Sand filtration is one of the most important basic technical processes in water treatment. It is a mechanical separation process in which suspended particles in water are retained in a filter layer (e.g. a layer of sand) through which water is passed. Raw water is generally filtered through filtration plant using sand as the filter layer. During the filtration process the pores in the filter layer become blocked by contaminants removed from the raw water passing through it. This leads to a gradually increasing drop in pressure.

The "back washing phase" begins once the minimum permitted pressure level is reached in the "operating phase". Here, the impurities are flushed out of the filter layer. During the operating phase, water passes downwards through the filter, while during the back-washing phase, it travels back up through the filter layer.

The layer of sludge which has built up on the surface of the filter layer is broken up at the start of the back washing process, creating a fluidised bed. The rotating motion of the grains of sand removes the dirt particles which have become attached to the surface of the granules and they are carried away from the filter with the rising flow of water.