Utility Week

UTILITY WEEK | APRIL 2023 | 31 Water Got a minute? Scan the QR code to take the Utility Week membership survey today Your Utility Introductory Complete the survey by 6th April for the chance to win one of four £50 Amazon vouchers We'd love to know what matters to you Probes fitted to these large tanks study N2O produced through the processes to understand how alterations in temperature, flow and other variables affect it. "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 gases, reveal 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 says: "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, but Perry explains that there are options to drive down N2O amounts by stabi- lising and optimising the system. Although the initial monitoring revealed higher levels of N2O than the carbon work- book estimated, encouragingly the mitiga- tion techniques could also be more effective than anticipated. Perry adds: "The total magnitude of emis- sions is higher than our estimates in previ- ous years, but this is crucial to revealing the true scale of the problem we're solving." Final settlement Final settlement tanks are monitored with flux boxes indicating miniscule levels of N2O released at this stage compared to the ASP. This is a similar amount to the primary stage because little nitrification occurs, suggest- ing 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 cam- paign on sludge cake has shown it is respon- sible for low levels of methane emissions Droning on Moving on to the final step, the company kit- ted a drone out with lasers and cameras to monitor any emissions from the final stage of processing prior to renewable biomethane leaving the plant as a grid export. The drone can find a leak and guide tech- nicians to repair it quickly, thus preventing losses of methane. Turner's drone methane sensor uses a laser to pick up particles of one in a million particles as it flies over the works. Leak detection was previously done manually and it could take days to patrol the vast network of pipes moving gas around the tanks. The drone, programmed by pilot Dun- can Turner, flies its route in 14 minutes, alert- ing Turner to any gas 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. 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. Ulti- mately, though, all these process emissions need to be addressed, and the clock is tick- ing. "We need to make some quite substan- tial inroads to reducing process emissions by 2030," Perry says, and says that will require investment. "No-one in the world has treated waste- water with no emissions, but with the right investment we're optimistic there are ways we can get this number down substantially before 2030," he says. Ruth Williams, water correspondent

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