Variable versus constant supernatant water layer in slow sand filtration

The water level in slow sand filters may be kept constant or variable during the filter run time. In the first case, an outlet control valve is usually needed to maintain the supernatant water layer constant during filtration, whilst in the second mode of operation, the water level varies from a minimum level above the top of the sand to a maximum pre-fixed level. Research studies performed by several authors may be found in the literature of slow sand filtration with the use of pilot filters controlled by the two methods and it seems nobody has questioned yet the validity of using one or the other method. This research study shows the results of an experimental comparison between the performance of slow sand filters operated according to both methods of control.