Geothermal heat pumps and wastewater

﷯It is wonderful to be in the plumbing and HVAC industries, particularly if you pay attention to trends, and there’s another one coming into the mainstream for us. “Waste-water thermal energy extraction” is gaining fast in popularity and practice. Yes, this is about the sewer.  It sounds messy, but don’t worry; someone else has already done most of the messy work for us.  This technology is related closely to energy recovery ventilators (ERVs) in principle, and will quite likely be a code requirement in coming years.  

Lynn Muller, chairman of International Wastewater Systems was among the speakers at an ASHRAE Geothermal Workshop in El Paso on April 11. As a disclaimer, I’m a neutral geothermal industry consultant. This story was selected solely on its value to the industry.

I’ve been following his efforts for quite some time, and it’s time to showcase a little bit of what he’s doing. A total of 350,000,000,000 kWh of usable energy goes down the drain each year in the U.S. according to the Department of Energy (DOE).  That is far more recoverable energy than ERVs have the potential of recovering from building exhaust fans. That’s probably enough energy to heat a not-so-small country.  

With 350 billion kWh of energy you can:

  • Heat 5 billion average-sized homes in the dead of winter, for an entire day (24 hours)
  • Heat 23 billion (50) gallon domestic hot water (DHW) tanks up three times from room temperature
  • Lynn’s company introduced the SHARC some years ago and had great success, so much success that the company has gone public. 

﷯The principle is simple in essence. Whenever you take a shower, run the washing machine or dishwasher, the temperature of the water going down the drain is going to be warm, at least as warm as the indoor temperature. That even counts for flushing toilets, because the tanks usually have time indoors to assume room temperature. 

The SHARC extracts the heat by running it through the SHARC and sending it through an exchanger, where a geothermal heat pump (GHP) does the job of heating domestic water, and even providing space heating and cooling, or just about any other heating and cooling function that may be needed.  

﷯Just like an ERV in principle, the heat going down and out of the building is captured and sent back to a mechanical device, in this case a GHP as a source for heating and cooling. In this process, the energy never really escapes the building, or in other words, is re-used. Taking a look at the energy consumption in U.S. buildings, it’s easy to see that domestic hot water runs second only to the energy consumption for HVAC.

Thermal energy extraction is easy!

It’s important to understand some of the ways that energy can be recovered.  It’s easy to convert waste heat, or thermal energy to a warmer temperature by use of a fluid/water sourced heat pump, more often called a geothermal heat pump or GHP. A GHP's job is to move thermal energy uphill, or in other words, increase the temperature.  

With 70 F wastewater, the jump to 130+ degrees is efficient and easy, boasting a coefficient of performance (COP) of 4 or 5, or 500 percent efficiency. A COP of 5.0 simply means that for every unit of electricity used, 5 units of heat transfer occur. Here’s a picture below to simplify how a GHP works.

﷯More about geothermal heat pumps

Geothermal heat pumps (GHPs) are the central component of the thermal extraction/rejection portion of energy recovery. GHPs use available energy in liquids between 25 F and 110 F and are able to absorb or reject heat to/from them.  GHPs are “thermal energy pumps”, concentrating heat energy through the Carnot Cycle principle, delivering final temperatures from well below freezing to 140+ for uses such as space conditioning, refrigeration, or domestic hot water.

Wastewater temperatures are favorable for the process of thermal transfer with GHPs. The average temperature of wastewater for all buildings/dwellings is between 65 F to 75 F, right in the middle of the highest efficiency ranges fo

A GHP uses liquid (like the water entrained in pipes, underground or even from a sewer line) as a heat source or a heat sink. There is a lot of energy in 70 F water, but at that temperature, the water is also able to effectively absorb vast quantities of energy. When the water is giving off heat, it’s a heat source. When it’s absorbing heat, it’s a heat sink.

Hydronic systems are amazing because they effectively channel BTUs within a pipeline, unlike air source systems. Geothermal heat pumps (GHPs) make the magic happen, by simply manipulating BTUs to whatever temperature is needed at the time. GHPs are the center of the energy universe for renewable and sustainable energy systems.

Jay Egg is a geothermal consultant, writer, and the owner of EggGeothermal. He has co-authored two textbooks on geothermal HVAC systems published by McGraw-Hill Professional. He can be reached at


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