Rapid Sand Filter: -
In this type of filter, the size of sand grain is increased to increase the rate of filtration. Here water is pretreated by coagulation and sedimentation for settlement of suspended solids before filtration.
It consists of watertight rectangular tank having a height of 2.5-3.5 meters. The area required is much less as compared to slow sand filter. The rate of filtration is about 100-150 LPM/Sqm.of filter area. Filter media consist of gravel and sand layers. The gravel is laid in 5-6 layers and its thickness varies from 0.6-0.9m. The effective size of sand is 0.35-0.50 mm. and The Uniformity Coefficient is1.2-1.65. The underdrainage system consists of Manifold and laterals. It serves two purposes- a) receives and collects the filtered water. b) allows backwashing for cleaning the filter. It should be capable of passing the wash water upward at high rate. Upward velocity ranges between 0.3-0.9 m/min. The rate of back wash is about 300-900 LPM/Sqm. of filter area.
Back washing: -Normal clean bed head loss varies from 15-30 cm and it increases with filter run, due to which rate of filtration starts reducing considerably. The other major drawback is the formation of mud balls, negative pressure and cracking of filter. To overcome these difficulties backwashing is necessary. For back washing inlet valves are closed. Then compressed air and wash water is forced upward through the underdrainage system. Compressed air assists the cleaning process of the filter. After washing, dirty water is collected in wash water gutters and carried by inlet chamber to the wash water drain. Then inlet valve is opened and the filtered water is led to wash water drain till we get the clear water from the filter media. Backwashing is usually done once in a day. The whole process of backwashing require approximately 15 minutes. The quantity of back wash water is about 2-4 % of the total quantity of treated water.