Why can rapid sand filters handle higher filtration rates compared to slow sand filters?

• A. Finer sand
• B. Coarser sand
• C. Much higher water level above the filter's media, thus higher head pressure
• D. Media not as thick

Answer: (B)

Rapid filtration can operate at higher flow rates mainly because of the media size. Coarser sand has larger pore spaces, which increases porosity and permeability, so water can pass through more quickly without forcing particles through tight spaces. This lets the filter handle higher velocities while still removing particles by physical filtration and adsorption, with less tendency to clog. Slow filtration uses finer sand, which creates smaller pore throats and relies more on a biologically active surface layer and long contact time; at higher rates this layer can’t develop properly and the bed becomes retreating, reducing effectiveness. So the bigger pore structure of coarser sand is what enables rapid filters to run at higher filtration rates.