A. M. BUSWELL AND M. LEBOSQUET State Water Survey, University of Illinois, Urbana, Ill.
A
NAEROBIC fermentation under especially
controlled conditions has been recommended for the treatment of heavy wastes such as distillery slop (3, 6-8, IO),milk and whey (1, 11, 14)> packing-house wastes, etc. (2, 4). This process has two limitations, as previous authors have pointed out. Anaerobic fermentation cannot be economically used with wastes containing less than about 1 per cent dry weight of digestible organic matter. When the wastes contain 2 per cent or more, the methane recovered will carry most or all of the cost of treatment. In some cases this limitation of the process can be overcome by suitable changes in plant operation to avoid diluting the wastes with clean water (e. g., cooling water). The second limitation of the process is that, although anaerobic treatment is capable of removing 80 to 90 per cent of the organic matter in the wastes, the remaining liquor is still about ten times as strong as domestic sewage. I n many cases this degree of purification is sufficient-for example, where t h e effluent can be discharged into a sewer or large stream or where five or six volumes of cooling or condenser water are available for dilution. Where complete purification (i. e., t o less than 100 parts per million B. 0. D.) is required, some other method of treatment must be applied. Early in the summer of 1935 the authors were asked to set up and operate a pilot plant to determine whether anaerobic digestion plus some subsequent treatment could be used for complete treatment of distillery wastes. Numerous earlier experiments with various chemical coagulants (not reported) had failed to produce any substantial improvement in the liquid remaining after anaerobic digestion. Since Naylor (15) had shown that similar wastes after septic action could be treated on aerobic bacterial filters and Hoover (9)had successfully treated diluted wastes on trickling filters, this form of secondary treatment was chosen.
Digestion Tanks The major units of the demonstration plant, as shown on Figure 1, consisted of two steel digestion tanks, A and B, and one tank, 3, in which was located a trickling filter. Each of the digestion tanks, A and B, was 9 feet in diameter and 7.5
Pilot plant results confirm laboratory data on the purification of distillery waste by anaerobic fermentation. The effluent from anaerobic fermentation when diluted 1 to 5 with trickling filter effluent can be successfully stabilized on a trickling filter at the rate of 250,000 gallons of the undiluted digestion liquor per acre per day. The high nitrate content in the recirculated filter effluent prevents odor nuisance. The sludge is small in amount and contains the phosphate of the grain as magnesium and calcium ammonium phosphate. 795
FIGURE 1. UNITSOF
THE
DEMONSTRATION PLANT
Complete Treatment of
Distillery Wastes feet deep, with a capacity of 3600 gallons, making a total capacity of 7200 gallons or 960 cubic feet. These tanks were already available and required considerable adaptation in order to serve their purpose. Steam coils were placed in the center of the tanks a t the bottom, and covers, with gas domes 3 feet in diameter and 3 feet high, were welded in place over the tanks. The process consisted essentially of passing the slop through two digestion tanks in series and applying the digested effluent, diluted with a part of the finally purified waste, to a trickling filter. The flow through the digestion tanks was as follows (Figure 2) : The raw thin slop was pumped to the cooling and feed tank, 1. Although provision was made in this tank for cooling the slop to the proper temperature (130" F.), the cooling facilities were seldom used. The heat of the slop could ordinarily be utilized in heating the first tank, A . After a dose had been measured and sampled, it was allowed to Row into the primary digestion tank, A . This tank was heavily loaded, accomplishing the greater part of the digestion and yielding most of the gas generated. From tank A the partially digested slop was forced by succeeding doses t o secondary digestion tank B where a further digestion at a decreasing rate was accomplished. A back-circulation pump, 5, made it possible t o return liquid from secondary tank B t o primary tank A for seeding and dilution. This is an important control measure, which, in conjunction with other control tests, made it possible to place on the primary tank, loadings of the order of ten times those used in common sewage-sludge digestion practices.
