A circular economy for heat

01 December 2025

How recovering heat from sewers, industry and waste treatment can save money and reduce carbon emissions

Staying warm this winter is about to get more expensive, with the energy regulator Ofgem having announced last week that gas and electricity bills for many households will go up again from January 2026. And of course, the cost of energy is not just financial but also environmental – burning fossil fuels releases carbon emissions that contribute to global warming, with devastating results for people and planet.

Yet throughout our society, heat is going to waste, whether as a byproduct of industrial processes or from our homes, where we send countless litres of hot water down the drain every day. This month on the Planet Possible podcast, host Niki Roach is exploring heat networks, an innovative way of repurposing that heat.

To discuss the role that heat networks could play, as we think about the energy mix that we want in the UK moving forwards, Niki is joined by Polly Cook, chief officer for climate, energy and green spaces at Leeds City Council, as well as David Riley, head of carbon and climate adaptation at Anglian Water and Michael Avant Smith, senior vice president for decarbonisation and growth strategy at the UK-based technology company BMA. David and Michael tell Niki about TORCH, an Ofwat Innovation Fund project on recovering heat from sewers.

If you enjoy this taster, you can listen to the full conversation here or search for Planet Possible in your favourite podcast app. Follow the podcast to stay updated on our monthly deep dives into pressing environmental challenges.

Now, over to Niki, Polly, David and Michael…

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Niki Roach: Let's start at the very beginning. What is a heat network?

Polly Cook: The most basic way of explaining it is if you think about your home heating system. So we have a heat interface unit, which in a house is the gas boiler, and then you have pipes running around your house. If you imagine that on a city scale, it's exactly the same. So we have pipes running around the city, transporting heat, they connect to a building with a heat interface unit, and it takes the heat from that system and heats the individual buildings. The heat source can vary, so it can be anything from an energy-from-waste plant to industrial processes like glass manufacturing. Data centes is a new one that's being looked at, or sewers, as we're about to hear about later.

Back in 2023, about 2-3% of our heat nationally was provided by district heating. And the ambition is by 2050, when we have our net zero goal as a country, we're aiming for 20%.

NR: Tell me about the network in Leeds

PC: There's 30km of pipes in the city as part of Leeds PIPES. It's connected to the energy-from-waste plant out to the east of the city, and that's where all the black bin waste from the city is taken and burned. It produces electricity, but also produces waste heat. That waste heat is then piped into the city center, and currently it heats 4,100 residential properties, and that could be student flats, social housing in tower blocks, but also 29 non-domestic buildings. So things like our really amazing historic buildings, like the Civic Hall, Town Hall, the Art Gallery, all of those are heated by district heating. One of the great benefits for us is that those buildings would have been really difficult to decarbonise. So actually having that heat just available has made that much, much easier.

We invested over £60 million in the heat networks. It's not a cheap project, but we've had funding support from the combined authority, from central government, even from the European Development Fund, when we first started. And then the council has probably invested about 50% of that money, because there's a business case behind that to support it as well as the environmental benefits. We've saved about 6,500 tonnes of carbon a year, and obviously that will grow as more and more people connect.

There's been social benefits associated with it as well, so it's supported 430 green jobs, 30 apprenticeships. Over half of that money has been spent in Leeds.

NR: Are Leeds pioneering in this space?

PC: When we started, we had lots of people who were really nervous about the technology. But actually places like Scandinavia have been doing it for decades. They're refining it and improving how they do it, but the technology is proven. And there are networks scattered around the country, but they're older networks.

Ours was one of the new generation. There hadn't been this big development and focus on district heating networks so ours was one of the first to really start that second revolution in it. But it's certainly not cutting edge in terms of new technology. The big challenge is how you get all the pieces to come into place in terms of investment and commercialisation, and planning policy that you have within your own place and nationally.

NR: What does policy and regulation look like for heat networks?

PC: The government has been really good in terms of the way that they’ve funded heat networks. It's been probably one of the most well thought through bits of policy in terms of how they've managed to get that market going. And they're currently in the process of developing advanced zoning policy – that will mean certain areas are allocated as district heating. The way that they've got to that position is they've determined where are the heat sources in a city. So there's no point in saying that a really residential area is going to be a district heating network if there's no waste heat nearby. So they've looked where there are industrial clusters or there's energy from waste plants and they will be designated as district heating zones. There will be controls about how that works and who gets precedent over developing those networks and what the role of the combined authority and the local authority is. So that policy will be really, really useful. Fingers and toes crossed that that will come in in 2026.

