Laboratory Composter for Simulation Studies

A self-heating laboratory composter comprised of a covered, double-walled, insulated tank containing water, an air-tight cylinder submerged in the tank, an inner screen mesh cylinder that held the organic material, a heater-circulator, and a differential temperature control system was built and tested for use in studies simulating the biochemical changes that occur during composting. A mixture of raw, limed sewage sludge and woodchips was composted, and the weightloss data obtained were similar to that from the pilot plant composting facility at Beltsville, Md. The mixture lost 33% of its wet weight, and 9% of its dry weight during 28-50 d of composting. Six to ten percent of the total N was lost. There were two major temperature peaks, which correlated with COi increases. Peak NH3 evolution occurred when COi evolution decreased. All preliminary data indicate that the laboratory composter provides a comparable simulation of a large composting facility. Additional Index Words: self-heating, sewage sludge, weight loss, nitrogen loss. Sikora, L. J., M. A. Ramirez, and T. A. Troeschel. 1983. Laboratory composter for simulation studies. J. Environ. Qual. 12:219-224. Composting stabilizes organic waste material through the decomposition of organic constituents. Heat, a byproduct of decomposition, is conserved in the mass, raising its temperature and causing the evaporation of water. Several studies have been conducted to evaluate the decomposition of organic wastes during composting, and the experimental units have been piles of windrows (Willson and Walker, 1973), externally heated bench composters (Bagstam et al., 1974; Suler and Finstein, 1977; Willson, 1971; Ashbolt and Line, 1982), and self-heating or adiabatic composters used for laboratory studies (Nell and Wiechers, 1978; Mote and Griffis, 1979; Norman et al., 1941; Walker and Harrison, 1960). Large piles or windrows preserve the selfheating concept of composting and the microbial suc1 Contribution from Biological Waste Management & Organic Res. Lab. , Agric. Environ. Quality Institute, USDA-ARS, Beltsville, MD 20705. Received 18 Feb. 1982. 'Microbiologist, USDA-ARS, and Agronomists, Maryland Environ. Service, Annapolis, MD 21401. cession that occurs during the heating and cooling trends, but mass balances on constituents are difficult to obtain in such open air systems. Bench composters are enclosed systems, externally heated according to a predetermined program, either to match heating cycles from open air piles, or to evaluate decomposition at a preset temperature range. Mass balances are obtained, but the time-temperature programs and microbial succession are criticized as possibly being artificial. The adiabatic composter preserves the self-heating concept, and is enclosed so mass balances can be calculated. To achieve the self-heating concept, the laboratory composter is designed to minimize heat loss by insulating the organic material, using some medium such as water, oil, or air. A temperature comparison mechanism senses the temperature of the decomposing organic material and adjusts the temperature of the insulating medium to within 0.1-0.2°C below that of the mass. A system with this design results in temperature decreases or increases, according to the decomposition rate and heat output of the organic material. Compost piles perform similarly; therefore, these self-heating and cooling systems are excellent tools for studying the composting process. The temperature comparison mechanisms differ primarily in their temperature-sensing devices. Mote and Griffis (1979) used a system of Cu-constantin thermocouples and a comparator circuitry to measure the temperature of the mass, and subsequently heat the insulating bath to just below the temperature of the mass. Walker and Harrison (1960) and Norman et al. (1941) also used thermocouples and galvanometer amplifiers with a light beam that, when deflected onto a photoconductive cell, actuated the insulating water bath. Nell and Wiechers (1978) used an electronic control system consisting of closely matched transitors. Nearly all laboratory composters pass humidified air through the compost mass to maintain aerobic conditions without drying out the mass. The Mote and Griffis (1979) design included an O2 replenishment system that was actuated when the volume of the head space over the mass decreased a set amount. J. Environ. Qual. , Vol. 12, no. 2,1983 219 ~ BASE ACID ROTACOMP~ ~_. ~-THERM~STO~ H20~TRAP ~ TRAP~-" METER~AIR ,HE . O UP S I CON SER