EPA 832-F-00-018 - September 2000

Chemical precipitation is a widely used, proven technology for the removal of metals and other inorganics, suspended solids, fats, oils, greases, and some other organic substances (including organophosphates) from wastewater. Generally speaking, precipitation is a method of causing contaminants that are either dissolved or suspended in solution to settle out of solution as a solid precipitate, which can then be filtered, centrifuged, or otherwise separated from the liquid portion. A voluminous precipitate can capture ions and particles during formation and settling, in effect “sweeping” ions and particles from the wastewater. (Tchobanoglous and Burton, 1991). Precipitation is assisted through the use of a coagulant, an agent which causes smaller particles suspended in solution to gather into larger aggregates. Frequently, polymers are used as coagulants. The long-chain polymer molecules can be either positively or negatively charged (cationic or anionic) or neutral (nonionic). Since wastewater chemistry typically involves the interaction of ions and other charged particles in solution, these electrical qualities allow the polymers to act as bridges between particles suspended in solution, or to neutralize particles in solution (Amirtharajah and O’Mella, 1990; Jacangelo, 1987). The specific approach used for precipitation will depend on the contaminants to be removed, as described below.