Water from washing machines and showers will no longer drain into a brook after a two-year investigation.
Two years of work by Thames Water, the Environment Agency and Harrow and Brent borough councils means dirty water from misconnected appliances will no longer flood into Wealdstone Brook.
Extensive detective work began in 2012 after dirty water, which should have been directed to sewer pipes and treatment works, was found to be running into the stream.
An investigation by Thames Water into 1,750 homes in the Kenton area between 2012 and 2014 found that 339 household appliances - including 50 toilets - were emptying straight into the stream.
Thames Water and council environment teams have worked with householders to make sure their pipes were properly connected.
Mathew Reed, environment officer at the Environment Agency, said: “The Environment Agency is really pleased that Thames Water’s hard work has helped clean up the flow of wastewater to the brook.
“Washing machines, showers and toilets were regularly discharging into the watercourse and reducing the water quality for the wildlife and creating an unpleasant environment in the brook.”