IKDUSTRL4L AND ENGINEERING CHEMISTRY
796
A 8 I
PRIMARY DIGESTION TANK SECONDARY DIGESTION TANK COOLING AND FEEDING TANK 2 OVERFLOW STORAGE TANK 3 TRICKLING FILTER 4 TRICKLINGFILTER EFFLUENT 5 BACK-CIRCULATING PUMP 6 FINAL SED. TANK
VOL. 28, NO. 7
DILUTE LIQUID SED. TANK PUMP DIGESTED SLOP FEED RECEIVER ROTARY DISTRIBUTOR FLOAT-CONTROLLED CONSTANT-HEAD ORIFICE FEED 12 TRICKLING-FILTER UNDERDRAIN SYSTEM 13 DRIP TEE 7
8 TRICKLING-FILT.ER
9 IO II
TRICKLING FILTER
_--_- _ - _ _ . ~
STEAM COIL, SAMPLE,+'
COCKS 6 TANK THEW' =--:ie.v>
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MOMETER NOT SHOWN I N EITHER DIGESTER
F E
WATER METER
PUMP
- DIAGRAM OF DEMONSTRATION
PLANT - DIGESTION TANKS &, TRICKLING FILTER-
1
FIQURE2 A 1000-gallon, digested liquid overflow tank, 2, made it possible to place the full 24-hour load on the filter in 8 hours, or during one shift. The trickling filter, 3, which required little or no attendance, was operated at a constant rate over the 24 hours; the necessary equalizing storage was furnished by overflow tank 2.
Trickling Filter Dilution of some sort had to be provided in the design of trickling filter 3 in order to reduce the oxygen demand of the liquid supplied to the filter. If, a t the same time, this dilution could result in a reduction of the odors, the filter could be operated without causing a serious nuisance. Trickling filter 3 was constructed in a circular steel tank. The stone bed was 9 feet in diameter and 7 feet deep. Although the filter was of an experimental nature, its size would be sufficient for a small dairy and could almost be considered a plant-scale demonstration. The effluent from trickling filter 3 was used to dilute the digester effluent fed to filter ll. This idea was used elsewhere in treating a concentrated industrial waste (16). I n order to accomplish the desired result, the effluent from the trickling filter was discharged into a small box, 4, with two outlets. One outlet led to a h a 1 sedimentation tank, 6, the overflow from which was the final effluent The other outlet led to a diluting liquid sedimentation tank, 7. The discharge line from this tank was conducted to the suction of trickling filter pump 8. Also connected to this suction was the digested distillery slop feed line, 9. By this arrangement pump 8 dosed filter 3 a t a rate equal to a rate of raw feed, plus a quantity of trickling filter effluent. For example, when pump 8 was operated a t a rate of 2 gallons per minute and digested distillery slop (2 and 11) was fed a t a rate of 0.5 gallon per minute, the pump automatically made up the difference (1.5 gallons per minute) from diluting liquid sedimentation tank 7 . With the arrangement used, feed 11 could be turned off entirely, in which case the pump would take its entire pumpage from diluting liquid sedimentation tank 7. A closed system would then result, the trickling filter effluent being pumped in its entirety back onto filter 3 again and again. Sedimentation tank 7 was installed for the purpose of removing settleable solids from that portion of the trickling filter effluent which was used for deodorizing and diluting the
feed of digested distillery slop. This was important in reducing the amount of suspended matter put on the trickling filter. As a consequence no troubles due to filter clogging were encountered. Other details of the design included a distributor of the rotary type, 10, and an orifice, 11, to accomplish a constant rate of feed of digested liquid. A wooden false bottom, 12, was constructed in trickling filter tank 3 as an underdrain system. Adequate vents were supplied. The filter material used was 1.5-3 inch blast furnace slag. The 9-foot trickling filter had an area of 63.6 square feet or 0.001462 acre. A rate of 1,000,000 gallons per acre daily, therefore, would be 1462 gallons daily, or slightly over 1 gallon per minute. J
Digestion Tank Loadings Difficulties were encountered during the early stages of the demonstration in obtaining high rates of feed on the digestion tanks. Since the digestion tanks were placed in operation less than 2 weeks after the decision to proceed, there was insufficient opportunity of preparing adequate seeding material. As a consequence it was necessary to use digesting sludge from a sewage treatment works for this purpose, where the digestion takes place a t ordinary sewage temperatures, whereas the slop was digested within the thermophilic range or a t 130" F. The first four weeks of operation elapsed before the sludge was conditioned sufficiently for appreciable amounts of slop to be fed. After this time (September 10) the feed had been increased to 100 gallons per day. This greater feed was continued until September 20 when it was possible to increase the feed until, by September 29, feed had been increased to 250 gallons per day. At this point both of the tanks were covered with a proper insulating material to reduce the heat loss and, therefore, greatly reduce the number of times steam had to be applied. A 4-inch layer of sawdust was used. It was then possible to increase the feed further until, on October 5, 350 gallons per day were being fed. This dose was maintained until October 17, a t which time the volatile acids had risen so high that it was necessary to curtail the feed until, on October 24, only 100 gallons per day could be fed.
