The heat recovery potential of 'wastewater': a national analysis of sewage effluent discharge temperatures

Final sewage effluent (FSE) is typically warmer than the rivers it is often discharged to. The excess heat energy could be recovered and utilised to help meet climate change targets. Using data from England for 2000-2019, we show that FSE temperatures were on average 2.2 degrees C higher than river temperatures, with a corresponding annual heat recovery potential of similar to 18.3 TW h which could meet similar to 3.6% of the UK's heat demand. Crude sewage temperatures were on average 1.5 degrees C higher that FSE temperatures, implying that a further similar to 12.5 TW h is lost annually during treatment prior to discharge. The largest temperature differences between FSE and rivers, and crude sewage and FSE, occurred during the autumn and winter months, meaning that the greatest seasonal heat recovery potential coincides with the greatest heat demand. The temperature difference between FSE and rivers increased at an average rate of similar to 0.03 degrees C per year from 2000 to 2019. Therefore, and in addition to predicted population growth, wastewater heat is a growing resource. The largest temperature differences between FSE and rivers would generally be expected to occur in northeast England. However, FSE discharges with sufficiently large temperature differences between FSE and rivers were demonstrated to exist across England and were not restricted to one region or water company. Wastewater treatment works discharge effluent continuously and occur nearby to domestic settlements, which account for the majority of the UK's heat demand. Therefore, there is clear local potential to recover heat and meet national emissions targets whilst further reducing environmental impact on rivers.