Composting under controlled conditions.
Three cylindrical reactors, each with a working capacity of approximately 200 litres, were used to investigate composting. The process was optimised and conditions were controlled so that composting on a laboratory-scale thermally resembled that occurring in the core of large open heaps. A baseline flow of humidified air aerated the reactors in five-minute bursts. The reactors operated as closed systems with facilities to analyse the composition of the off-gas for ammonia, oxygen and carbon dioxide. Temperature was used to monitor the progress of the process. Heat loss from the reactor surface was compensated for with an external heat source. A basic model of radial conductive heat losses showed that 53 watts per square-metre would be the maximum heat flux needed to keep the temperature difference across the reactor to within a degree when running at 60 degrees C. A heating cable was used, which could supply 150 watts per square-metre, and the radial temperature difference was reduced to within a degree in more than 60% of the recorded temperatures in the case studies presented. The temperature of the composting material was held at 60 degrees C using a high flow rate 'cooling' aeration with temperature feedback. This, however, led to a mean vertical temperature difference of at least 10 degrees C. The aeration strategy resulted in a well-aerated material, which favoured aerobic microbial activity and the temperature increased as a result of the internally generated heat associated with composting. Three-quarters of the ammonia was emitted in the first week.