The process of filtration of impure water is said to be a result of the following mechanisms.
1.
Mechanical Straining:
The suspended particles in water whose size are bigger
than the voids in the filter media cannot pass through them and therefore get
arrested in the top portion of the filter media thus making the water free from suspended impurities.
2.
Sedimentation and Flocculation:
It has been found that the filters remove the
particles whose size is smaller than the size of the voids. This can be
explained by assuming that the voids of the filter-media act as tiny coagulation
and sedimentation tanks. The colloidal particles act as a gelatinous mass and therefore
attract finer particles these finer particles then settle down in the voids
and get removed.
3. Biological
Filtering:
Certain bacteria and micro-organisms are generally present in the
voids of the filter. They may either reside initially as a coating in the sand
grains or they may be caught in the initial process of filtration. Nevertheless,
the organisms need biological impurities such as algae, plankton, etc. as their
food for survival. These organisms, therefore, utilize this impurity and convert
them into harmless compounds by the process of biological metabolism. These
harmless compounds thus formed appears in the top layer and is called as
schmutzdecke or dirty skin. This layer helps in absorbing and staining out the
impurities.
4. Electrolytic
Changes:
The purifying action of the water can also be explained by the theory of
ionization. According to this theory, a filter helps in purifying water by changing
the chemical characteristics of water. This can be explained by the fact that
the sand grains and the filter media carry some electric charges opposite in
nature. When these oppositely charged particles and the impurities come in contact,
they neutralize each other and therefore change the chemical characteristics of
water. After some time, interval the electric charge of the filter media gets
exhausted and has to be restored by replacing the top layer of the filter
media.
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