Glossary

Learn about the different types of contaminants that might put your family at risk.

particle size
As used in industry standards, the size of a particle suspended in water as determined by its smallest dimension, usually expressed in microns.
parts per million (ppm)
A common basis for reporting the results of water and wastewater analyses, indicating the number of parts by weight of a dissolved or suspended constituent, per million parts by weight of water or other solvent.  In dilute water solutions, one part per million is practically equal to one milligram per liter, which is the preferred unit.  17.12 ppm equals one grain per U.S. Gallon.
pathogen
An organism which may cause disease.
permanent hardness
Water hardness due to the presence of the chlorides and sulfates of calcium and magnesium which will not be precipitated by boiling.  This term is largely replaced by “noncarbonated hardness.”
pH
The reciprocal of the logarithm of the hydrogen ion concentration.  The pH scale is from zero to 14, and 7.0 is the neutral point, indicating the presence of equal concentrations of free hydrogen and hydroxide ions.  pH values below 7.0 indicate increasing acidity, and pH values above 7.0 indicate increasing base concentrations.
pOH
The reciprocal of the logarithm of the hydroxide ion concentration.  The pOH is related to pH by the expressions: pH + pOH = 14.
pollution
“Pollution is an impairment of quality such that it interferes with the intended usages.” (House Report 2021.)
porosity
A measure of the volume of internal pores in filter media and ion exchangers, sometimes expressed as a ration to the total volume of the medium.
positive charge
The electrical charge on an electrode or ion in solution due to the removal of electrons.
postchlorination
The application of chlorine to a water following other water treatment processes.
potable water
Water which is safe and suitable for human consumption.
ppm
The abbreviation for “parts per million.”
prechlorination
The application of chlorine to a water prior to other water treatment processes. 
precipitate
To cause a dissolved substance to form a solid particle which can be removed by settling or filtering, such as in the removal of dissolved iron by oxidation, precipitation, and filtration.  The term is also used to refer to the solid formed and to the condensation of water in the atmosphere to form rain or snow. 
precipitation
The phenomenon that occurs when a substance in solution is chemically transformed into an insoluble form.  The conversion of dissolved solids into suspended solids which may be concentrated subsequently by flocculation and sedimentation.
precoat
The application of a granular filter medium, such as diatomaceous earth, to a membrane prior to the service cycle of a filter.
preliminary filter
A filter used in a water treatment plant for the partial removal of turbidity before final filtration.  Such filters are usually of the rapid type, and their use allows final filtration at a more rapid rate or reduces or removes the necessity of other preliminary treatment of the water.  Also called contact filter, contact roughing filter, roughing filter.
pressure differential
The difference in pressure between two points in a system due to differences in elevation and/or pressure drop due to flow.
pressure drop
A decrease in water pressure during flow due to internal friction between molecules of water, and external friction due to irregularities or roughness in surfaces past which the water flows.
pressure tank
A tank used in connection with a water distribution system, for a single household, for several houses, or for a portion of a larger water system, which is airtight and holds both air and water, and in which the air is compressed and the pressure so created is transmitted to the water.