Kinetics of CaCO₃ precipitation in seeded aeration softening of brackish water desalination concentrate

Desalination of brackish water is energetically attractive, but the concentrate produced poses a major environmental issue. Intensive research has been carried out to develop demineralization techniques of such concentrates to facilitate their utilization and disposal. Aeration was reported to allow chemical-free and effective demineralization through CaCO₃ precipitation. When applied in concentrate treatment, the process was assumed to follow first order kinetics, but second order kinetics were proposed when the process was applied in potable groundwater treatment. In the current study the effect of aeration rates and seed concentration on reaction kinetics was studied in real brackish water desalination concentrates (BWDC). In the absence of or at low aeration, CaCO₃(s) precipitation followed second order kinetics characteristic of simple CaCO₃(s) precipitation. With increasing aeration rate, the reaction shifted toward first order kinetics, characteristic of gaseous CO₂ release. Both aeration rate and seed concentration linearly increased the gas mass transfer coefficient. Under the experimental conditions, maximal removal rate was 1.05 g calcium-CaCO₃ L^-1 h^-1. Calcium removal reached 73% and was found to depend on raw concentrate composition and reaction time. The composition of the softened concentrate corresponded well with the geochemical PHREEQC code simulation of the solution at equilibrium. The precipitates comprised at least 92% calcite, which may be of economic significance. The ability to use the intrinsic precipitates as seed particles may be advantageous in terms of CaCO₃ recovery and simplicity of operation.