Continuous Thermophilic

SCHULZE, K. L. (Michigan State University, East Lansing). Continuous thermophilic composting. Appl. MIicrobiol. 10:108-122. 1962.Under complete mixing conditions, aerobic decomposition of mixed organic waste materials has been maintained continuously in the thermophilic phase in a 55-gal rotating drum. Temperatures ranged between 53 and 70 C. Raw material was added daily or every second day in amounts up to 18 lb per 100 lb of decomposing material. The weight of material removed ranged between 42 and 60 % of the raw material added. Factors influencing the operation of the composting unit were studied in detail. In recent years, the urban disposal of solid organic waste material such as garbage and refuse has become more and more of a problem. According to the Bureau of the Census, about 64/c of our population live in cities and smaller communities classed as urban. This means that about 100 million people in the United States produce garbage, with each person contributing a daily average of approximately /2 lb that must be disposed. Thus, there are about 25,000 toIns of garbage to be handled per day. A recent survey by the International City Managers Association showed that most American cities dispose of their refuse by land fill. San]itary land fill was practiced by 484 cities, 371 cities had open dumps, 110 were feeding garbage to hogs, 109 used incinerators, and 34 employed salvage methods. In a few places, garbage was ground centrally and added to sewage sludge for anaerobic digestion. Composting was mentioned only once. The hog feeding has been discontinued because of health hazards. Open dumps are objectionable from a public health poinlt of view. Flies, mosquitos, and rodents breed on exposed waste material; odor and smoke problems are common. Even in sanitary land fill it is difficult to control odors, flies, and rodents. In addition, sites available for land fill disposal have become scarce due to the extensive development of suburban residential areas. Incineration has advantages insofar as the operation can be highly mechanized, requires a relatively small space, and is hygienically effective. The process, however, is expensive and increases atmospheric pollution. I Project supported by grants no. RG 4180, C1-C5, from the National Institutes of Health, Bethesda, Md. 2 Patents applied for. Against this background it is surprising that composting, so far, has not been used to any great extent in the United States. There is no doubt, however, that this method will become increasingly important in future years. In Europe and in Asia there are already a considerable number of large-scale plants in operation, especially in Holland, Germany, Switzerland, the Isle of Jersey, Japan, and Thailand. Several of these plants have been described by Nesbitt (1958), Wiley (1961), and in Compost Science, a new journal exclusively devoted to the field of composting (Teensma, 1961; Gothard, 1961). M\ost of the large-scale plants use the windrow method in which the refuse is stacked into long piles approximately 6 ft high and 10 ft wide. Occasionally these piles are turned over to provide air and mixing. Decomposition is slow and there is very little control with regard to process conditions such as moisture, temperature, and air supply. Consequently, 4 to 8 months are needed before a finished compost is produced. In smaller, more mechanized installations, bins, silos, and rotating drums are being used. A description of the various types of equipment developed for composting purposes has been given by Gotaas (1956) in his comprehensive book on composting. Since the rotating drum appeared to offer a high degree of control over the process parameters, we developed a laboratory model composter which consisted of a closed rotating Plexiglass cylinder, 10 in. in diameter and 19 in. long. The unit had a loading capacity of about 16 lb of freshly ground garbage at 50 to 60 % moisture. The apparatus proved very useful in establishing quantitative data for the relationship between oxygen uptake rate and temperature, the respiratory quotient, and for the effect of moisture on the composting process under batch operation. It was found that the oxygen consumption rates increased directly with the temperature between 80 and 145 F, with values ranging from 1 to 5 mg 02 per g initial volatile matter per hr. The temperature coefficient of the oxidation reaction was Qlo = 1.9, which is characteristic for biological reactions such as growth or respiration. The respiratory quotient (RQ) showed an over-all value of 0.9. This mea-ns that, on a molar basis, slightly more CO2 is produced than oxygen