Hardness removal processes are very pH-dependent, especially for removal of magnesium (Mg) and silica (Si). Bench-scale tests were conducted with a groundwater that was supersaturated with carbon dioxide and contained calcium (Ca), Mg, and Si. The purpose of this work was to assess and optimize several softening processes to reduce chemical use (i.e., sludge production) and improve turbidity removal. Optimal dosing of lime and soda ash (OLSA) removed 79% of Ca and Mg hardness and 23% of Si hardness. Iron salt addition during OLSA improved the rate of turbidity removal, had minimal effect on Ca or Si removal, but decreased Mg removal unless the pH was readjusted to offset the pH decline that resulted from iron hydroxide formation. Sodium aluminate addition during OLSA improved the rate of turbidity removal and increased settled sludge viscosity, but did not affect Ca, Mg, or Si removal. In separate semibatch aeration softening tests (without lime or soda ash addition), Ca removal increased as a function of aeration rates; Mg and Si were not removed.The addition of a nuclei seed increased dissolved Ca removal from 60% without the nuclei seed to > 80% in the presence of 3 g/L of nuclei seed. Results indicate that the aeration softening process would remove hardness, decrease chemical consumption, and reduce sludge production. alcium (Ca) and magnesium (Mg ) are abundant alkaline earth metals that can significantly affect water quality, treatability, sludge production, and the economics of using a water supply for domestic or industrial applications (Batchelor et al, 1991). Ca and Mg are divalent cations present in igneous rock minerals as silicates (e.g., feldspar, olivine), in sedimentary rock as carbonates (e.g., calcite, dolomite), or in sandstone and detrital rock as cement between particles. Weathering of these rock types results in mineral dissolution and solubilization of Ca, Mg, silica (Si), and carbonate species (Hem, 1992). C