of Filtration Terms
The pressure above an absolute vacuum. One atmosphere (14.70
psi) greater than gauge pressure. Symbolized as psia when the pressure is
expressed in psi units.
(gen) The taking in,
incorporation or reception of gases, liquids, light or heat. (phys/chem.) Penetration of one substance into the inner structure of another (cf.
adsorption, in which one substance is attracted and held on the surface of
another). Occurs between a gas or vapor and a liquid. (pharm.) The
process of movement of a drug from the site of application into the
extracellular compartment of the body.
Charcoal activated by heating to 800-900ºC
to form a material of high adsorptive capacity for many gases, vapors,
organics, etc. Has a large internal surface area (approx 1,000 m2/g).
Commonly used in the pharmaceutical industry to remove organic contaminants.
Can be used either as an additive in granular form which is then filtered
out or as a filter media in a filtration device itself.
Retention of gas, liquid or solid on a surface due to
positive interaction (attraction) between the surface and the molecules of
the adsorbed material.
Organisms which require oxygen to live.
A dispersion of small liquid particles in a gas.
Refers to "common" environmental conditions in which
experiment is conducted. For example: 14.7 psia and 20º to 25ºC (room temperature).
Organism capable of growing without the presence of oxygen.
ANISOTROPIC (ASYMMETRIC) MEMBRANE:
A membrane in which the pore size and structure are not the
same from one side of the membrane to the other. Such membranes are usually
considered "directional" because of the difference in flow characteristics
depending on which side of the membrane faces the feed stream.
Positive pore or electrode of an electrolytic system.
Similar to or resembling water. In reference to solution made
Refers to an operation performed in a sterile environment
designed to prevent contamination through introduction of bacteria.
Analytical procedure to determine purity or concentration of
a specific substance in a mixture.
A chamber for sterilizing with saturated steam filters or
equipment by using constant high temperature and pressure (121ºC,
15 psi). One method of ("terminal") sterilization using saturated steam.
A backward surge of pressure from downstream to upstream of
the filter. Can be the result of closing a valve or air entrapped in a
Reversal of a fluid flow through the filtration media, as an
attempt to clean or "regenerate" a filter.
Free living simple celled, microscopic organisms having a
cell wall and characteristic shape (e.g., round, rod-like, spiral or
filamentous); lack a defined nucleus.
Term used when testing the bacterial retention of a filter.
A unit of pressure. One bar = 14.5 psi.
Measurement of filter retention efficiency. Ratio of
particles exposed to a filter (as feed stream) to particles present in the
The load or level of microorganisms in a substance to be
Biological refuse, possibly pathogenic in nature.
Biological safety or non-toxicity of a substance to a living
organism by passing tests as listed in the United States Pharmocopeia.
Analogous to "chemically inert." For filters used in biological and health
care application, Plastic Class-VI tests apply, which include Systemic
Injection, Intracutaneous and Implantation Tests.
The continuous zigzag motion of suspended minuscule
particles. The motion is caused by impact of the molecules of the fluid upon
BUBBLE POINT PRESSURE:
A test to determine the maximum pore size openings of a
filter. The differential gas pressure at which a wetting liquid (usually
water) is pushed out of the largest pores and a steady stream of gas bubbles
is emitted from a wetted filter under specific test conditions. Used as
filter integrity test with specific, validated, pressure values for specific
pore-size (and type) filters.
A Nitrile rubber seal compound. This is a generic term
covering many formulations.
Solids deposited on the filter media.
Negative pole or electrode of an electrolytic system.
(N s/m2; N = Newton) A unit
of absolute viscosity. One centipoises equals 0.01 stoke.
A unit of kinematic viscosity (m2/s).
One centistokes equals 0.01 stoke.
Process of separating two substances of differing densities
by high speed spinning to create centrifugal force. Typically used to
separate suspended particles from liquid.
The separation of substances in a mixture based on their
affinity for certain solvents and solid surfaces.
To clear a liquid by filtration, by the addition of agents to
precipitate solids, or by other means.
CLASS 100 ENVIRONMENT:
A room environment maintained by air conditioning and
filtration so that fewer than 100 particles of size 1µm
or larger are found in a cubic foot of air.
Removal of all bacteria by filtration through a sterilizing
grade 0.2µm absolute filter.
Tube or cylinder containing the chromatographic bed or
stationary phase, usually in the form of beads.
Term used in relation to the non-reactivity of filter
materials with the substance to be filtered.
An apparatus or method for removing some of the water from a
sample to concentrate the substances dissolved or suspended in it; usually
used to concentrate solutions of biological macromolecules, e.g., proteins
and nucleic acids.
