Assessment and Optimization of chemical and physicochemical Softening Processes

Abstract

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

Ambient iron-mediated aeration (IMA) for water reuse

“…Of note, EC in the IMA group was slightly increased from 222 to 240 ms/cm from 9 to 48 h, probably because some water was lost as a result of the enhanced evaporation due to continuous bubbling, so that dissolved chemicals were concentrated. While these results are a function of the aqueous ionic balance, and hence specific to the water matrix, they are consistent with those of Chao and Westerhoff (2002) who termed the process aeration softening.…”

Enhancing calcium carbonate precipitation by heterogeneous nucleation during chemical softening

“…The use of return sludge as seed material has been studied previously, and it has been found that the dosage of crystal seed is important in the seeding process (Judkins & Wynne, 1972). Another study found that the addition of seed particles increased the removal of dissolved calcium (Chao & Westerhoff, 2002). However, neither of these studies specifically explored the size or surface area of the crystal seeds.…”

Modeling the Economic Feasibility of Large‐Scale Net‐Zero Water Management: A Case Study

“…The IMA process was described in detail previously (Deng et al, 2013;Englehardt et al, 2013). Briefly, an IMA process consisting of aerated electrocoagulation is assumed, primarily to provide aeration softening (Chao and Westerhoff, 2002), in which calcium carbonate precipitates through equilibration with atmospheric carbon dioxide and coagulation. Secondary benefits include phosphate precipitation and organics reduction.…”