Utility Week

30 | APRIL 2023 | UTILITY WEEK Water Analysis Measure by measure 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. In the distance are sludge cake mounds, and behind us the complex network of pipes and anaerobic digestion processes to extract biogas. Raw sewage enters the plant every day and departs as solids, liquid and gas – and each stage has the potential to release potent gases into the atmosphere. The company is leading the world on measuring and moni- toring the amount of N2O, methane and car- bon released during wastewater treatment. Monitoring equipment has been installed 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 sites are being moni- tored this year. Based on these results, the team devised a company-specific emissions factor for the first time. Each plant is unique, so 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 varia- tions and to understand what's driving these emissions," Perry explains. The work is being mirrored by projects in Denmark and Australia. So far, seasonal var- iations align with Severn Trent's preliminary results. Sri says that despite the geographical and climate differences, the bacteria appear to react in similar ways in the three coun- tries, with a springtime spike. Ammonia enters with the influent and is converted to N2O during the nitrification pro- cesses. "Every point where there is sewage is potential for emissions," Sri says, "but our work is narrowing it down to the most impor- tant points so we can plan action to reduce these emissions." Flux monitors on the first settlement tanks – where effluent settles to allow solids to be siphoned off – show very low emission rates. This, Sri explains, is because ammonia has yet to be broken down, while the relative stillness of the tank means traces of N2O stay in the solution. As the water moves through the activated sludge process tanks to aerate the settled sewage, bacteria and micro-organisms break down the ammonia from waste to produce nitrate. N2O can be produced as a by-product. Under settled conditions N2O can stay dis- solved 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 says the aerated tanks are a cost-effective, reliable treatment that works in any British weather. The bubbles promote the growth of bacteria, which carry out bio- logical 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." M ind the geese, they can be aggres- sive," I'm warned. This wasn't included in the mandatory health and safety briefing video but I'm assured it should be. On a bright winter day wagtails swoop overhead and gulls bob around hap- pily on the water while geese keep a respect- ful, 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: how to reach net-zero emissions when no-one knows the full scale of gases being released. Severn Trent's energy manager, Howard Perry, and process emissions manager, Bha- rani Sri, explain how the team set to work understanding methane, nitrous oxide (N2O) and carbon from treating wastewater. "No-one has fully cracked this yet, but that w2makes it so exciting to find out," Perry says of the company's efforts to com- pile meaningful information on the gases. Working to the adage that you can't man- age 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. As we walk around Minworth, the scale of what the team is working to achieve becomes clear. Settlement tanks stretch out with acti- vated sludge plants (ASPs) bubbling behind. "

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