October 10, 2012

Mining sewage for heat is catching on

Construction Corner | Korky Koroluk

We're hearing more and more about mining sewage systems to recover heat. They're far from common so far, but they are cropping up here and there around the world.

There’s one in a condominium building in Vancouver, several in China, a few in Paris, one in Philadelphia —the list goes on.

It’s clearly a trend, but it’s a trend within a trend.

Tapping sewers to obtain heat is just like geothermal energy, but instead of using the ground or a lake as a source, you use the sewers, which is why it’s become known simply as sewage geothermal.

Most of us never think about the sewage being carried in pipes right beneath our feet.

It’s usually pretty warm and we can capture that heat through heat exchangers, just as a ground-source geothermal system uses exchangers to bring heat into our homes.

A Vancouver firm, International Wastewater Heat Exchange Systems Inc., has a system that filters suspended solids, and then sends the filtered wastewater to a heat exchanger.

There, heat is either extracted from the wastewater (for heating) or transferred to the wastewater (for cooling). Then the wastewater is discharged back to the sewage main pipe.

In Philadelphia, a firm called NovaThermal Energy LLC is doing something similar.

It has built a plant at a sewage treatment plant for easy access to wastewater.

The company also plans to market a Chinese system that’s somewhat like the one in Vancouver.

They plan to sell it to any large buildings located near a major trunk sewer that carries a steady flow of wastewater still warm from its previous uses.

Many industrial processes use hot water, which is one reason wastewater is usually pretty warm.

International Wastewater says it can reach an average temperature of just over 25°C where it leaves buildings.

In septic drains, the average is about 15°C.

What we’re seeing is part of a larger trend: district heating and cooling.

We’re going to see more and more of it as construction costs climb, as energy costs soar, and more ways are sought to limit emissions of greenhouse gases.

District heating and cooling is hardly new. In Northern European countries, district energy systems are an important part of heat production.

More than half of Denmark’s buildings are on district energy systems. In its capital, Copenhagen, 98 per cent of buildings are supplied by district energy.

Even in North America, it’s not uncommon to see entire industrial or academic campuses heated from a central plant. In Ottawa, a central plant heats the government buildings on and around Parliament Hill.

At one time, it could be even be found in small towns.

Growing up in small-town Alberta, I was familiar with the gas-powered steam plant that provided heat to the small central business district.

But, it was shut down in the name of progress, and individual buildings had to install their own small gas boilers.

In recent years, we’ve seen the growth of Enwave Energy Corp., which now provides district heating and cooling to something like 140 buildings in Toronto’s downtown core.

Just north of Toronto, Markham District Energy now has three combined heat and power plants in service, with a fourth under construction.

District energy is growing, so it makes sense for the systems to tap in to any energy source that’s handy, and that often means a sewage plant or a large sewer main.

All this will mean that an increasing share of our heating and cooling needs will be met by small facilities serving a relatively small area—a subdivision, an industrial park, a neighbourhood, a town. It will mean more work for electrical and mechanical engineers and contractors as a changing climate forces us to seek out new ideas to replace those that have become too shop-worn to keep.

Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments or questions to editor@journalofcommerce.com.

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