Definitions and Classification of Filtration

Filtration is the separation of a fluid-solids mixture involving passage of most of the fluid through a porous barrier which retains most of the solidparticulates contained in the mixture. Filtration is the term for the unit operation. A filter is a piece of unit-operations equipment by which filtration is performed. The filter medium or septum is the barrier that lets the liquid pass while retaining most of the solids; it may be a screen, cloth, paper, or bed of solids. The liquid that passes through the filter medium is called the filtrate.

Filtration and filters can be classified several ways:

1. By driving force
   The filtrate is induced to flow through the filter medium by hydrostatic head (gravity), pressure applied upstream of the filter medium, vacuum or reduced pressure applied downstream of the filter medium, or centrifugal force across the medium. Centrifugal filtration is closely related to centrifugal sedimentation, and both are discussed later under “Centrifuges.”
   
   
2. By filtration mechanism
   Although the mechanism for separation and accumulation of solids is not clearly understood, two models are generally considered and are the basis for the application
   of theory to the filtration process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within the pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration.
   
   
3. By objective
   The process goal of filtration may be dry solids (the cake is the product of value), clarified liquid (the filtrate is the product of value), or both. Good solids recovery is best obtained by cake filtration, while clarification of the liquid is accomplished by either depth or cake filtration.
   
   
4. By operating cycle
   Filtration may be intermittent (batch) or continuous. Batch filters may be operated with constant-pressure driving force, at constant rate, or in cycles that are variable with respect to both pressure and rate. Batch cycle can vary greatly, depending on filter area and solids loading.
   
   
5. By nature of the solids
   Cake filtration may involve an accumulation of solids that is compressible or substantially incompressible, corresponding roughly in filter-medium filtration to particles that are deformable and to those that are rigid. The particle or particleaggregate size may be of the same order of magnitude as the minimum
   pore size of most filter media (1 to 10 μm and greater), or may be smaller (1 μm down to the dimension of bacteria and even large molecules). Most filtrations involve solids of the former size range; those of the latter range can be filtered, if at all, only by filter-mediumtype filtration or by ultrafiltration unless they are converted to the former range by aggregation prior to filtration.
   
   These methods of classification are not mutually exclusive. Thus filters
   usually are divided first into the two groups of cake and clarifying
   equipment, then into groups of machines using the same kind of driving
   force, then further into batch and continuous classes. This is the
   scheme of classification underlying the discussion of filters of this subsection.
   Within it, the other aspects of operating cycle, the nature of
   the solids, and additional factors (e.g., types and classification of filter
   media) will be treated explicitly or implicitly.