GLOSSARY OF TERMS
AAMI grade water: Water meeting
quality standards established by the Association
for the Advancement of Medical Instrumentation
and used mainly for hemodialysis systems.
Adsorption: Adsorption is the
binding of a molecule to surface (solid or
liquid) by non-specific physical forces. For
example, the removal of free chlorine and
chloramines by activated carbon is through the
mechanism of adsorption.
Algae: A group of single-celled
plants, which includes both seawater and fresh
Alkalinity: A measurement of the
quantity of chemicals present in water which can
neutralize acids. These include carbon dioxide,
bicarbonate, carbonate and hydroxides. See also
Alum: See aluminum sulfate.
Aluminum sulfate: An aluminum salt
commonly used as a flocculant by municipal water
Anions: A negatively charged ion
Bacteria: Bacteria are single cell
microorganisms capable of replicating on their
own. They can be divided into two broad
categories, aerobic (requiring oxygen) and
anaerobic (not requiring oxygen). Bacteria can
live in a very broad range of habitats. Some,
for example pseudomonads, can thrive in
environments containing a very low level of
nutrients. These bacteria are frequently slime
producers and are a major problem in water
treatment systems. Other bacteria, which adhere
to surfaces, secrete a gelatinous material which
serves to protect the bacteria from chemical
disinfectants. This combination of bacteria and
their protective coating is sometimes referred
to as biofilm. The concentration of bacteria in
water is commonly given in terms of colony
forming units (cfu) per ml. A colony forming
unit is a viable bacterium able to replicate to
form a whole colony when incubated in a given
Brine: A strong solution of salt(s)
(usually sodium chloride and other salts too)
with total dissolved solids concentrations in
the range of 30,000 to 300,000 or more
milligrams per liter. Potassium or sodium
chloride brine is used in the regeneration stage
of cation and/or anion exchange water treatment
equipment. Sodium chloride brine saturation in
an ion exchange softening brine tank is about 26
percent NaC1 by weight at 60° Fahrenheit.
Cation: A positively charged ion
Cellulase: An enzyme which causes
the decomposition of cellulose.
Cellulose Acetate: A synthetic
polymer derived from naturally occurring
cellulose and widely used in the fabrication of
membranes. The polymers used for water
purification membranes may be diacetate,
triacetate or blends of these materials.
Chloramines: Chemicals used to
disinfect municipal water. They are formed by
reacting ammonia and free chlorine and may occur
naturally when free chlorine combines with
ammonia arising from the breakdown of
vegetation. Chloramines are strong oxidants.
Chlorinated hydrocarbons: A group of
organic chemicals formed by reacting petroleum
derived chemicals with chlorine. Such chemicals
include pesticides (insecticides) and herbicides
and are frequently potent carcinogens.
Chlorine: Chemical used to disinfect
Chlorophenoxyls: See chlorinated
Coagulant: A chemical which causes
dispersed colloidal particles to become
destabilized thereby aiding in their removal
during municipal water treatment. Aluminum and
iron salts are commonly used for this purpose.
Coagulation: A practice common in
municipal water treatment in which a chemical
(coagulant), most commonly alum, is added to
water in order to destabilize colloidal
particles by neutralization of their electrical
charges. Coagulation is used, together with
flocculation, as a process for colloid removal.
submicron-sized suspended particles which are
well dispersed in a solution and will not
readily settle out on standing.
Compaction: The undesirable physical
compression of a reverse osmosis or ultra
filtration membrane which results in reduced
flux rates. The phenomenon is accelerated at
higher temperatures and pressures.
Conductivity: The ability of an
aqueous solution to carry electric current
depends on the presence of ions in the
solution. Conductivity is a quantitative
measure which describes this ability. Solutions
of inorganic ions are relatively good conductors
(and exhibit high conductivity), whereas
solutions of organic molecules are rather poor
conductors (and exhibit low conductivity).
Highly purified water is also a poor conductor.
Conductivity is expressed in units of Siemen/cm
(also known as mhos/cm)Conductivity measurements
are are frequently encountered in monitoring the
performance of reverse osmosis equipment.
Conductivity is temperature dependent and should
be measured with a temperature-compensated
meter. The usual reference temperature is 25C.
