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In 2012, King County moved forward with a Request for Expression of Interest and Information (RFI) that invited local developers and commercial property owners to submit ideas for privately-owned district energy systems that could extract and recover heat contained in the Wastewater Treatment Division's conveyance system.

How does it work?

King County Average Temperatures (calculated from 3 years of data, 2009 – 2011)
  • Sewage is a great source of heat because of all the hot water that goes down our drains each day from showers, laundry and dishwashing. Flowing beneath our feet right now are millions of gallons of warm wastewater. The technology can extract and use the heat from the pipes in the same way that a car radiator extracts heat from the engine to warm up the car interior in cold weather.
  • Are there odors? No. The system is “closed-loop”, so there is no actual contact with raw sewage.  
  • Wastewater traveling through underground pipes maintains an annual average temperature of about 65-70 degrees Fahrenheit - warm enough to provide an extractable source of thermal energy that can be piped to nearby buildings, replacing the need for boilers, furnaces, or air conditioners.  WTD is proposing to work with local real estate developers to prove how we can tap into this thermal energy asset.
  • 70% of US energy consumption is to heat/cool space & water (U.S. Energy Information Administration, Residential  Energy Consumption Survey )
  • Reclaiming wasted thermal energy is more efficient than any form of energy generation. Thermal energy contained in the world’s sewers is an enormous untapped resource.  US Department of Energy estimates that 350 billion KWH are discarded annually down drains in the form of heated water.
  • Thermal energy is captured from sewage using a heat exchanger. Similar to a geothermal application, heat pumps then boost temperatures from the warm sewage supply to a higher temperature range useful for residential space heating and domestic hot water.
  • Sewer heat recovery has efficiency and cost advantages when compared to typical geothermal installations, due to higher heat source temperatures and lack of expensive well-field installations.  During the coldest days of the year, systems tend to be augmented by high efficiency back-up natural gas boilers.

Where has this already been happening?

  • New technologies are already proving success in Vancouver BC, Norway, Switzerland, Japan, and Paris.  While several projects are in the planning and development phase in the United States, no U.S. projects are currently doing sewage heat recovery from the conveyance process.

Could an energy system tied to the sewer provide both heating and cooling?  Could it eliminate the need to put in cooling towers in new buildings?

  • Yes, heating and cooling applications are possible using sewage heat recovery technologies - capturing heat from data centers or cooling towers works.  This not only saves energy, but saves water as cooling towers use an evaporative process to dissipate heat.  

Most projects like this are dependent on hydronic building systems.  How can we make hydronic building systems more prevalent so District Energy projects will work?

  • Most district energy projects utilize hydronic building systems.  Part of the intent of King County’s 2012 RFI was to begin working with designers and developers at very early in the conceptual design phase to allow for consideration of sewage heat recovery.


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