Source-separated wastewater – enabling higher energy recovery

This study quantifies the impact of source-separation in heat recovery by comparing greywater in source-separated wastewater treatment plants (WWTPs) versus mixed wastewater in conventional WWTPs as a heat source using an idealized Lorenz heat pump model. The results reveal that greywater requires less electricity input than mixed wastewater below a certain heat recovery percentage, defined relative to the thermal energy required for domestic hot water heating. This percentage has a functional relationship with the temperature gap between greywater and tap water. This relationship serves as a delineation function, identifying when greywater or mixed wastewater is advantageous for heat recovery under typical wastewater conditions across heat recovery percentage. Beyond conceptual calculations, this study investigates heat recovery from treated greywater using measured temperature data from a 207-household pilot source-separated WWTP with a CO₂ heat pump model. Various heat recovery scenarios are examined, including different supplied hot water temperatures, whether reusing treated greywater after heat recovery, and different heat loads. The standard scenario without water reuse can recover 9.7 kWh of thermal energy from 1 m³ of treated greywater and save 10% of household energy demand. With this heat recovery level, 11 out of 12 months, greywater is a better heat source compared to mixed wastewater, according to the delineation function. This work gives guidance for future wastewater heat recovery projects on selecting proper heat source between greywater and mixed wastewater. Furthermore, it addresses knowledge gap in heat recovery from treated greywater in source-separated WWTPs and provides a reference for future net-zero neighbourhoods.