When the occasional snowfall dusts the streets of Vancouver in the midst of winter, the layer of white can quickly become punctuated by steaming openings where it has already melted. The access holes to the drains below ground are caused by the heat flowing through the city's sewers, warming up the pavements.

"There's enough heat in the sewerage system to literally heat up neighbourhoods," remarks Derek Pope, manager of neighbourhood energy for the city of Vancouver, Canada. "That's what we've been doing here in False Creek since 2010."

The residents of False Creek, a recently redeveloped neighbourhood of Vancouver, on the west coast of Canada, get their energy from a rather unusual renewable source – their sewage wastewater. Increasingly, municipalities around the globe are harnessing this underground form of excess heat as they decarbonise their energy networks.

Residents in the 6,210 apartments in the False Creek neighbourhood get their heat from renewable energy sources, with sewage heat being the largest contributor.

Everything that goes down our drains ends up as sewage water – from what we flush down the toilet, to what comes out of our baths and washing machines.  Down the pipe it flows, eventually ending up in a wastewater treatment plant where it is chemically, biologically and mechanically cleaned and treated, ready to be recirculated back into our homes once again. But the heat that's generated from the dishwasher or a long, hot shower is generally forgotten about once it washes down the plughole, explains Pope. Instead it heads underground, and straight into the sewer systems – escaping out of vents and melting through winter snow.

Heat in water is relatively easy to harness once it's in the sewage system because it's contained – and no, any heat recovered isn't going to smell. By comparison, heat in the air quickly escapes out of windows, doors, and roofs. In addition, there's plenty of hot wastewater to work with. In 2020, experts at the London South Bank University estimated that energy from the UK's daily 16 billion litres of sewage wastewater could, in theory, provide more than 20TWh of heat energy annually – enough to provide space heating and hot water to 1.6 million homes. Over in the US, Americans flush an estimated 350TWh of energy down the drain each year – the equivalent of heating 30 million homes a year.

There's enough heat in the sewerage system to literally heat up neighbourhoods – Derek Pope

Tucked under a Vancouver bridge, an energy centre sits on top of the existing sewage pumping station so heat can be captured before sewage reaches the treatment plant. Heat pumps cool down warm sewage that's around 20C (68F) in temperature and concentrate that heat to produce scalding hot water which can be as high as 80C (176F), Pope explains. "What's really exciting is that our heat recovery system operates at efficiencies of over 300%, so for every unit of electricity that we put in to run the heat pump, we get over three units of thermal energy or heat out of it." Sewage is also consistently warm so heat pumps continue to operate efficiently even on cold wintry days when heat demand is highest – providing a constant source of renewable energy.

In the EU, buildings are responsible for 40% of energy consumption. Heating, cooling and domestic hot water account for 80% of the energy that we, as citizens, consume. In Vancouver, Pope explains, buildings are responsible for more than 50% of the city's greenhouse gas emissions, because the main way the city heats its buildings and produces hot water is by combusting natural gas. "So utilising waste heat is one of the tools in our toolbelt to transition away from that," he says. "This is a really good platform for reducing emissions at the neighbourhood scale, especially in areas in dense populations."

Harnessing sewage heat is a "missed opportunity", according to Semida Silveira, professor in systems engineering at Cornell University in the US. "There's a lot of heat in the world that we just throw away. Today, we have a lot of energy inefficiency," Silveira explains, pointing out that energy efficiency could resolve half of the targets for carbon reductions in the US.

Energy efficiency is overlooked because it's invisible, Silveira adds. Yet, the International Energy Agency, the global authority for the energy sector, refers to energy efficiency as "the first fuel" – a simple and cost-effective way to reduce demand and strengthen energy security. Excess heat is the world's largest untapped energy source according to a 2023 report published by global engineering firm Danfoss. The report highlights how surplus heat produced in the EU – the heat released from transport networks, factories, data centres, sewage systems and so on – could provide heat for almost the entire region if harnessed effectively.

Excess heat within the sewage system is being successfully captured and repurposed in some towns and communities, like False Creek.

Once the heat running through False Creek's sewage pipes is recovered by heat pumps a five-mile-long (8.3km) pipe network, known as a thermal grid, distributes heat back to the district's 44 buildings. "In each building, heat exchangers transfer heat out from the closed-loop water system into the buildings' heat system and domestic hot water pipes, then cool water recirculates back to the energy plant," Pope explains.

At False Creek's sewage heat recovery system, construction is underway to expand. Heat pump capacity is being tripled from 3 megawatts (MW) to more than 9MW. "That's a big milestone for the utility," says Pope who has noticed "an explosion" of district energy systems across the Greater Vancouver region, with many using or planning to use sewage heat as their primary source of energy, as opposed to fossil fuels such as natural gas.

