Evaluation of the life cycle sustainability of a wastewater treatment plant using the eco-efficiency index for optimizing the energy-economy-environment nexus

Despite significant progress in the sustainability of wastewater treatment plants (WWTPs), key challenges remain. This paper assesses the sustainability of a WWTP under various scenarios in Iran, considering different electricity-generating technologies powering the WWTP and sludge management systems. The eco-efficiency (EE) index, combining life cycle assessment (LCA) and economic assessment, is used to assess the energy-economy-environment nexus. The scenarios are based on the electricity sources and sludge management systems of the WWTP. Furthermore, an optimization model is proposed to determine optimal energy tariffs and assess the feasibility of transitioning towards energy self-sufficiency. The results indicate that scenarios utilizing the landfill for sludge management and energy generation have the highest environmental impacts, while the scenario employing wind power and anaerobic digestion (AD) system has the least impact. From an economic perspective, scenarios utilizing AD and selling biogas yield lower profits than the alternatives. Also, the scenario utilizing wind power and compost production from the sludge has the highest sustainability, with an EE index equal to 60.68 $/Pt. Furthermore, the results show that when the tariffs for electricity and heat are 0.86 $/kWh and 0.052 $/MJ, energy production will be economically and environmentally justified. The feasibility assessment suggests that when only the grid electricity option is available, it is preferable for the WWTP to utilize combined heat and power (CHP) systems, enabling it to meet its needs independently and create a benefit. This study provides critical insights for WWTP sustainability, guiding efficient wastewater management practices.