Pilot study on the softening rules and regulation of water at various hardness levels within a chemical crystallization circulating pellet fluidized bed system

Chemical crystallization circulating pellet fluidized bed reactor (CPFBR) systems are widely used in water softening applications, and softening parameters and effects may be governed by certain rules for the optimum treatment of water at different levels of hardness. In this study, pilot tests were conducted on a CPFBR system with water at eight different levels of hardness. Specifically, the effects of the differences in hardness, alkalinity, and calcium and magnesium concentrations on the softening rules and regulation of the CPFBR were analyzed. The results showed that when Na2CO3 and NaOH were added simultaneously, the removal rate of Ca2+ was positively correlated with the dosage of Na2CO3, while pH was positively correlated with the dosage of NaOH. Under the experimental water quality conditions, when Ca2+ <180 mg/L, Mg2+ <70 mg/L, and pH < 11, no fine calcium carbonate or magnesium hydroxide pellets were discharged with the water, and the turbidity of the effluent did not increase. For water with a higher concentration of Mg2+ than that of Ca2+, the morphology of calcium carbonate crystals, element contents on crystal pellet surfaces, and fine pellets flowing out with water were all affected by Mg2+. Based on the analyses and summary of the crystallization softening rules for water at various hardness levels, key supportive data and regulation methods are provided for the engineering design and application of such CPFBR systems.