OVERVIEW
Rapid Sand filters are typically designed as part of the mult-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. The Filtration system requires a relatively small land area in proportion to the population served, and the design is less sensitive to change in raw water quality. e.g. turbidity, than slow sand filters.
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 alum. Since media other than silica sand can be used in such filters, a more modern term is "rapid filtration" instead of "rapid sand filtration". The unfiltered water flows through the filter medium under gravity or under pumped pressure and the floc material is trapped in the sand matrix.
Mixing, flocculation and sedimentation processes are typical treatment stages that preced filtration chemical additives, such as coagulants, are often used in conjunction with the filtration system.