"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.
Advantages include:
Much higher flow rate than a slow sand filter; about 150 to 200 million gallons of water per acre per day
Requires relatively small land area
Less sensitive to changes in raw water quality, e.g. turbidity
Requires less quantity of sand
Disadvantages include:
Large pore size will not without coagulant or flocculent remove pathogens like Crytosporidium smaller than 20 microns
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 odor problems.
Produces large volumes of sludge for disposal.
Requires ongoing investment in costly flocculation reagents.
Treatment of raw water with chemicals is essential.
Skilled supervision is essential.
Cost of maintenance is higher.
It cannot remove bacteria."
http://en.wikipedia.org/wiki/Rapid_sand_filterOn a small scale, rapid sand filtration is part of a system that reduces total load of pathogens.