Larry D.Benefield, Ph.D.
Professor
Department of Civil Engineering
Auburn University, Alabama

Joe M.Morgan, Ph.D.
Associate Professor
Department of Civil Engineering
Auburn University, Alabama

Chemical precipitation is an effective treatment process for the removal of many contaminants. Coagulation with alum, ferric sulfate, or ferrous sulfate and lime softening both involve chemical precipitation. The removability of substances from water by precipitation depends primarily on the solubility of the various complexes formed in water. For example, heavy metals are found as cations in water and many will form both hydroxide and carbonate solid forms. These solids have low solubility limits in water. Thus, as a result of the formation of insoluble hydroxides and carbonates, the metals will be precipitated out of solution.

Although coagulation with alum, ferric sulfate, or ferrous sulfate involves chemical precipitation, extensive coverage of coagulation is given in Chapter 6 and will not be repeated here. The discussion of the application of chemical precipitation in water treatment presented in this chapter will emphasize the reduction in the concentration of calcium and magnesium (water softening) and the reduction in the concentration of iron and manganese. Attention will also be given to the removal of heavy metals, radionuclides, and organic materials in the latter part of the chapter.