In recent years, production of biogas by an anaerobic digestion method has been reconsidered as an alternative means for producing clean fuel. Thus, research on various aspects of biogas production by anaerobic digestion has become important for its widespread utilization. One of the important aspects is energy balance of the process. The energy balance is affected by a heat demand, whereas the heat demand is affected by ambient temperature. In the present study, in order to clarify the effect of ambient temperature on the energy balance of anaerobic digestion, typical municipal sewage sludge anaerobic digestion in an actual wastewater treatment plant was adopted as analysis model and was studied under various temperature conditions. It was found that the heat demand for heating the sludge was temperature-dependent and that 26～39% of the total amount of heat demand was heat losses from digester tanks. From the energy balance, it was found that net energy can be produced by the anaerobic digestion in any region and season in Japan, and this shown that it has a possiblity to be a new energy resource. It was also shown that, when ambient temperature became closer to the digestion temperature, more efficient anaerobic digestion can be obtained. These results were confirmed by indexes of energy balance that were defined in this study, with notable proportional relations being found between indexes and ambient temperature. Ratio of energy of heat demand to biogas produced had an inverse proportional relation with ambient temperature, whereas ratio of energy of net energy to biogas produced had a proportional relation, and ranges of these values in all of Japan were 0.13～0.44 and 0.45～0.84, respectively.
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