HOME > Glossary


Assimilation of molecular species throughout the bulk of the solid or liquid


Accumulation of the molecular species at the surface rather than in the bulk of the solid or liquid


An aqueous solution with a pH greater than 7


A measure of hydrogen ion concentration, a measure of the acidity or alkalinity of a solution

Parts Per Million (ppm)

A dimensionless quantity and measurement unit that is used to express a very dilute concentration level of a solvent or particulate matter in a solution, such as when describing the level of pollutants in the water or other fluids.

Grain per gallon (gpg)

Water hardness defined as (1) grain (64.8 milligrams) of calcium carbonate dissolved in (1) US gallon of water (3.785412 L). It translates into 1 part in about 58,000 parts of water or 17.1 parts per million (ppm).


The amount of dissolved calcium and magnesium in the water.

Scale (2 types of hardness)

Temporary = Ca(HCO₃)₂, or Calcium bicarbonate

Permanent = CaCI₂, or Calcium chloride

Calculate ppm to grains of hardness

Divide the ppm hardness value by 17.1

The result is the water hardness expressed in grains per gallon.

For example, say you have a water hardness value of 180 ppm. Work out 180 ÷ 17.1 = 10.526.

Total dissolved solids (TDS)

Comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.

TDS in drinking-water originate from natural sources, sewage, urban run-off, industrial wastewater, and chemicals used in the water treatment process, and the nature of the piping or hardware used to convey the water, i.e., the plumbing. In the United States, elevated TDS has been due to natural environmental features such as mineral springs, carbonate deposits, salt deposits, and sea water intrusion, but other sources may include: salts used for road de-icing, anti-skid materials, drinking water treatment chemicals, stormwater, and agricultural runoff, and point/non-point wastewater discharges.

For drinking water and fountain beverages, a TDS of up to 500 is acceptable, but for boiler-based steam ovens, TDS should be kept very low (< 100 ppm). With respect to trace metals, an elevated total dissolved solids may suggest that toxic metals may be present at an elevated level. It is important to keep in mind that water with a very lower TDS concentration may be corrosive and corrosive waters may leak toxic metals such as copper and lead from the plumbing, this also means that trace metals could be present at levels that may pose a health risk.

Micron (carbon and sediment)

The openings between pieces of the filter media. For example, a 20-micron filter has larger openings than a 5-micron filter. Consequently, the 20-micron filter element will let larger particles pass through the filter than the 5-micron media would.

Bacteria range in size from 0.2 to 2 microns in width or diameter and from 1 to 10 microns in length for the nonspherical specie, so a 1-micron filter will remove most bacteria and cysts. As a general rule, the smaller micron rating for a filter is better, but there is a trade-off. Flow capability usually drops off as the micron rating gets smaller, especially if the water has a lot of sediment, which well water often has.