Theory of filtration
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| source :pamirtimes.net |
Filters are classified as -
Gravity filter
Pressure filter
In gravity filters the head required to flow through filter media
is provided by the head of water over the filter media but in pressure filter head is provided by pressure applied from outside.
Theory of filtration
when water is passed through the filter medium removal of impurities takes place by the following mechanism.
Mechanical Straining
Sedimentation
Biological Changes
Electrolytic Changes
Mechanical Straining : Particle which are larger in size than the pores of filter are strained out . The arrested impurities including coagulated flocks forms a mat on the top of sand layer,which has pores sizes smaller than that of the sand grain.
Hence even the particle smaller than the pores sizes of sand gets removed .
Sedimentation : Due to the removal of the turbulence from the water up -to certain extent some particles settle down on the surface of sand grains making the filtered water free from suspended solids .
Biological Changes : During first few days upper layer of sand grain gets coated with sticky deposits of partially decomposed organic matter together with iron, manganese aluminum etc , which contains the algae ,plankton etc that uses nitrogen and phosphorous as nutrient and grow ,and due to photosynthetic reaction produces oxygen.
Electrolytic Changes : The sand grain in filter and impurities in water carry opposite charges ,thus because of their interaction the chemical characteristic of water changes and it becomes free from dissolved impurities.
During washing process these neutralized material are removed from the sand bed .The oxygen is utilized to oxidize the organic matter which are in particulate form or dissolved form ,thus a stable harmless compound is formed .The process
takes place in the layer termed as SCHMUTZDECKE.
Due to scarcity of organic matter ,bacteria by endogenous respiration also destroys each other and hence bacterial load is also reduced .
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