Conductivity measurements are sometimes used to
estimate total dissolved solids in water. While
convenient, this practice is imprecise. (See
Concentrate: A term used in
distillation, electrodialysis, reverse osmosis,
and ultrafiltration to describe that portion of
the incoming feedwater that has passed across
the membrane but has not been converted to
product water and is being sent to the drain.
Also called brine or retentate.
Deionization: Removal of ions from
water by exchange with other ions associated
with fixed charges on a resin.
Disinfection: Disinfection is the
process of killing micro-organisms, usually by
one of a variety of chemical agents, such as
formaldehyde and sodium hypochlorite.
Disinfection lowers the number of
micro-organisms without necessarily killing all
those present. Although total killing of all
organisms is virtually impossible, Sterilization
can generally only be achieved routinely by
heat, gamma irradiation, ethylene oxide, and, in
certain cases, special filtration.
Distributor: A fitting, usually
installed at the top and bottom of the tank in a
loose media system, which is designed to produce
even flow through all sections of an ion
exchanger or filter media bed and to function as
a retainer of the media in the tank. May also
be called diffuser.
Empty Bed Contact Time: The empty
bed contact time (EBCT) is used as a measure of
how much contact occurs between particles, such
as activated carbon, and water as the water
flows through a bed of the particles. As the
EBCT increases, the time available for the
particles to adsorb solutes from the water also
increases, as does the amount of solute removed
from the water during its transit through the
bed. EBCT is calculated from:
Where Vm is the volume of
particles in the bed and Q is the volumetric
flow rate. A consistent set of units must be
used when calculating EBCT with this equation.
For example, if Vm is given in ft, then Q must
be expressed in ft/min for the EBCT to have
units of minutes. Values of Q can be converted
from other units, such as GPM, to ft/min using
the conversion factors.
Feed Water: Water entering a
purification system or an individual piece of
purification equipment, such as an ultrafilter
or reverse osmosis system.
Flocculant: A substance, used in
combination with coagulants, which causes
submicroscopic suspended matter (colloids) to
aggregate into larger particles which can be
removed by settling or filtration.
Flocculation: A practice common in
municipal water treatment in which destabilized
colloidal particles are formed into larger
particles (flocs), usually by stirring. The
floc is removed from the water by settling or
filtration. The process may also incorporate
the addition of such compounds as synthetic
polyelectrolytes which increases the size of the
flocs, thereby making them more easily removed
by settling or filtration. Removal of colloids
by flocculation is done in combination with
Fluoride: A salt of hydrofluoric
acid which may occur naturally in water supplies
or be added by municipal processes for the
prevention of dental cavities. Fluoride is
considered toxic in the hemodialysis setting and
has been implicated with renal bone disease.
Flux Rate: The rate per unit of area
at which water passes through a semi-permeable
membrane, such as those used for ultra
filtration or reverse osmosis.
Fouling: The deposition of insoluble
materials, such as bacteria, colloids, oxides
and water-borne debris, onto the surface of a
reverse osmosis or ultra filtration membrane.
Fouling is associated with decreased flux rates
and may also reduce the rejection rates of
reverse osmosis membranes.
Fulvic Acids: Acidic substances
which are found in humic (organic) soils and
which may become suspended in water.
Fungus: A parasitic plant which
produces no chlorophyll and is dependent on
other life forms for its existence.
Glauconite Sand: A mineral which is
frequently used in depth filters.
Grains of Hardness: Although the
theoretical hardness of water is the sum of the
concentrations of all metallic ions, other than
the alkali metals, it is commonly expressed as
the equivalent concentration of calcium
carbonate in grains. Ionic concentrations can
be expressed in terms of their combining
potential (Eq/L), the number of moles present
(mol/L), or their masses in any of several
conventions. In the English system masses are
expressed in terms of pounds (avoirdupois) which
contain 7000 grains each. Although considered
outdated in most of the world, the US water
purification industry continues to express
hardness in units of grains/gal expressed as
calcium carbonate. Grains/gal expressed as
calcium carbonate can be converted into metric
units (mg/l) by multiplying the former by 17.1.
Grains/gal expressed as calcium carbonate can
also be converted into mEq/L of a univalent ion,
such as sodium (Na+) by multiplying by 0.342.
Care must be taken in using these conversion
factors to size equipment based on ion exchange
principles since the ionic content of the water
will depend on the type of ions present as well
as their total mass.
