3.5. Energy analysis
The energy analyses included energy consumption for UF-RO system, membrane combined system, and energy recovery (kWh/m3) from biogas methane. Although full-scale anaerobic digesters require heating to improve biogas yield in the warm climate zone, the heat energy term was assumed negligible because the suggested process was evaluated under ambient temperature conditions (23 ± 3 °C) in this study. Table 2 shows the estimated energy consumption and recovery associated with the combined systems. The membrane combined system was an energy-neutral unit for organic matter recovery and nitrogen removal, with energy consumption of 0.30 kWh/m3 and energy recovery of 0.21 kWh/m3, resulting in the net energy consumption of 0.09 kWh/m3. However, the net energy consumption of 1.11 kWh/m3 for conventional MBRs was significantly higher than the proposed combined process in spite of considering biogas production from anaerobic sludge digestion. Thus, the energy-neutral membrane combined process is not only related to biogas recovery and low energy input, but also closely related to the reduced wastewater quantity accounting for 1/4 of the original.