The foul sewer is designed to carry contaminated wastewater to a sewage works for treatment, whereas the surface water sewer carries uncontaminated rainwater directly to a local river, stream or soakaway.

What each sewer does:

Sewer system	Disposes of:	Transferred to:
Foul water	Toilet, bath, shower, kitchen sink, washing machine, dishwasher, etc	Local sewage treatment works
Surface water	Rainwater from roof, driveway, patio, roads, etc	Local watercourse

Across the Thames region, plumbing mistakes result in up to one in ten households misconnecting their waste appliances to the surface water system and in some parts of London this is as many as one in three homes.

As a result of these misconnections, every day the equivalent of 1.8 Olympic-size swimming pools of wastewater wrongly enter the region's rivers and streams.

We are working in partnership with the Environment Agency to reduce the number of misconnections across our region.