CROSSFLOW (TANGENTIAL FLOW) FILTRATION:
A filtration system in which the feed stream flows across the
filter media and exits as a retentate stream. The retentate stream is
recycled to merge into the feed stream, while a portion of it passes through
the filter media, resulting in concentration of the feed stream (referred to
DELTA () P:
See "Differential Pressure".
DIATOMACEOUS EARTH FILTRATION:
A filtration method that employs a medium consisting of
microscopic shells of single celled plants known as diatoms.
Deionized water; water processed through an ion exchange
process by passing through both cation and anion exchange resin beds, or a
mixed resin bed to remove both positive and negative ions. The purity of
water is measured by its electric resistance. High quality DI water has a
minimum resistance of 18 megohm per cm at 25ºC.
DEAD END (CONVENTIONAL) FILTRATION:
Feed stream flows in one direction only, perpendicular to and
through the filter medium to emerge as product or filtrate.
A matrix of randomly distributed fibers creating a tortuous
path with pores of undefined size and shape.
The difference in pressure between the upstream and
downstream sides of the filter. Also called P, psid or pressure drop. May be
modified with applied, available, clean, dirty, initial, or maximum.
In gas filtration, at low gas flow velocities, very small
particles <0.1µm are subject to Brownian motion. Thus
they can move out of the gas streamlines and become intercepted by the
DIFFUSIONAL FLOW TEST:
A test to determine the integrity of a filter. The test is
based on the measurement of diffusive (diffusional) flow of a gas through a
wetted filter. Either the gas or the downstream liquid, displaced by the
gas, may be measured. In addition, the transition from diffusional flow to
bulk flow (i.e. bubble point) can be determined.
In gas filtration, particles larger than the pores are
removed by direct interception with the filter surface. Some particles
smaller than pores can be removed as well depending on the probability,
which is proportional to their size, of hitting the surface.
Amount of dirt or debris retained by a filter in grams per
unit area of the filter medium.
Drug Master File. A written document that explains the
formulation of an active ingredient, and is referenced in an Investigational
New Drug (IND), New Drug Application (NDA), or Amendment to New Drug
Application (ANDA) from a company.
Dioctyl phthalate, a plasticizer that can be aerosolized to
particles of extremely uniform size of the order of 0.3µm.
Retention of DOP aerosol is used a s standard procedure for pore size rating
of air filters.
DOWNSTREAM SIDE (OF FILTER):
The filtrate or product stream side of the filter.
DRY HEAT STERILIZATION:
Sterilization at or above 180ºC using a
convection or forced air oven without moisture; may concurrently
depyrogenate if adequate time and elevated temperature are employed.
Escherichia coli; The most prevalent bacteria in the
gastrointestical tract of humans and animals. It occurs in solids and water
as a result of fecal contamination.
Chemical sterilization using ethylene oxide usually 12:88
(12% ETO in Freon). Employs a slightly elevated temperature, 66ºC
(150ºF), and high relative humidity to facilitate
permeation of the ethylene oxide into the material being sterilized.
EFFECTIVE FILTRATION AREA:
The portion of filter that fluid flows through during the
filtration (EFA) process.
The fluid which has passed through a filter (syn: filtrate or
product stream); also, outflow from other types of treatments such as
wastewater treatment plants.
Environment Protection Agency regulates environmental
monitoring. Establishes and enforces its guidelines.
Chemicals which may be leached from a filter during a
filtration process; usually tested for by soaking in water under controlled
conditions; may be removed by pre-flushing with suitable liquid.
Fabrication fabrication area (e.g., in electronics industry).
Food and Drug Administration.
Generally referred to as enzymatically controlled breakdown
of an energy rich compound (as a sugar to produce ethyl alcohol, carbon
dioxide, and energy) by the action of yeasts which carry the necessary
enzymes (bacterial fermentations also occur).
A device for carrying out filtration which consists of the
filter medium and suitable holder for constraining and supporting the filter
in the fluid path.
To pass a fluid containing particles through a filter medium
whereby particles are removed from the fluid.
A measurement of how well a filter retains particles. Usually
expressed as the percentage of retention of particles of a specific size by
a filter; see also "Beta Ratio" and "Log Reduction Value."
FILTER MEDIA MIGRATION:
Problem caused by a filter medium which is constructed of a
non-continuous or fibrous polymeric matrix such that portions of the filter
change structure causing undefined pore size/distribution, as a function of
The permeable material that removes particles from a fluid
The effluent of a filtration process. The filtered product.
The process by which particles are removed from a fluid by
passing the fluid through a permeable material.
