Wastewater treatment: Reducing emissions while adding value

The water sector is hurtling towards its self-imposed 2030 net-zero goal. There is much work to be done to drive down the energy intensity of its processes, most notably in wastewater treatment. Process emissions from sewage treatment currently account for around two thirds of the total greenhouse gases output by water and sewerage companies. The journey to net zero will require them to vastly lower the energy consumed across their organisations, add renewable supplies and credibly offset where necessary.

Companies face a challenge, therefore, to reduce emissions by modernising processes and where possible reusing biproducts, which have previously been regarded as waste, to get secondary benefits from waste or heat energy. There is potential to use this energy by the company as a renewable source or supply it to other organisations also working to decarbonise.

Industry experts speaking at the Utility Week Live Summit, sponsored by Capita, discussed the ways companies are making treatment less energy intensive and the opportunities to recycle the waste products to other sectors. This does not come without obstacles – both from financial and regulatory viewpoints – and participants at the summit shared insight on overcoming these.

For the 2030 commitment, Water UK created a routemap that acknowledges as things stand not all emissions can be neutralised and a significant amount of offsetting may be needed. Overall, 10 megatons of carbon will be cut within the next decade based on a 2018-19 baseline.

Part of the issue, as explained by Peter Vale, technical lead on innovation at Severn Trent, is that wastewater treatment hasn’t really changed in the past 100 years, which means it’s prime for innovation.

“(These processes) have served us really well, they’re robust and enable us to treat water to the highest standard, but they are energy intensive, so the argument to change is becoming more compelling,” Vale said.

Reliability aside, these linear treatment flow sheets are not so compatible with the low-carbon biodiverse future the sector needs to move to and embrace. Vale said: “Transforming to a circular way of managing assets is key but will demand a real re-think about how we treat wastewater.”

Understanding of the output gases is growing each day and research on how to drive out nitrous oxide and methane – natural biproducts from treatment processes – over the coming decades is underway.

Although damaging to the environment, these biproducts can be used for heating and as an energy source themselves. Energy from biogases can be harnessed as a renewable supply not just for water companies. Innovative schemes to recover heat and energy to use in heating networks have led to symbiotic partnerships between water companies and other industries to recycle the carbon intense emissions.

This re-think means updating and innovating treatment processes. Severn Trent, as part of its triple carbon pledge, now generates more than 50 per cent of its energy from renewables, which has allowed the organisation to cut its net-carbon emissions by 40 per cent in the past five years. However, Vale recognises the need to go further and faster to meet targets.

One area Severn Trent is exploring, with the Water Research Centre (WRC) is capturing carbon dioxide via enhanced mineral weathering. David Hodkin from the WRC explained that mineral capture offers longer term storage of carbon than trees or flora that could be cut down or destroyed – thus releasing the carbon.

As certain minerals weather they release metal ions such as magnesium and calcium into solution. These in turn can increase the alkalinity of water by generating bicarbonate and carbonate ions, which results in the storage of CO2 that can then be transferred to and stored in the oceans.

Hodkin’s work is to increase the rate of weathering of these minerals by increasing their surface area or temperature to increase the amount of carbon drawn down and stored as alkalinity in the seas.

The work is currently at an early stage in the study, and Hodkin said funding for innovation is crucial but so is the acknowledgement that not every idea will succeed.

Sharing the risk

He praised Ofwat’s approach to funding innovation through its £200 million competition during AMP7 that allows and encourages companies to shoulder the risk involved in projects. He said that while everyone wants something totally new from innovation, they also want to know for sure that it will work. Changing that attitude to accept known risks and factor those into investment decisions is a necessary step, he added.

Spreading the risk became a clear theme throughout the event, with participants acknowledging the greater gains that could be made through collaboration and support from regulators. Partnerships outside of the water sector open up opportunities to recycle biproducts from treatment works. At Thames Water, teams have been working with the Greater London Authority (GLA) to see where and how waste from the Hogsmill Sewage Treatment works in southwest London can heat 2,000 homes in Kingston.

The site could generate up to up to seven gigawatt hours of low-carbon heat per year supplied via a sealed network of pipes to the district heating system at a new housing estate.

The project with Kingston Council and Kingston University was driven by the need to decarbonise heat at the housing estate with the chance to save around 105 kilo tonnes of carbon dioxide equivalent in emissions over 30 years.

The Climate Change Committee has forecast a need to increase district heating networks by nine times to satisfy 18 per cent of the nation’s heat demand in this way in the future.

Aram Wood, director of renewables at Thames, said the company was “on the cusp” of this important step towards decarbonising heat but several challenges remain.

The project uses energy recovered from the sewage treatment process and if successful could be scaled up to heat homes across the country from other treatment plants.

Wood pointed to the multiple benefits of the approach – allowing water companies to extract the heat from wastewater treatment and, because sewage water is so warm, leaving heat pumps or heat exchangers with less work than from a cold source. The heat pump concentrates thermal energy and supplies it to the districting heating system via an on-site energy centre.

The GLA, which funded the feasibility study, is mapping where sources of heat from industrial processes such as the sewage treatment works can be matched with demand.

Thames calculated there’s a massive opportunity of around 5TWh in the treatment and sewer networks annually that is broadly equivalent to heating 300-350,000 homes in London. Wood explained the displaced gas would equate to 900,000 tonnes of carbon dioxide being saved each year, if those 5TWh can be recovered.

