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The Water Doctors designed this glossary to help you understand more about water, and the many things that can affect the quality of the water you drink. We hope you find it useful!

absolute filter rating 
Refers to the smallest particle size that a filter will trap 100 percent. For example, a 5-micron absolute filter will trap all particles 5 microns and larger. See nominal filter rating.

activated carbon 
Carbon that has been specially treated to enhance its ability to trap certain chemicals.

adsorption 
The process by which carbon filters trap chemicals.

aeration 
The process of exposing water to large amounts of oxygen in order to remove certain kinds of chemicals.

aggressive water 
Water that, because of its purity, aggressively reacts (chemically) with materials.

aquifer  
A naturally occuring underground reservoir.

backwashing, backflushing 
Reversing the flow of water through a filter in order to cleanse it of accumulated particulate matter.

CA membrane 
Cellulose acetate membrane. A type of membrane used in reverse osmosis.

carcinogen 
A substance that causes or contributes to the onset of cancer.

chloramine 
A combination of chlorine and ammonia used to disinfect many public water supplies.

chlorine 
A chemical used to disinfect water.

colloid  
A tiny particle that remains in suspension in a liquid.

contact time 
The amount of time that water contacts with a filter while flowing through it.

contaminant  
A substance in water that is harmful or otherwise undesirable. Same as pollutant.

corrosion  
A chemical process by which water attacks metal surfaces and weakens or destroys them.

CTA membrane 
Cellulose Tri-acetate membrane. A type of membrane used in reverse osmosis.

de-ionization  
(DI) A process that removes minerals from water by ion exchange.

disinfection  
The process of killing bacteria and other micro organisms in water.

dissolved solids  
Particles that have dissolved in water and are in solution.

distillate 
Water that has been vaporized by boiling and then returned to liquid form.

distillation 
The process of boiling water, capturing the steam, and cooling it so that purified water is produced.

effluent 
The water now that exits from a device or system.

feed, feedwater 
A solution that enters a device or system for a specific purpose, as in a chlorine feeder.

filtrate  
In a treatment device, the water flow after it has passed through a filter or membrane.

finished water 
Water that has been improved by a water treatment plant and is ready to be delivered to customers.

flocculent 
A substance that is added to water to make particles clump together in order to achieve better filtration.

giardia 
A micro organsim that occurs in water in the form of cysyts; a cause of gastrointestinal disorders.

gpm, gph, gpd  
The rate of water flow in gallons per minute, per hour, or per day.

ground water 
Water which has an underground source.

hardness  
The amount of calcium and magnesium in water, which causes it to clean inefficiently.

heavy metals 
The toxic metals in water, such as cadmium, lead and mercury.

hydrogen sulfide 
A toxic gas in water that smells like rotten eggs.

influent 
The water flow input to a device or system.

ion  
An electrically charged atom.

ion exchange 
A process by which undesirable ions in water are exchanged for more beneficial ones.

leaching 
The process by which chemicals from the walls of a container enter the water in the container.

MCL 
Maximum Contaminant Level; the maximum level recommended by federal law for a particular water pollutant.

MCLG Maximum Contaminant Level Goal; the optimum maximum contaminate level in the future, for a particular water pollutant.

membrane 
a thin material that is porous enough to reject pollutants while allowing pure water to pass through.

mg/l Milligrams per liter 
A measure of the amount of a substance in water. The equivalent to parts per million.

micron 
A shortened term for one micrometer. One millionth of a meter.

mutagen  
A substance that causes or contributes to genetic mutation.

NDWR  
National Drinking Water Regulations, as established by the Safe Drinking Water Act of 1974.

NOC 
Naturally occuring organic chemical.

nominal filter rating 
Refers to the smallest particle size that a filter will trap most of. For example, a 5-micron nominal filter might trap 95 percent of all particles 5 microns or larger.

NSF 42 Standard 
The National Sanitation Foundation's standard which tests for the reduction of chlorine, iron, phenol, sulfate, hydrogen sulfide, and zinc, as well as scale control.

NSF 53 Standard  
The National Sanitation Foundation's standard which tests for the reduction of arsenic, barium, cadmium, copper, flouride, herbicides, lead, mercury, nitrate, pesticides, selenium, trihalomethane, TCE and VOC.

oxidizing filter 
A filter that removes pollutants from water by chemical reaction.

ozone 
A toxic form of oxygen that is used to disinfect water.

particulate 
Particles in water.

permeate  
That portion of the water that passes through any RO membrane.

pH 
The measure of alkalinity/acidity in water. A pH of 1 to 7 is acidic, 7 - 14 is alkaline, and exactly 7 is neutral.

pollutant  
A substance in water that is harmful or otherwise undesirable. 
 

pore size  
This refers to the smallest substance that an RO membrane will reject most of. For example, a .001-micron membrane might reject approximately 90 percent of all substances .001 microns and larger.

PPB 
Parts per billion. A measure of the amount of a substance in water. One part per billion is equivalent to one billionth of a gram per liter. 
 

PPM  
Parts per million. A measure of the amount of a substance in water. Equivalent to milligrams per liter (mg/l).

pre/post filter 
A secondary filter attached either before the main filter, to prevent particulate matter from entering the main filter; or after, for final "polishing" of water exiting the main filter.

Primary Regulations National Drinking Water Regulations that pertain to harmful water pollutants.

psi 
Pounds per square inch of water pressure.

recovery rate 
The ratio of pure water produced to total water used in the process of reverse osmosis.

regeneration 
A process by which a filter medium is cleansed of accumulated pollutants, such as by reversing the flow of water through it.

rejection  
The process whereby certain substances cannot pass through a membrane and are drained off.

resin 
A specially prepared mineral that is used in de-ionization and in some kinds of filters.

reverse osmosis (RO) 
A process by which contaminants are rejected by a membrane while pure water is allowed to pass through.

scale 
A hard, whitish buildup of mineral deposits on surfaces that contact water, such as the inside of water pipes or the boiling chamber of a distiller.

Secondary Regulations National Drinking Water Regulations that pertain to the aesthetic and convenience qualities of water.

SOC 
Synthetic organic chemical.

softness 
The quality of water, desirable for efficient cleaning and minimal corrosion, that results from very low amounts of calcium and magnesium.

teratogen 
A substance that causes or contributes to birth defects.

TDS 
Total Dissolved Solids. The standard measure of minerals dissolved in water.

TFC 
membrane Thin Film Composite membrane. A type of membrane used in reverse osmosis.

THM 
Trihalomethane. A type of toxic chemical, commonly formed when organic chemicals in water combine with chlorine.

turbidity 
A measure of the opacity, or cloudiness of water that is caused by particulates.

Ultraviolet disinfection (UV) 
A process by which intense ultraviolet light is used to kill bacteria and other micro organisms.

VOC 
Volatile Organic Chemical. A class of chemicals that evaporate easily and which can be absorbed through the skin.