Water Filtration is to collect or gather impurities from the incoming water flow. When the pores of filters logged, it has to be cleaned by forcing out the accumulated particles by reversing the flow and increasing the velocity of water passing back through the filter. It is called backwashing. We can improve the quality of water by doing it at regular intervals.
First the filter is taken off line and the water is drained to a level that is above the surface of the filter bed. Next, compressed air is pushed up through the filter material causing the filter bed to expand, breaking up the compacted filter bed and forcing the accumulated particles into suspension for 20-30 minutes.
After the air scour cycle, clean backwash water is forced upwards through the filter bed, causing the filter bed to expand and carrying the particles in suspension into backwash troughs suspended above the filter surface. In some applications, air and water streams are simultaneously pushed upwards through the granular media. Backwashing continues for 30 minutes to one hour time, or until the turbidity of the backwash water is below an established value. A filter bed should have as much expansion as possible upto 30 to 50 percent of bed volume without loosing media or displacing the support gravel. At the end of the backwash cycle, the upward flow of water is terminated and the filter bed settles by gravity in its initial configuration.
After allowing sufficient time for the filter bed to settle by gravity, rinsing can be done with clear water for 30 minutes until the crystal clear water comes out as the output. Then water to be filtered is passed through the filter and the service cycle can be from 16hrs to 20hrs.
SILT DENSITY INDEX (SDI)
In order to measure the degree of the fouling problem, a measure called Silt Density Index (SDI) is used. Here filtration rates are calculated by exposing a 0.45-micron filter to the feed water under pressure. An SDI of less than 5 is typically considered acceptable for a reverse osmosis systems.(RO). If the value of SDI is higher than 6, stop passing the feed through RO membranes and the pretreatment cycle should be carried out to bring down the SDI levels to less than 4.
In any pretreatment system the SDI reduction in multi grade filters (MGF) should be brought down significantly. If the size of the impurities in the raw water is less than 25 microns, suitable coagulants/flocculants should be added in the raw water to increase the size of the particles (Impurities) and improve the filtering efficiency of MGF.
In carbon filters (CF) SDI reduction across the CF should be minimum. If a significant reduction in SDI is seen across the CF, it will foul very frequently. The remedy is to improve the particle removal efficiency of the MGF.
A turbidity spike can occur if the filter bed is not allowed to settle by gravity for a reasonable time. For small plants back washing can be carried out at the end of the day and allow the filter to settle by gravity at night time.
A carefully designed pretreatment plant aimed at reducing the SDI to as low as possible will go a longway in trouble free operation of the RO plants.