同©大学 ③ration Filter Influent Waste(or Spent)Filter Backwash(FBW) Water Above Filter Filter Media Filtration Rate Control Equal(Constant)Rate Underdrain Filter Effluent Declining Rate System Backwash Water Supply Filter to Waste (Backwash Air Supply) Fig.4 A typical gravity filter cross sectionTongji University (3) Rapid filtration Fig.4 A typical gravity filter cross section