Before revision: 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.
After revision: Rapid sand filters use relatively coarse sand and other granular media , such as anthracite, in beds of 0.6 to 1.2 metre depth to(0.5 to 1.0 mm) remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals—typically alum.