Improving compost process efficiency by controlling aeration, temperature and pH

Biowaste composting is rapidly increasing, and many composting plants in Scandinavia have had problems with low pH during the process. The aim of this thesis was to develop methods to improve process efficiency in large-scale composting. The investigations focused on alleviation of acid-related process inhibition and the interrelationships between decomposition rate, temperature, aeration, evaporation and oxygen concentration. Composting experiments were performed at laboratory-, pilot- and full-scale, and the microbial and physical processes were modelled and simulated. In composting of food waste a prolonged initial acidic phase can occur, resulting in low degradation rate. In successful composting, the initial phase is followed by high-rate composting at pH values above neutral. A combination of temperature above 40 °C and pH below 6 severely inhibits the composting process. Experiments at large-scale composting plants showed that it is possible to increase the activity and shorten the acidic phase by increasing the aeration rate, even though the temperature remains above 40 °C. When composting source-separated household waste in controlled 200-litre reactor experiments, the decomposition of organic matter during the high-rate phase was faster at 55 °C than at 40 or 67 °C. The ammonia emissions at 67 °C were more than double those at 55 or 40°C. Experiments were also performed in rotating 3-litre reactors, which were fed daily with fresh waste and water. In reactors with a starting culture more than four times the size of the daily feed, a well-functioning process was established. In reactors with a starting culture less than twice the daily feed, the pH decreased and the composting process failed. A dynamic model was developed for the initial self-heating phase of batch composting. It was possible to describe the process in simulations with a mesophilic microbial community tolerant to low pH and a thermophilic community inhibited by low pH. Simulations showed that in large-scale composting, the water loss is mainly determined by the accumulated decomposition. The process temperature has very little effect on the water loss.