After 14 years of living on a houseboat we have now moved into an apartment in a private home. There are many wonderful things about this apartment — the lack of dry rot and any concerns about the wind direction are two of them. Perhaps my favorite is the radiant floor heat.
Radiant heat, hot water piped under the floors, keeps your toes and the rest of your body toasty. Water is great for this as it has a high specific heat, i.e., it takes a lot of energy to heat it up and then it holds the heat very efficiently. That is one reason that you can sometimes swim in the ocean in October even if you don’t live in Miami. The water absorbs the heat from the summer and holds onto it well into the fall. It is also the reason that snow is rare in Seattle, a city surrounded by water. The water is warmer than the air and keeps our precipitation as liquid. Here it is important to remember that the heat in the water is a form of energy storage. We have the ability to use that heat either directly as warm water or indirectly by using the energy that is stored in the heat.
Radiant floor heat is a wonderful thing in my apartment but it can be an even better thing for a neighborhood. District Energy Systems is the term used to describe radiant floor heating on a community scale. Hot water or steam is produced at a central plant and then piped underground to buildings. The heat from the water or steam is used to warm the air and the water in the connected buildings. In the summer months, chilled water is substituted for the heated water and provides cooling. These types of systems are energy efficient, reduce carbon emissions and reduce worry and maintenance in individual buildings. There are more and more of these systems in the U.S. and internationally.

“Instant” Infrastructure

Now I bet everybody reading this wants toasty toes in the winter and a cool floor in the summer. And here’s the awesome news: the infrastructure for these systems is one we already have! Our wastewater systems carry warm water to treatment plants that then treat the water and release warm effluent to natural water bodies. The water in these pipes is typically 65° to 70°, warm enough to provide energy to heat buildings, melt snow on sidewalks, and even heat swimming pools. These pipes already run under every city street — from every home and building connected to the municipal wastewater system.
Right now this is waste heat and can even be environmentally detrimental. Effluent discharges into streams at temperatures higher than water temperature can damage aquatic life within the rivers and streams. Take that excess heat and put it into a swimming pool and the people in the pools will smile and swim longer. If you happen to be swimming laps in the 14th Arrondissement of Paris, your water has been warmed by heat exchangers in the sewage pipes. The metal plates capture the warmth and not the odor. The warmth is then used to transfer the heat via a heat pump to the water.  A similar system using effluent from the treatment plant was put into use in the Olympic Village in Whistler, British Columbia.
According to the U.S. Energy Information Administration, 70 percent of our energy use is to heat or cool space and heat water. What this means in Whistler is that it has up to a 95 percent reduction in carbon emissions associated with heating and cooling. The U.S. Department of Energy estimates that 350 billion kilowatt hours of energy are generated and not used each year in the form of warm wastewater. What that means in the U.S. is we have a ready-made resource under our feet.

Making It Happen

The pool in Paris and the Village in Whistler are the only two cases I could find where this resource is actually being used. King County (WA), where I live, has been trying to get some parties interested for a few years. So why don’t we all use waste heat from wastewater to keep our toes warm in the winter and cool in the summer?
First, the pipes that carry our wastewater and effluent are generally decades past AARP eligible. These systems are old and leaky and were built long before the technology that makes District Energy Systems possible. In order to set them up to allow for heat transfer we would need to dig up a lot of streets and also convert heating systems on the receiving ends.
Second, these same pipes are typically owned and operated by municipalities, whose primary mission is to keep us healthy by killing the pathogens in our wastes, not by making sure our homes are warm enough. Changing this perspective to broaden benefits and potentially allow for alternative sources of revenue will require visionaries in municipal governments.
Third, this heat is a waste and has traditionally been considered as such. For those concerned with effluent discharges, temperature regulations are another hurdle to deal with alongside nitrogen and biological oxygen demand limits. It is a big step to go from waste and nuisance to resource, particularly when to date there has been limited motivation to do so. This type of change can be encouraged by national incentives and/or public and private partnerships.
We are getting to the point where waiting to replace our infrastructure will not be an option. We have seen bridges collapse. While it is harder to see sewer pipes fail, they are already leaking like sieves. Before we make the major investments to replace these systems, it would make a lot of sense to consider how to design them to maximize their utility. And that requires a broader lens to understand their potential purposes, including district energy. This can also mean lowering the potential costs to ratepayers. If rebuilding our water and wastewater infrastructures carries a high price tag, cutting or eliminating our heating and cooling costs are great ways to reduce the pain.
A recent article noted that some municipalities are turning to private capital and management to rebuild their water infrastructures. These private firms get the job done but typically at higher than projected costs with a company profit included in the price tag. Their focus is likely more on the profit than on the vision. Putting trust back into municipal systems and municipal officials could likely reduce the price tag. Having officials with the foresight to find the value in waste makes the whole deal even better. It may not increase the profits but it would more than double the benefits.
Capturing the heat and the potential benefits make sense from a cost perspective, a carbon perspective and a toasty toes perspective. Just consider that and don’t let yourself get cold feet.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle and a member of BioCycle’s Editorial Board.