Absorption – A process by which a substance is taken up chemically or physically in bulk by a material (absorbent) and held in pores or interstices in the interior.

Activated Carbon – A highly porous form of carbon used for sorption of organics and removal of chlorine and chloramines.

Adsorption – Adherence of molecules, atoms and ionized species of gas
or liquid to the surface of another substance (solid or liquid) as the result of a variety of weak attractions.

Anion Exchange Resin – An ion exchange resin with immobilized positively charged exchange sites, which can bind negatively charged ionized species, anions.

Azeotrope – A blend of two or more components with equilibrium vapour phase and liquid phase compositions that are the same at a given temperature and pressure.

Bactericide – A chemical or physical agent that kills bacteria.

Biocide – A chemical or physical agent that kills microorganisms.

Biofilm – A layer of microorganisms enclosed in a glycoprotein polysaccharide matrix, which are adherent to each other and/or to surfaces.

Carbon Fines – Very small particles of carbon that may wash out of an activated carbon bed.

Cartridge – A pre-packed disposable container for housing a water purification resin, media or membrane.

Cation Exchange Resin – An ion exchange resin with immobilized negatively charged exchange sites, which can bind positively charged ionized species (cations).

CFU/ml – Colony Forming Units per milliliter. A measure of viable microbial populations.

Colloid – A stable dispersion of fine particles in water that have a typical size less than 0.1 μm. Colloids containing iron, aluminum, silica and organics are commonly found in natural and potable waters.

Concentrate – The liquid containing dissolved and suspended matter that concentrates on the inlet side of a membrane and flows to drain.

Condenser – The stage of a distillation system that removes sufficient heat from a vaporized liquid to cause the vapor to change to a liquid phase.

Conductivity – Conductivity is the reciprocal of resistivity. For water purification systems, conductivity is usually reported as micro Siemens per centimeter (μS/cm) at 25°C.

Degassing – The removal of O2 and CO2 from water, usually by transfer across a hydrophobic membrane. CO2 is removed to increase downstream ion exchange capacity.

Deionization (DI) – Removal of impurity ions from water. Usually
used to refer to ion exchange – see Ion

Distillation – A purification process that takes advantage of changing the phase of a substance from liquid to vapor and back to liquid usually at the boiling temperature of the
substance, in order to separate it from other substances with higher or lower boiling points.

Electrodeionization (EDI) –Technology combining ion exchange resins and ion-selective membranes with direct current to remove impurity ionized species from water.
Endotoxin – A thermally stable lipopolysaccharide component from the cell wall of viable or non-viable gram-negative microorganisms. Can act as a pyrogen.

Endotoxin Units (IU/ml or EU/ml)
– A quantification of endotoxin levels relative to a specific quantity of reference endotoxin. 1 EU/ml is approximately equal to 0.1 ng/ml.

Epifluorescence – Method of fluorescence microscopy which can be used to detect bacteria after filtration and staining.

Feed water – The water that is introduced into a purification process.

Filtration – A purification process in which the passage of fluid through a porous material results in the removal of impurities.

Fines – particulates released from a bed of material such as ion exchange resins.

Fouling Index – See Silt Density Index.

Gram-negative – refers to bacteria that do not absorb a violet stain originally described by Gram.

Hardness – The scale-forming and lather-inhibiting qualities of some water supplies, caused by high concentrations of calcium and magnesium. Temporary hardness, caused by the presence of magnesium or calcium bicarbonate, is so called because it may be removed by boiling the water to convert the bicarbonates to the insoluble carbonates. Calcium and magnesium sulfates and chlorides cause permanent hardness.

Ion – Any non-aggregated particle
of less than colloidal size possessing either a positive or a negative electric charge.

Ion Exchange (IX) – The process of purifying water by removing ionized salts from solution, by replacing hydrogen ions for cation impurities and hydroxyl ions for anion impurities.

Line Cell – An electrode assembly inserted into a water stream by
which the conductivity or resistivity is measured.

Microorganism – Any organism that is too small to be viewed by the unaided eye, such as bacteria, viruses, molds, yeast, protozoa, and some fungi and algae.

Off-line – In water monitoring systems, referring to measurement devices that are not directly coupled to the water stream.

On-line – In water monitoring systems, referring to measurement devices directly coupled to the water stream.

Particulates – Discrete quantities of solid matter dispersed in water.

Permeate – The purified solution which has been produced by passage through a semi-permeable reverse osmosis membrane.

Regeneration – The method by which exhausted ion exchange resins are reactivated by treatment with strong acid or alkali.

Resistivity – The electrical resistance between opposite faces of a one- centimeter cube of a given material
at a specified temperature. Resistivity is the reciprocal of conductivity. For water analysis, resistivity is usually reported in megohm-centimeters
(MΩ-cm) and corrected to the value at
25°C. All resistivity values referred to in this guide are at 25°C unless otherwise stated.

Reservoir – In water purification systems, a container holding quantities of purified water.

Reverse Osmosis (RO) – A process in which water is forced under pressure through a semi permeable membrane leaving behind dissolved organic, dissolved ionic and suspended impurities.


Sanitization – Chemical and/ or physical processes used to kill microorganisms and reduce
Contamination from microorganisms.

Silt Density Index – also called the Fouling Index (FI) is a test used to estimate the potential of the water to block filters, derived from the rate of blockage of a 0.45 μm filter under standard conditions.

Softening – A water treatment process whereby cations, notably hardness- forming calcium and magnesium ions, are exchanged for sodium using cation exchange resins in the sodium form.