Glossary of Water Treatment Terms & Definitions - M


Macroporous Resin

Ion exchange resins produced in both cation and anion versions with 12 percent or higher cross-linkage. They offer a higher resistance to oxidation and organic fouling.


One of the elements making up 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 (Mn)

A element sometimes found dissolved in ground water, usually with dissolved iron but in lower concentrations. It causes black stains in laundry and plumbing fixtures at concentrations higher than 0.05 mg/l. It is removed the same way as iron, by ion-exchange or oxidation and filtration.

Manganese is very similar to iron in its properties. In fact, they both often occur together with manganese composing a small part of the combination. Manganese can occur in a clear water state, latter turning to a colored water state. It can come from the well already oxidized due to dissolved oxygen in the water. There are also manganese bacteria which are similar to iron bacteria. . Manganese staining does often appear to be darker than iron staining (sometimes black)

The distinction between iron and manganese is usually not that important because most methods which successfully treat iron will also treat manganese. However, when a larger amount of manganese is present, modifications and additions to iron removal equipment may become necessary.

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. It is regenerated by the use of two to four ounces of a weak solution of potassium permanganate per cubic foot of manganese greensand.


Maximum Contaminant Level. A drinking water standard. The maximum amount of a contaminant allowed in drinking water.


Maximum Contaminant Level Goal. The goal set for the maximum amount of a contaminant to be allowed in drinking water. Has not been approved to become the MCL.

Mechanical Filter

A filter primarily designed for the removal of suspended solid particles, as opposed to filters that remove contaminants by chemical means.

Microgram per Liter

Also known as parts per billion (ppb). The common symbol for microgram per liter is µg/l.


One millionth of an ohm. A unit of measurement used to test the electrical resistance of water to determine its purity. The purer the water, the greater its resistance to conducting an electrical current. Water of absolute purity has a resistance of eighteen million ohms across one centimeter at a temperature of twenty-five degrees Celsius.


One millionth of a mho. Used to measure the conductivity and the approximate TDS content of water. Absolute pure water has a conductivity of 0.055 micromhos per centimeter at 25 degrees Celsius. Also known as micro Siemens. The specific conductance is the reciprocal of resistance, therefore MHO is OHM spelled backwards.


A linear measure equal to one millionth of a meter, or .00003937 inch. The symbol for the micron is the Greek letter "µ". The smallest particle visible to the human eye is 40 microns. Most types of bacteria range from 0.05 to 10.0 microns in size.

Micron Rating

The term applied to a filter or 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.

Milligram per Liter

(mg/l) A unit concentration of matter used in reporting the results of water and wastewater 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.


A term applied to inorganic substances, such as rocks and similar matter found in the earth's 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 incorrectly applied to ion exchangers, even though most of the modern materials are organic ion exchange resins.


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.