抄録
Objective. We have developed the “Methane Fermentation System with Hyper Thermal Solubilization (Methane Fermentation System – H.T.S)” that consists of an 80℃ solubilization tank and a 53℃ methane fermentation tank. Organic matter (sludge) is solubilized by a specific microorganism, Anoxybacillus beppuensis MU3, which produces thermophilic protease. We isolated this microorganism from Beppu hot spring located in the Kyushu Island of Japan. The system can produce 6,500 L/day biogas from 0.5 m3/day sewage sludge (65.7% digestibility in VSS equivalent). It generates approximately 2-fold higher biogas than the general single-tank methane fermentation system and seems more environmentally friendly, although it requires relatively more resources (e.g., pumps, electricity, and tubes) for its operations. This study, therefore, comparatively evaluated the environmental impacts of both these systems by LCA method.
Results and Discussion. The Methane Fermentation System – H.T.S requires more propane gas than the single-tank methane fermentation system to maintain the temperature of the solubilization tank at 80℃; thus, the environmental impact of this system on fuel consumption increases. Therefore, the results of the Environmental Impact Assessment have shown that the Methane Fermentation System – H.T.S has a more significant impact on human health and social assets than the single-tank methane fermentation system. Similar results were obtained with regard to the single index. On the other hand, the Methane Fermentation System – H.T.S had a smaller impact on biodiversity, whereas both systems showed nearly the same degree of impact on primary production, as the impact on fuel consumption was lesser than that on biodiversity and primary production. Furthermore, it was observed that the reduced use of thermal energy generated by burning the biogas (methane gas) produced, for keeping the solubilization and fermentation tanks warm, could significantly reduce the consumption of propane gas and, consequently, the impact on human health, social assets and single index.
Conclusion. From the above results, it can be concluded that although the Methane Fermentation System – H.T.S can generate more biogas than the single-tank methane fermentation system, efforts must be made to reduce the propane gas consumption in order to reduce the environmental impact of such biogas production.
