Key factors of membrane electrolysis control the ordered crystallization of calcium and magnesium in nanofiltration concentrate
To address the complexity of the product types in the municipal nanofiltration(NF)concentrate treatment,CaCO3 and Mg(OH)2 are recovered with low energy consumption with the help of high salt and low resistance characteristics of NF concentrate based on the principle of cath-ode discharge polarization in membrane electrolysis.The cation exchange membrane(CEM)elec-trolysis was optimal for the ordered crystallization of calcium and magnesium,and the separation and recovery of calcium and magnesium could be achieved at pH 10.5 and 11.5,respectively.Com-pared with continuous-flow CEM electrolysis,the sequential batch system consumed less energy,produces water faster,and recovers calcium carbonate with a purity of 96.77%and magnesium hydroxide with a purity of 81.95%.Membrane electrolysis power was changed slightly during multi-cycle operation with total energy consumption lower than 9.2 kWh/m3.In addition,the membrane fouling on the cathode and anode sides showed different,and the cathode side was due to the adhesion of calcium and magnesium precipitates generated in the cathode chamber,and the fouling on the anode side was the result of the combination of inorganic and organic substances in the anode chamber.The results showed that the membrane electrolysis system can be one of the strategies for the recovery of calcium and magnesium in NF concentrate by ordered crystallization.