Experimental and data-driven modeling of multistage freeze desalination for high-salinity reverse osmosis brine...

This study investigates multistage freeze desalination (MSFD) for the treatment of reverse osmosis (RO) brine, targeting high-purity water production with improved overall recovery efficiency. Experiments were conducted using indirect batch freeze crystallization of seawater samples at - 15 °C, examining the effects of freezing configuration (i.e., top-bottom, and bottom-up). A wide range of initial salinities (i.e., 2, 4, 8, 17.5, 35, 70, and 120 g/L NaCl) was evaluated to determine their impact on water recovery and the minimum number of stages required to achieve potable water quality. Directional melting was employed to resolve salinity gradients across 32 discrete layers, enabling detailed characterization of salt diffusion and brine entrapment during freezing. Based on the experimental data, a predictive mathematical framework was developed to optimize the MSFD process performance. A three-dimensional response surface model, fitted using a second-order polynomial and artificial neural network (ANN), successfully correlated initial salinity, crystallinity, and accumulated salinity, achieving an excellent coefficient of determination (R² = 0.9978). Top-down freezing exhibits the lowest salinity accumulation and thus the highest desalination efficiency, outperforming bottom-up freezing. Results also show that increasing initial salinity necessitates additional crystallization stages, with water recovery decreasing sharply from 39.3% to 5.28% (top-bottom) and from 40% to 1.3% (bottom-up) as salinity increased from 8 to 120 g/L to reach potable water quality (~ 0.95 g/L). The top-bottom configuration consistently outperformed the bottom-up arrangement, producing lower cumulative salinity and more uniform gradients. To further enhance recovery, the concentrated brine rejected from the MSFD process was examined through thermodynamic modeling of eutectic freeze crystallization (EFC), allowing prediction of phase behavior, salt precipitation, and maximizing water recovery. The integrated MSFD-EFC system achieved total water recoveries of 92.52% (top-bottom) and 81.72% (bottom-up). These findings demonstrate that MSFD-EFC coupling enables progressive purification with high yield and minimal waste, offering a sustainable near-zero-liquid-discharge solution for high-salinity brine treatment.