Rapid Sand Filters
Sand filtration is a very basic and cost effective method of treating contaminated water. Sand filtration utilizes the filtering properties of sand and produces clean water that is perfectly safe for consumption. Moreover, as this process requires little or no mechanical power, chemicals or replaceable parts or even minimal operator training, but only demands for some amount of periodic maintenance, it is the best option of water treatment for people living in the economically challenged and isolated areas.
In this process, typically, the water is let to pass through a bed of sand that makes the larger suspended particles settle in the top layers of sand. Smaller particles of organic sediment left in the sand filter are eaten by microscopic organisms including bacteria and protozoans which 'stick' in the layers of slime that form around the sand particles. The clean water which passes through the filter is safe to drink.
There are three types of sand filters: rapid sand filters, upflow sand filters and slow sand filters. The most commonly used type are the rapid sand filters. These filters employ a layer of activated carbon or
anthracite coal above the sand to remove organic compounds, thereby enhancing the taste and odor of raw water. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough. The particles that escape through the surface layers are trapped in the subsequent layers of sand. It has to be noted that if the surface layer sieves all the particles, the filter will quickly clog.
Rapid sand filters can be cleaned by passing water upward through the filter after blowing compressed air up through the bottom. This air blowing process breaks up the compact media filter.
- Filters out much smaller particles than paper and sand filters can.
- Filters out virtually all particles larger than their specified pore sizes.
- They are quite thin and so liquids flow through them quite rapidly.
- They are reasonably strong and so can withstand pressure differences across them of typically 2–5 atmospheres.]
- They can be cleaned (back flushed) and reused.