Energy efficiency assessment of China wastewater treatment plants by unit energy consumption per kg COD removed

Unit energy consumption per kg pollutant removed (kWh/kgCOD_removed) is used for the first time in assessing and ranking the sustainability of main treatment technologies of 1215 wastewater treatment plants (WWTPs) in China. The metric better measures the sustainability of main treatment technologies in WWTPs than unit energy consumption per cubic meter treated (kWh/m³). The energy consumption data of these WWTPs were selected from the database of 1399 WWTPs to evaluate the energy efficiency of different treatment technologies. 80.3% of the WWTPs applied anaerobic-oxic plus anaerobic–anoxic–oxic, oxidation ditch, and sequencing batch reactor as main technologies. Statistical analysis shows that the unit energy consumption of WWTPs decreases with increasing design flow rate, operation loading rate, and influent COD concentration. For example, the average unit energy consumption of SBR decreases from 2.76 kWh/kgCOD_removed to 0.83 kWh/kgCOD_removed when the design flow rate increases from less than 10,000 m³/d to 100,000–200,000 m³/d. The mean unit energy consumption of SBR decreases from 1.71 kWh/kgCOD_removed to 1.32 kWh/ kgCOD_removed and 2.85 kWh/ kgCOD_removed to 0.63 kWh/kgCOD_removed as the operation loading rate and COD removal increase from 40% to 100% and from less than 150 mg/L to over 450 mg/L, respectively. SBR has the lowest unit energy consumption among all the technologies. Therefore, SBR might be the most appropriate technology in small and medium-scale WWTPs in China. Regression equations were developed to predict the unit energy consumption for sustainable design treatment trains by input variables such as design flow rate, operation loading rate, and influent COD concentration.