Waste not, want not: Turning human waste into a renewable-energy source for buildings

In our continual search for renewable energy usage in our projects, we mostly look up to the sun and often to the wind. But we don’t often look underground to the renewable resource that is a human by-product—sewage. On average each person in developed communities produces approximately 400 liters (100 gallons) of wastewater per day.

Sewage is an energy source that starts in wastewater streams of residential, commercial, and industrial buildings and resides in our municipal sewer mains. We know there is an abundance of sewage, but what does it do to fight climate change? Sewage water is high-temperature water (roughly around 60°-68°F). A portion of the heat contained in wastewater can be recovered and used for heating a building during the winter and cooling it during the summer. Essentially, the sewer main can be used as a heat rejection device.

Sewage water can also be treated for non-potable water demands like toilet flushing and irrigation, thus becoming a key engineering strategy for meeting energy and water goals. And if the building site has a sewer main in its vicinity—then BINGO—you have found gold.

So, how does it work?

The sewer heat recovery system treats the sewer main as a large heat sink. It filters and pumps sewer water through a heat exchanger, providing a closed-loop heat exchanger that can be connected to both heat pumps for heating and cooling. By closed loop, it means that none of the sewage elements come in contact with the building systems—only the heat gets transferred. This system enables the use of heat pumps for heating, which drastically reduces the energy required to heat the building.

Heat pumps can save more than 50% of the heating energy consumption of conventional systems. And the fuel source for heat pumps is electricity, which can be offset by on-site photovoltaic panels—taking advantage of another renewable energy source.

The same principle applies to domestic hot water. It takes a lot less energy to heat 65°F water than 50°F water. During summer months, when there are cooling requirements, these systems can reverse their heat pumps and use the 65°F sewage to dump excess building heat and reduce the air-conditioning costs. This is a similar process to geothermal systems that use the earth’s temperature for heating and cooling, but geothermal systems are considerably more expensive than these systems.

As municipalities take action toward their carbon goals and move toward less on-site combustion, electrification of systems, and increased renewable-energy sources, the combination of localized renewable-power production and efficient electricity-driven HVAC systems reduces greenhouse gas emissions and results in less air pollutants in urbanized areas. Sewage as a renewable energy source is still largely untapped. However, through education, awareness, and outreach, architects, engineers, and developers are beginning to understand the potential of sewage as a resource of energy.