17 DEC 2003
The terminology used in describing filtration systems is not always consistent. Different
manufacturers and filtration professionals may use the same term to describe different
concepts and filtration functions. When discussing a particular application with different
equipment manufacturers, the design professional should verify that the terms being
used have the intended meaning. Terms used in this design guide and the filter industry
Adsorption--The process of transferring a substance from a liquid to the surface of a solid
where it is bound by chemical or physical forces. (See DG 1110-1-2 Adsorption Design
Backwash--A high-rate reversal of flow for the purpose of cleaning or removing solids from
a filter bed or screening medium.
Bed volume--The volume occupied by filter media in a filter.
Capacity refers to a filtration system's ability to perform at acceptable levels until it is
economical to end the filtration cycle and remove the accumulated solids. Capacity can be
expressed as units of time, volume of liquid fed, or solids collected before terminating the
Coagulation--The destabilization and initial aggregation of finely divided suspended solids
by the addition of a polyelectrolyte or a biological process. (See EM 1110-1-4012
Effluent--Partially or completely treated water or wastewater flowing out of a basin or
Filter, as a technical term can be used as a verb or as a noun. As a verb it means to
pass a mixture of particles suspended in a fluid through a permeable medium. As a
noun, filter refers to equipment or hardware (e.g., the canister or vessel that holds the
filter medium or directs the liquid flow through the medium). Filter does not refer to the
medium itself. Instead the medium may be referred to as the filter medium, the filter fab-
ric, the filter cloth, etc. Filtration system may be used to refer to the totality of the
equipment, hardware, structure, permeable medium, piping, controls, etc., encompass-
ing the filtration process.
Filter aid is a material added to the filtration process to prolong the useful life, or capacity,
or improve the retention of the filtration system. It is often added as a precoat either with
the influent liquid or with a pre-applied clear liquid where it deposits on the filter medium or
the septum to then act as a filter medium collecting finer sized particles in the influent liquid.