Hardness: Hardness was originally
defined as a measure of the ability of water to
precipitate soaps made from fatty carboxylic
acids. These “soaps” precipitated in the
presence of calcium and/or magnesium ions.
Today, hardness is used to describe the total
concentration of calcium and magnesium,
expressed as mg/L of calcium carbonate. It is
generally calculated from measurement of calcium
and magnesium ion concentrations, using:
Hardness (mg CaCO'/L)= 2.497 x Ca (mg/L) + 4.118
x Mg (mg/L)
Homogeneous membranes: See
Hydrolysis: A chemical process
resulting from reactions with water; frequently
used in reference to the breakdown of polymers.
Hydrophilic: Pertaining to a
substance which readily absorbs water
Hydrophobic: Pertaining to a
substance which does not readily absorbs water
An atom or molecule having either a positive or
negative electrical charge. Positively charged
ions are referred to as cations and ions having
a negative charge are termed anions.
Ion Exchange: Ion exchange is based
on the principle of electroneutrality, that is,
charged species are stable only when they exist
as balanced pairs of positive and negative
charges. Ion exchange resins, the materials
used to carry out the process of ion exchange,
are particles which contain fixed charges on
their surface. To maintain electroneutrality,
each of these charges has an ion of equal and
opposite charge held to it; these ions are
called counter ions. The counter ions are
mobile and can leave the fixed charge if some
other counter ion is available to replace it.
The replacement ion must
be of the same charge as the initial counter ion
in order to maintain electroneutrality. The
initial counter is established by washing the
resins with a concentrated solution of the
desired counter ion. For example, the softener
resins are cation exchangers containing
carboxylic acids on their surfaces. If these
resins are washed with strong NaCI solutions,
the predominant cation in solution is Na+ and it
will become the counter Ion. In use, the
perfusing water will provide competing counter
ions, such as Ca2+. Because of the preference
of carboxylic acids for Ca2+ over Na+ in dilute
solutions, the water will be depleted of Ca2+ in
exchange for the Na+ initially present.
Langelier Saturation Index: The
precipitation of calcium and magnesium
carbonates in water purification systems is a
serious cause of system failure. The
insolubility of these compounds are a complex
function of the pH of the water, the dissolved
carbon dioxide content, the carbonate content,
the presence of other salts, and the
temperature. The Langelier Saturation Index is
a method of predicting whether or not carbonate
deposits will form under given conditions.
Calculation of the Langelier Saturation Index is
complex and will not normally be done by
hemodialysis personnel. Reverse osmosis venders
may use the index in determining the maximum
recovery and rejection rates that can be
obtained from a reverse osmosis system before
carbonate deposits will seriously reduce water
quality and recovery. It should be noted that
the utility of such determinations is limited to
those situations in which a softener is not used
as part of the pre-treatment scheme for reverse
Lignin: A polysaccharide found in
the cell walls of plants; a breakdown product of
decaying vegetation which may be present in
surface water supplies.
Membranes: Membranes are thin films
made with structures designed to provide
selective transport of solutes. In general, the
selectivity of a membrane is based on its
ability to pass or exclude species according to
their size. Membrane structures may become
homogeneous or asymmetric. Homogeneous
membranes have structures which are uniform in
cross-section, at lease to a magnification of
100 x. Most homogeneous membranes have been
developed for micro-filtration and hemodialysis.
Membranes reduce not only
the flow of undesirable solutes, but also the
flow of solvent. In order to minimize the
reduction in solvent flow, asymmetric membranes
have been developed. These membranes are made
with asymmetric cross-sections, that is, they
consist of two parallel layers. The resistance
to flow of the skin layer, which gives the
membrane its filtration selectivity, is
minimized by reducing its thickness. The
resistance to flow of the ticker support layer,
which provides structural strength, is minimized
because of its open pore structure. These
different layers may be made from the same
material, as in asymmetric cellulose acetate
membranes, or from different materials, as in
thin-film composite membranes.
Membranes used in water
treatment equipment are fabricated in two forms,
as flat sheets or as hollow fiber.
Microporous: In the context of
purification, membranes having an average pore
size which is between 0.1 and 1.0 microns in
Monovalent Ion: A cation or anion
having a single electrical charge.
