ABSTRACT
Filtration with granular media has long been applied in the treatment of municipal and industrial waters as well as for wastewater treatment both in upgrading existing conventional plants and in designs of new advanced treatment facilities. Granular media filtration involves passing the water through a bed of granular material with resulting deposition of solids. The most important characteristic in equipment costs is the filtration rate, which determines the filter size. Operating costs depend on filtration rate, terminal head loss, media characteristics, and backwash design. Media is cycled downward through the bed, withdrawn at the bottom, externally washed, and returned to the top. Biological flocs tend to be significantly stronger or more resistant to shear than chemical flocs, at least those from alum or iron coagulants. In contrast to flocs from other coagulants, calcium carbonate precipitates are strongly removed at the filter surface where they may form a dense compressed layer that is hard to remove during washing.
