Feasibility of small scale heat recovery from sewers

Warm wastewater enters the sewer system at many places. Most warm water discharges occur irregularly during short periods. All warm water used in homes ends up in the sewer system. Moreover, in many cities hot water is discharged into the sewer from for instance industrial water users. Examples are industrial textile laundry or food industry.
Thermal energy is an important factor in the total energy consumption related to the water cycle. Research has indicated that for domestic use (shower, bath, laundry machine, dishwasher etc.) the primary energy required to heat the water is about ten times higher than the operational energy for producing and distributing drinking water and collecting and treating wastewater (Sukkar et al. 2009a, Sukkar et al. 2009b, Kluck et al. 2011, Hofman et al. 2010). Using the average drinking water consumption at households (approximately 130 L per person per day) and the total heat used to produce warm water (60 MJprimary/m3 water), the average temperature of domestic wastewater entering the sewer system can be calculated. In the Netherlands this temperature is between 25 and 30°C.