# Pressure Sand filter

## Principle

The principle of the Pressure Sand filter is a type is depth filtration and the mechanism is impingement and entanglement of solids on account of low-pressure differential. The slurry enters the sand bed and the clear liquid is collected from the bottom outlet.

## Construction of Pressure Sand filter

The construction of a standard pressure sand filter is given in Figure 1. It is a closed system and consists of a cylindrical tank with a bottom containing a number of brass strainers. These are mounted on a false bottom or connected to a manifold embedded in concrete. The strainers have narrow slots sawed in them. A layer of moderately coarse gravel is placed on the top. Above this, a 6 to 12 m depth sand layer is placed, which acts as a filter medium. Baffles are arranged at the point of the feed inlet, in order to prevent the disturbance of sand by the direct stream. Provision is made at the bottom for collecting the filtrate.

## Working

The feed is introduced from the top at pressure through the baffle. The feed is passed smoothly at a uniform rate onto the sand medium. Filtration is achieved by straining mechanism through the sand medium. Then, the feed is passed through a strainer, so that further filtration takes place and particles are retained. The precipitate is discarded. The filtrate is collected from the bottom.

When the precipitate clogs the sand (indicated by reduced filtration rate), backwashing permits the regeneration of the filter bed. During this operation, water moves up through the sand bed and moves out from the inlet.

### Uses of Pressure Sand filter

A pressure sand filter is used for filtration of boiler feed or water for similar purposes. Sand filters are mainly used when the solids are little and a large volume of feed is to be handled. Such operations can be achieved at minimum cost. Pressure sand filters are applicable only to the separation of precipitates that can be removed from the sand.

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