“That is a massive opportunity, probably the greatest untapped resource Thames Water has and can play a significant role to contribute to the decarbonisation of heat in an urban area like London,” he said.

The motivation has grown in recent years as a wide range of organisations, including local authorities, have declared climate emergencies. These activities are trickling through to regeneration schemes, housing development and municipal or public buildings.

Thames is in discussions with multiple groups to use effluent from its assets in such a way.

“It’s an extremely exciting time, the question is how to make it happen,” Wood added. Before being able to scale up, the pilot scheme in Kingston is “trying to make the economics stack up”.

The cost remains a stumbling block because heat recovered from effluent through heat pumps is currently more expensive than combined heat and power from traditional gas supplies.

Although a project like this cannot compete on price, it can help these organisations with their climate targets by recycling heat that is being generated anyway. Closing loops like this have societal and environmental benefits but resolving the cost issue would require government support in the form of subsidies or grants to get such projects off the ground. Wood describes it as an untapped resource, which extrapolated across the whole industry soon adds up to considerable carbon savings as well as decarbonisation of other organisations in urban areas.

As well as cost considerations, there are regulatory issues, such as whether an effluent permit would be required and whether Ofwat or Ofgem be the body to oversee the energy produced from such a project.

Circular economy

Cost is not the only factor however – the societal benefits of a scheme that closes the loop on emissions can’t be calculated.

In Scotland, the government is working on a circular economy strategy to “build a strong economy, protect resources and support the environment”. It is a tenet of Scottish Water’s objectives for its current regulatory cycle SR21, which runs to 2027, to assess the potential for resource recovery from sewerage.

The biological treatment processes to clean wastewater generate nitrous oxide and methane equivalent to 43kt CO2 each year. The top contributors for the company’s total process emissions are from wastewater aeration – nitrous oxide 34 per cent; wastewater sludge treatment – nitrous oxide 22 per cent; and sludge storage – methane 20 per cent. Over the coming decades the company has forecast it can reduce these by 20 per cent through optimisation of the processes and aims to maximise the energy it recovers from bioresource.

Mark Williams, business strategy and climate change manager at Scottish Water, said: “Energy intensity is crucial to get to grips with and drive that down – from the power used on sites, be as efficient as possible, move away from energy intense activities, explore how to maximise energy recovery and other value from bioresources, sludges and other products wastewater treatment centres have.”

He told the audience the company has been working with its assets to add more efficient pumps and reduce power as well as installing more monitoring equipment to understand how the kit is working and find ways to make it more efficient.

The company is now involved in a number of projects focused on reducing energy consumption and recycling where possible. At the moment, aeration is around 70 per cent of total site energy used at activated sludge plants. Driving down the energy used here is key so the company has begun replacing blowers and systems with more efficient ways of getting air into the treatment processes.

Williams explained the next step: “One of the biggest individual energy saving opportunities is moving to real-time control of the process and looking at how to manage aeration levels and at various stages within the process, managing biomass to minimise the input to achieve the same levels of performance.

At some of the largest works energy consumption has been taken down by about 15 per cent and is now being applied to the wider asset base.

“This is adding up to an ambitious programme of energy efficiency over the next 10 years to try to take 30, 40, 50 GWh of energy out of all the water treatment processes,” Williams said.

“Sludge has been a source of energy and value across the sector for a number of years and Scottish Water has done a lot to advance the digestion technologies. This is where our investment over the next 10 years hopes to move this forward for Scottish Water and we hope to increase the amount of power we’re getting to around 90GWh.”

Williams said innovation is needed to develop recovery of ammonia from wastewater to avoid process emissions and then to find opportunities to use that recovered ammonia in line with circular economy intentions.

Overcoming barriers

At present there are more questions than answers as companies work to update century-old techniques. Lack of regulatory incentives was identified as a stumbling block and workshop participants called for these to be strengthened to drive innovation in wastewater treatment.

Seminar discussions on how to strengthen regulatory incentives noted there is something of a grey area around creating value out of energy resources: should it be Ofwat or Ofgem regulating?

One person said: “It’s important to modernise the regulatory environment including the incentives for the next phase of decarbonisation and resource recovery.”

Modernisation, it was suggested, would need to address the misalignment between funding and regulatory cycles. One person commented that five-year regulatory cycles inevitably lend themselves to “short-termism of regulation”, and suggested longer timeframes, as have been adopted for water resource management plans, would be more relevant.

Likewise, any incentives that are offered “appear for an AMP cycle then evaporate after year five”, making it harder to create a business case to get energy from waste projects off the ground.

Another participant countered that creating a viable revenue stream could be one approach to funding projects if regulatory incentives aren’t sufficient. However, this value would then need to be distributed between customers and shareholders.

One participant proffered a timeframe of 10-15 years to get waste heat recovery projects off the ground, which they suggested should be factored into the periodic review period with longer-term incentives added to create certainty while such schemes are launched. They said taking a long-term view and consistent approach, with alignment between Ofwat and Ofgem, “would help a lot.”

Providing an essential service means wastewater treatment can never be paused, so trials or new approaches must be in addition to core business and will not happen overnight. Ofwat has actively encouraged innovation through its fund as a way to enable companies to share risk. This may allow them to overcome the dilemma that people demand innovation, something no one has done before, but they also want to know for sure it will work.

The societal and environmental benefits may outweigh the financial benefits but economics still must stack up or projects will never progress. The sector is facing monumental challenges but the opportunity to reimagine century-old processes with a focus on decarbonisation will lead to step changes in treatment for the 21st century.