Literature Review - 2

2.1.2 Rapid sand filters
In rapid sand filters, also called as mechanical filters, coarser and more uniform sand media (effective size 0.4 to 0.6 mm, uniformity coefficient 1.3 to 1.7) is used and the filters are equipped with mechanical back_ washing facilities. This is the most widely adopted filter system and major part of the research and experimental work in water filtration is on this type of filter. Pair et al. (1966) discussed the design of such filters in detail. The filter is generally designed',for filtration rate of 73.5 to 98 1pm/sq.m. and the filter run is terminated when the headloss through the filter reaches maximum allowable value which is usually about 250 cm. Filter is then backwashed by upf low of wash water with or without auxilliary air scouring.

Unavoidable stratification of bed due to back-washing is a major drawback of rapid sand filters. Is the stratification finest sand comes at the top and gives a fine to coarse grain grading with increasing depth. As most of the turbidity gets arrested in the top few cm of fine sand, the filter gets choked much earlier with conse-quent increase in headloss and hence the filter is required to be backwashed even if lower sand layers are not clogged and are still capable of removing turbidity.

2.1.3 Pressure filter:-
This filter is to slight modification of rapid sand filter. It operates at somewhat higher rates than rapid sand filter as the water can be forced through the media under pressure. Due to requirement of large and expensive pressure tanks, their use is generally confined to small water treatment plants like those of industries and swimming pools.

2.2 Development of filtration theory
Based on the extensive field observations and data collected for the slow sand and rapid sand filtera, norms of design and operation were decided mainly on the basis of ensuring better filtrate quality and high rates of filtration were thought to be detrimental to filter efficiency. However, increasing research in the filtration theories revealed functioning of slow sand and rapid sand filters and suggested new possibilities of improvement.

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