SEWER HEAT RECOVERY
An enormous amount of thermal energy is lost when the liquid effluent from wastewater treatment plants is discharged to surface water bodies. Existing sewer heat recovery technologies are available to capture this wasted energy and put it to productive use to heat and cool buildings or to provide domestic hot water.
Several European cities — including Oslo, Stockholm, and Zurich — as well as Tokyo and Sapporo in Japan are successfully utilizing sewer heat recovery systems to provide heating and cooling for buildings. This technology has been grossly underutilized; sewer heat represents an untapped source of “wasted” energy that can and should be utilized.
Sewer Heat Recovery Technology
A sewer heat recovery system consists of two parts:
1) a heat
exchange system in the sewer pipes themselves and
2) a heat pump
station.
The heat exchange system is typically a series of closed-loop tubing containing a water-alcohol mixture. The mixture is warmed by the sewage and sent to a heat pump station. Heat pumps then use the warm water to generate heat for buildings or provide domestic hot water use. Sewer heat can be recovered either from individual sewer mains (raw sewage) or from the treatment plant itself (raw or treated sewage). (Photo credit: Kristian Vinkenes)
Case Study: Rabtherm Panels
The most prominent turnkey vendor of sewer heat recovery systems is Rabtherm, a Swiss firm. Rabtherm offers prefabricated heat exchange panels that are placed directly in sewer inverts — heat pumps are purchased separately. The firm has overseen the installation of seventeen major sewer heat recovery systems worldwide.
- Rabtherm's system generates 3-7 kW of useful heat per m^2 of heat exchanger.
- 1 m^3 of sewage (about 5 bathtubs worth) extracts 2.3 kWh of energy.
- 420 liters of sewage are needed to generate 1 kWh of energy.