Performance of fabric-protected slow sand filters treating a lowland surface water

The performance of slow sand filtration can be substantially improved by the application of a non-woven synthetic fabric layer to the surface of the sand. By means of pilot-scale experiments, using the River Thames as the source water and pretreatment by sludge blanket clarifiers, the comparative performance of fabric-protected slow sand filters has been evaluated over an eight-month period. Under conditions designed to simulate poorly-controlled pretreatment, a correctly-specified fabric type, configuration and thickness can extend filter run times by a factor of 3-5 compared to a conventional slow sand filter. Furthermore, this can be achieved, together with the avoidance of any significant change in hydraulic behaviour of the underlying sand, thereby avoiding the need to remove and clean sand. Fabric washing is relatively simple and efficient.