Filtration plays an important role in the natural treatment
of groundwater as it percolates through the soil. It is also a major part of
most water treatment. Groundwater that has been softened, or treated through
iron and manganese oxidation, requires filtration to remove floc created by
coagulation or oxidation processes. Since surface water is subject to run-off
and does not undergo natural filtration, it must be filtered to remove
particles and impurities. Filtration can be compared to a sieve or
micro-strainer that traps suspended material between the grains of filter
media.
However, since most suspended particles can easily pass through the
spaces between grains of the filter media, straining is the least important
process in filtration. Filtration primarily depends on a combination of complex
physical and chemical mechanisms, the most important being adsorption.
Adsorption is the process of particles sticking onto the surface of the
individual filter grains or onto the previously deposited materials. Forces
that attract and hold particles to the grains are the same as those that work
in coagulation and flocculation. In fact, coagulation and flocculation may
occur in the filter bed, especially if coagulation and flocculation before
filtration was not properly controlled. Incomplete coagulation can cause
serious problems in filter operation.
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