As part of Vancouver's climate emergency action plan, the False Creek Neighbourhood Energy Utility, which is owned and operated by the city, is developing a roadmap for heat sources to be fully renewable by 2030. By then, Pope envisages most systems will use a combination of waste heat, thermal storage, and other renewables to meet decarbonisation targets.

The question is, who's going to pay for that? – Semida Silveira

The biggest challenge is a lack of integrated, forward-thinking urban planning, argues Pope: "We need to start viewing waste heat as a resource and find ways to incorporate that into our city planning." While he acknowledges there's some upfront capital cost, Pope explains the neighbourhood energy unit in False Creek is set to recover its outgoings; residents pay utility rates and the cost of low-carbon energy is relatively low, plus the recent expansion has been part-funded by a grant that supports projects reducing greenhouse gas emissions. Next, he wants to see more municipal governments make that step to implementation.

Investing in infrastructure that uses this form of excess heat as a resource makes financial sense because wastewater treatment is such an energy-intensive process. Water treatment plants eat up an estimated 30-40% of municipal electricity bills, according to a 2017 report by the Congressional Research Service, a US-based public policy research institute. By harnessing sewage heat, wastewater treatment plants could become energy producers rather than energy consumers. In the Danish city of Aarhus, the Marselisborg wastewater treatment plant produces more energy than it needs to treat and distribute water for the 200,000 people it services, by recovering waste heat and processing waste sludge to produce biogas. The potential in excess heat from wastewater treatment plants in Denmark, a country of 5.8 million inhabitants, corresponds to 600-700MW, according to the Danish Water and Wastewater Association. That's the equivalent of two fairly large power plants, meaning a potential to heat about 20% of all households with carbon-neutral heat sourced from sewage.

Change, Silveira believes, is on the horizon. "Urban planners always used to think about land use, transport, functionalities, but today many municipalities are thinking in terms of energy too," she says. But well-designed policy is crucial. Silveira says the key question for municipalities is whether to capture heat from sewage water at a local level (by capturing hot shower water in buildings, for example) or at a more centralised level (once sewage water reaches the treatment plant). While property owners might want to recapture excess heat within the buildings directly to reduce their energy bills, water treatment plants might prefer more heat to reach them via warm sewage so they don't have to buy-in extra heat. The technology exists for homeowners to capture excess heat – for example, the steam from a hot shower – which can be funnelled back into the home's own heating system. However, it's costly. "If you're going to solve the problem, you're going to have to make an investment, so the question is, who's going to pay for that?" Silveira asks.

In Sweden, a cold country with high heat demands, district heating systems like False Creek's are commonplace. In densely populated areas, they are "a fantastic way to distribute waste heat" from a variety of sources, including sewage, says Silveira, who has been studying the wastewater heat recovery system in Stockholm that provides heat to about 800,000 residents. "The Stockholm system is totally underground, you don't see or hear anything, there's no smell," she explains.

Like in Aarhus, Stockholm's sewage heat recovery produces biogas from digested sludge. That biogas is used as bus fuel by the city's public transport network. "You can do a lot to improve the circularity of a city if you have these centralised solutions," says Silveira who emphasises that Sweden's pioneering carbon tax system has also been "instrumental" because it provided the incentive for the heating sector to use waste heat and move away from fossil fuels.

So successful harnessing of surplus heat from sewage is not just about technology, as Silveira explains. There has to be buy-in at government-level. "Political organisation and regulations influence which direction the decision makers choose to go, and political will depends on what they perceive as possible." She explains that in Sweden, centralised solutions like district heating have worked well because municipalities have the power to make these kinds of long-term decisions.

But even in countries like the UK where local councils don't have as much sway, or where urban areas aren't yet set up for district heating, sewage heat can still be repurposed and used to operate water treatment plants and provide clean water and sanitation. "If water treatment plants can use that energy internally, that's a big thing that most cities can do," says Silveira. Already, wastewater heat recovery projects are underway around the world, including in London's Kingston-upon-Thames, Oslo in Norway and Beijing, China.

Back in False Creek, there are tangible benefits for residents. Noisy heat pumps and boiler equipment operate within the energy centre, so rooftops of residential buildings that would otherwise have been home to heating infrastructure are being repurposed as communal gardens and green roofs. Maintenance is centralised, carried out by professionals and paid for by the neighbourhood energy unit – plus customers are protected from fuel price volatility, Pope says. "By connecting to a system that operates at a very high efficiency, you're sheltered from price swings in gas or electricity."

Pope hopes that False Creek will inspire other municipalities to decarbonise their thermal grids. "The local emission reductions that we're realising are great but where we can really amplify the effect of what the city is doing is by sharing what's worked and what hasn't, then help others make confident investments in low carbon energy."

Correction: This story originally stated that False Creek was a suburb of Vancouver, which is incorrect. It has also been clarified to note that winter snowfall in Vancouver happens only occasionally, where the original article suggested it was more common.

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