Thermal use of urban wastewater: Some case studies

Urban sanitation networks in the 21st century must go beyond transporting and treating wastewater; they should also prioritize energy efficiency. Wastewater, accounting for over 70 % of water consumption, is collected through sewage systems and treated at wastewater treatment plants (WWTPs) before discharge. With temperatures between 10 and 23 °C, it represents a valuable thermal energy source. This study explores two approaches: drying sludge using treated WWTP effluent via heat pumps and recovering heat from urban sewers for building heating. In the first case, a WWTP treating 7045 m³ day −1 employed a 7.2–10.4 kW water-to-water heat pump at 45 °C (COP 4.2–4.5), reducing sludge moisture from 78.6 % to below 30 % and cutting disposal costs by €15,414 year −1 . The €15,000–€16,000 investment was recovered in 2.2–3 years, yielding an IRR above 40 %. In the second case, heat from a sewer collector (300 L s −1 , 19 °C) was used to heat an 8500 m² sports center. Three 140 kW heat pumps delivered 420 kW, reducing CO₂ emissions by 150 t year −1 and heating costs by €44,336 year −1 , with a 2.5-year payback (excluding the cost of installing underfloor heating). These results show that wastewater heat recovery can provide reliable, cost-effective, and sustainable thermal energy, supporting urban energy efficiency and the circular economy, and promoting the vision of a globally applicable “sustainable city.” • Heat recovery from WWTP effluent improves plant energy efficiency. • Wastewater heat used to dry sludge, reducing disposal costs by over 50 %. • Lab tests define optimal sludge thickness and drying temperature to select heat pump. • Recovering sewer heat is more cost-effective at larger application scales.