Chapter 21 - Filtration Systems

Abstract

Filtration is the process of separating suspended solids from liquid by means of a porous medium. Filters may be classified by combinations of: method of liquid movement through the filter, method of medium handling, and type of medium. This chapter discusses the filters that can be classified according to type of medium, that are, screen, cartridge, cloth or paper, coarse granular, and filter aid. Screen filters have fixed pore sizes generally equal in size and not of great depth. Therefore, all solids above the fixed pore size are prevented from passing through. Cartridges are removable filter elements, preassembled or molded into the proper shape. They depend on depth filtration, removing solids within the labyrinth path of flow. Filter cloths are made of paper, textiles, plastics, rubber, and various other materials or combinations thereof. Although filter cloths are used in cartridge filters, they are more generally used in filter presses or fiat-bed indexing filters. Granular filters all depend to a varying degree on depth filtration. The selection of a filter to suit a specific process requires evaluation of many factors that are: required clarity of effluent, maximum size of particle in effluent, variation of inlet flow and/or solids concentration, requirements for medium replacement or backwashing, disposal of removed solids and backwash liquid, use of pretreatment, and operating economics.