Classification of filters:

The media used for filtration is mostly natural silica sand. In addition to this, crushed anthracite (hard) coal, diatomaceous earth, powdered or granular activated carbon, plastic spheres, rings and granules and metal fabrics are also used as filter media. In the lower portion of bed well washed gravel are used to support sand layer. Activated carbon is prepared from coal with a special manufacturing process to make it highly porous with high absorbing capacity. The supporting gravel layer permits the filtered water to move freely to the under-drains and allow the wash water to move uniformly upward.

The filters can be classified based on several criteria as shown below:

For treatment of community water supply, fifth type of classification is normally adopted. Accordingly filters under this category are classified as:

• The slow sand gravity filters, and
• The rapid sand gravity filters.