Recycling Sewer Water to Save Energy and Money

Recycling sewer water does not sound appealing; however, it is already used around the world for heat, air conditioning, and even drinking water.  When the American Geophysical Union decided to renovate its headquarters in Washington DC with a goal of net zero energy use, a wastewater heat exchange system was researched as a potential solution for achieving net zero energy.  Read on to learn more about their findings, the energy and money saving potential, and how the system actually works.

There are currently over 500 wastewater heat pumps used around the world with the first installation constructed over 20 years ago. According to studies from Switzerland and Germany (where the majority of these technologies are currently being used) approximately 3% of all buildings could use wastewater heat pumps for heat and air conditioning. “Compared with other, traditional sources of energy for heat pumps, wastewater from local residential drainage systems exhibits relatively high temperatures during the heating season. Values under 10 degrees Celsius (50 F) are rare. Wastewater therefore offers an ideal basis for the use of heat. In summer, the wastewater temperatures climb to over 20 degrees Celsius (68 F). This also allows use for the generation of cold for air conditioning” (Schmid, 2). Not only does this technology save money, but it is also environmentally friendly.

Hot water is wasted every single day that could otherwise be recycled.  “According to the U.S. Department of Energy, 400 billion kilowatts of hot water goes down the drain annually in the U.S., which is roughly 40 billion dollars’ worth of energy at an average price of $0.10/kW.” Every day approximately 1 million gallons of sewage is created per 10,000 people. This sewage is 70 degrees Fahrenheit, which is an ideal temperature for heat pumps. “Sewage is free, it’s abundant, and it’s widely untapped” (Lyons Hardcastle).

The American Geophysical Union (AGU) is performing a $41.7 million renovation of its headquarters, 62,000 square feet in Washington D.C., all of which will meet net zero energy goals.

What does net zero mean? “Net zero means the total amount of energy used yearly by a building is equal to or less than the amount of energy created onsite through the use of innovative technologies and renewable power generation” (AGU).

Varying technologies like solar PV, radiant cooling system, green wall, and sewer heat exchanger are being implemented to achieve the net zero goals.  In order to learn as much as possible about wastewater heat exchange systems, the AGU team traveled to Germany and Switzerland to check out real-world working examples.

So how exactly will the sewer heat exchanger work at the AGU’s headquarters?

• Wastewater will be brought to the wet well through gravity and filtered for solid materials and then pumped into the building
• “Wastewater is pumped into the heat exchanger tank and condenser water fills the tubes inside the heat exchanger tank and transfers heat without coming into direct contact with one another.”
• The condenser water is then pumped to a chiller and the wastewater returns back to the sewer system

In addition to recovering heat for pumps, additional value provided includes:

• “During periods of the year when the sewer temperatures are cool, the building’s radiant cooling system will operate in “free cooling” mode using the water from the sewer heat exchange system allowing the building’s chiller to be turned off” (AGU).
• There is no need for a cooling tower on the roof, which saves a substantial amount of fresh water.
• “Noise and unsightly plumes usually associated with cooling towers are eliminated” (AGU).

According to various wastewater heat capture system installers/manufacturers, estimated emissions and cost reductions are between 30% to 85%, depending on the system and the building use.  Additionally, these systems can qualify for LEED points for sustainable building design.

Thought this was interesting? Read on to learn how other renewable technologies, like Electric Vehicles, will grow over the next few years.

Sources:

Lyons Hardcastle, Jessica. Environmental Leader. (2016, February 24). Using Sewage to Provide Buildings’ Heating, Cooling. Retrieved 2017, August 11, from https://www.environmentalleader.com/2016/02/using-sewage-to-provide-buildings-heating-cooling/

Huber Technology Waste Water Solutions. Huber Heat Exchanger RoWin. Retrieved 2017, August 11, from http://building.agu.org/2017/03/29/agu-seeks-international-best-practices-for-sewer-heat-exchange/Huber T

Patke, Samantha & Perry, Great. AGU- American Geophysical Union. (2017, March 29). AGU Seeks International Best Practices for Sewer Heat Exchange. Retrieved 2017, August 11, from http://building.agu.org/2017/03/29/agu-seeks-international-best-practices-for-sewer-heat-exchange/

Schmid, Felix. Energy-engineer FH, SwissEnergy Agency for Infrastructure Plants Gessnerallee 38a, CH-8001 Zurich, Switzerland. Sewage Water: Interesting Heat Source for Heat Pumps and Chillers.