parts per million (ppm) - A common basis for reporting the results of water and waste water 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 precipitate by boiling. This term is largely replaced by "noncarbonate hardness". (See noncarbonate hardness.)
permanganate - Generally refers to potassium permanganate, a chemical compound used in water treatment. (See potassium permanganate.)
pH - The negative logarithm of the hydrogen ion concentration. The pH scale ranges from zero to 14 with 7 as the neutral point, indicating the presence of equal concentrations of free hydrogen and hydroxide ions. pH values below 7.0 indicate acidity, with 0 most acid; pH values above 7 indicate basicity, with 14 most basic, or alkaline.
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, or voids, in ion exchangers and filter media; sometimes expressed as a ratio to the total volume of the medium. (See void volume.)
portable exchange - A term applied to water softeners, filters and deionizers 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 net electrical charge on an electrode or ion in solution due to the removal of electrons. (See electron, cation.)
postchlorination - The application of chlorine to a water following other water treatment processes. (See prechlorination.)
potable water - Water which is suitable for human consumption.
potassium permanganate - An oxidizing chemical commonly used in processes for removing iron, hydrogen sulfide and manganese.
ppm - The abbreviation for "parts per million".
prechlorination - The application of chlorine to a water prior to other water treatment processes. (See postchlorination.)
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.
precoat - The application of a granular filter medium, such as diatomaceous earth or powdered activated carbon to a membrane or screen or other filtration surface, prior to the service cycle of a filter.
pressure differential - A difference or change in pressure detected 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.