JULY, 1936
Ih-DUSTRIAL Ah-D ENGIKEERIYG CHEMISTRY
In previous experiments the sludge occasionally compacted during the earlier stages and required agitation until the fermentation was well established. Accordingly, arrangements were made to increase the circulation and agitate the sludge from time to time. Following the agitation, it was possible to increase the feed from 100 to 550 gallons per day in the course of 6 days. This rapid rate of increase is significant as indicating the rate a t which it is possible to start the tanks when the right type of installation is available.
I.
797
_ PROPOSED _ _LAYOUT _ FOR 1500-BUSHEL DISTILLERY
Gas Production The gas production confirmed results of previous laboratory-scale work and the work on a larger scale a t the sewage treatment works of Peoria Sanitary District (11). A uniform rate of 11 cubic feet of gas was obtained per pound of volatile solids fed to the tank.
Analytical Data It was not possible, with the personnel and equipment available, to make complete daily analyses. A large number of samples were analyzed a t the plant, and several samples were brought to Urbana for check analyses. The raw screened slop ran from 3 to 4 per cent solids (80 per cent volatile) with a B. 0. D. of 15,000 to 16,000 p. p. m. and an organic nitrogen content of 1900. The effluent from the digestion showed a B. 0. D. of 1500 to 2000 p, p. m. The effluent from the trickling filter, while operating a t a rate of 250,000 gallons per acre per day (based on the undiluted digester effluent fed), had a B. 0. D. of 138 p. p. m. (average of sixteen field samples). The four samples of trickling-filter effluent brought to Urbana for analysis showed B. 0. D. values of 39,98,80, and 104 p. p. m., respectively. These samples showed nitrate nitrogen values ranging from 60 to 820 p. p a m. and nitrite nitrogen from 25 to 150 p. p. m. Tightly stoppered samples showed no putrefaction after several weeks. Kiby (10) reported 200 p. p. m. nitrate nitrogen and 70 p. p. m. nitrite nitrogen in the effluent from a trickling filter treating similar wastes.
Odors Anaerobic fermentation always results in production of hydrogen sulfide and other odorous compounds. I n this installation the escape of odors is avoided by completely sealing the fermentation tanks. The gas which carries the odors is burned for heat or power. Odors from the digester effluent as it discharged to the trickling filter were anticipated. However, as soon as nitrification had become established, the high nitrates in the recirculated filter effluent stabilized the digestion effluent so that no odor was produced when the usual 4 or 5 to 1 mixture of the two liquids was sprayed onto the trickling filter. This stabilizing effect of nitrates has been known for many years and was once suggested as a means for treating raw sewage (12). An analytical method for determining oxygen demand by means of nitrates was once in use
FIGURE3
(IS) This early work, together with the results of the present experiment, amply justify the conclusion that this process can be operated without odor nuisance.
Sludge Since the fermentation resulted in 90 per cent gasification, the amount of sludge produced was relatively small. Some sludge was carried over with the effluent so that actual accumulation of sludge was relatively negligible when compared with the sludge from other types of material. No accurate measure could be made within the period of this experiment. Practically all of the phosphorus of the grain was recovered as magnesium and calcium ammonium phosphate. One sample of sludge showed 3.3 per cent phosphorus and another 4.2 per cent on a dry basis.
Literature Cited (1) Barrett, ??. W., Chemistry & Industry, 55, 48T (1936). (2) Boruff, C. S., IND.ENG.CHEM.,25, 703 (1933). (3) Boruff, C.S., and Buswell, A. M., Ibid., 24, 33 (1932). (4) Boruff, C.S., and Buswell, A. M., Sewage Works J.,4,973 (1932). (5) Buswell, A. M., IND. ENG.CHEM.,22, 1168 (1930). Wafer Works and Sewerage, 82, 135 (1935). (6) Buswell, A. M., and Boruff, C. S., U. S. Patent 2,029,702(Feb. 4, (7) Buswell, A.M., 1936). (8) Buswell, A. M., Boruff. C. S., and Wiesman, C. K., IND.ENQ. CHEM.,24, 1423 (1932). (9) Hoover, C.R.,and Burr, F. K., Ibid., 28, 38 (1936). (10) Kiby, W., (?hem.-Ztg., 59,600 (1934). (11) Kraus. L.S., Sewane W o r k s J . , 5,623(1933). (12) Lederer, Arthur, j.Infectious Diseases, 13,236 (1913). (13) Ibid., 14, 482 (1914). (14) Muers, M. M., Chemistry & Industry, 55, 71T (1936). (16) Naylor, W., “Trade Wastes,” p. 153,London, Charles Griffin & Go., 1902. RECEIVED M a y 6, 1936.
