Strategy for introducing sewage sludge energy utilization systems at sewage treatment plants in large cities in Japan: A comparative assessment

Abstract

Abstract The sewage sludge recycling strategy in Japan is shifting to a phase focusing on energy utilization. At present, the treatment of dewatered sludge depends on high-temperature incineration at 850\xa0°C, and it is necessary to switch to energy conversion technologies to recover energy. Sewage treatment plants equipped with digestion tanks can recover energy by decomposing some of the volatile solids in the sludge, but decreasing the volatile solid content of the dewatered sludge in the solid matter also decreases the efficiency of the dewatered sludge conversion process. Therefore, in this study, we developed a tool for evaluating sewage sludge energy conversion systems based on a precise heat balance analysis. We also focused on how the presence or absence of digestion affects each energy conversion technology. The targeted sewage sludge energy conversion technologies were three incineration systems that included waste heat power generation (power generation from low-pressure steam and high-temperature water, power generation from high-pressure steam, and power generation from low-pressure steam and sludge drying) and two solid fuel conversion systems (dry granulation and low-temperature carbonization). The evaluation indices were energy balance, greenhouse gas emissions, and total project cost. The results showed that 1) not using digestion is a superior option in the incineration with waste heat power generation systems, and 2) using digestion is superior in the solid fuel conversion systems.