Rapid sand filtration is a purely physical drinking water purification method. Rapid sand filters (RSF) provide rapid and efficient removal of relatively large suspended particles. Two types of RSF are typically used: rapid gravity and rapid pressure sand filters. For the provision of safe drinking water, RSFs require adequate pre-treatment (usually coagulation-flocculation) and post-treatment (usually disinfection with chlorine)
As explained in the previous chapter, filtration is the process whereby water is purified by passing it through a porous material (or medium). In rapid filtration sand is commonly used as the filter medium but the process is quite different from slow sand filtration. This is so because much coarser sand is used with an effective grain size in the range 0.4-1.2 mm, and the filtration rate is much higher, generally between 5 and 15m3/m2.h (120-360 m2.day). Due to the coarse sand used, the pores of the filter bed are relatively large and the impurities contained in the raw water penetrate deep into the filter bed. Thus, the capacity of the filter bed to store deposited impurities is much more effectively utilised and even very turbid river water can be treated with rapid filtration.
