In water treatment plants chemical-physical processes play an essential part in the process chain. They are successfully employed in the pre- and post-treatment of waters of different origins, as well as of other process liquids. In this regard the removal of particles by filtration is a central step. Classical filtration processes are used for particles ranging from 1 μm to 100 μm, i.e. bacteria, yeast cells, sand. Smaller particles are separated by membrane processes, whereas larger ones are removed by screening, sedimentation or flotation.
As early as in 2000 BC the construction of a cistern with an added filtration was described. Rain water from roofs und rocks was collected and led into a basin. By covering the cistern, the water was kept clean of dirt and algae. In a sandwich-like stone wall filled with clay there was a wooden tube with a sleeve valve. Subsequently the water could be led over a bed of sand or gravel, and so it was filtered before being used. In special cases the filter bed was extended by a layer of carbon.
Filtration by means of granular materials is the oldest technical method of water purification. At first the filter material has to be chosen for the required filtration task by an expert. Apart from the suitable filtration process also the filter dimensions are important to optimize the life-cycle-costs.