Rapid Gravity Filters:

One of the drawbacks of the slow sand filter is its requirement of large area for installation. This makes it costly particularly for the places where cost of land is high. In order to reduce the requirement of the space, rate of filtration is to be increased, which could be achieved by increasing the size of filter media or allowing the water to pass under pressure through filter media. The first principle is used in the construction of rapid gravity filter while later is achieved in pressure filters.

The rapid gravity sand filters are most commonly used nowadays. These are mostly operated with coagulants and often follow settling basins. They produce best result with incoming water having turbidity in the range of 10-20 JTU.

It is basically a bed of sand supported in a bed of gravel embedded in which there is a system of underdrains. The whole system is enclosed in an open concrete chamber.  The sand layer has a thickness of around 0.45 to 0.75 m. The sand grains are fairly uniform in size lying between 0.4 mm and 1.2mm. About 10% of sand may pass an aperture of 0.45 mm and about 60% may pass an aperture of 0.65 mm.  The co-efficient of uniformity should vary between 1.2 and 1.7 and the sand should be sharp, hard, clean and siliceous.