Comprehensive Evaluation of a Sewage Treatment Plant as a Base for Recirculation of Materials and Energy in the Region

Abstract

Based on the concept that a sewage treatment plant should serve as a base for the circulation of materials and energy in a region, a combined electric power generation system using both digestion gas and sludge incineration for power generation was studied in the use of organics in influent. To function as a base for the circulation of materials, the plant was assumed to employ phosphorus recovery, which was estimated individually for an ash alkali process and a MAP process. Using a model treatment plant with a capacity of 48,000 m³/day, it was found that introducing high-efficiency solid-liquid separation to recover solid organics proved effective in increasing the electric-power self-supply ratio attained in normal power generation. With digestion gas this ratio was raised from 6.2% to 13.0%, and with combined power generation, the ratio was raised three times higher to 18.6%. Phosphorus recovery was increased by 10% to 30% over the conventional process in an ash alkali process by introducing an AO process using the transfer of phosphorus to sludge as part of the wastewater treatment process. When various measures are evaluated in terms of water eco-efficiency from a viewpoint of environmental performance in the sewage treatment plants, power generation using sludge incineration was 1.96 kg/kWh, compared to the benchmark, which was 1.62 kg/kWh of power generation with digestion gas, and phosphorus recovery was 2.08 kg/kWh. The target for water eco-efficiency may be set to 3 kg/kW/h when various measures are combined.