Material and Energy Flow Analysis in Sewage Sludge Incineration and Composting Treatment Processes

This study aims to clarify the material and energy flows in a well-known sewage sludge treatment plant in Japan; the treatment processes include composting, drying, incineration, and incineration with ash melting. We used not only statistical data but also data such as carbon, nitrogen, and moisture content in sludge obtained through our on-site observations. Material and energy flow analysis of sewage sludge composting is seldom reported in comparison with the analysis of other recycling and treatment methods. Therefore, we have performed material and energy flow analysis on sewage sludge composting to estimate factors such as environmental load and energy balance. We investigate the energy consumption of sewage sludge composting in one company that produces manure through aerobic fermentation of sewage sludge. In addition, we compare sewage sludge composting with incineration and evaluate their respective characteristics regarding sewage treatment and production of manure. It has been found that the use of anaerobic digestion of sewage sludge can reduce the overall energy consumption in sewage sludge treatment if power generation using biogas is adopted; however, the digestion of sewage sludge lowers the heating value of the digested sludge, requiring more energy to operate the digester. The energy consumption in a wastewater treatment plant accepting returned water from sludge digestion was increased by 10% to 20% compared to the process without sludge digestion. The overall energy consumption in both wastewater treatment and sludge digestion should be considered. The electric power was 60% of the overall energy consumption and 70% of the energy consumption in the plant's production sector.