dechlorination - The removal of chlorine residual.

deionization - The removal of all ionized minerals and salts from a solution by a two-phase ion exchange process. First, positively charged ions are removed by a cation exchange resin in exchange for a chemically equivalent amount of hydrogen ions. Second, negatively charged ions are removed by an anion exchange resin for a chemically equivalent amount of hydroxide ions. The hydrogen and hydroxide ions introduced in this process unite to form water molecules. The term, commonly abbreviated as DI, is often used interchangeably with demineralization. (See demineralization, ion exchange.)

demineralization - The removal of ionized minerals and salts from a solution by a two-phase ion exchange procedure, similar to deionization, and the two terms are often used interchangeably. (See deionization, ion exchange.)

density - The mass of a substance per specified unit of volume; for example, pounds per cubic foot. True density is the mass per unit volume excluding pores; apparent density is the mass per unit volume including pores. (See specific gravity.)

detergent - Any material with cleaning powers, including soaps, synthetic detergents, many alkaline materials and solvents, and abrasives. In popular usage the term is often used to mean the synthetic detergents such as ABS of LAS. (See alkyl benzene sulfonate, linear alkyl sulfonate, soap.)

D.I. or DI - Abbreviation for "deionization".

dialysis - The separation of components of a solution by diffusion through a semi-permeable membrane which is capable of passing certain ions or molecules while rejecting others. (See electrodialysis, semi-permeable membrane.)

diaphragm pump - A type of positive displacement pump in which the reciprocating piston is separated from the solution by a flexible diaphragm, thus protecting the piston from corrosion and erosion, and avoiding problems with packing and seals.

diatomaceous earth - A processed natural material, the skeletons of diatoms, used as a filter medium.

diatomite - Another name for diatomaceous earth.

dielectric fitting - A plumbing fitting made of, or containing, an electrical nonconductor, such as plastic; used to separate dissimilar metals in a plumbing system to control galvanic corrosion.

differential pressure - See pressure differential.

digestion - The process in which complex materials are broken down into simpler substances; may be due to chemical, biological or a combination of reactions. (See sterilization.)

disinfection - A process in which vegetative bacteria are killed; may involve disinfecting agents such as chlorine, or physical processes such as heating. (See aerobic, anaerobic.)

dissociation - The separation of molecules into positively and negatively charged ions; occurs when salts dissolve in water. (See ionization.)

dissolved solids - The weight of matter in true solution in a stated volume of water; includes both inorganic and organic matter; usually determined by weighing the residue after evaporation of the water at 105 or 180oC.

distillation - The process in which a liquid, such as water, is converted into its vapor state by heating, and the vapor cooled and condensed to the liquid state and collected; used to remove solids and other impurities from water; multiple distillations are required for extreme purity.

distributor - A device or system designed to produce even flow through all sections of an ion exchanger or filter bed, and to retain the media in the tank or vessel; usually installed at the top and bottom of loose media systems. (See collector.)

dolomite - A specific form of limestone containing chemically equivalent concentrations of calcium and magnesium carbonates; the term is sometimes applied to limestones with compositions similar to true dolomite.

domestic - A term sometimes applied to water conditioning equipment designed for household use.

downflow - A term designating the direction (down) in which water or a regenerant flows through an ion exchanger or filter during any phase of the operating cycle.

drain - A pipe or conduit in a building plumbing system which carries liquids to waste by gravity; sometimes the term is limited to liquids other than sewage.

drain line - A tube or pipe from a water conditioning unit that carries backwash water, regeneration wastes and/or rinse water to a drain or waste system.

dynamic - Active, alive, or tending to produce motion, as opposed to static, resting or fixed.

dynamic system - A system or process in which motion occurs, or includes active forces, as opposed to static conditions with no motion.