Health Hygiene & Sanitation

Lesson 26 : Water Purification

Purification Of Water On A Large Scale

Filtration

Filtration is the second important stage in the purification of water as it aids in the removal of 98-99 per cent of bacteria. Two types of filters are in use, the "biological" or "slow sand" filters and the "rapid sand" or "mechanical” filters.

Slow sand or biological filters

Fig 1 : Slow Sand Filter

It is the standard method of water purification even today.

Elements of a slow sand filter are (Fig 1):

1. Supernatant (raw) water
2. Sand bed
3. An under-drainage system
4. A system of filter control valves