Heated wastewater can be harnessed to pre-heat the inlet to hot water systems, offering considerable savings and the downsizing of hot water unit capacity.

Until recently, recovering heat in wastewater discharged from fixtures and appliances in dwellings, commercial buildings and industrial plants was a largely neglected source of energy.

However, with the rising cost of energy and the need to reduce greenhouse gases, greater attention is being given to this readily available and clean energy source.

Several heat-recovery systems are now available, particularly from companies in Canada, and new systems are also being developed in Europe.

Andre Cayer is president of Watercycles Energy Recovery, which is based in the Canadian province of Saskatchewan. The company manufactures the Watercycles heat-recovery system and exports to the United States and Europe.

Cayer says the system is designed to recover energy from water being flushed down the drain in residential and commercial applications. It uses no energy itself, and there are no moving parts.

“The Watercycle, which has a copper coil system and forms part of the wastewater pipe, extracts heat from the wastewater. This heat is then conveyed to the incoming fresh water before it reaches the water tank, thus pre-heating the supply. The system will extend the life of a hot water tank and reduce the stress on a tank-less water heater.

“In residential situations, the Watercycle requires no maintenance and has no effect on lifestyle while saving money and reducing greenhouse emissions. The system will recover its cost in two to seven years, and this is based on shower use only.

“For commercial applications, the main opportunities are in areas such as multiple shower installations in fitness centres and swimming pools, and dishwashers in commercial kitchens. The cost is greater due to the fittings and installation time involved, but savings are also greater because of the higher volume of hot water consumed in a commercial setting.

“In the case of dishwashers, the water temperature is also a factor, due to the delta T (change in temperature). The greater the difference between incoming fresh water and heated wastewater, the faster the heat travels, and therefore there is greater effectiveness.

“There is a C$100 (US$98) government grant across Canada for home-owners who retrofit, and in Saskatchewan the grant is C$200 (US$197), which is 30% of the purchase cost. There are also grants in Saskatchewan and Ontario for installations in new homes. In addition, several natural gas companies are offering grants to builders and home-owners in various provinces.

“Despite the grants, our largest market is the eastern US, where energy costs are three times those of Canada, so people can’t afford to throw energy away. Our main goal is to have Watercycle installed with tankless and solar water heating systems.

“Tankless systems are designed for groundwater temperatures of about 20ºC (68ºF). In locations where the water is 10ºC (50ºF) or less they have difficulty meeting the demand and a larger unit is required, which will of course consume more energy.

“The Watercycle will put water into the tankless system over 20ºC no matter how cold the incoming water is (for our standard 58 long unit). This wastewater heat-recovery system will also increase the amount of days the solar system can provide 100% of the hot water needs. It will provide more energy per dollar spent than increasing the size of the solar collection system.”

Renewability Energy, based in Ontario, manufactures the Power-Pipe drainwater heat-recovery system, which the company says is installed in many commercial, industrial, multi-unit apartment and residential applications in Canada, the US and Europe.

According to Renewability Energy vice-president marketing Walter Urban, the US Department of Energy and Natural Resources has identified water heating as the second largest energy use in American and Canadian homes, accounting for 20-30% of home energy costs.

“Showering is typically the highest hot water load, and about 90% of the energy used to heat water in a home is wasted to the sewer,” Urban says.

“A standard water heating system can be downsized by about 40% with the Power-Pipe because of the additional water heating capacity provided. A typical family of four will reduce greenhouse gases by one ton (1 tonne) per household per year by using a Power-Pipe.

“The patented Power-Pipe consists of copper tube wrapped tightly around an inner copper drainpipe. The unit becomes part of the drainage stack, usually in the basement of a building, by cutting the drainpipe and using the supplied connectors.

“The exterior coils become part of the freshwater supply line by diverting it to the Power-Pipe.

“A key feature of its operation is the way drainwater clings to the inside pipe wall and falls quickly, creating a ‘falling film’ that readily imparts its heat to the pipe wall. Efficiency of the system depends on the length and diameter of the Power-Pipe installed and the water flow rate.

“Manifolded systems are available for industrial and commercial applications to split large flows across multiple Power-Pipe units. Because industrial and commercial systems run for a large part of the day, energy savings can be substantial. Payback on industrial systems typically ranges from four to 16 months.”

Urban says there are numerous examples in industry where the installation of Power-Pipe has resulted in substantial energy savings, including at the Unilever Ragu food processing facility in Peterborough, Ontario.

