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Eliminating the use of combined sewer overflows (CSOs) could cost £100 billion and nature-based solutions alone would not be able achieve the goal, the Environmental Audit Committee has been told.

In written evidence submitted to the EAC regarding river pollution, Water UK said the sector will invest £1.1 billion in the next five years towards improving CSOs, but estimated the cost for complete elimination would be “somewhere in the region of £100 billion” taking a large package of measures beyond just nature-based solutions.

This should include legislative and policy changes to reduce the increasing volume of water entering sewers.

Water UK suggested new approaches to groundwater infiltration into sewers and changes to how consumers dispose of products that can cause blockages would also be necessary.

The trade body said environmental regulators must have sufficient resources to understand the condition of waterbodies and all sectors’ impacts on them; quickly locate sources of harm; and take enforcement action accordingly – even against diffuse polluters.

The head of the Environment Agency warned in January that underfunding has left it unable to properly monitor activities of private companies, including the water sector.

Ofwat wrote in its submission that the cost of eliminating harm from CSOs would require an end-to-end and systems-orientated approach which considers nature-based solutions and consumer behaviour change as well as water company infrastructure. The latter, the regulator added, clearly has a role to play but cannot solve the problem alone. The associated costs would be “economically unacceptable” and the measures would not be enough to address the growing pressures of controlling what enters the infrastructure.

The Chartered Institute of Water Environment Management agreed that a multi-faceted approach would be essential and focussing too heavily on any one solution could result in poor deliver.

It said that education, engagement, cooperation and commitment of wide range of parties – regulators, governmental departments, planning and highways authorities, the water industry, developers, drainage engineers, architects, product manufactures and the public – all need to be part of the solution.

“There is a fundamental need to look at this complex problem in the round. Stormwater overflows resulting in sewage pollution of watercourses is a symptom of a far wider mismanagement of water within our urban landscape,” CIWEM said.

It noted its strong advocacy for nature-based solutions and urged Ofwat to ensure its requirements do not discourage their use, even where performance may take multiple investment rounds to fully optimise.

On behalf of the sector, Water UK recommended that the work of the Storm Overflow Taskforce was built upon to develop a national register of all assets discharging materials into waterways – including local authorities and private individuals, particularly in sensitive catchments to ensure action to rectify problems can be taken.