These are small rapid sand filters placed in closed vessel horizontally or vertically in which water is passed under pressure greater than the atmospheric pressure. Water from sedimentation tank is pumped into these filters.
The pressure varies from 3 to 7 kg/sq cm. The diameter varies from 0.3-2.75 m and the height varies from 2-2.5 m. in case of vertical pressure filters. In case of horizontal filters the diameter varies from 2-3 m and length up to 9m.
The rate of filtration is 6000-15000 LPH/sqm of the filter area. These are less efficient than rapid sand filters in removing turbidities and bacteria.
The quality of water from pressure filter is not as good as that of slow or rapid sand filter and therefore preferred for treating small quantities of water and best suited for swimming pool, railway stations, private estates and individual industries.