SEE North American Industry Classification System.


A membrane treatment process which falls between reverse osmosis and ultrafiltration on the filtration/separation spectrum.     The nanofiltration process can pass more water at lower pressure operations than reverse osmosis, can remove particles in the 300 to 1,000 molecular weight range such as humic acid and organic color bodies present in water, and can reject selected (typically polyvalent) salts.     Nanofiltration may be used for selective removal of hardness ions in a process known as membrane softening.

Nanometer (nm)

One billionth (10-9) of a meter, and equal to one millimicron or 10 angstroms.

National Pollutant Discharge Elimination System (NPDES)

The NPDES permit is the regulatory agency document issued by either a federal or state agency which is designed to control all discharges of pollutants from point sources in U.S. waterways.     NPDES permits regulate discharges into navigable waters from all point sources of pollution, including industries, municipal treatment plants, large agricultural feed lots, and return irrigation flows.

Natural Softening

Naturally softened water comes to us in many ways: rain, snow, sleet, hail, fog, mist, dew and melting glaciers.  This is a result of the normal hydrologic cycle of evaporation and condensation that separates mineralized water from its salts.

Natural Sparkling Water

Carbonated water whose carbon dioxide content is from the same source as the water itself.

Naturally Soft Water

A term that classifies naturally occurring low hardness water such as a surface water or rain water as “soft” even though the hardness content is usually greater than 1 gpg but often less than 3 gpg.  Naturally soft waters generally have a high carbon dioxide content with an acidic pH and are prone to cause corrosion.  This characteristic is not true for “softened” water which has been treated to reduce the hardness content using ion exchange.

Near the first service connection

Pertaining to public water system monitoring at one of the 20 percent of all service connections in the entire system that are nearest the water supply treatment facility, as measured by water transportation time within the distribution system.


Death of cells or tissue.

Negative Charge

The electrical potential of an atom which gains one or more electrons, therefore leaving it with more electrons than protons. Anions carry negative charges.

Negative Head

A condition of negative pressure or of partial vacuum.

Negative Pressure

A pressure below that of the existing atmospheric pressure taken as a zero reference.


Members of the phylum Nematoda, commonly called roundworms.     Nematodes have tiny cylindrical thread-like bodies that are pointed at both ends and covered with tough cuticle or skin-like membranes. Many are free-living dwellers in soil and water; others, like hookworms, Ascaris, pinworms, trichina worms, and filaria worms are parasites of man, animals, and plants.


An abnormal growth or tissue, as a tumor.


An instrument for measuring turbidity in water using a photometric analytical technique for measuring the light scattered by finely divided turbidity or colloidally dispersed particles suspended in water.


A means of measuring turbidity in a sample by using an instrument called a nephelometer. A nephelometer passes light through a sample, and the amount of light deflected (usually at a 90-degree angle) is than measured.

Nephelometric Turbidity Unit (NTU)

The standard unit of measurement used in the water analysis process to measure turbidity in a water sample. The NTU has replaced the formerly used Jackson turbidity unit (JTU).


Exerting a destructive or poisonous effect on nerve tissue.


In water chemistry, the midpoint (neutral) reading of 7.0 on the pH scale, indicating that the solution (water) producing the neutral reading will produce neither an acid nor alkaline reaction. A 7.0 reading on the pH scale means that there are an equal number of free hydrogen (acidic) ions and hydroxide (basic) ions. As relates to electricity, a condition of being uncharged because of a balance between negative and positive charges. The condition of neutrality means that there is neither an excess nor a lack of electrons.


A trade name for a calcite mineral product (crushed southern limestone) which is used in loose media bed filters to modify the pH of low pH water sources.


The addition of either an acid to a base or a base to an acid to produce a more nearly neutral solution.     The use of alkaline or basic materials to neutralize acidity of some water is common practice in water processing.     Neutralization does not always mean the attaining of pH 7.0. When a strong acid reacts (is neutralized) with a weak base, the resulting pH may remain less than 7.0; when a strong base reacts with a weak acid, the pH may remain greater than 7.0.


1. An alkaline substance such as calcium carbonate (calcite) or magnesium oxide (magnesia) used to neutralize acidic waters or an acidic substance such as acetic acid or dilute hydrochloric acid used to neutralize alkaline waters.    2. The term "neutralizer" is commonly used to refer to calcite or magnesia acid-neutralizing filters used to neutralize acidity and/or reduce free carbon dioxide in water and thereby raise the pH of acidic water.


A fundamental particle found in the nucleus of an atom. A neutron has a mass equal to that of a proton but carries no charge.


The force necessary to give acceleration of one meter per second to one kilogram of mass.


A natural nitrogen compound (NO3-) sometimes found in well or surface waters. In high concentrations, nitrates can be harmful to young infants or young livestock.


The biochemical transformation of ammonium nitrogen to nitrate nitrogen.

Nitrification Inhibitor

A chemical that slows down the conversion of ammonium to nitrate nitrogen.

Nitrogen Fixation

The biological or chemical process by which elemental nitrogen, from the air, is converted to organic or available nitrogen.


A term used to describe chemical compounds (usually organic) containing nitrogen in combined forms. Proteins and nitrates are nitrogenous compounds.

NOAEL - No observed adverse effect level

The highest dose in a toxicity experiment which did not produce an observable adverse effect.


Chemically unreactive, especially toward oxygen or resists chemical action such as corrosion caused by air, water, or (to a lesser degree) acids.     Gold, silver, platinum, palladium, and mercury are said to be "noble metals" because they don't rust and are resistant to acid damage. Certain gases are called "noble gases" because they are inert (chemically inactive and stable.)

Noble Metal

Chemically inactive metal (such as gold). A metal that does not corrode easily and is much scarcer (and more valuable) than the so-called useful or base metals.


