Spatially informed multi-objective decision-making tool for retrofitting municipal wastewater treatment plants

The resources embedded in wastewater effluents and residual biosolids have motivated the conversion of traditional energy-intensive wastewater treatment plants (WWTPs) into net energy-positive water resource recovery facilities. Recent food waste regulations, including the New York State (NYS) Food Donation and Food Scraps Recycling Law, created new opportunities to integrate food waste (FW) co-digestion with innovative waste energy and resource recovery technologies at WWTPs. This study develops a spatially informed decision-making tool to facilitate the selection of retrofit design for WWTPs, using NYS as an example. Retrofit configurations for each WWTP was optimized based on both economic and environmental objectives. FW co-digestion is consistently included in the optimal strategy when considering both economic and environmental objectives. For Effluent Thermal Energy Recovery (ETER), local heating/cooling yields a higher net present value (NPV) than biosolid drying but is only cost-effective for WWTPs with a flow above 2.2 MGD. For biosolids upcycling, hydrothermal liquefaction (HTL) generates a higher NPV than hydrothermal carbonization (HTC) but is only cost-effective for WWTPs with a flow above 60 MGD. Co-digestion, ETER for heating/cooling, and Power-to-Gas (P2G) are the most effective strategies for greenhouse gas reduction. If the allocated FW has a high undigested solids content, adding HTC to the process can further reduce GHG emissions. When considering life-cycle environmental impact, co-digestion, ETER for heating/cooling, and hydrothermal processes are the most effective options. HTL can produce greater overall environmental benefits than HTC due to its lower non-GHG-related impact. Finally, four WWTPs were identified as high-priority candidates for retrofitting based on favorable economic and environmental performance, and environmental justice considerations. To our knowledge, this is the first spatially informed decision-making model for WWTP retrofitting that not only recommends optimal strategies for energy and resource recovery at individual WWTPs, but also identifies statewide priority for retrofitting based on economic, environmental, and social considerations. This study demonstrates a holistic approach to support decision-making processes at WWTPs, and offers potential applicability to any municipality exploring food waste management and other resource recovery opportunities.