consumed. Warburg experiments made in connection with these studies demonstrated that the activity of the composting process as measured by the oxygen uptake rate increased directly with the moisture content from a minimum of zero at moistures below 20 % to a maximum at a moisture of 60 %O. Detailed reports of these studies have been published previously (Schulze, 108 on N ovem er 2, 2017 by gest ht://aem .sm .rg/ D ow nladed fom CONTINUOUS THERAIOPHILIC COMIPOSTING 1958, 1960, 1961). The batch experiments confirmed the concept that aerobic decomposition normally proceeds through a series of distinct phases: (i) fermentation, (ii) acid formation, (iii) thermophilic activity, and (iv) temperature decline. During the first two phases, mesophilic microorganisms such as yeasts and bacteria are predominant and the temperature of the decomposing material increases from ambient to about 45 C (113 F). The third phase is characterized by the activity of thermophilic bacteria which produce temperatures from 45 to 71 C (119 to 160 F) due to their intense respiration. In the fourth phase, actinomycetes and fungi take over, accompanied by a decline in temperature back to ambient. The results of these studies led to the idea that under properly controlled conditions it might be possible to operate a composting process continuously in the thermophilic phase, i.e., in the range of 120 to 160 F. This procedure would eliminate the lag periods associated with the two initial phases and would provide a rapid decomposition of organic waste material. Accordingly, an attempt was made to operate the I'lexiglas laboratory unit continuously in the thermophilic phase by daily additions of ground garbage. The experiments as reported by Moore (1958) showed that temperatures from 120 to 140 F were reached but could not be maintained for more than about 10 days. The main reason for the steady decline in temperature appeared to be the small size of the rotating drum. The heat losses connected with the daily removal and addition of composting material were probably greater than the heat produced in the unit. Therefore, it was decided to build a larger pilot plant using an insulated, rotating 55-gal steel drum as the reactor vessel. It was hoped that this unit could be operated for months at temperatures between 120 and 160 F. MATERIALS AND METHODS The composting unit was built from a 55-gal steel drum, equipped with support, rotating mechanism, electric timer, air compressor, air flow recorder, temperature recorder, and Beckman3 oxygen analyzer as shown in Fig. 1 and 2. Figure 1 is a sectional diagram of the pilot plant and Fig. 2 a photograph of the unit in operation. An opening (11 by 17 in.) was cut into the side of the drum for the addition and removal of material. During operation, this opening was closed by a hinged cover and a fitted rubber gasket. Insulation was provided by a 1-in. layer of Styrofoam taped to the sides and the ends of the drum. Air was supplied by a small 5 ft3 per min (cfm) piston compressor. The air flow recorder had a range of 0 to 30 ft3 per hr (cfh) and was adequate for all runs. The exhaust gas was led downward into a water trap so that condensate could collect and overflow into a drain. Through a T-piece the exhaust line was connected to a Beckman oxygen analyzer, model D-2. This allowed periodic analysis of the exhaust gas with respect to its residual oxygen concentration. Rotation of the drum was at 1 rev/min and regulated by an electric timer which permitted varying the rotating time from 0 to 50 % per 1,' hr. Normally, however, the drum was rotated only for about 5 min before and after each feeding. Garbage from a university campus dormitory was used as basic raw material. The garbage consisted of table scraps containing meat, pastry, bread, potatoes, vegetables, citrus fruit, tea leaves, coffee grounds, and paper. The material was ground in a W-W4 garden shredder, model 2 X BE with 34-in. screen openings. Since it was determined early in the operation that garbage alone was too moist and too dense for direct composting, a number of conditioning materials were used to establish suitable mixtures: (i) dewatered digested sewage sludge from the East Lansing sewage treatment plant; (ii) airdried digested sewage sludge; (iii) Vermiculite;5 (iv) waste paper, mostly newsprint; and (v) air-dried compost from previous runs. The dewatered sewage sludge was taken in 50-lb batches 3Beckman Instruments, Inc., Fullerton, Calif. 4 W-W Grinder Corporation, Wichita, Kan. 5 Zonolite Corporation, Chicago, Ill. FIG. 1. Diagram of rotating drumin composter 109 on N ovem er 2, 2017 by gest ht://aem .sm .rg/ D ow nladed fom