Rapid sand filter

The rapid sand filter or rapid gravity filter is a type of filter used in water purification and is commonly used in municipal water treatment facilities.

Rapid sand filters use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals - typically salts of aluminium or iron. Water and flocs flows through the filter medium under gravity or under pumped pressure and the flocculated material is trapped in the sand matrix.

Rapid sand filters must be cleaned frequently, often several times a day, by backwashing, which involves reversing the direction of the water and adding compressed air. During backwashing, the bed is fluidized and care must be taken not to wash away the media.

The output from rapid sand filters requires further treatment before it is suitable for supply as drinking water. Unlike slow sand filters, rapid sand filtration has very little effect on taste and smell and dissolved impurities unless activated carbon is included in the filter medium.

Advantages

• It has a much higher flow rate than a slow sand filter;
• Requires relatively small area of land
• Is less sensitive to changes in raw water quality
• It delivers about 150 to 200 million gallons of water per acre per day.

Disadvantages

• The rapid sand filter is not an adequate treatment on its own.
• It requires greater maintenance than a slow sand filter and generally requires mechanical pumping of water and air, at least for backwashing. For this reason, it is not usually classed as an appropriate technology.
• Generally ineffective against taste and odour.
• Produces large volumes of sludge for disposal.
• Requires on-going investment in costly flocculation reagents.
• High flow rates cause little or no Schmutzdecke formed, hence, little biodegaradtion takes place in these filters. Thus, higher bacterial count is obtained in the filtered water. This increases the necessity for disinfection before supply.