Batch co-digestion of two-component mixtures of agro-wastes

One of the key constraints to development in rural areas of the developing world is the lack of suitable energy supplies. One possible solution is to promote the generation of biogas from agricultural and other organic wastes. In practical terms, the amounts of organic waste generated at particular sites may not be sufficient to make digestion cost-effective. The concept of co-digestion is well established. What is not clear is whether some wastes would have adverse effects when added to a stable digester or when used in conjunction with another waste. This paper examines some initial studies into the use of anaerobic digestion for two-component mixtures. The experiments were designed using mixture design mathematics so that any synergism or antagonism could be identified. Three batch co-digestion trials were undertaken to derive optimum mixtures for the co-digestion of poultry manure with molasses (A-1), with sheep and goat manure (A-2), and with thickened waste activated sludge (A-3). The results from Trial A-1 showed that during the batch co-digestion of poultry manure (a high-nitrogen waste) with molasses (a highly degradable waste) synergistic or antagonistic interactions could not clearly be deduced. The results of Trial A-2 showed that the best results were obtained with a mixture of 75% poultry manure and 25% sheep and goat manure. The conclusions which were drawn from the results of Trial A-3 were that synergistic and antagonistic interactions were not apparent and that the methane yields generally increased with increased substitution of waste activated sludge with poultry manure.