Literature Review - 1

2. LITERATURE REVIEW
Research and experimentation in the area of filtration is so vast and varied in character and line of approach, that it is difficult to give a comprehensive outlook covering all aspects of filtration. However, the work can be grouped in two major categories, based on practical and theoretical approaches. All the experimentation and field studies conducted for establishment of engineering and design parameters and operational norms can be inclUded in one category,whereas the theoretical investigations in mechanism and kinetics of filtration can be considered as of other category. Field observations provided valuable data on filter performance but did not help in changing the established filtration practice to any significant level. Research on filtration mechanisms, however, gave a rational basis for the filter design and helped in developing new high rate filters varied in type and operation. Field data collected from these new filter units are serving as feed back fat further improvements In design.

2.1 Filtration practice
Slow sand filters, resembling more or less to natural filtratio of water through ground strate and the mechanized and more compact rapid sand filters fort-td the most common features of the filtration practice prevalent in almost all parts of the world till recently.

2.1.1 Slow sand filter
This is an earliest and simplest type of filter with no requirement of any mechanical appliances; but the filtration rate is very low and therefore is more suitable for rural communities where plenty of land is available at cheap rate. In a report of W.H.O., Huisman (1970) gave detailed account of the design procedures and suggested methods for their effective operation and control. Use of fine sand (effective size of 0.25 to 0.35 mm) with less restriction on uniformity in size (uniformity coefficient of 2 to 3) is main design feature in such a filter. This reduces pore size and enhances simple straining of the suspended -particles in the filter media. Formation of a surface biological mat called as 'Schmutzedecke' was considered to be very important for effective filtration in such filters. Chemical pretreatment to water was not recommended for these filters due to possible interference in the 'Schmutzedecke formation'. As a result, these filters were operated at exceedingly low rates of about 133 1/hour/sq.m. Agarwal (1973) brought out falacy in preventing pretreatment to water applied to these filters and showed that the filtration rate can be increased upto 1000 1/hour/sq.m. Than relaxation in operational restraint helped in augmenting capacity of existing slow sand filters.

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