No-observed-effect level; dose level at which no effects are noted.

Nominal Diameter

An approximate measurement of the diameter of a pipe. Although the nominal diameter is used to describe the size or diameter of a pipe, it is usually not the exact inside diameter of the pipe.

Nominal Filter Rating

Filter rating indicating the approximate size particle, the majority of which will not pass through the filter. It is generally interpreted as meaning that 85 percent of the particles of the size equal to the nominal filter rating will be retained by the filter.

Noncarbonate Hardness (permanent hardness)

The hardness content in water associated with strong acid anions of chloride, sulfate, and/or nitrate. Noncarbonate hardness does not form hard scale upon heating.

Noncommunity Water System (NCWS)

A public water system that is not a community water system.     There are two types of NCWSs: transient and nontransient.

Nonconventional Pollutant

Any pollutant which is not statutorily listed or which is poorly understood by the scientific community.


Resistant to decomposition or decay by biological means such as bacterial action or from chemical or physical causes such as oxidation, heat, sunlight, or solvents.

Nonhardening Salt

Salt containing amounts of agents such as calcium or magnesium chloride which become soft or liquid by attracting and absorbing moisture from the air and preventing salt caking and bridging.

Nonionic Polymer

A polymer that has no net electrical charge.


Not disease-producing.

Nonpoint Source

Pollution sources which are diffuse and do not have a single point of origin or are not introduced into a receiving stream from a specific outlet.     The pollutants are generally carried off the land by stormwater runoff. the commonly used categories for non-point sources are; agriculture, forestry, urban, mining, construction, dams and channels, land disposal, and saltwater intrusion.


Water that may contain objectionable pollution, contamination, minerals, or infective agents and is considered unsafe and/or unpalatable for drinking.

Nonsettleable Solids

Very small, fine suspended solids, typically colloidal particles of less than 0.1 microns in diameter, which will not settle out of calm nonturbulent water, sewage, or other liquids in what is considered a reasonable time of about two hours.

Nontransient Noncommunity Water System (NTNCWS)

A public water system that regularly serves at least 25 of the same nonresident persons per day for more than six months per year.

Normal Flow Filtration

The flow of the entire feedwater stream in one direction directly through the filter media. The flow is usually "normal" or perpendicular to the media surface area.

Normal Solution

A normal solution contains one-gram equivalent weight of dissolved substance in a liter of solution. The equivalent weight of an acid is that weight which contains one-gram atom of ionizable hydrogen or its chemical equivalent. For example, the equivalent weight of sulfuric acid (H2SO4) is 49 (98 divided by 2 because there are two replaceable hydrogen ions). A one N solution of sulfuric acid would consist of 49 grams of H2SO4 dissolved in enough water to make one liter of solution.

North American Industry Classification System (NAICS)

The standard statistical classification system, adopted in 1997 by the United States Office of Management and Budget (OMB), that assigns an industry number to businesses and business units by type of economic activity.  The NAICS has been harmonized with and also adopted by Mexico and Canada.  It replaces the Standard Industrial Classification (SIC) system used in the U.S. from 1938 and until 1997.  The NAICS is a standard industry classification system that facilities the collection, tabulation, presentation, and analysis of data relating to establishments and ensures that data about the U.S. economy published by U.S. statistical agencies are uniform and comparable among the North American countries.  The NAICS uses a six-digit numerical coding system to identify particular industries and their placement in the hierarchical structure of the classification system.  The first two digits group establishments (or locations at which an economic activity occurs) into one of 20 sectors, such as for example, Agriculture, Mining, Construction, Manufacturing, Wholesale Trade, Retail Trade, and Administration and Support Services.  The remaining digits designate one of the 1,170 industries identified in NAICS.  When any employer applies for an Employer Identification Number (EIN), information about the type of activity in which that business is engaged is requested in order to assign a NAICS code.    In addition, statistical agencies such as the Census Bureau and the Bureau of Labor Statistics assign NAICS codes based on information reported to them.  Water treatment equipment manufacturing, for example, is in NAICS 333319, water softener and water conditioning direct selling and service providers are in NAICS 454390, water softening and conditioning equipment wholesaling is in 421720, water softening and conditioning compounds and materials wholesaling is in 422690, and water softening and conditioning support services are in 561990.  Bottled water manufacturing (purifying and bottling) is in NAICS 312112, bottled water stores are in NAICS 445299, bottled water wholesaling is in NAICS 422490, and other water treatment and distribution is in NAICS 221310.

No-salt Water Conditioner

SEE saltless water treatment device

No-salt Water Softener

SEE saltless water treatment device; water softener


Acquired after admittance to a hospital.

Not Detectable

A term used in reporting test results to mean that the substance being tested cannot be detected by the equipment or method being used for this particular test. This term implies that it is possible that trace amounts may be present in quantities too small to be detected by the test equipment or method.

NPL (National Priorities List)

A list of Superfund sites chosen for immediate attention.


National Secondary Drinking Water Regulations.


See nontransient noncommunity water system.


National Toxicology Program


Nephelometric turbidity unit.


A term used in reporting test results to mean that the substance being tested cannot be detected by the equipment or method being used for this particular test. This term implies that it is possible that trace amounts may be present in quantities too small to be detected by the test equipment or method.


Any substance that is assimilated (taken in) on organisms and promotes growth.     For example, carbon dioxide, nitrogen, phosphorous, potassium, and numerous mineral elements are essential nutrients which promote the growth of plants.     Water and oxygen are also included in this definition.

Nutrient Pollution

Contamination of water resources by excessive inputs of nutrients: in surface waters, excess algae production is a major concern.

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Updated on Mon, 22 Jun 2020 by Jonathan

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