The choice of biological waste treatment method for urban areas in Japan—An environmental perspective

Biological treatment of organic waste is environmentally friendly and a wide range of treatment methods exists. Integrated biological treatment systems with additional equipments, such as pre-treatment, wastewater treatment and deodorisation processes are currently in use. To promote and spread the application of biological waste treatment, a life cycle assessment (LCA) study was conducted on six biological treatment methods: integrated wet anaerobic digestion (AD), integrated dry AD, simple wet AD, simple dry AD, integrated composting and simple composting systems. The impacts of operating rate and wastewater treatment, which affect GHG emissions, were also quantitatively analysed. Integrated wet AD showed the highest total GHG emissions due to the high energy consumption by additional equipments which occupy 80% of the whole process. Integrated composting also presented higher GHG emissions than simple composting because of the higher electricity consumption. Additional equipments are necessary for integrated systems installed in urban areas, and this study suggests that the reduction of energy consumption for these additional equipments is an important issue. Among the additional equipments for AD, wastewater treatment largely affected the GHG emissions. Dry AD normally generates less wastewater due to low moisture content in the waste. Thus, effective treatment of wastes with low environmental loads can be achieved by dry AD, where energy consumption from wastewater treatment is low. On the other hand, methane yield from food waste by dry AD is generally smaller than wet AD. Installing an advanced dry AD reactor with additional functions such as long solid retention time, and adjusting the moisture content of input waste by mixing paper waste will contribute to the efficient treatment of organic waste in urban areas.