Glossary of Terms

Here’s a list of water-related terms that might help you understand our site better. It is compiled from a number of sources and should not be considered an “official” U.S. Geological Survey water glossary.

A

ACTIVATED CARBON:
A form of elemental carbon whose particles have a large surface area with adsorptive quality, primarily used to remove chlorine, objectionable tastes and odors, and numerous toxic organic compounds from water.

ADSORPTION:
The process by which particles and molecular impurities adhere to the surface of activated carbon. This is typically done by electrochemical attraction.

B

BACTERIOSTATIC:
A feature of a carbon filter that is supposed to inhibit the growth of bacteria within the filter, usually done by adding silver.

BRACKISH WATER:
Water containing between 1000 and 1500 mg/L of dissolved solids.

BRINE:
See also: Reject Water. One of two streams of fluids generated by a reverse osmosis unit. It contains the impurities removed from the feed water.

C

CELLULOSE ACETATE (CA) AND CELLULOSE TRIACETATE (CTA):
A family of synthetic materials based on cellulose used to make RO membranes. While CTA is superior to CA, under adverse water conditions both are effective in removing a wide spectrum of impurities from water. The disadvantage of Cellulose type membranes is that they are subject to bacterial attack, particularly in chlorinated water supplies. CTA has superior bacterial resistance.

CHLORAMINES:
Chemical complexes formed from the reaction between ammonia and chlorine. They are presently being used to disinfect municipal water supplies because unlike chlorine, they do not combine with organics in the water to form potentially dangerous carcinogens such as trihalomethanes (THM). Chloramines can exist in three forms, the proportions of which depend on the physical and chemical properties of the water. Water containing chloramines cannot be used for fishtanks or kidney dialysis equipment.

CHLORINE:
A chemical in the form of liquid or gas used to disinfect water. It is known to react with organic matter in the water to form trihalomethanes (THM’s), are a suspected carcinogen.

CONDITIONED WATER:
Generally, water that has been treated by a water softener to remove hard minerals like calcium and magnesium.

CONDUCTIVITY:
A measure of the ability of a substance to transmit an electrical current. The conductivity imparted to water by dissolved solids is a function of both the amount and composition of the salts and of the temperature of the water.

CONTACT TIME:
The length of time water is in direct contact with activated carbon. This is a major factor in determining how effectively organic impurities will be removed.

D

DEIONIZED WATER:
Water which has had the Total Dissolved Solids removed by the ion exchange process using acid, caustic chemicals, and plastic resin beads.

DISINFECTION:
Process of making water biologically safe for human consumption by using chemicals, ultraviolet, ionization, etc., which destroys harmful microorganisms.

DISTILLED WATER:
Water which has been purified using the evaporation-condensation cycle. It generally contains less than 5 ppm of TDS.

E

EPA (USEPA):
The United States Environmental Protection Agency. This federal agency is responsible for safeguarding our environment, including public water supplies.

F

FEED PRESSURE:
The pressure at which water is supplied to the RO module.

FEED WATER:
A term which refers to the water supply that is put into a water treatment system for processing (removal of impurities).

FLUX:
The flow of water through reverse osmosis membranes, measured per square foot of surface.

FOULING:
The process of depositing impurities on the membrane surface, thus impeding normal function. This can be due to the presence of suspended solids or precipitated salts of biological growth. This causes a decrease in both the amount of water produced and the quality of the water.

G

GROUNDWATER:
Water located in the underground aquifers. It is the supply tapped by wells.

H

HARDNESS:
The amount of calcium and magnesium in the water measured in grains per gallon (expressed as calcium carbonate). This level is important to control during the RO process to prevent scaling. Each grain is equal to 17.1 ppm of calcium and magnesium (expressed as calcium carbonate).

HYDROLYSIS:
The chemical degradation of an RO membrane in water due to certain conditions such as high pH. Cellulose-based membranes are quite susceptible to hydrolysis while the TFC types are virtually immune.

I

ION:
An atom of any Total Dissolved Solid as it exists in water with a net electrical charge, positive (called cations) or negative (called anions).

M

MECHANICAL FILTRATION:
The process of removing suspended particles from water by a straining action. The finest mechanical filters can remove bacteria about 0.2 microns.

MICRON:
A unit of length, one-millionth of a meter. The smallest particle that can be seen by the naked eye is about 40 microns across. The smallest bacteria are about 0.2 microns across. 1 micron = .00004 inches.

