The rapid sand filter or rapid gravity filter is a type of filter used in water purification and is commonly used in municipal drinking water facilities as part of a multiple-stage treatment system. Rapid sand filters were first used in the United States in 1896 and were widely used in large municipal water systems by the 1920s, because they require smaller land areas compared to slow sand filters.
Design and operation
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.
Mixing, flocculation and sedimentation processes are typical treatment stages that precede filtration. Chemical additives, such as coagulants, are often used in conjunction with the filtration system.
A disinfection system (typically using chlorine or ozone) is commonly used following filtration. Rapid sand filtration has very little effect on taste and smell and dissolved impurities of drinking water, unless activated carbon is included in the filter medium.
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.
Advantages and Disadvantages
Rapid sand filters are typically designed as part of multi-stage treatment systems used by large municipalities. These systems are complex and expensive to operate and maintain, and therefore less suitable for small communities and developing nations.
- Much higher flow rate than a slow sand filter
- Requires relatively small land area
- Less sensitive to changes in raw water quality, e.g. turbidity
- Requires greater maintenance than a slow sand filter. For this reason, it is not usually classed as an "appropriate technology," as the term is applied in less-developed countries.
- Generally ineffective against taste and odour problems.
- Produces large volumes of sludge for disposal.
- Requires on-going investment in costly flocculation reagents.