Nitrate: An anion comprised of one
nitrogen atom and three oxygen atoms. Nitrates
are considered are harmful to infants when
Osmotic Pressure: When a solution,
such as salt water, is separated from pure water
by a membrane which is impermeable to the salt,
a flow of water will occur from the pure water
to the salt solution. The driving force for
this flow is called the osmotic pressure and its
magnitude depends on the number of salt
particles in the solution. Note that the
osmotic pressure depends on the number of
particles and not on the total mass of
particles. For example, 1 g/L of a small
solute, such as sodium chloride, will exert a
greater osmotic pressure than 1 g/L of a large
solute, such as a protein. For water to flow
from the salt solution to the pure water, the
solution must be exposed to a hydrostatic
pressure greater than its osmotic pressure.
This is the principle of reverse osmosis.
Oxidants (oxidizing agents):
Chemicals which provide oxygen and accept an
electron in an oxidation-reduction reaction.
Free chlorine and chloramines are oxidants which
are widely used for disinfection.
Ozone: An extremely active oxidizing
agent which consists of three oxygen atoms. It
is formed by the action of a high voltage
electrical field on oxygen or air.
Parallel: In water purification, an
arrangement of equipment in a side-by-side
configuration such that water flow is divided
and passes through one or both of these
Permeate: See product water.
PH: Water (H2O) can dissociate into
two ions: hydrogen (H+) and hydroxyl (OH).
These ions can also be added to water in
combination with other oppositely charged ions.
Thus, a solution of hydrochloric acid added to
water provides both H+ and the chloride anion,
C1-. The concentration of H+ in the water is a
measure of water’s acidity and the concentration
of OH-, a measure of its alkalinity.
To simplify quantitation
of H+ differences, where numbers with a wide
range of exponents are encountered, scientists
devised a logarithmic scale called pH. The pH
values range from 1 to 14. A pH value of 7 is
considered neutral. Lower values of pH indicate
acidic conditions and higher pH values indicate
alkaline conditions. Because pH is a
logarithmic scale, an increased of 1 pH unit
corresponds to a ten-fold change in acidity.
Phenols: Weak aromatic acids which
are indicative if industrial pollution of water
supplies. When combined with chlorine, they
produce an objectionable taste and odor.
Polisher: A treatment stage placed
at the end of other treatment to bring the water
to a more highly conditioned and more perfect
state. For example, a mixed bed of ion exchange
media installed as the final treatment step in
the deionization process to remove last traces
of undesirable ions.
Polyamide: A synthetic polymer of
the nylon family used in the fabrication of
reverse osmosis and ultra filtration membranes.
Polysulfone: A synthetic polymer
used in the fabrication of reverse osmosis and
ultra filtration membranes which are
characterized by extreme thermal stability and
Polyvalent Ion: A cation or anion
having a multiple electrical charge.
Polyvinyl Chloride (PVC): A
thermoplastic material produced by the
polymerization of vinyl chloride. Used
extensively in the U.S. for piping, food
packaging, and injection molded plastic parts.
PVC is the most common pipe material used in the
U.S. for dialysis applications.
Potassium Permanganate: An oxidizing
agent commonly used for the regeneration of
manganese greensand iron filters and
occasionally used as a disinfectant.
Pressure Drop: Expenditure of a
certain amount of energy is required for a fluid
to flow through any channel, such as a pipe,
particle bed, or membrane. The pressure at any
point is a measure of the energy content of the
fluid at that point. Since some of this energy
is expended in flowing to a second point
downstream, the pressure at the downstream point
is less than at the original point. The amount
of energy expended, and hence the decrease in
pressure (or pressure drop), is depended on the
flow rate and viscosity of the fluid, and the
size and shape of the channel. Pressure drops
are usually expressed in terms of lb/In2 or PSI,
or in the SI system, kPa (kilopascals) or
Kg/cm2. Pressure drop is sometimes referred to
colloquially as “delta P”.
Pretreatment: Any water treatment
step performed prior to the primary treatment
process, such as filtration prior to
Product Water: The purified water
stream from purification equipment, such as
reverse osmosis units and ultra filters.