MILLILITERS/MINUTE:
A common measurement for the flow rate of smaller RO systems usually measured with a graduated cylinder. One-thousandth of a liter per minute Milliliters/Min. x .38 = gallons per day.

MINERAL SALTS:
The form in which minerals from dissolved rock exist in water— same as Total Dissolved Solids. These are inorganic forms of minerals. In excess, they cause water to have a disagreeable taste. Some are harmful to human health.

MOLECULAR WEIGHT (MW):
The sum of the atomic weights of the individual atoms (from a periodic chart) that make up a molecule of a particular substance. Cellulose-based membranes can remove substances as light as MW of 300, while TFC-type membranes remove substances as light as MW of 200.

O

ORGANICS:
Any of the compounds whose molecular structure is based on carbon. (E.g. carbon dioxide, wood, sugar, protein, plastics, methane, THM, TCE, etc.).

OSMOSIS:
Osmosis: The natural tendency for water molecules to pass through a semipermeable membrane from the side that’s low in dissolved impurities, to the side that’s high in impurities.

OSMOTIC PRESSURE:
The pressure created by the tendency of water to flow in osmosis. Every 100 ppm of TDS generates about 1 pound per square inch (psi) of osmotic pressure. This osmotic pressure must first be overcome by the water pressure for the RO membrane to be effective.

P

PARTS PER MILLION (ppm):
The parts of Total Dissolved Solids per one million parts of water (e.g. one pound of mineral salts dissolved in one million pounds of water will give one part per million of TDS).

pH:
The acidity or alkalinity of water due to dissolved solids and measured on a scale of 1 to 14; 1 being most acid, 14 being most alkaline, and 7 being neutral.

PRETREATMENT:
Whatever alterations of the raw feed water required to prevent damage to the membrane.

PRIORITY POLLUTANTS:
Those pollutants that post the most serious health hazards determined by the Environmental Protection Agency (EPA).

PRODUCT WATER:
Pure water that has been separated from the feed water stream by the RO membrane.

R

RECOVERY:
The amount of product water as compared with the total amount of feed water. This will give a measure of the efficiency of operation. For example, 10 gallons of feed water (if we separate 6 gallons into product water and reject 4 gallons), is a recovery of 60%.

REJECTION:
The percentage of TDS removed from the feed water. Typically, more than 90% rejected is achieved with reverse osmosis.

REJECT WATER:
See also: Brine. The portion of the feed water that does not pass through the RO membrane, and which carries the remaining impurities to the drain.

REVERSE OSMOSIS (RO):
A reversal of the natural phenomenon of osmosis brought about by application of hydraulic pressure greater than the osmotic pressure in water to cause the water molecules to flow through the membrane away from the dissolved substances.

S

SEDIMENT:
See also: Turbidity. The sum of particles of dirt, clay, silt, and vegetation which floats on (or are suspended in) water and can be removed by mechanical filtration.

SEMIPERMEABLE:
Applies to special materials (both natural and synthetic) which allow certain substances —such as water —to pass through, while blocking or rejecting the passage of other substances such as dissolved solids and organics.

SOFTENER:
A water treatment device in which hard water is passed through a cation exchange material (resin) so that the calcium and magnesium are replaced by sodium, making it more desirable.

SOFT WATER:
Water containing less than 1 grain-per-gallon of dissolved calcium and magnesium.

SPIRAL WOUND:
The most common practical configuration for an RO membrane in which sheets with a large surface area are wrapped in a spiral fashion to fit in a small space.

T

TCE (TRICHLOROETHYLENE):
One of the more common toxic organic contaminants found in water. It is common in many homes, industrial environments, dry cleaning solvents, and cooking pans.

TFC (THIN FILM COMPOSITE):
The most advanced membrane made with a polyamide base polymer. It exhibits superior performance, immunity to adverse water conditions and is the only membrane material that is truly bacteria proof.

TRIHALOMETHANES (THM’s):
A group of suspected carcinogenic organic compounds formed in water when chlorine (used as a disinfectant) reacts with naturally-occurring organic matter. For example, one of the most common THM’s is chloroform.

TOTAL DISSOLVED SOLIDS (TDS):
Generally, the total amount of minerals and metals which are dissolved in water.

TOXIC ORGANICS:
Carbon-based chemicals which are frequently found in our water supplies and are harmful to human health. They are usually from agricultural and industrial effluents and hazardous waste dumps (e.g. TCE, PBC, DCBP, pesticides, etc.).

TURBIDITY:
See also: Sediment. Suspended biological, inorganic, and organic particles in water which may be insufficient amount to make the water seem cloudy.