Filters
In gravity filters,water flows by gravity through the sand bed.In pressure filters,water is sent under pressure through the filters bed.
The gravity filters are further divided as
a.Slow sand filters and
b.Rapid sand filters
a.Slow sand filters:
Slow sand filters was first invented in england by james simpson in 1892.They were popular and wiedly used in treatment plants untill the introduction of rapid sand filters.Now a days only a few treatment plants have slow sand filters and being replaced by rapid sand filters.
Construction:
Slow sand filters consists of a open water tight tank, rectagular in plain, constructed with masonry or concrete.The surface area of the tank varies from 100 to 2000sq.m and depth about 2.5 to 4m.
It contains a sand bed of 60 to 100 cm thickness supported by a gravel bed 30 to 50 cm thick laid in 5 to 6 layer.The sand has an effective size of 0.35 mm and uniformity coeffcient of about 1.75 to 2.0.The top 15cm layer of sand is sand filters than the bottom layer.
An under drain system is placed below the gravel layer consisting of open jointed lateral drains.The lateral drains slope towards the central main drain.It also consists of an outlet chamber provided with a rectagular telescopic arrangment to draw water.
Operation:
Water from sedimentation tank is taken through a submersibe inlet chamber and is applied uniformly over the bed of sand.The water precolates through sand bed during which the development of dirty skin called "schmutz decke".
Cleaning:
During the filtration the sand bed gets clogged and resistance to flow is developed.This result in the increase loss of head.When this loss of head reaches permisible value, filters is stopped dried for few days and the top layer of 2 to 3 cm is scrapped and washed with a hope pipe.
b.Rapid sand filters:
Rapid filters were developed in the last decade of nineteenth century by G.W.Fuller in U.S.A.The main disadvantages of slow sand filters is that they requries a large space for installation as the rate of filtration is very low.
Construction:
These filters units are smaller in size and are located in a building called filters house.number of filtration unit are constructed in one or two rows.Each unit is a rectagular block of size 5m × 4m to 10 × 8m.Depth of the tank is about 3m.Ratio between length and width may be 1.25 to 1.33.
A gravel bed of 0.45 to 0.6m supported the filters medium.The size of gravel is from 5 to 25mm the smaller size being placed at top.Gravel prevents sand from entering into drains and this causing them to choke.
Cleaning:
After sometimes of filtration the filters bed is clogged and loss of head increase with corresponding fall in rate of filtration.When the loss of head reaches about 2.5m,it requries cleaning.As the of loading is very high,the filters bed clogs qucikly.There is no possibility of the development of dirty skin in the case of rapid and filters.
2.Pressure filters:
pressure filters is similar to rapid filters but in the case water is filtered under pressure greater than that of atmosphere.Pressure is above 3 to 7 kg/cm and is developed by pumping.It consists of a closed horizontal or vertical steel tank in which filter medium,gravel bed and under drainage system are provided.The diameter of tank varies between 1.5 to 3m and length between 3.5 to 8m.The water mixes with coagulation is directly fed to the filter.The filtered water is collected through under drainage system.
The rate of filtration is very high and is about 6000 to 15000 of filters area.The efficiency of removal of suspended matter and bacteria is low and hence not suitable for treatment of public water supply.
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