Filter strips are strips of ground where water running off a site can pass, allowing some or all of it to soak away. The rest often enters a swale - shallow drainage channel - or another sustainable drainage system (SUDS) component. This strip of ground can also be used to filter grit and other particles from the run-off.
Filter drains or filter trenches can be used beside roads and other impermeable surfaces, but should be avoided at busy road junctions or where rainwater can become heavily contaminated. Filter drains allow the run-off to soak away into the surrounding soil. Filter drains are filled with stones or gravel. This stone fill collects particles and helps to prevent pollutants from entering groundwater. You can use a filter strip or swale to reduce the level of pollutants entering the filter drain.
You can use filter drains for source control and to convey run-off between SUDS measures. See a diagram of how a filter drain works.