The other bit of regulation is from the customer perspective. The regulator will be Ofgem, the same as it is for gas and electricity.

NR: We’re now going to hear about a really innovative project called TORCH, which is aiming to shine a light on recovering heat from sewers. David, tell me what TORCH stands for and what it's aiming to achieve.

David Riley: It stands for ‘tool for optimising decisions for recovery of sewer catchment heat’.

If you think about the natural by-product, about what goes down the drain and what is flushed down the toilet, there is normally a heat associated with that. And that heat is contained within the sewer. It's an important part of the treatment process as well. If we can extract ready-made heat that already exists, is generally lost to the environment, is classed as a waste, that means that we are closing the cycle in terms of avoiding carbon emissions in the future. In terms of scale, when we were developing the project, we estimated around about 3.6 million tonnes of carbon that can be saved through the opportunity of obtaining heat. Now, will this TORCH Project deliver 3.6 million tonnes? No, it won't. But what it does do, it just shines that light on the scale of opportunity that does exist.

NR: Michael, how are you involved in TORCH?

Michael Avant-Smith: “So BMA have really been playing a central role in terms of the foundational platform. That's the technology platform that brings all the different sources of information, all the different data together into one place so that we can understand what that looks like in terms of the distribution of heat across the wastewater networks by the individual companies but more broadly as well. This is really looking at the national potential of wastewater heat recovery as well, not just a specific area.

And then what we're doing is matching that into the energy system. It's all very well having the source of heat, but how could you effectively use that as part of the overall energy system? Because obviously when we're talking about that heat recovery, we're talking about replacing heating from other sources. So that could be from fossil fuels, you know, traditional boilers, it could also be as an alternative to heat pumps, which obviously are seen as a very efficient way of providing heat.

Heat networks as a whole are probably the most efficient overall. But only if you can recover heat in the right place, move that recovered heat and upgrade that recovered heat efficiently. And that is the connectivity back into broader energy systems. So you still need to upgrade that heat with heat pumps, for example. That means more electricity demand. So it's a big puzzle that we're really piecing together.

NR: How engaged are the energy sector?

MAS: The levels of collaboration have been exceptionally good. There's actually more interest than we feel we can handle as part of a single project at the moment, which I think is very different to a lot of innovation projects where you struggle to get that engagement. Engagement is high from a variety of stakeholders, so the water companies, obviously, also local authorities, but also energy companies, particularly when you talk about the electricity distribution network operators.

They are very interested and, in some cases, concerned about the scale up of demand for heat pumps, for example. So anything that they can understand that will maybe take some of that load off or shift the load, they're very interested in understanding. Those levels of engagement will only increase, but I think we're only scratching the surface in terms of what that collaboration looks like, what that new value chain looks like.

The devil's in the detail there, in terms of what really makes sense for end customers as well. That's something we shouldn't lose sight of: this is ultimately about, does this make sense for the consumer to use heat recovered from the sewer network as part of a heat network? Is that going to be better, more reliable and, importantly, more affordable?

NR: Where's the maturity here?

MAS: This is a techno-economic study. You've got to consider what's technically viable. How do the technologies fit together, but also the commercial models. How does this get paid for? It will involve significant investment across that complete value chain, from recovery all the way through moving that heat, upgrading that heat, distributing that heat to different users.

NR: And if this works, what does scaling look like?

DR: In terms of scalability, running this through the Ofwat Innovation Fund is fantastic because what that allows us and pushes us to do, quite rightly so, is understand how we can raise awareness of this project. The reason why we've got that funding is that we are going to be sharing the results. We are going to be working with lots of academia to prove the concept, technology companies whose platforms show us where our sewers are and where that heat is, councils.

The key thing right now is to have the tool in place, the modelling in place and the desire in place to ensure that we know how this can work. Once that's in place, as long as we've got government support, policy support, this project is starting to look at loads of opportunity to have a brighter future where we’re extracting more and more heat from the sewers (where it's not impacting on our infrastructure), and where it's a benefit to those who use it and us who are extracting it.

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There’s much more where that came from – listen to the full conversation here. You can also search for Planet Possible in your favourite podcast app.

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