“At that facility, hot that is used for cleaning product containers, and was previously wasted, is now recovered by a Power-Pipe system and pre-heats steam feed water used in the cooking process.

“This turnkey engineered heat-recovery system, which was installed at a cost of about C$27,500 (US$27,000), saved more than C$26,000 (US$25,600) in the first year of operation, resulting in a payback of just over a year. The system includes a heat meter to accurately quantify energy savings and operational performance.”

ReTherm Energy Systems, based in the Canadian province of Prince Edward Island, produces the ReTherm heat-recovery system, which is aimed particularly at the residential and small-business sectors.

General manager Kevin Coulson says the system is capable of recovering up to 30% of drainwater heat, is easy to install, does not need maintenance, and requires zero space because it forms part of the wastewater drain pipe. “In addition, ReTherm has a stay-clean design, as the fast moving water on the vertical centre section continually scours the heat transfer surfaces, thus preventing build-ups.

“This system is also easy to install. In homes under construction, installation can be completed in one or two hours, and retrofitting typically requires four to six hours.

“We have found that many businesses commonly waste enormous amounts of hot water energy every year, and they often have a water heating system that is substantially and needlessly oversized.

“By pre-heating fresh water via a heat-recovery system, it’s possible to double or even treble overall water heating capacity. Large volumes of drain water can be accommodated with multiple ReTherm units that can be configured for specific needs.”

Waterfilm Energy in Medford, New York, invented the GFX shower water heat-recovery system incorporating a gravity-film heat exchanger made from standard DWV copper drainpipe surrounded by a coil of copper water tube.

The inventor, Dr Carmine Vasile, says about 80% of the heat leaving a typical showerhead is carried away in the drainwater.

“We offer six standard heat exchanger models for recycling 40-75% of the drainwater heat. Actual savings depend on the shower flow rate, plumbing system and type of GFX system installed.

“For a two-bathroom house with two showers, two bathroom sinks, kitchen sink, dishwasher and clothes dryer, one Model S3-60-LC or S4-60-LC will do the job at normal cold water supply pressures.

“Best efficiency is achieved by providing pre-heated water to the entire house, but it can also just be fed to the water heater and/or the cold water input of a shower. If plenty of pre-heated water is mixed with hot water at the shower, far less hot water is needed to provide a comfortable shower.”

Developments in Europe

In the March 2007 edition of WPR we reported on the development of a heat-recovery technology called LowHeat, which is designed to capture low-grade heat in wastewater from the whole of a commercial or domestic building.

The project is being advanced by a consortium of 12 small and medium-sized enterprises in six European Union countries.

Alastair Green, research and development engineer at UK-based consortium member AK Industries Limited, says patents to protect the LowHeat technology are now in place and promotion can now proceed.

“Setting up a test installation is the top priority, and the main focus will be on industrial and commercial applications, rather than residential.

“The potential for large cost savings is much higher in these instances. Businesses such as laundrettes or dyeing facilities are prime candidates, as they produce huge amounts of hot water that is just poured down the drain.

“Recovering even a small portion of this heat energy would result in substantial cost savings and would highlight the real potential of this technology.

“Another benefit with industrial and commercial applications is that the installation itself tends to be less complicated. This type of property will often use a single drain outlet for most of the wastewater, whereas with residential applications there can be outlet drains all around the property, making installation more difficult.

“That said, once we have a test installation set up, and customers in the industrial and commercials areas are seeing real benefits, we do have plans to explore the residential market.

“At AKI we are also developing another wastewater heat-recovery device called Warmit, which is aimed at recovering heat energy directly from a shower. In effect, LowHeat is the follow-on project from Warmit, taking the idea to the next level by recovering wastewater heat energy from an entire property.”

New product in the Netherlands

In the Netherlands, the Itho Company has developed an updated version of its WTW shower water heat-recovery system.

Itho’s Ronald Jense says the new DWTW system features an aluminium double-wall drainpipe section so that if there is a leak the wastewater cannot come in contact with the fresh water, and any leak can be detected from outside the unit.

“This very lightweight unit, which is constructed of aluminium and PVC, has an efficiency of about 60%. By passing through the DWTW, the shower drainwater cools from 38ºC to 25ºC (100ºF to 77ºF), and the incoming cold water temperature increases from 15ºC to 25ºC (59ºF to 77ºF).”

The company expects substantial demand for the new system, particularly in the dwelling construction market.