Emission possible

At its vast Minworth water treatment plant, Severn Trent is pioneering efforts to pinpoint exactly where in the treatment process potent greenhouse gasses are created. Ruth Williams visited the site to discover first hand what progress has been made.

“Mind the geese, they can be aggressive,” I’m warned. This wasn’t included in the mandatory health and safety briefing video but I’m assured it should be. On the bright winter day wagtails swoop over, gulls bob around happily on the water while geese keep a respectful, non-menacing, distance.

As well as a habitat to many birds, the sprawling site is an innovation hotspot for Severn Trent. Minworth is the company’s largest sewage treatment works and home to its industry-leading work to solve one of the global wastewater sector’s most pressing challenges: Reaching net zero emissions when no one knows the full scale of potent gases being released.

Severn Trent’s energy manager, Howard Perry, and process emissions manager Bharani Sri, explain how the team set to work understanding methane, nitrous oxide (N2O) and carbon from treating wastewater.

Working to the adage of you can’t manage what you can’t measure, the company was the first in the UK to add monitoring devices to understand what was happening at different stages of treatment.

When the company began its work to track emissions – the great unknown in the battle against greenhouse gases in the water sector – Sri explains he joined from the energy team. “There were no job titles at that stage, colleagues who felt drawn to the project were chipping in,” he says of how the team was first formed.

“No one has fully cracked this yet, but that makes it so exciting to find out,” Perry says of the company’s efforts to compile meaningful information on the gases.

As we walk around Minworth, the scale of what the team has been working to achieve becomes clear. Settlement tanks stretch out with activated sludge plants (ASPs) bubbling behind. In the distance are sludge cake mounds, and behind us the complex network of pipes and anaerobic digestion processes to teat sludge to make biogas.

From raw sewage entering every day, to its departure as solid, liquid and gas – each stage has the potential to release potent gases into the atmosphere. The company is leading the world on measuring and monitoring N2O, methane and carbon so it can minimise the risks of harm.

Measure by measure

The company installed monitoring equipment at three wastewater treatment works on the activated sludge process and one sludge treatment centre over the past two years to build data sets. More site are being monitored this year.

Based on results, the team came up with a company-specific emissions factor for the first time but because every plant is unique it was important to understand how applicable the measurements would be to other locations.

“It is essential to observe the output of gases through the year for seasonal variations and to understand what’s driving these emissions,” Perry explains.

This work is being mirrored by projects in Denmark and Australia. So far, seasonal variations align with the preliminary results at Severn Trent’s sites. Sri says despite the geographical and climate differences, the bacteria appear to react in similar ways in the three countries with a springtime spike.

The good news is the work is narrowing down the sources of these emissions. For example, in the first settlement tanks – where effluent settles to allow solids to be siphoned off the bottom –flux monitors showed the emission rates from this part of the process were very low.

Sri explains monitoring showed negligible quantities of N2O, because the ammonia is yet to be broken down to produce any, while the relative stillness of the tank means any N2O stays in solution.

Ammonia enters with the influent and is converted to N2O during the nitrification processes. “Every point where there is sewage is potential for emissions,” Sri says “but our work is narrowing it down to the most important points so we can plan action to reduce these emissions”.

Biology doesn’t always do what we tell it to

As the water moves through the activated sludge process tanks that aerate the settled sewage, bacteria and microorganisms break down the ammonia from waste to produce nitrate. But N2O can be produced as a by-product. Under settled conditions N2O can stay dissolved in the liquor and may even be broken into nitrogen eventually via denitrification, however the turbulence caused by aeration ends up stripping the N2O into the gas phase. Therefore, the secondary tank is the prime source of these emissions, which are 273 times more potent than carbon dioxide.

Severn Trent believes these aerated tanks are a cost-effective, reliable treatment that works in any British weather. The bubbles promote the growth of bacteria, which carry out biological processes. As ammonia oxidises, N2O is released as a by-product, which the team are working to understand how to suppress.

“Compared to a chemical process,” Sri explains, “the biology does not always do what we tell it to.”

Probes are fitted to these large tanks to study the N2O produced through the processes to understand how it is affected by alterations in temperature, flow and other variables. “We want to understand the conditions that cause the nitrous oxide to spike,” Sri says.

There are two sensors to measure levels of dissolved N2O in the liquor and the team can calculate how much will be stripped out into the gaseous phase.

At Minworth, there are five activated sludge process tanks, each with four lanes. The sheer size of these open tanks, which are continually releasing potent gases, reveals the scale of the challenge faced by every wastewater treatment plant.

Severn Trent alone has around 80 sites with ASP lanes of varying sizes.

The N2O release is an unavoidable part of treating ammonia from wastewater and will be present in all processing plants large or small. Perry adds: “Data sets with seasonal data help us understand where processes can be optimised to reduce emissions and how we can approach mitigation solutions by investing in the assets”.

Aeration is essential to the treatment process, so cannot be stopped, but Perry explains there are options to drive down N2O amounts by stabilising and optimising the system.

Although the initial monitoring revealed higher levels of N2O than the carbon workbook estimated, encouragingly the mitigation techniques could also be more effective than anticipated.

Perry says: “The data we have now is one hundred times better than we had with the carbon accounting workbook. The total magnitude of emissions is higher than our estimates in previous years, but this is crucial to revealing the true scale of the problem we’re solving.”

Final settlement tanks are monitored with flux boxes indicating miniscule levels of N2O released at this stage compared to the ASP. This is similar amounts to the primary stage because little nitrification occurs, suggesting the vast majority of N2O is released from the ASPs.

Moving around the site, following the journey of the treated water we move to the advanced sludge treatment stage. Treated sludge goes on to produce biogas and a solid sludge cake, a bioresource used as a natural fertiliser. Severn Trent’s monitoring campaign on sludge cake has shown it is responsible for low levels of methane emissions.

Eyes in the sky

Moving on to the final step, the company is using a drone kitted out with lasers and cameras to detect and monitor any emissions from the final stage of processing prior to renewable biomethane leaving the plant as a grid export.

Leak detections were previously done manually and could take days to patrol the vast network of pipes moving gas around the tanks. The drone, programmed by pilot Duncan Turner flies its route in 14 minutes alerting him to any points gas is detected. Currently it detects the presence of methane but does not quantify the leak, which is a separate project being worked on by Cranfield University to more precisely assess quantities lost.

To avoid accidents, methane is sometimes released intentionally to prevent dangerous build-ups but there are also points where releases of methane are unintended. Using the drone can find a leak and guide technicians to repair it much more quickly, thus preventing losses of methane.

Turner’s drone is fitted with a methane sensor with a laser that picks up particles of one in a million measurement as it flies over the works.

Planning investment is another benefit of this monitoring. The team explains older assets tend to leak more, so by monitoring and identifying problems the company has a better insight into asset health and can make smarter decisions on when and where investment is needed.

Net zero?

In 2018 the sector set itself a bold ambition to achieve net operational emissions of zero by 2030 – 20 years ahead of national targets. A degree of credible offsetting is anticipated to meet that date using renewable exporting, tree planting and sequestration work. Ultimately though, all these process emissions need to be addressed, and the clock is ticking.  “We need to make some quite substantial in-roads to reducing process emissions by 2030,” Perry says and underlines that will take investment.

“No one in the world has treated wastewater with no emissions, but with the right investment we’re optimistic there are ways we can get this number down substantially before 2030.”