Image description: The pull handle on a lavatory. Image by Andrew Martin from Pixabay

A hidden enforcement gap

Misconnected household pipes, washing machines, dishwashers, sinks or toilets accidentally plumbed into surface water drains rather than the foul sewer, let untreated wastewater flow directly into rivers, streams and the sea. Although discharging foul water into a surface water sewer is unlawful under Section 109 of the Water Industry Act 1991, the offence is rarely visible to building inspectors and the Department for Environment, Food & Rural Affairs has previously estimated that between 150,000 and 500,000 UK households may have some form of misconnection, a figure that has barely changed in years. Recent investigations are starting to put a sharper number on the problem: Natural Resources Wales uncovered more than 30 cases in Cardiff after pollution reports on the River Taff and River Ely, and in London alone Thames Water identified 2,294 misconnections in just five years.

Volunteer eyes, missing oversight

Ben Morris, founder of the Clean Up the River Brent campaign, told the BBC that the consequences were “disastrous for nature” and that the faults were often only discovered as a result of volunteer action. Force trustee Rob Gray described it as a “hidden pollution scandal” with significant public health and environmental impact. The Rivers Trust has highlighted the issue as one of the major threats to river health, with public awareness of it remaining limited. The Zoological Society of London (ZSL) developed the Outfall Safari in partnership with Thames Water, the Environment Agency and other partners, first deployed on the River Crane in May 2016; volunteers walk riverbanks scoring outfalls for visible pollution, with high-scoring outfalls reported for further investigation. Between 2020 and 2025, Outfall Safari volunteers identified1,314 incidents of misconnected appliances or defective pipework.

Who is responsible, and what next

Property owners are liable for misconnections within the boundary of their property. Thames Water runs a Surface Water Outfall Programme to prioritise pollution hotspots, but campaigners argue it remains too dependent on volunteer reports. A national data system in development under the Catchment Systems Thinking Cooperative (CaSTCo) is expected to see full release of an updated Cartographer database for the 2026 Outfall Safari season, potentially enabling more systematic identification of misconnections across the UK, and starting to close the long-standing enforcement gap.