"I never thought I'd be saying the words that 'Sewage heat recovery is the coolest thing,'" said Jessie Israel, resource recovery manager at King County's Wastewater Treatment Division.

The technology is simple. Wastewater, which consists of what gets flushed down toilets but is mixed with millions of gallons of hot water from showers, dishwashers, washing machines, and more, maintains a fairly constant temperature as it travels through sewers to the treatment plant—typically about 60°F (15.6° C), though this varies by geography and season.

In a sewage heat recovery system, a heat pump is used to capture the warmth of wastewater and transfer it to the clean water stream that is entering homes and businesses. It all operates as a closed-loop system, meaning