M alkalinity - Methyl orange alkalinity. (See total alkalinity.)

macroreticular - A term applied to ion exchange resins that have a rigid polymer porous network in which there exists a true pore structure even after drying. The pores are larger than atomic distances and are not a part of the gel structure.

magnesium - One of the elements in the earth's crust, the compounds of which when dissolved in water make the water hard. The presence of magnesium in water is a factor contributing to the formation of scale and insoluble soap curds.

manganese - An element sometimes found dissolved in ground water, usually with dissolved iron but in lower concentration; causes black stains and other problems similar to iron. It can be removed by a water softener or it can be precipitated by chlorine at a pH of 9.5 or above.

manganese greensand - Greensand which has been processed to incorporate in its pores and on its surface the higher oxides of manganese. The product has a mild oxidizing power, and is often used in the oxidation and precipitation of iron, manganese and/or hydrogen sulfide, and their removal from water (see greensand, manganese zeolite).

manganese zeolite - Synthetic gel zeolite which has been processed in the same manner as manganese greensand, and used for similar purposes.

MBAS - Abbreviation for "Methylene Blue active Substance".

MCL- Abbreviation for "Maximum Contaminant Level"; the maximum allowable concentration of a contaminant in water as established in the U.S. EPA Drinking Water Regulations.

mechanical filter- A filter primarily designed for the removal of suspended solid particles, as opposed to filters with additional capabilities.

media- The plural form of "medium".

medium- A material used in a filter bed to form a barrier to the passage of certain suspended solids or dissolved molecules.

Methylene Blue Active Substances - Chemical compounds which react with methylene blue to form a blue compound which can be used to estimate the concentration by measurement of the depth of color. Substances measured include ABS and LAS types of detergents, thus the term is commonly used as an expression of detergent concentration. (See detergent.)

mg/L - The abbreviation for milligrams per liter.

micrometer - Formally known as micron. A linear measure equal to one millionth of a meter or .00003937 inch. The symbol for the micrometer is "um".

micron- See micrometer.

micron rating - The term applied to a filter medium to indicate the particle size above which all suspended solids will be removed throughout the rated capacity. As used in industry standards, this is an "absolute" not "nominal" rating.

mil- One thousandth of an inch.

milli- The prefix used with units of measure to indicate one thousandth of the unit. Example: a milliliter is one thousandth of a liter.

milligram per liter (mg/L) - A unit concentration of matter used in reporting the results of water and waste water analyses. In dilute water solutions, it is practically equal to the part per million, but varies from the ppm in concentrated solutions such as brine. As most analyses are performed on measured volumes of water, the mg/l is a more accurate expression of the concentration, and is the preferred unit of measure.

millimicron - (archaic) See "nanometer".

mineral- A term applied to inorganic substances such as rocks and similar matter found in the earth strata, as opposed to organic substances such as plant and animal matter. Minerals normally have definite chemical composition and crystal structure. The term is also applied to matter derived from minerals, such as the inorganic ions found in water. The term has been applied to ion exchangers, stemming from the early use of natural zeolite. The term is inappropriate to the modern organic ion exchange resins.

mineral acidity- Acidity due to the presence of inorganic acids such hydrochloric, sulfuric and nitric acids, as opposed to acidity due to carbonic acid or organic acids.

mole - 6.02 x 10²³ atoms of an element or 6.02 x 10²³ molecules of a chemical compound. The weight of one mole of an element is equal to its atomic weight in grams; the weight of one mole of a compound is equal to its molecular weight in grams.

Molecule - The simplest combination of atoms that will form a specific chemical compound; the smallest particle of a substance which will still retain the essential composition and properties of that substance, and which can be broken down only into atoms and simpler substances.

most probable number (MPN)- The term used to indicate the number of microorganisms which, according to statistical theory, would be most likely to produce the results observed in certain bacteriological tests; usually expressed as a number per 100 ml of water.