Overview

The filter medium is that critical component which determines whether or not a filter will perform adequately. Within the context of solid/liquid separation the term filter medium can be defined as any material that, under the operating conditions of the filter, is permeable to one or more components of a mixture, solution or suspension, and is impermeable to the remaining components (Purchas and Sutherland, 2002). The principal role of a filter medium is to cause a clear separation of particulates (which may be solid particles, liquid droplets, colloidal material, or molecular or ionic species) from the liquid with the minimum consumption of energy. In order to achieve this, careful selection of the medium must take into account many factors; criteria by which a medium is assessed include the permeability of the clean medium, its particle retention capability and the permeability of the used medium. Serious loss of permeability may follow plugging or blinding of pores in the filter medium, and can determine the lifetime of the medium if an uneconomic filtration rate results. Permeability and particle retention are dependent on the structure of the medium, but interaction of media structure with the shape and size distribution of the particles challenging the medium is also of crucial importance.

A vast variety of materials in diverse forms are used as filter media for which Purchas (1981) produced a guideline classification, which is reproduced in Table 2.1. Alternative methods of classification are available, but there always exist media that cannot be fitted...