How waste water can be used to generate heat

With the UK set to phase out gas boilers, a district-wide network in Scotland is showing how low-carbon technology can be used to heat local buildings


  • Donald MacBrayne

10 August 2021

Combined heat and power engine

© Scottish Water Horizons

A district heating network (DHN) in Forthside, Stirling, is the first in the UK to use heat from waste-water extraction technology while generating electricity using a combined heat and power (CHP) engine.

The technologies are housed at an energy centre on the Stirling Waste Water Treatment Works site, which provides it with much of the heat source needed to operate.

The £6m project is led by Scottish Water Horizons – a subsidiary of the utility focusing on renewable energy – in partnership with Stirling Council, and it is funded by the Scottish government's Low Carbon Infrastructure Transition Programme. The network lowers carbon, helps to tackle fuel poverty and contributes to Scottish Water's efforts to meet its net-zero emissions targets, while supporting the council's commitment to enabling sustainable growth.

The heat generated by the system is sold to the council by Scottish Water Horizons, and is typically 10% cheaper than the mains gas it has replaced. This saving is then passed on to users, including nearby offices, leisure facilities and a secondary school.

The DHN has also been designed to support the area's expansion, with proposals to service future commercial properties and residential development.

How the technology works

Sewage heat recovery technology filters waste water to remove any solids, and then passes it through a heat exchanger to extract the heat, at around 10–15C.

Using a closed loop, this heat is transferred to clean water on the other side. It is boosted to temperatures of around 60C suitable for space heating and domestic hot water by using a water-source heat pump.

The heated clean water is then ready to be distributed to users through 2.5km of DHN pipes, which are highly insulated to ensure maximum efficiency.

When maintenance is required, back-up gas-fired boilers at the energy centre ensure that there are no interruptions in heat supply.

The system has been operational since 2019 and has continued to supply heat and electricity throughout the pandemic, albeit at reduced volumes due to lower demand from some buildings that have been empty during lockdown.

Stirling Council and Scottish Water Horizons are actively pursuing new customers for the network. There is potential to extract more energy from the waste water if required, which means the DHN can be expanded to meet future demand.

Opportunity at the door

With Holyrood's target for net-zero emissions in Scotland by 2045, there is an urgent need to move away from traditional gas boilers and find sustainable, carbon-saving methods for generating energy, such as waste-water heat recovery.

The Stirling project uses heat recovered from waste-water treatment works and transferred directly to the energy centre. However, energy could also be recovered upstream in the sewerage network. Developing extraction points to do so would enable distributed energy projects to be installed closer to users, consequently improving efficiency.

Scottish Water operates more than 30,000km of sewerage network, which represents a considerable opportunity to develop local, low-carbon energy generation. In many cases, the heat is literally passing the customer's door.

The Forthside project is the second such in the UK, following one at Borders College in Galashiels also developed by Scottish Water Horizons. The team, comprising staff from the utility and council, have learned a tremendous amount from these innovative schemes. For instance, they understand how best to integrate the different technologies into a single scheme, retrofit district heating into existing buildings and structure contracts for such arrangements.

On this basis, two more projects are currently under construction, and both are due for commissioning by the end of this year. As each progresses the team are continuing to learn valuable lessons, enabling them to grow, adapt and improve as more schemes are developed.

Local benefits

Exploring carbon-saving technology and its implications is key to enabling the growth and distribution of green heat.

As the energy source is warm at the outset, for instance, it offers greater efficiencies than traditional water-source heat pumps. This means that building surveyors can specify such systems to meet requirements for efficiency and carbon reduction as set in modern building standards.

Unlike the use of fuels such as biomass and gas, the technology means there are no emissions from combustion, so it is ideally suited to use in areas where air quality needs improving or is subject to management. The heat pump function can also be reversed during the summer months to provide cooling.

'Exploring carbon-saving technology and its implications is key to enabling the growth and distribution of green heat'

Challenges at several scales

Challenges for the technology can also be quite site-specific. For example, the system is best suited to fourth-generation heating, which runs at around 60C, or fifth-generation systems, which operate at ambient temperatures that can be increased to the required level at each individual building.

This means that if the building is being retrofitted to connect it to a DHN, then emitters such as radiators may need to be upgraded: lower-temperature systems such as fourth-generation networks may need larger radiators with a greater surface area to emit the same amount of heat as a traditional system running at 80C. This is not always the case, though; for instance, the Galashiels project did not require a single emitter to be upgraded because they were already oversized.

Depending on the location, the sewers may be deep and old as well; in such circumstances, it tends to be technically more challenging and costly to access them for energy extraction.

But the main challenge to date has been changing the attitudes of surveyors, architects, consultants and energy managers, so they are prepared to consider something that sounds quite left-field compared to conventional gas boilers.

Another significant challenge has been economic. The systems are currently subsidised by the UK government's non-domestic Renewable Heat Incentive. However, this scheme is due to close in March next year, and the government has not yet announced what if anything will replace it.

In most circumstances, the technology will still not be able to compete with mains gas in terms of cost. However, with the focus of many businesses, local authorities and surveyors on the commitment to net-zero carbon, the importance of sustainable energy must not be overlooked.

Donald MacBrayne is a business development manager at Scottish Water Horizons
Contact Donald: Email

Related competencies include: Sustainability

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