Recovery (percent recovery): A
measurement applied to reverse osmosis and ultra
filtration equipment which characterizes the
ratio of product water to feed water flow
rates. The measurement is descriptive of
reverse osmosis or ultra filtration equipment as
a system and not of individual membrane
elements. Expressed as a percentage, recovery
is defined as:
% Recovery = (Product flow rate/feed flow rate)
Rejection (percent rejection): A
measure of the ability of a reverse osmosis
membrane to remove salts. Expressed as a
percentage, rejection is defined as:
% Rejection = (1-Product concentration/Feed
concentration) x 100
Resistivity: Resistivity is a
measure of the current-resisting characteristics
of a substance when an electrical charge is
applied (and is the reciprocal of
conductivity). The standard unit of resistance
is the Ohm. Because of the variable nature of
water, a distance between measuring probes must
be maintained if accurate measurements are
desired. The almost universal standard distance
for this is the centimeter, hence the “Ohm-cm”.
Resistivity measurements, like conductivity
measurements, can be used in many ways to
improve the management of a water purification
system, and are commonly used to asses the
quality of water produced by deionizers.
Because temperature effects resistivity of
water, temperature compensating devices are
frequently used. These adjust the resistance
meter to indicate what the water resistance
would be at one temperature, usually 25°C.
Salt Passage Rate: A measurement of
the passage of salts through a reverse osmosis
membrane. Salt passage is related to rejection
% Salt passage = 100 - % Rejection
Scaling: In reference to reverse
osmosis equipment, scaling is the precipitation
of sparingly soluble salts, such as calcium
carbonate, onto the surface of a membrane.
Scaling is associated with decreased flux and
reduced reverse osmosis rejection rates.
Sedimentation: The process by which
solids are separated from water by gravity and
deposited on the bottom of a container or basin.
Semi-permeable: Descriptive of a
material, such as a reverse osmosis or ultra
filtration membrane, which allows the passage of
some molecules and prevents the passage of
Series: In water purification, an
arrangement of equipment in a successive or
Silt density Index: The silt density
Index (SDI) is a measure of the ability of water
to foul a membrane or plug a filter. SDI is
measured using an apparatus which typically
consists of an inlet pressure regulator and
pressure gauge followed by a filter holder
containing a 0.45 micron microporous membrane
filter. Commercial test kits, complete with
instructions on how to calculate the index, are
Sorbent: See adsorption.
Sterilization: A physical or
chemical process that reduces the number of
organisms to a safe predetermined level.
Surge Tank: A type of pressurized
water storage vessel also known as a bladder
tank. Used almost exclusively on residential
well water systems, they provide a small amount
of pressurized water to downstream valves or
equipment without requiring a pump. When
installed on a piston type pump, they reduce
violent pressure spikes. Surge tanks typically
have large areas of stagnation that offer
opportunistic bacteria a favorable environment
Total Dissolved Solids (TDS): The
sum of all organic, inorganic and ionic contents
in a solution (excluding all dissolved gasses).
Since a TDS meter cannot measure organic content
of water, most TDS readings are an
approximation. TDS measurements are widely used
in the water and waste water industries to
monitor final water quality. The TDS meter
derives its’ values from resistivity and
conductivity measurements of the product water.
Total Organic Carbon: Organic
compounds dissolved in water are characterized
by their carbon content. Total organic carbon
is the mass of carbon present in a water sample,
excluding the carbon present as CO2 and/or
carbonates. The values are determined by
catalytically oxidizing (burning) all dissolved
carbon (after CO2/CO3-removal by acidification)
to CO2. The resulting CO2 may be measured
directly by infra-red absorption, or it may be
reduced in a furnace with hydrogen to form
methane, which is measured by flame ionization
Turbidity: Turbidity is a measure of
the presence of colloidal matter in the water
that remains suspended. Suspended matter in a
water sample, such as clay, silt, or finely
divided organic and/or inorganic material will
scatter the light from an incident light beam.
The extent of scattering is expressed in Jackson
or Nephelometric turbidity units (JTU and NTU,
Ultra filters: A membrane based
filtration system in which the pore sizes range
from 0.001 to 0.1 microns.
||Gallons Per Day
||Liters per Minute
||National Pipe Thread
||Pounds per Square Inch
||Poly Vinyl Chloride
||Volts Alternating Current
||Water for Injection
||Terminal Barrier Strip
||Total Dissolved Solids
||Colony Forming Units
||Limulus Amebocyte Lysate
||Gallons per Minute
||Empty Bed Contact Time
||Reverse Osmosis Unit made by