A conceptual demonstration of direct contact membrane distillation and freeze desalination hybrid process for reverse osmosis brine treatment

• Hybrid RO-DCMD–FD enhances water recovery and salt extraction from RO brine. • CFD and thermodynamic models optimize flux, crystallization, and energy use. • Thermal integration lowers SEC from 403 to 299 kWh/m³ of freshwater. • Raising FD temperature to –10°C reduces SEC to 235 kWh/m³ with trade-offs. • Integrated system advances ZLD goals with ∼75% salt recovery efficiency. Growing volumes of hypersaline reverse osmosis (RO) brine generated by desalination plants pose significant operational and environmental challenges, underscoring the need for energy-efficient treatment pathways that support zero-liquid-discharge objectives. Freeze desalination (FD) and membrane distillation (MD) have individually shown promise, yet existing hybrid FD–MD concepts remain limited by high thermal demands and by process configurations that typically apply FD before MD. This study proposes an enhanced hybrid desalination system for the treatment of RO brine, in which direct contact membrane distillation (DCMD) is performed prior to freeze desalination. The MD-first configuration is designed to maximize water recovery, increase brine concentration toward eutectic conditions, and improve selective salt-hydrate recovery. The DCMD unit is evaluated using a computational fluid dynamics (CFD) model to optimize flux and module performance, while thermodynamic modelling is employed to predict ice and salt crystallization behaviour across different operating conditions. A key innovation of this work is the introduction of a thermal-recovery optimization framework that quantifies and integrates two internal energy-recovery pathways: (i) recovery of cold energy released during ice melting in FD, and (ii) reuse of sensible heat from the MD brine stream. This integrated heat-reuse strategy significantly reduces the specific energy consumption (SEC) of the hybrid process. System performance is assessed across MD reject salinities of 70, 100, and 130 g/L and FD freezing temperatures of −10, −15, and −30°C. The proposed system achieves an SEC of 403 kWh/m³ of freshwater without heat recovery, which decreases to 299 kWh/m³ with thermal integration. Increasing the FD freezing temperature from −30°C to −10°C further reduces SEC to 235 kWh/m³, with an expected trade-off in water recovery (85% to 55%) and salt extraction (∼75% to ∼15%). This integrated approach offers a promising and energy-efficient pathway toward zero liquid discharge in sustainable brine management.