同海大学 inlet pipe used to introduce backwater for Tongji University sand cleaning filter gravel 3)Rapid filtration outlet under drain Replaced slow sand filters in 20th century; Much higher loading rate than slow sand filters,more than 100 X,typically 5-15 m/hour; Media are coarser,more uniform,(often multiple); Removal is not by physical straining on the surface as primary mechanism; Conceptually,rapid filtration is like sedimentation; Particles need pre-treatment with a coagulant to destabilize electrical charge;Destabilized particles adhere to grains in filter medium and are removed; Turbidity of outflow changes with time;Tongji University (3) Rapid filtration • Replaced slow sand filters in 20th century; • Much higher loading rate than ; slow sand filters, more than 100 X, typically 5-15 m/hour; • Media are coarser, more uniform, (often multiple); • Removal is not by physical straining on the surface as primary mechanism; • Conceptually, rapid filtration is like sedimentation; • Particles need pre-treatment with a coagulant to destabilize electrical charge; Destabilized particles adhere to grains in filter medium and are removed; • Turbidity of outflow changes with time;