Often considered expendable, wastewater from commercial buildings and sewage treatment plants (STP) has value as a heat source, offering a sustainable and in some cases economical avenue to a lower energy footprint of facilities. Today, heat recovery from wastewater has been successfully implemented at STPs, in residential or commercial buildings, and in large sewage pipelines. About 25% of a building's energy is used to heat water, which ends up in shower drains and dishwasher or laundry water, typically discharged to sewers at about 18 °C to 20 °C (64 ºF to 77 ºF).
North America and Europe together produce about 300 million m3/day of wastewater flowing through sewers, with a potential to replace 1.5 billion MWh of natural gas used for space heating and heating domestic water. The U.S. DOE estimates that annually, 350 billion kW of hot water goes down the drain, roughly $35 billion worth of energy (at $0.10 per kW). A challenge, however, has been to introduce heat recovery solutions that are sized for the existing infrastructure and provide a reasonable ROI for wider adoption. One company that appears to be succeeding in both areas is Sharc Energy Systems, which has developed a system that combines solids removal and a standard heat pump. It claims that operating at domestic wastewater temperatures makes the heat pumps 25% to 30% more efficient than ground source heat pumps.
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