17 DEC 2003
Porosity is, strictly speaking, how much of a specific material is comprised of void
space. It is the ratio of the non-solid volume to the total volume of a material. However,
among filtration professionals, porosity may be used to describe the filter medium's re-
tention. For example, a cartridge may be described by its manufacturer as having a
porosity of 5 microns when what they mean to say is that the filter will retain some per-
centage (e.g., 90% or 95%) of particles 5 microns or larger. Other manufacturers may
be used porosity when the correct term is permeability. For example, a certain filter fab-
ric may be described as having a porosity of 10 cubic feet per minute at a certain pres-
sure drop. The designer should be aware, therefore, that whenever porosity is being
discussed in terms other than as a percentage or as a ratio, it is likely that some prop-
erty other than porosity is being described.
Retention is a measure of the efficiency of removal. It describes how much of what
sized particle is removed. For example a manufacturer may refer to the retention of a
filter being 95% of particles 5 microns and larger. As with permeability, these numbers
are generally derived in the laboratory and may vary with application and over the cycle
life of a filter during operation.
Septum is used to describe either the filter medium on which a filter aid collects as a
precoat or as the actual interface between the flowing liquid and the stationary particles. In
this Design Guide septum will be used to describe the filter medium on which the filter aid
collects or the filter cake forms as it becomes the filter medium. Often the septum will be a
rigid medium, such as a wire mesh, whose purpose is not so much to act as a filter medium
as it is to act as a structure on which the filter medium can form.
Suspended solids--(SS) milligrams of dry solids per liter of solution captured by a stan-
dard glass-fiber filter. Determined by Method 2540 D AWWA, 1998.
Total dissolved solids (TDS) is the weight per unit volume of all volatile and non-volatile
solids dissolved in a water or wastewater after a sample has been filtered to remove
colloidal and suspended solids.
Total solids (TS) is the sum of dissolved and suspended solids in a water or wastewater.
Matter remaining as residue upon evaporation at 103 to 105 degrees C.
Total suspended solids (TSS) is the measure of particulate matter suspended in a sample
of water or wastewater. After filtering a sample of a known volume through a glass wool
mat or 0.45-micron filter membrane, the filter is dried and weighed to determine the residue
retained. (EPA Test Method 160.2)
Turbidity means a qualitative measurement of water clarity that results from suspended
matter that scatters or otherwise interferes with the passage of light through the water.
Ultrafiltration (UF) means a low pressure, 200-700 kPa (20-100 psi), membrane filtration
process that separates solutes in the 20 to 1000 angstrom (up to 0.1 micron) size range.
Underdrain--Flow collection and backwash water distribution system used to support the
filter bed in most granular media filters. Also called filter bottom.