Foul water drainage is the system of pipes that carries wastewater from toilets, sinks, showers, washing machines, and other domestic fixtures away from a building for treatment or disposal. It is different from surface water drainage, which handles rainwater and runoff.

How it works

  • Wastewater flows from the home into internal pipework and soil stacks, then into underground drains.
  • From there, it usually goes to a public sewer and then to a treatment works, or to a private system such as a septic tank or cesspool if the property is not on mains drainage.

Why it matters

  • It helps keep wastewater safely contained so it does not contaminate the ground or the environment.
  • Good foul drainage also reduces blockages, leaks, and hygiene problems.

Simple example

If you flush a toilet or drain a sink, that water enters the foul water drainage system, not the rainwater system.