Abstract: A granular media filter has an underdrain system for use in a granular media filter comprising perforated lateral conduits arranged for the distribution of air and water for scouring and backwashing of the filter. Each conduit is in the form of a single linear conduit containing two or more separate axially parallel ducts, each duct having an external wall in common with the conduit, which wall is provided with axially spaced apertures for emission of water or air, there being no intercommunication between the ducts. The underdrain system itself also forms a feature of the invention.