Thames Water has identified the source of pollution in a Northolt watercourse following a two-year investigation.

Sixty four properties were found to be incorrectly plumbed into the sewer network, resulting in foul waste finding its way into Costons Brook, a tributary of the River Brent.

Among the 127 appliances which had been connected to the wrong pipe during installation, there were 32 washing machines, 10 dishwashers, 10 baths and – worst of all – two toilets.

With more than 3,500 properties in the catchment area, tracking down the source of the misconnections inside homes was a huge challenge. But after months of tests, which included putting environmentally friendly dyes down sinks to see where it emerged, the case was solved.

The vast majority of appliances have now been correctly plumbed, with the local authority taking action against a handful of properties which have not complied.

Thames Water’s Jonny Evans said:

“After two years of hard work, we’re delighted to have this resolved. The waste coming into the brook could have had a real long-term environmental impact so we were determined to get to the bottom of this complex issue.

“Misconnections can be a big problem for local rivers and streams so we would urge anyone having a new appliance installed, such as a washing machine, to make sure they check it has been plumbed in correctly.”

Since 2015, Thames Water has removed more than 11,000 appliances which have been misconnected and leaking into nearby streams and rivers. Of these, 22 per cent have been washing machines. Overall responsibility for misconnections lies with the property owner.

Thames Water’s five-year business plan covering 2020-25 will deliver an 18 per cent reduction in pollution incidents as part of an industry-leading £11.7bn investment programme, which includes £2.1bn to boost resilience and reduce leakage by 15 per cent from its network of water pipes.