New heat pump technology harnesses the energy of wastewater

Cities around the US are trialling a pioneering new technology that reduces the need for traditional heating by up to 70%

Sewage heat recovery systems may not sound sexy, but one day they could mean your dirty bathwater helps provide you with more efficient and lower-carbon central heating.

Every day the potential energy contained in vast quantities of warm wastewater disappears down countless drains in the UK and across the world, but thanks to a pioneering new system, grey water – from baths, showers, washing machines, dishwashers and sinks – can now be exploited to help create a more sustainable future.

The temperature of wastewater travelling through sewers to treatment plants varies geographically and according to the season, but is typically around 15.6C (60F). The new system uses heat pumps to capture the heat that remains in grey water after it leaves our homes and transfers it to clean water streams. This heated water is then ready to be fed back to households and businesses.

It all works on a closed-loop basis, meaning only the heat is transferred and there is no risk of the dirty water contaminating the clean water supply. Tapping the warmth of the sewage reduces the amount of energy needed from conventional sources to heat water to the required temperature for household needs.

As well as heating water that comes out of the hot tap or goes to washing machines or dishwashers, the system can also heat water for radiators in the same way.

“Sewage heat recovery systems may not sound sexy, but one day they could mean your dirty bathwater helps provide you with more efficient and lower-carbon central heating”

And in yet another benefit, buildings with the system – which is sometimes referred to as sewage geothermal – can reverse their heat pumps in summer to cut air conditioning costs.

The systems have been trialled in a handful of locations such as Oslo, Tokyo, Beijing and Vancouver. One of the most notable early examples is in Beijing, where it is used to heat and cool a hotel, a train station and a high-rise apartment block.

Another high profile example operates at the Southeast False Creek site, which was Vancouver’s Olympic Village during the 2010 Winter Games. It is the first major system of its type in North America and serves a community of up to 16,000 people, which aspires to be one of the greenest in the world.

The 3.2 megawatt system supplies around 70% of the community’s heating energy needs, with the rest coming from three natural gas boilers. First, sewage is filtered before being pumped through a heat exchange system. Water-to-water heat pumps then tap thermal energy from the wastewater. It is then transferred to pipes, which carry a separate loop of clean water into radiators for heating and into local homes for general use.

According to its operators, the use of sewage heat recovery cuts greenhouse gas emissions by more than 50% compared to conventional heating. Its five exhaust stacks have even been turned into a piece of public art, featuring lighting that changes from blue to red as demand rises.

Meanwhile, a system that went online in Chicago in May 2012 has cut the costs for heating and cooling by 50%. Other locations in the US considering adopting the technology include Philadelphia and King County in Washington State. Good news, as according to one estimate, Americans currently flush 350 billion kWh of energy into the sewers each year – roughly enough to power 30 million US homes.

However, there are no firm signs yet that the system could be adopted in the UK soon. Water UK, which represents all major water and wastewater service suppliers, said it did not know of the concept and could not say whether any individual members had considered it.

According to the body, the UK water industry collects and treats more than 16 billion litres of wastewater each day, while Waterwise, an NGO focused on decreasing water consumption in the UK, states that domestic water heating is responsible for 5% of UK CO₂ emissions and up to 25% of household energy bills.

However, one groundbreaking project is currently being pursued in Scotland, where researchers at Glasgow Caledonian University are aiming to transfer heat from water in abandoned coal mines to provide up to 40% of the city’s heat.