Optimizing the recovery pathway of a net-zero energy wastewater treatment model by balancing energy recovery and eco-efficiency

As more countries commit to carbon neutrality, the concept of carbon-neutral or energy self-sufficient wastewater treatment plants (WWTPs) is attracting more attention. Optimizing energy recovery evaluation from both economic and ecological perspectives contributes to promoting this concept into practice. In this study, the energy recovery and eco-efficiency balance in a WWTP in China with the weekly operating status as the decisionmaking unit was explored. In addition to the chemical energy recovered by anaerobic digestion with combined heat and power (CHP), the thermal energy in wastewater and solar energy can be recovered through the heat pump (HP) and photovoltaic (PV) modules and form optimal pathways in the net-zero energy wastewater treatment model. Additionally, with three inputs and seven outputs covering both economic and ecological indicators, a slacks-based measure in data envelopment analysis was conducted to evaluate eco-efficiency. The results show that the water quantity of WWTPs can be influenced by seasonal variations, causing low ecoefficiency in summer; for water quality, carbon source shortages can hinder the efficiency. The most efficient performance was achieved under tech-portfolios of CHP + PV or CHP + PV + HP. The eco-efficiency distribution exhibited a similar normal distribution to energy recovery, with optimal eco-efficiency achieved at energy recovery rates of 85-90%. With further improvements in energy recovery, the marginal benefit of the energy yield decreased. This study provides optimized energy recovery pathways in WWTPs by balancing energy recovery and eco-efficiency, which is significant for planning technical renovation in WWTPs for energy recovery.