The process of briefly heating a beverage to destroy
objectionable enzymes and microorganisms. See "Pasteurization".
Decrease in flow rate as a result of filter plugging or
FLOW DECAY TEST:
An experiment to determine flow rate and throughput of a
filter type or combination of filters on a specific liquid, usually by using
a small area filters, to determine the sizing of a filter system by
It is the speed at which a liquid flows and is measured in
gallons or liters per minute. Flow rate of a liquid can be affected by the
liquids' viscosity, differential pressure, temperature and type of filter
FORWARD FLOW TEST:
An integrity test measuring air diffusion. See "Diffusional
In densitometry, the division of peaks into fractions in
order to quantitate the electrophoretically separated bands. In chemistry,
separation of a mixture of components into different portions (fractions).
Gas Chromatography; similar to HPLC except that mobile phase
is an inert gas such as helium.
Material inserted between contact surfaces of a joint to
ensure a fluid-tight seal.
The pressure measured by a pressure gauge. Pressure above
ambient pressure. Symbolized as psig when the pressure is expressed in psi
Gas Liquid Chromatography.
Good Manufacturing Practices. Regulations promulgated by the
Food and Drug Administration governing the manufacture of drugs (Ref. Code
of Federal Regulations 21 CFR 210 & 211), medical devices (21 CFR 820), and
Large Volume Parenterals (21 CFR 212 proposed).
Gallons per hour.
Gallons per minute.
Process of collecting and analyzing groundwater in areas
where contamination is suspected such as dumpsites and landfills. Look for
pesticides, dissolved metals, etc.
Health Industry Manufacturer's Association. A trade
association, whose membership includes both pharmaceutical manufacturers and
filter manufacturers, that defines and sets standards governing the
validation of filters for sterilizing liquids.
Also called Retention Volume. Volume of fluid retained in a
filter and/or housing after purging the assemble with air or suitable gas.
High Pressure Liquid Chromatography allows separation and
analysis of very small quantities of complex mixtures with high resolution
and great sensitivity. Purpose: identify nature of a compound or measure
amount or concentration of a compound.
Having an affinity for water; a membrane which will wet with
Literally, fearing water; a membrane which cannot be wetted
by and repels aqueous and other high surface tension fluids; when pre-wetted
with low surface tension fluid, such as alcohol, the filter will then wet
Retention mechanism in gas filtration. Also called Inertial
Collection and Inertial Impact. As the gas stream lines bed in the vicinity
of the filter, the carried particles continue in a straight line due to
their inertia and impact the filter. Effective primarily for particles about
0.3µm and larger, at high gas velocities and low
Latin for "in place." Sterilization or integrity testing of a
filter in the system rather than as an ancillary operation such as in
autoclave or bubble point stand.
Chemical inactivity; unable to move; totally unreactive.
The pressure entering the inlet side of the filter. Also
called upstream pressure or line pressure.
A non-destructive test which is used to predict the
functional performance of a filter. The valid use of this test requires that
it be correlated to standardized bacterial or particle retention test.
Examples: Bubble Point Test, Diffusion Test, Forward Flow Test, Pressure
See "Direct Interception."
ION EXCHANGE COLUMNS:
Vessels filled with ion exchange resin (anion, cation, or
mixed) for producing conditioned or DI Water. Also, type of column used for
Ion Exchange Chromatography (IEC).
ISOTROPIC (SYMMETRIC) MEMBRANE:
Membrane in which the pore openings are the same diameter
throughout the thickness and on both sides of the membrane. Such membranes
are non-directional, i.e., their flow characteristics are independent of
which side faces the feed stream.
K or k:
The symbol for kilo or 1,000. As in kilogram (kg = 1,000g) or
kilometer (km = 1,000m). In information systems, and computers, 1K means
1024 bits of information. A 64K memory stores 65,536 bits.
The ratio of absolute viscosity (poise) to the specific
gravity of a fluid. The unit of kinematic viscosity is the stoke. See "Centipoise"
The pressure in the supply line. Also called inlet pressure,
LIVE STEAM STERILIZATION:
Sterilization by flowing saturated steam through a vented
vessel or system, usually at 125ºC and 20 psi (but can
be performed up to 140ºC and 35 psi.)
LOG REDUCTION VALUE (LRV):
The logarithm to the base 10 of the ratio or organisms in the
feed to organisms in the filtrate. Example: Log10(109/2)
Also used as a ratio of in/out bioburden in other
sterilization methods such as autoclaving.
Large Volume Parenteral. Intravenous injection packaged in
containers of 100 – 1000 mL used to correct electrolytic imbalances, replace
body fluid and provide general nutrition.
