OVERVIEW

Although rapid sand filters predominate, the two types of filters covered in this chapter may be effective in many applications where source water quality permits. In general, the combined costs of constructing and operating slow sand and diatomaceous earth (DE) filters may be considerably less than the cost of rapid sand filtration plants for the same capacity.

The principal mechanisms for separation of solids in all porous media filters are attachment and straining or entrapment. Because of the relationship of the somewhat large pores in rapid sand filter media compared with particulates, the primary mechanism for separation is attachment. In the case of slow sand and DE filters, however, the pore-particulate size relationship results in more substantial separation by entrapment.

During the initial operation period of slow sand filters, the separation of organic matter and other solids generates a layer of biological matter on the surface of the filter media. Once established, this layer is the predominant filtering mechanism. Solids are removed from water by a DE filter as the source water flows through a precoat layer of powderlike DE.

For all practical purposes, most solids are separated at the surface of the media in both actions. Because of the small pore size of the media, particulate separation is ideal for removing the cysts of Giardia and Cryptosporidium. In most situations, neither type of filter requires previous conditioning...