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.
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 "noncarbonate 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.
Portable Exchange - a term applied to water softeners and filters which are designed for connection to a water system with special fittings and disconnection and transport to a central station or plant for regeneration or servicing.
Positive Charge - the electrical charge of an electrode or ion in solution due to the removal of electrons.
PPM - the abbreviation for part per million.
Precipitate - to cause a dissolved substance to form a solid particle which can be removed by settling or filtering, such as 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.
Pressure Differential - the difference in pressure between two points in a system due to the 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 toughness in surfaces past which the water flows.