ABSTRACT
Precipitation converts dissolved pollutants into solid particles that can be removed by some simple physical separation method like settling or filtration. Controlled precipitation is used to remove metals from industrial wastewater, remove phosphorus from wastewater, and to soften drinking water and boiler feed water. Uncontrolled precipitation causes scaling and clogging in boilers, hot water heaters, coffee makers, and pipes. Precipitation reactions are used to remove a soluble substance from solution by transforming it into an insoluble form. The minimum solubility of the sulfides is lower than the hydroxides and the sulfide precipitates form at much lower pH levels than the hydroxides. The hydroxides have the interesting property of having a minimum solubility at a particular pH. This occurs because these metals exist in different ionic forms, depending upon solution pH, and these forms have different solubility. Precipitation tests should be done at varying pH levels and with different chemicals to learn which produces the best result.
