Water Microbiology

Water Microbiology

Rapid Sand Filter

Rapid sand filters are constructed in a manner similar to that of slow sand filters. They also consist of layers of sand, gravel, and rock. Water is pretreated before filtration by a coagulant such as alum or ferrous sulphate.

The water passes through a settling tank in which most of the precipitate settles out, and the remainder is pumped on to the filter. Rapid sand filters soon become clogged and are cleaned by forcing cleaned water backward (back washed) through the bed of gravel and sand, and bubbling air through them.

The back water rises through the filter and carries the accumulated material to the sewer. The wash water is thus wasted, Care is taken in this backwashing procedure to see that the fine sand on the surface is not lost.

Rapid sand filters are usually operated in batteries, so that some may be in operation, while others are being cleaned.
They are nearly as effective as slow sand filters but operate 50 times faster than slow sand filters. Rapid sand filters are capable of delivering 150 to 200 million gallons of water per acre, per day. They require a much smaller area of land for more water filtrations and cost much less to install and maintain.

Many other filtration devices such as pressure filters, diatomite fitters, membrane filter, reverse osmosis etc, are employed to remove various impurities in water. Recovery of potable water from the sea and from domestic and. industrial sewage is also undertaken by the use of filtration techniques.

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