From wastewater to resources: Membrane-free electrochemical separation coupled with crystallization-granulation for selective removal and value-added recovery of hardness ions

Abstract

Current water treatment technologies primarily focus on removing Ca²⁺ and Mg²⁺ from wastewater, but struggle to achieve efficient selective precipitation and resource recovery. This paper presents a method that couples membrane-free electrochemical separation water softening (MFES) with crystallization-granulation (CG) to effectively remove and recover Ca²⁺ and Mg²⁺ from circulating cooling water. The coupled process consists of two main parts: removing and recoveringCa²⁺ and Mg²⁺. Each part employs a series connection of MFES and CG systems. The MFES controls the effluent pH, while the CG system enables rapid precipitation and formation of dense CaCO₃ and Mg(OH)₂ products. Experimental results indicate that by adjusting electrochemical parameters and using a stainless-steel filter with a pore size of 3 × 1.8 mm as the cathode, the effluent pH can be maintained at around 9.50. This results in a Ca²⁺ removal efficiency of over 94.0 % without affecting Mg²⁺ hardness. Adjusting the electrochemical parameters and employing a sintered nickel-plated copper powder filter with a pore size of 90 μm as the cathode can increase the effluent pH to about 11.50, achieving an Mg²⁺ removal efficiency of over 84.0 %. The MFES system ensures high hardness removal rates, while the CG system produces dense, high-purity CaCO₃ and Mg(OH)₂ products, offering practicality and economic benefits. The proposed MFES-CG process offers a simple and efficient approach for removing and recovering hardness ions from water, providing new insights into effective water softening and resource recovery.