MEAN FLOW PORE MEASUREMENT:
The theoretical diameter of the mean pore. It is calculated
as the diameter of the pore of a wetted membrane partially voided of liquid
such that air flow of the partially wetted membrane is equal to ý the dry
In filtration, the material through which fluid passes in the
process of filtration and which retains particles. Also, the nutrients
containing solutions in which cells or microorganisms are grown.
Migration of the materials making up the filter medium. May
cause contamination of the filtrate.
A continuous matrix with pores of defined size.
Separation of particles ranging from 0.1µm
to 10µm from a fluid by passing the fluid through a
membrane. Used for clarification, sterilization or to detect or analyze
bacteria and other organisms and particulate matter.
Also referred to as "micron." It is a 1/1,000,000 of a meter
(1µm = 10-6µm =
25.4µm = 0.001 inch;
60µm = approximately the diameter of a
A unit of measure equal to one thousandth of an inch. 1 mil =
0.001 in = 0.025 mm.
MINIMUM BUBBLE POINT PRESSURE:
Also referred to as minimum critical bubble point pressure,
it is a filter specification derived from diffusional flow – bubble point
curves for a number of filters. It is a diffusional flow pressure just
before the onset of bulk flow.
MIXED CELLULOSE ESTERS:
Synthetic materials derived from naturally occurring
cellulose. First materials used in the manufacture of membrane filters.
Mixed cellulose esters membranes are used in a wide variety of applications,
e.g., concentration of bacteria in water analysis (GN-6) and sampling of
Non-fiber releasing. A filter which will not release fibers
into the filtrate.
National Institute of Occupational Safety and Health develops
basic methodology for analytical test procedures.
A thermoplastic, polymeric material that has high mechanical
strength & compatibility with many different kinds of chemicals. When used
as a membrane it is hydrophilic.
Original Equipment Manufacturers.
Related to or derived from a living organism. Always contains
The pressure exiting the outlet side of the filter. Also
called downstream pressure.
Branching a filtration setup so that two assemblies of the
same pore size are in parallel, to increase flow rate or simplify filter
Any discrete unit of material structure; a discernible mass
having an observable length, width, thickness, size and shape.
Relating to or occurring in the form of fine particles.
Partial sterilization of a substance and especially a liquid
(as milk) at a temperature and time of exposure that destroys objectionable
organisms without a major chemical alteration of the substance. Maintaining
the high temperature for only a short period of time is referred to as
A pump functioning by alternate pinching and release of
tubing which drives the fluid forward in a pulsing action. The major
advantage's are that the peristaltic pump is noninvasive, i.e., the pump
does not contact the fluid being filtered, only the inner wall of the tubing
contacts the fluid and the low shear imparted.
The degree to which a fluid will pass through a permeable
substance under specified conditions. The space or void volume between
molecules allowing fluid flow.
The fluid which passes through a membrane.
The inverse (negative) logarithm to the base 10 of hydrogen
ion concentration. Measure of a substance's acidity or alkalinity with 7
being neutral. Measure of hydrogen ion concentration.
POISE (ABSOLUTE VISCOSITY):
Numerically equal to the force required to move a plane
surface of one square centimeter over another plane surface at the rate of
one centimeter per second when the surfaces are separated by a layer of
fluid one centimeter in thickness (dyne sec/cm2).
A thermoplastic polymeric material which is resistant to a
broad range of chemicals. When used as a membrane, polypropylene is
Commonly used membrane material. Has excellent flow rates,
high mechanical strength, resistant to a broad range of temperatures (can be
sterilized) and is hydrophilic. Is not resistant to exposure to many organic
Diameter of pore in membrane.
PORE SIZE-ABSOLUTE RATING:
The rated pore size of a filter at which particles equal or
larger than the rated pore size are retained with 100% efficiency.
The percentage of the filter volume which is void space (syn.
Void volume). Also, number of pores per square centimeter of filter area.
Adsorption of a protein to a surface such as a cellulose
nitrate or nylon membrane due to several types of interactions between the
protein molecules and the surface.
A type of bacteria used in sterility testing. One of the
smallest bacteria (0.3µm in diameter), used to
challenge a sterilizing grade filter during validation testing. Under HIMA
challenge conditions (107 c.f.u./cm2 EFA), sterilizing grade filters must retain all 100% of P. diminuta.
Polytetrafluoroethylene; More commonly known as Teflon.
Highly durable and resistant to a broad range of temperatures and chemicals.
PTFE is hydrophobic.
