Process emissions: Sizing up the challenge

Although it is widely accepted that process emissions account for the largest share of greenhouse gas emissions in the wastewater sector, at present there are only estimates of how much methane and nitrous oxide is released at the more than 7,000 sewage plants in England and Wales. Howard Perry, energy manager at Severn Trent, speaks to Utility Week about its efforts to begin monitoring theses gases as the first step towards reduction.

To reach the water sector’s target of reducing greenhouse gas emissions to net zero by 2030, Severn Trent has adopted an approach of reduce, remove, replace and offset.

Like other wastewater companies, it recognises the magnitude of the challenge, estimating it will require around £500 million extra investment from its 2025-30 PR24 business plan to reach the net zero goal, some generated from non-regulated business activities.

The company compiled a baseline of its current carbon equivalent emissions of 470,000 tonnes per year. Of this, an estimated 190,000 tonnes are from process emissions and an estimated 128,000 tonnes from secondary treatment processes at wastewater treatment sites.

Wastewater treatment works have multiple, very large, activated sludge process (ASP) lanes that each emit nitrous oxide. Perry says these are working just as they were designed to but contribute heavily to emissions.

“They have all the treatment benefits we have relied on for many years so to fundamentally change that or to capture all the gas from one area and get rid of it seems like a pretty big challenge.”

Severn Trent led a bid to the Ofwat Breakthrough Challenge to roll out mass monitoring of wastewater treatment sites to accurately record emissions. Despite the urgent need to act if the sector wants to reach its self-imposed target, the bid was not successful.

“We know it’s a big problem, but we need to understand the problem first. We’re doing some monitoring already, but it needs a much broader set of monitoring that needs to begin across a range of sites. The earlier we start that work the better because the more likely to be able to find where emissions come from and how they can be solved,” Perry says.

The bid was turned down by the panel as a project that should be funded by other sources. Although Severn Trent and other companies will continue with their own self-funded monitoring projects, Perry believes the work needs to be wider to provide more data: “The more data we have, the more likely we are to find a solution.”

He says both Anglian and Thames have embarked on monitoring, also with an agreement to share the results. However, Perry says the innovation fund bid could have enabled more wide-scale, quicker, and more ambitious monitoring.

“We need to be collecting that data and science to propose the solutions for 2030 and beyond,” he states. “One issues with process emissions is they are very seasonal – they vary over time with weather temperature so we would need a year of monitoring to see the footprint and what types of things are likely to happen at different times of year, plus years can be different with hotter or wetter weather impacting differently. The earlier that starts, the better and the more science we can do in the area.”

He says monitoring is already underway at some of Severn Trent’s sites to measure when, where and how much emissions occur, with methane emissions being picked up using sensors at sludge treatment areas and thermal imaging from drones, and nitrous oxide emissions being gauged using sensors in secondary treatment areas.

“Even over the past few months of monitoring that has allowed us to identify leaks and fix them, and find spots within the process that have methane losses we didn’t know about before. Tangible reductions to greenhouse gas emissions are already happening.”

Perry says drones are flown over wastewater treatment works at a height of 50 metres and shine down lasers that can gauge methane concentrations within their beams and measure the temperature. The company has employed drones for asset inspections and as a tool in its leak detection arsenal since 2017.

He says digestor maintenance has been instigated off the back of these inspections, which are a first for the sector.

“The drone will fly over the site and then gives a graph that shows the methane concentration. That, aligned with a visual camera of where that was and a ground-based walkaround survey to look at the assets in more detail, can result in addressing leaks,” Perry explains.

The company intends to increase its monitoring and carry on building up its evidence base over time, with the nitrous oxide and methane monitoring sensors to be in place and operational up to 2030. Perry says these can be used to keep track of how actions are affecting emissions.

“As we make improvements and changes these monitors will show that in the data. The survey is also a regular process now that we want to keep up and running to understand emissions and track any improvements.”

Of course, the key question is, after all that monitoring and data gathering, what will the next step be to start to bring emissions down?

“We don’t have all the answers to that yet. It’s helpful to get the message out there so suppliers and other companies are thinking about this. If we have more collective brains on the problem the solutions will emerge.”

He says existing research shows that efficient operational control of secondary treatment processes may be particularly helpful for controlling emissions but this will take more time and monitoring to prove.

“We believe some of the work we have done for energy efficiency and process control is improving emissions from what they would otherwise be. Maintenance and repair of leaks is already helping and will continue to help.”

Optimisation of assets, as well as finding and fixing where gasses may be leaking, are changes the industry can begin to make now, Perry says, but he admits that beyond that remains an unknown.

“We have to start to think in terms of capital solutions for an alternative to the process that doesn’t result in those emissions or to capture those greenhouse gasses. What these are and how much they would cost we don’t know, but it is something we need to figure out before the next price review.”

Severn Trent recently hosted a hackathon with suppliers to solve these challenges.  The worked showed there would be “no silver bullet” to the issues the company and sector face. “There’s some suggestions of where we might go but no single solution, which is why we need all brains on board.”

He says the next price review will be critical for setting out the investment and funding needed to get net zero by 2030. “It will either give us the investment that we need to hit that goal, or it won’t. How much we fund ourselves and how much is funded by the customer is key for PR24.”

At present, the company believes there will be a mixture of approaches – some of which have already begun at Severn Trent to optimise assets – to get the emissions down as much as possible but with 2030 looming large, what can be expected this decade?

“Again, we don’t know yet,” Perry admits. “It will take us longer than 2030 to get greenhouse gas emissions down to nothing. This will take multi-AMP asset replacement cycles or big investment in capture and destruction.”

Although it may sound like a relatively short amount of time to bring real change to processes that have served the sector largely unchanged for a century, Perry believes the high level of ambition will drive improvements.

“It may look insurmountable, but we aren’t going to wait until AMP9 or later. We will do as much as we can to understand the problem and to do what we can,” he says. “Nine years doesn’t seem like enough time to do what we need to do, but a lot can change in nine years.”