ABSTRACT
This chapter focuses on deep granular filters, although some attention gives to precoat filters. Filtration involves a complex variety of mechanisms, which includes transport, attachment, and detachment. Silica sand, anthracite coal, garnet, and ilmenite are the most common types of medium used in granular bed filters. Effective size and the uniformity coefficient are employed in the United States to describe the filter medium. The shape and roundness of the filter grains are significant because they affect the backwash flow requirements, the fixed-bed porosity, head loss for flow through the medium, filtration efficiency, and the ease of sieving. A slow sand filter generally consists of a watertight basin containing a layer of sand over a layer of gravel. Dual-stage filtration, in packaged form, is a good alternative to conventional rapid rate filtration for small systems. Microfiltration and ultrafiltration have been studied for the removal of protozoan cysts and viruses.
