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  • by Karma Loveday

Tackling storm overflows is never cost beneficial at national level

At national level, it is not cost beneficial to tackle sewage spills from storm overflows under any scenario, according to the Storm Overflow Evidence Project.

The first of a kind research, commissioned by Defra’s Storm Overflows Taskforce and conducted by Stantec, found “that taking into account social, public health and ecological benefits, none of the policies and scenarios examined are cost-beneficial when assessed nationally”.

The most expensive option considered was the complete separation of wastewater and stormwater systems (eliminating storm overflows); that was costed at between £350bn and £600bn, increasing household bills between £569 and £999 a year.

The research went on to say that the costs of retaining storm overflows discharging to inland waters but limiting their operation vary widely, depending on how frequently they operate. It modelled nationally applied policies and scenarios costing between £5bn (40 spills average) and £280bn (0 spills average), where the equivalent benefits were £2bn and £39bn. This would impact annual household bills by between £9 and £495 respectively.

Other scenarios modelled included those relating spill frequency to river type, river sensitivity and bathing use. The project also found that nationally, wastewater network upgrades would be cheaper without the addition of sustainable drainage schemes, and that SUDS alone even with the highest ambition were “unlikely to reduce spills to 40 per year or fewer on its own”.

Despite the national picture, the project pointed out there could be viable cost beneficial solutions locally. It also argued more work should be done to refine our understanding, and that the economics could change – for example costs could “reduce significantly if a major deployment programme incentivised the supply chain to invest, innovate and achieve better economies of scale. Another source of savings is from co-delivery with aligned infrastructure improvements”.

Other findings

All policies and scenarios assessed carry a significant cost in carbon. Achieving a national average of 10 spills per year would emit five times the amount of carbon involved in constructing the Thames Tideway project. Getting to zero spills would emit 33 times the amount of Thames Tideway.

Over a third of the public surveyed in May 2021 ranked pollution related to sewage as a ‘top three’ environmental issue. The overwhelming majority (70%) would like remedial action focused on river ecology (including its plants and animals) rather than its aesthetic (13%) or to support safe swimming (8%).

If we do nothing new on storm overflows, up to 83 additional water bodies could fail to achieve good ecological status by 2050 because of their impact; an increase of 13% from today’s baseline.

Investment in spill-based approaches should take into account the opportunity cost and equivalent benefits that would derive from targeting alternative environmental drivers.

The Storm Overflows Taskforce is made up of Defra, the Environment Agency, Ofwat, CCW, Blueprint for Water and Water UK.


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