit, but such precautions have previously not been found necessary for digestion of Champaign-Urbana sewage solids. The character of the fresh solids fed, however, was quite different from that usually obtained from the local sewage. This was undoubtedly due to the fact that, during the first part of the experiment, the flow of a small stream was diverted to the sewer to facilitate the building of bridges. The sewage treated by the nidus tank was obtained from this sewer. As shown in Table I, this mixture of sewage and creek water deposited a sludge quite different from the normal domestic sewage. The first column gives the average composition of the fresh solids over the entire period of this experiment. The second column is the average composition of the solids from previous experiments. The fresh sdids used in this experiment have a somewhat higher ash and crude-fiber content and a lower protein content than the average fresh solids. The greatest difference, however, is in the grease and the undetermined fraction. The grease is abnormally low and the undetermined fraction quite high. The low grease content is of especial interest in view of the numerous cases of acid digestion which have bern caused by a too-high grease content. Beside cellulose, the undetermined fraction (at least that including the sludge from the creek) would probably contain considerable humus-like material.
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16.3 per cent. The ratio of carbon dioxide to methane was only slightly higher than the ratio obtained from alkaline digestion of sludge of the same source. The percentage of nitrogen was high because of the absence of the diluting effect of the larger volumes of carbon dioxide and methane usually produced by normal digestion. of Acid Digestion on Sewage Solids A M o I i N T A M O U N T AMOUNT AMOUNTAMOUNT AMOUNT R E DIP R O - DIRBADDED C O V E R E D G E S T E D D C C R D G E S T E D C O V E R E D Grams Grams Grams Grams % % Total solids“ 1116.4 1091.5 24.9 .... . . . . 101.9b Volatile matter 750 7 32.5 718 2 .... .... .... Azh .... 305 7 373 3 .... . . . . 102.1 Grease 67.3 . . . . 47.2 142 6 75 3 .... Proteinc 149 9 .... 46.1 .... 80 8 69.1 C r u d r fiher 65 7 . . . . 50.5 .... 138 8 70.1 Ammonia .... 2 7 12.1 .... .... 14 8 Acetic acid 96 9 . . . . 71.6 .... .... 25 3 .... 45,s GdS 45 8 Includes ammonium compounds. Table 11-Effect
....
b Incliides gas o
(Total organic N-ammonia
....
....
N X 6.25)
The data in Table I1 give some idea of the gross changes in organic matter subjected to acid digestion. This type of digestion is usually (1, 6 ) considered t o be favorable to the decomposition of grease and unfavorable to protein decomposition. In this experiment, however, although all decomposition was greatly retarded as compared to alkaline digestion, the decomposition of grease, or a t least the initial steps in the Table I-Composition of Solids Fed and Average Champaignbreakdown, seemed to be retarded to a greater extent than Urbana Sewage Solids was the decomposition of proteins. Only 47.2 per cent of the AVERAGE SOLIDS CHAMPAIGU-URBANA grease was decomposed in this experiment as compared with FED SEWAGE SOLIDS c, 90.3 in a previous alkaline-digestion experiment (4), whereas % /O 27 4 24 7 A.;h 46.1 per cent of the protein was decomposed in this experiment Volatile matter 72 6 75 3 and 63.5 per cent in the same previous experiment. Grease 17 8 32 5 Protein 14 3 18 0 In such comparisons, however, the limitations of the Crude fiber 14 S 10 8 Undetermined 25.7 14 0 analytical methods employed must be kept in mind. AlThat the adjustment of the pH failed to bring on normal though the methods are identical, the results may not be digestion indicates that the lime treatment is not a certain comparable. The protein calculation (total N-ammonia N remedy for sour tanks, although it is undoubtedly of aid in x 6.25), for example, assumes that the organic nitrogen is associated wholly with protein. The analysis of the material many cases. fed and that recovered indicate that about half of this proNature of Process from Data Obtained tein fraction has been decomposed. As to the degree of deIncomplete recovery of ash in previous studies ( 3 ) was at- composition, the test indicates that deamination has octributed to loss through sampling error. Although the diges- curred. Reduction of amino acids t o free ammonia and a tion processes were not similar, the data in Table I1 substan- saturated fatty acid is the accepted reaction for the normal tiate this view. The analytical methods were identical and anaerobic decomposition ( 2 ) . However, the ammonia, when therefore not a t fault. The ash recovered checks the amount determined by distillation, includes amines, and in acid media fed to within 2 percent, whereas previous experiments indicated it is known ( 2 ) that amino acids may be decarboxylated to a 16 to 20 per cent loss of ash. Aside from the sampling error, amines. The protein fraction, therefore, may have been dethe chief error in the solids and ash determination is the loss of aminated or decarboxylated or both. This type of analysis volatile compounds in evaporating the sample for the total does not indicate which reaction occurred or how much further solids determination. These were partially corrected by sepa- the breakdown was carried. Similarly, about one-half of the rate determination of the ammonia which was added to the crude fiber and about one-half of the grease were not recoversolids as acetate and carbonate on the basis of the volatile acid able as such at the end of the digestion. A gain in the volaand alkalinity determinations. The dissolved carbon dioxide tile acids and gas, however, indicates that these substances was not determined separately, and was included in the were end products of some of the reactions. Data on the gross changes of the organic matter undergoing volatile-acid determination. Although the equivalent weight decomposition, nevertheless, give some idea of the nature of of carbon dioxide was considerably lower than that of acetic acid, the amount was small in this material in comparison the process, and are of value as a preliminary to more detailed with the acid, furthermore this gain was partly compensated studies. Studies of acid-sludge digestion are of practical for by the small fraction of acids higher than acetic which here value only in so far as they may lead to methods for preventwere calculated as acetic. For greater accuracy, however, ing or remedying this type of digestion. these should be determined separately. Most of the obserLiterature Cited vations made in this experiment conform to those previously made by others (1, 5 ) on the same type of digestion. The (1) Buswell, “Chemistry of Water and Sewage Treatment,” p. 264, Am. sludge was light in color, odorous, and very gelatinous. Chem. SOCMonograph Series No 38, Chemical Catalog, 1928. Foaming was persistent, but comparatively slow in rising, and (2) Buswell and Neave, State Water Survey, Bull. SO (1930). and Pearson, “Further Studies on Rapid-Stage Sludge Digesnot accompanied by rapid gas production. The composition (3) Buswell tion,” Sewage Works J., 3, No. 2 (April, 1931). of the gas was as follows: carbon dioxide, 28.0 per cent; hy- (4) Illinois State Water Survey, Bull. 29 (1930). drogen, 0.9 per cent; methane, 54.8 per cent; and nitrogen, (5) Neave and Buswell, INO. ENG C H E M . , 19, 1012 (1927).
