AWWA JAW56579

Hardness removal processes, especially for removing magnesium (Mg) and silica (Si), are very pH-dependent. A study was undertaken to determine the extent to which aeration could replace chemical addition as a means of calcium (Ca) removal. Bench-scale tests were conducted with a groundwater supersaturated with carbon dioxide (CO2) and containing Ca, Mg, and Si. Optimal dosing of lime and soda ash (OLSA) resulted in 79% removal of Ca and Mg hardness and 23% Si removal. Addition of iron salts during OLSA improved turbidity removal, had minimal effect on Ca or Si removal, and decreased Mg removal unless pH was readjusted. Sodium aluminate addition during OLSA improved turbidity removal and increased settled sludge viscosity but did not affect Ca, Mg, or Si removal. The authors found that removing Ca from water depends on the presence of "seed" particles onto which calcite minerals precipitate; the addition of a nuclei seed increased dissolved Ca removal from 60 to > 80%. Addition of polymers affects the density of the sludges that are produced. On the basis of these experiments, aeration stripping would appear most appropriate for waters supersaturated with CO2 and containing predominating calcium carbonate hardness. For utilities using these source waters, the article offers a better understanding of the effects of calcite chemistry on aeration and softening processes that should help them improve Ca removal while minimizing chemical addition and sludge production. Includes 20 references, tables, figures.