DICHLORO-S-TRIAZINETRIONE: a chemical compound. It is an oxidizer, bacteriocide, algicide, and cleaning agent that reacts with water to form hypochlorous acid, which is related to bleach.

DISINFECTANT:  Chemicals or processes which work to destroy vegetative forms of microorganisms and other contaminants.  Examples are chlorine, bromine, Soft-Swim, and ionizers; also included are copper and silver algaecides.

DIRECTIONS:  What you should read before using any chemicals.

DIRT DEMAND:  The demand that your pool has for dirt.  This level is inversely proportional to available time for cleaning.  If you remove the dirt from the pool, you have created a dirt deficit, and the pool will actually suck dirt out of the air to maintain its dirt demand.

DIVERTER VALVE:  Used in a twin port skimmer, a diverter allows the operator to manipulate the amount of flow from the main drain and skimmer to the pump.  

DPD:  A method of testing for chlorine levels in the pool water.  Unlike OTO, DPD testing allows determination of total and free available chlorine levels which through subtraction gives us combined levels.

DRAIN:  Also called the "main drain," this plumbing fitting is the start of one suction line to the pump and is usually situated at or near the center bottom of the pool.

DRY ACID:  Sodium bisulfate, a granular form of acid used to lower pH and alkalinity in the water.  It is safer and less caustic than muriatic acid.  Usually available as a "pH decreaser."