Ability of a filter to recover bacteria (or other defined
particles) from a solution. In Membrane Filtration Technique, expressed as
percent of bacteria originally present or observed on a comparable pour
Ability of a filter to retain particles (total number or
those of a specific size) suspended in a gas or liquid. Expressed as a
percent of particles originally present.
See "Hold-up Volume."
REVERSE OSMOSIS (RO):
A filtration separation method (usually crossflow or stirred
cell type) operating at 200-1500 psi to overcome osmotic pressure. Pore
sizes are typically in the order of 10-10 meters (107mm). Efficiency is usually
described in terms of percent salt rejection with 90% being common.
Standard cubic feet per minute, i.e. units of gas flow rate.
A standard cubic foot is measured as volume of gas at 760 millimeters of
mercury pressure (1 bar) and 0ºC temperature.
To make clean by removing dirt and other extraneous materials
with soap and general disinfectant so as to reduce possibility of growth and
spread of pathogenic organisms.
Filtration through two or more filters of decreasing pore
size one after the other to increase throughput, filtration efficiency, or
to protect the final filter.
A filter with straight-though capillary pores with identical
dimension, e.g. a screen filter.
Standard Operating Procedure. A written document that
explains how to complete a specific production-oriented task.
The process by which steam, compressed air, or gas is forced
into a liquid through perforations or nozzles in a pipe as part of
STANDARD (NORMAL) PRESSURE:
A pressure of 1 atmosphere (14.70 psi or 760 mm of mercury)
to which measurements of quantities dependent on pressure are often
STERILE, STERILITY, STERILIZATION:
To make or be free of any viable microorganisms. Demonstrated
by testing to show the absence of microorganisms.
A non-fiber releasing filter which produces an effluent in
which no microorganisms are demonstrable when tested by the method specified
in the current edition of the United Sates Pharmocopeia. Usually accepted as
0.2µm pore-size absolute rating.
Also "interfacial tension." Tendency of the surface of a
liquid to contract to the smallest area possible under the existing
circumstances. Defined as a force in dynes acting on a line 1 cm long lying
in the surface of the liquid.
A soluble compound that reduces the surface tension of a
liquid, or reduces interfacial tension between two liquids (causing
formation or micelles) or between a liquid and a solid, thereby functioning
as a wetting agent.
Small Volume Parenteral; Typically administered to a patient
as a bolus or single syringe injection.
A deduction of weight, made in allowance for the weight of a
container or medium; the initial weight of a filter.
Resistance to breaking as a function of tensile force
(tension). The amount of force required to break a membrane by stretching.
Usually accompanied by measurement of Elongation-at-Break, the total amount
of stretching realized at break, expressed as percent of the original
The amount of solution which will pass through a filter prior
TOTAL DISSOLVED SOLIDS:
Is the portion of the total solids in the sample that passes
through the filter and is indicated by the increase in weight in the vessel
after the filtrate has been dried at 180ºC.
The material residue left in the vessel after evaporation of
a sample and its subsequent drying in an oven at 103-105ºC.
The increase in weight over that of the empty vessel represents the total
solids. Used in analyzing drinking water.
TOTAL SUSPENDED SOLIDS:
Is the portion retained on the filter and indicated by an
increase in the weight of the filter after drying at 103-105ºC.
Used in analyzing drinking water.
An imaginary continuous course or path that can be traced
from a point on the upstream side of a filter to a point on the downstream
side. Pathway traveled by the liquid or gas during filtration.
United States Department of Agriculture.
United States Pharmacopeia/National Formulary.
A separation method operating at 50-200 psi in crossflow
filtration mode. Efficiency is approximately 90%. Used to separate large
molecules according to their molecular weight.
UPSTREAM SIDE (of filter):
The feed side of the filter.
The depression of pressure below atmospheric pressure.
Demonstration that a process or product does what it is
supposed to do by challenging the system and providing complete
A resistance to flow as a function of force, or gradual
yielding of force. Viscosity is in units of centipoises or centistokes. For
a given filter and differential pressure, flow rate will decrease as
viscosity increases; e.g. oil will have a flow rate much slower than water.
The viscosity of water is 1 centipoise.
Evaporates easily, converts easily from liquid form to gas.
WATER BREAKTHROUGH TEST:
An integrity test for hydrophobic filters in which the
resistance to water flow is overcome by a specific pressure such that water
will flow through a correspondingly specific pore size of the filter. Also
called a water intrusion test. Useful test to determine gross loss of
integrity (e.g., installation integrity) and filter hydrophobicity.
A surfactant added to a membrane to assure complete intrusion
(wetting) by a high surface tension fluid such as water.