The Storm Overflows Taskforce has published its independent research report on options, costs and benefits for reducing storm sewage discharges in England.

The research, the first assessment of its kind, was commissioned by the Storm Overflows Taskforce – made up of Defra, the Environment Agency, Ofwat, CCW, Blueprint for Water and Water UK – and funded by Water UK.

The independent research, carried out by Stantec, presents a detailed overview of potential approaches that will act as an important evidence base for government action.

Storm overflows are a legacy of sewer design and construction practices until the second half of the twentieth century – described in the report as “a pragmatic and affordable means of draining towns and cities.

There are 15,000 storm overflows in England and 13,350 discharge to inland rivers. Their use is “consistent with practice throughout Europe, which has some 650,000 overflows across the continent ,” the report says.

However, today many storm overflows operate more frequently than is acceptable to the public and other stakeholders in the community - even though they make “a relatively small contribution” to water quality standard failures overall.

The government-led Storm Overflow Taskforce commissioned the report from Stantec with the remit of exploring policy options that reduce the occurrence of storm overflow spills and any harm that is caused. Policies and scenarios covered in the report include limiting the annual average number of spills (the spill frequency) of 13,350 storm overflows discharging into inland rivers in England to 40, 20, 10, 5 and 0, as well as differentiating between universally applied national limits and more targeted ‘sensitive catchment’ ones.

The report has considered three delivery scenarios for each of the policies; storage, storage with a low uptake of sustainable drainage (SuDS) and storage with a high uptake of SuDS.

This is a national assessment to understand the typical investment required to meet a range of policy outcomes. The different policy options provide a significant range in the costs for different delivery scenarios which include a low estimate for the lowest cost scenario, and a high estimate for the highest cost scenario.

Key findings 

Key findings in the report include:

Costs and bill impacts

The complete separation of wastewater and stormwater systems (eliminating storm overflows) would cost between £350 billion and £600 billion. This could increase household bills between £569 and £999 per year and is also highly disruptive and complex to deliver nationwide.

The costs of retaining storm overflows discharging to inland waters but limiting their operation vary widely depending on how frequently they operate. The report has modelled nationally applied policies and scenarios costing between £5 billion (40 spills average) - £280 billion (0 spills average). The equivalent benefits are £2 billion and £39 billion. The impact on annual household bills could be between £9 and £495 respectively. The ranges depend on how policies are delivered and reflect uncertainties.

A refinement mixes the requirement for spill control depending on river type. A general limit of 40 spills on average per year, reduced to 10 spills in sensitive catchments would cost between £18 billion and £110 billion. The impact on annual household bills could be between £30 and £208 per year. This 40/10 spill policy is similar in cost and bill impact to the policy of 20 spills on average per year.

A policy focused on achieving 10 spills per year on average in sensitive rivers(such as chalk streams) would cost between £16bn and £82bn. The impact on annual household bills could be between £26 and £150 respectively. A policy focused on achieving 10 spills per year on average in rivers where storm overflows are observed to be the reason for not achieving good ecological status would cost between £13bn and £59bn. The impact on annual household bills could be between £22 and £108 respectively.

A policy focused on improving rivers known to be used for bathing to achieve an average spill frequency of five per year would cost between £8bn and £26bn.

The impact on annual household bills could be between £13 and £48 respectively. However, this policy ignores the costs of dealing with other sources of microbial and other contamination, which may be more significant and difficult to deliver.

Over a third of the public surveyed in May 2021 ranked pollution related to sewage as a ‘top three’ environmental issue. The report says that if we do nothing new on storm overflows, we estimate that 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.

This deterioration is due to reduced river flows, population growth, urban creep and changes in rainfall due to climate change. For the same reasons, rivers currently used for recreation will see around a quarter of their length become unsuitable for swimming.

All policies and scenarios assessed in report carry a significant cost in carbon

The report salso says that all policies and scenarios assessed in this report 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 – a £5bn “super sewer” and largest ever project undertaken by the UK water industry. Getting to zero spills would emit 33 times the amount of Thames Tideway.

The research has 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 report identifies a number of recommendations for further consideration and analysis; in particular on the need for common approaches to valuing benefits, for improving understanding of water company customers’ willingness to pay for reductions in storm overflow harm, and the effect of operational aspects on overflow performance.

Further analysis at both a local and national level could identify more efficient solutions and a greater understanding of benefits, the report says.

The Storm Overflows Taskforce will now consider the report and its recommendations to:

• support the development of the best mix of policy solutions
• inform the government plan on storm overflows to be published in September 2022.

Click here to download the report in full.