Wastewater Energy Recovery

It’s time to put to use the hot water from the sewer.

There are a lot of great ideas for saving energy, but first cost is usually a limiting factor for our customers. One of those has to do with wastewater energy recovery. I doubt that I’m alone when I have wondered about all of the hot water (and energy) running down the drain from  the shower/bathtub, washing machine, dishwasher and other fixtures and drains in the house.

Lynn Mueller of International Wastewater Systems Inc. (IWS) has been involved in geothermal heat pump systems for some 30 years. It was only a matter of time until he attached one to a plate and frame exchanger and made his own wastewater thermal extraction system. Maybe no one else would tackle the effort, but it’s a technology that has come of age. Mueller did well, and his company went public last year and continues to expand. It won the AHR 2016 Green Building Product of the Year with the newly released PIRANHA heat recovery system.

California is getting one thousand (1,000) PIRANHA thermal heat recovery systems, thanks to a joint venture between International Wastewater Systems Inc. and RENEW Energy Partners LLC. The PIRANHA is designed to provide for the domestic hot water (DHW) needs in multi-family housing from 50 to 200 units.
According to the DOE, 350 billion kWh of usable energy goes down the drain each year. That is enough energy to heat 5 billion average sized homes in the dead of winter for an entire day (24 hours) or, heat 69 billion DHW tanks up from room temperature to 130 F. This is a remarkably large quantity of energy that is not being recovered. 

Just as heat pump water heaters have been proven to be a great way to save energy on DHW bills, energy recovery with wastewater takes this concept to the next level. With a heat pump water heater, the compressor normally sits atop the tank and extracts heat from the air in the garage, or wherever it happens to be located. The by-product is chilled air, because the heat has been pulled out of the air stream and placed in the DHW tank. Essentially, the PIRANHA does the same thing with the wastewater that runs down the drain, recovering unused energy for other purposes such as DHW, heating or cooling; but the potential exists to save quite a bit more energy with wastewater than just air-exchange alone.

In Vancouver, Canada, the city built a wastewater energy plant to displace natural gas (NG) heating, and they are convinced that through the use of geothermal heat pumps (GHPs) and some smart strategies, they can meet their goals. This article has an example of a large wastewater heat recovery system and a video interview with Brian Crowe, Director of Water, Sewers and District Energy for Vancouver (a must see). Incidentally, Vancouver is Lynn Mueller’s hometown. 

The PIRANHA thermal heat recovery systems that are going into California are the entry level portion of this technology, recovering energy primarily for DHW needs. Another product of IWS introduced some years ago is the SHARC, having more applications, such as cooling and heating for residential, commercial and district applications. This is the junction at which wastewater heat recovery and mechanical professionals become fully engaged. There will soon be buildings and designs crossing our collective desks with these waste energy recovery systems in the design, and they’ll be a big part of the plumbing and HVAC systems.  

I spoke with Mueller recently, and he has some exciting things coming up. In the near future, IWS will be introducing a residential level wastewater energy recovery unit. This is another opportunity to help customers become more sustainable, save money and increase the profits for plumbing and HVAC professionals. Mueller told me that IWS is seeking professionals and companies that wish to be trained on this technology. Just give them a call.

GHPs are integral components of this technology. Unlike the air-source type, 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 and reject heat to/from them. GHPs are “thermal energy pumps,”  concentrating heat energy through the “Carnot Cycle,” delivering final temperatures from well below freezing to 140+ for uses such as space conditioning, refrigeration, or domestic hot water.  

Of course, these applications are well suited to be a hybrid of waste energy (as the primary source/sink) and earth coupled systems. The earth coupled portion of projects is reduced in both scope and cost by taking advantage of the wastewater thermal energy heat source and sink.

Hydronic systems are amazing because they effectively channel BTUs within a pipeline, unlike air source systems. GHPs make the magic happen, by simply managing BTUs entrained in liquids to whatever temperature is needed at the time. GHPs are the “center of the energy universe” for renewable and sustainable energy systems. 

Technologies that reduce greenhouse gas (GHG) emissions are in high demand; and wastewater energy recovery has a great potential to reduce GHG emissions and save energy in buildings of all kinds. This is a technology whose time has come. 

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 jayegg.geo@gmail.com.

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