Utility Week

16 | JUNE 2022 | UTILITY WEEK Water Analysis CSOs: we're all in this together In the first of a series of articles about storm overflows, Utility Week talks to three water companies who explain that this is a societal issue – not one that the sector alone can solve. W ater companies have made main- stream news over the past couple of years thanks to the growing awareness about combined sewer overflows (CSOs), which discharge into waterways dur- ing times of heavy rain. Although only one of many contribut- ing factors to poor river water quality, the media and public have jumped on CSOs as a scourge that needs eradicating. Government, regulators and companies have responded with pledges to eliminate the risk of harm from overflows in the coming decades. Indeed, the sector has been more ambi- tious than the targets proposed by the Department for Environment, Food & Rural Affairs (Defra) in its plan, currently under consultation. The experts all agreed that sig- nificant improvements could be made more cheaply than the eye-watering price tags attached to Defra's plan – and far sooner. Nick Mills, head of pollution and flood- ing resilience at Southern Water, sets out the challenge facing the sector: "The amount of rainwater entering our system during a storm shows the scale of the problem, and the sources are predominantly roof and road- run-off, which are assets we don't own so we have to work in partnerships to address the problem." During heavy rain, this can be a deluge of up to 30 times the normal flow rate hitting a treatment site in medium to high urban areas. "We've seen changes in flow peaking at up to 9,000 litres per second at treatment works that ordinarily handle around 300 litres/second. This can cause a host of prob- lems, not only that storm overflows have to be used. Those assets are absolutely ham- mered in those conditions – nine tonnes a second are hitting the infrastructure. When people hear that they accept that building more infrastructure isn't the answer." The costs mentioned in Defra's plan on tackling harm from overflows, which ranged between £350 billion to £600 billion, were the price of digging up roads to completely separate sewer systems across the country. In reality the costs could be far lower. Alternatives to network expansion Mills, who leads the company's CSO task- force, says: "We're trying to prove over the next two years that our approach is more affordable and we can do it quicker." He explains that this approach is about understanding the network and assets to optimise each part and improve how other networks such as highways meet with the sewer systems. The next interventions are to tackle surface water by slowing its flow from entering the network, which would reduce the use of storm overflows. This would entail sustainable drainage systems (SUDS) solutions. Using SUDS could minimise the impact of around 90% of rainfall events in a year while offering amenity benefits, biodiversity and ecological net gain, or carbon reduction ben- efits, which Mills says can open additional funding sources. "That is a critical point for pushing this route. We shouldn't be burden- ing customers for all of this when we can genuinely partner with others," he says. These options would not be as expensive as has been calculated, and the company is launching pilots this summer to prove that. By optimising existing assets the team wants to control what is a passive system. "This goes beyond our own systems to also consider highway interfaces and connections to us for ways to improve, as well as pumps and electrical equipment." This could mean operating the networks and pumps differ- ently in anticipation of weather events. "If we know heavy rain is due, we can empty the system to maximise existing storage; make sure the network gets as much hydrau- lic capacity as possible; operate pumping stations to maximise pumping capacity." The company wants to exhaust optimisa- tion and SUDS before it considers infrastruc- ture approaches such as building more storm tanks, sewers or treatment works capacity. "They're our last resort but o¡en as an industry we start there because they have guaranteed results. I don't think they repre- sent best value," Mills adds. Nicole McNab, who leads on engage- ment and communications for Southern, says: "These are the most cost-effective, and also the most sustainable solutions. If we go

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