Sand Filters

Moderately turbid waters can be clarified in the simplest way by being passed through a medium such as a bed of graded sand. The degree of filtration depends on the grain size of sand used in the filter and this in turn is a function of the filtration rate used in the filter vessel. The gradual build-up of the turbidity in the filter raises the back pressure and when the resulting pressure drop from the filter inlet to the filter outlet reaches a predetermined level, the filter has to be cleaned by backwashing. Backwashing is carried out by reversing the flow through the bed, thus lifting and expanding the filter bed. The grains of sand collide with each other and, in doing so, free the accumulated turbidity, which is swept away to drain with the backwash water. Sometimes air is injected prior to backwash water to break up the bed. This reduces the flow rate of backwash water required to effectively clean the filter bed.

Selection of sand filter sizes can only be performed after careful examination of water analysis, flow rates and required purity of filtered water.