The sand bed contains sand of different seizes such as silt,silicate,grand,small stones and big stone,water passes through all this stages before going to the activated carbon.
Grains of sand form a layer which is penetrated by the water and which will stop larger particles in the intervals between grains acting like a simple sieve.
Smaller particles will also be retained by the wall effect on the grain surface when they touch a grain as they pass through the filter.
The smaller the diameters of the grains, and hence the longer particles remain in the filter, the higher the filter’s stopping power will be.
There are three types of sand filtration :
- Rapid sand filters : Rapid sand filters need to be cleaned frequently, by back washing, which involves reversing the water direction.
- Semi-rapid sand filters
- Slow sand filters :
The first two require pumps and the use of chemicals (flocculation principle).
A flocculent is used which, through a chemical principle, traps suspended matter and particles to form large flakes which are deposited by sedimentation. (Sedimentation means that suspended particles stop moving and settle).
Unlike other sand filtration methods, slow sand filters use biological processes to clean water, and are non-pressurised systems.
They can treat water and reduce the presence of micro-organisms (bacteria, viruses, microbes, etc.) without the need for chemicals and, if necessary, using a layer of activated carbon.
Slow sand filters do not require electricity to operate.
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