Sewage Heat Recovery: Tapping into the potential energy of warm wastewater

By Lynn Mueller

Shower water is still hot when it flows into the drain. The same is true of the water we use for washing our dishes and our clothes. In fact, the typical North American flushes 75 liters of hot water down the drain every day. 

A recent study by the U.S. Department of Energy estimated that Americans flush 350 billion kilowatt hours of energy into the sewers each year. This wasted energy would be enough to supply 30 million homes. 

For a regular household, this can cost homeowners hundreds of dollars per year, as water heating is the second highest source of energy demand in a home. What you may not be aware of, however, is that it’s now possible to capture 95 percent of this wasted heat and recycle it back into our buildings using Sewage Heat Recovery (SHR).

Using SHR, the heat energy flowing down our drains never has to leave the building. Further, unlike solar or wind power, the technology doesn’t require a quantum shift in the way we live or the way we think, it can simply be plugged into our existing infrastructure.

Basically, water enters our buildings at 44.6-48.2 F and leaves at 68-77 F. SHR captures the heat in water that is leaving the building and uses it to re-heat our hot water tanks and the building itself. The technology is not complicated. 

First, a filter is used to separate out solids, which make up about 2-3 percent of our sewage. Then, with the help of a heat exchanger, the heat from the sewer water is transferred into clean water and this warm, clean water is sent back into the building. At the end of the cycle, the clear sewer water picks up the solids that were extracted at the start and flushes them back into the municipal sewer system. 

In the summer, buildings with SHR systems can reverse their heat pumps and use the wastewater to reduce a building’s air-conditioning costs. In this scenario, the pumps extract heat from the building and transfer it through the exchanger into the sewer water.

One of the reasons that the potential reusable heat in wastewater has been ignored is because touching sewage seems dirty. However, today’s SHR systems are hermetically sealed, meaning there is no associated smell. They are also designed to be clog-proof, with an automatic back flush to filter sewage simply and effectively. Moreover, an industry-leading computer monitoring system will flag any potential problems long before they become an issue. 

SHR is gaining popularity. Operations have started in Norway, Japan and even in China’s Beijing South Railway Station. North American cities are realizing that there is a valuable energy resource currently flowing under the city streets. Vancouver, Seattle and Philadelphia have all started experimenting with SHR systems. 

In Vancouver, International Wastewater Systems has installed SHR systems, called SHARCS, into several public and private buildings including the Gateway Theater in the city of Richmond. The Gateway installation will be the first application in Canada that will use raw wastewater directly from the municipal sewer, rather than the wastewater coming out of the building. 

Although SHR systems are applicable to any building, they work best with residential buildings of greater than 200 units or with institutional buildings like hospitals and prisons that have exceptional hot water usage. The most cost-effective time to introduce SHR systems into a building is while doing other energy upgrades or retrofits.

Worldwide, the option of using SHR on a district-wide scale is also being explored. District energy systems are large-scale, multi-building heating projects that can supply energy over a large area using either recovered energy from other buildings, industrial sources, waste, or by burning carbon-neutral fuels. SHR could easily plug in to district energy infrastructure. 

While sewage may not be as exciting as fuel cells or tidal energy, the fact that it has a payback period of 2-5 years makes it a cost-effective renewable energy system. SHR systems also work at 500-600 percent efficiency, meaning that for every dollar spent on operational costs, $5 dollars of heat is recovered. Moreover, current systems are demonstrating consistent energy saving performance of 76 percent. 

Looking back, at the turn of the 19th century sewage talk was related to public health and how to keep people from getting sick. During the 1970s, it became an environmental issue, as there was public worry about discharging pollutants into our waterways. Today, sewage is gaining attention in a different sort of way. Its potential as a renewable energy resource is undeniable. 

Lynn Mueller is president of International Wastewater Systems (IWS), Vancouver, British Columbia. Mueller has been a leading advocate of sustainable space conditioning energy systems for 25 years. He was previously president of WaterFurnace Canada, and president of Earth Source Energy Inc. IWS has successfully developed the SHARC technology and is well under way to implementing the system around the world. He can be reached at www.sewageheatrecovery.com or 604-475-7710.

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