Benchmarking energy consumption in municipal wastewater treatment plants in Japan.

Reduction of greenhouse gas (GHG) emissions is one of the most important tasks facing municipal WWTPs. Electric power consumption typically accounts for about 90% of the total energy consumption. This study presents a benchmarking analysis of electric power consumption. The specific power consumption (SPC) ranged from 0.44 to 2.07 kWh/m(3) for oxidation ditch plants and from 0.30 to 1.89 kWh/m(3) for conventional activated sludge plants without sludge incineration. Observed differences of the SPC can be attributed to the difference in the scale of plants rather than to different kinds of wastewater treatment processes. It was concluded that economical benefits by centralizing treatment had contributed significantly to the reduction of energy consumption. Further analysis was carried out on the plant that had shown an extremely small SPC value of 0.32 kWh/m(3). In this WWTP, a large amount of digestion gas was generated by anaerobic digestion. In particular, it was used to generate power using phosphoric acid fuel cells to generate approximately 50% of the energy consumed in the plant. It was calculated that this plant had reduced the overall SPC by 0.17 kWh/m(3). The effect of power generation using digestion gas demonstrated clearly the advantage of implementing energy recovery schemes.