Filtration is an a important process and plays a vital role in the treatment of water.The effluent of sedimentation tank still contains some of the suspended,colloidal and much of the bacteria impurities which escape the sedimentation tank due to higher terminal velocity, trubulence caused by scrapper etc.Further, sedimentation cannot remove the impurities due to the sewage pollution.
What are the objectives of filtration
1.To remove the residual suspended and colloidal particles in water.
2.To remove bacteria pollution.
3.To change the undesirable chemical characteristics of water.
4.To remove colour, odour and taste.
5.To remove iron and maganese from water.
What are the mechanisms of filtration
Filtration is an a complex process in which many mechanisms act simultaneously and remove the impurities the important mechanisms which are responsible for filtration are listed below.
A.Mechanical straining:
It remove the partical of suspended matter that are too large to pass through the voids of sand medium.Hence the sand medium act as a strainer or sieve.
B.Sedimentataion and adsorption:
The voids between sand grains act as small sedimentation tanks which the partical settle and gert absorbed on to the surface of sand particles.The surface of sand particles contain gelatinous containing formed by previosly deposited bacteria and floculent matter due to the coagulation.
C.Biological metabolism:
This is the growth and life process of living micro organisms the sand grains are coated with zoogleal film having vital bacteria activity.These living organisms frends on the organic impurities in water resulting in some important changes in chemical and biological characteristics of water.
D.Electrolytic action;
The sand grains in the filter medium and impurities in water have opposite electric charges over them.When the suspended and dissloved impurities come into contact with sand grains,the charges are neutralized and thus the chemical characteristics of water are charged.Ultimately electrical charge on the filter is exhausted and the filter must be cleaned to renew these charges.
Filter sand:
The sand used in filter beds should be clean free from day,loam and organic matter.If the sand is to fine,it clogs the filters soon causing greater loss of head.If it is too coarse the efficiency of removal of impurities will be decreased,so it is necessary to specify the sand to be used in the filter,the specification is made in terms of a.effiective size and b.uniformity coefficient.
Effective size:
It is defined as the sieve size in milimeters which perimits 10% of the sand by weight to pass through it, effective size is a measaue of the size of sand particles.
Uniformity coefficient:
It is defined as the ratio between the sieve size which passes 60% of sand by weight and the effective size uniformity coefficient is a uniformity of sand.
If we denote effective size as D10 and the sieve size passing 60% as D60.
Uniformity coefficient = D60 divide by D10
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