Brine concentration using air gap diffusion distillation: A performance model and cost comparison with membrane distillation for high salinity desalination

Zero liquid discharge (ZLD) has emerged as a treatment process to address the challenges with brine disposal from desalination plants. To achieve ZLD, a brine concentrator is commonly used to extract water until a high salinity approaching saturation is reached. However, state-of-the-art brine concentrators are energy-intensive thermal evaporators comprising expensive metal alloys and complex maintenance. This study presents the design of a new brine concentrator referred to as air gap diffusion distillation (AGDD) that operates without a membrane up to a high salinity. A holistic analytical framework is developed to evaluate the system performance as a function of its energy efficiency (gained output ratio, GOR) and water flux (represented as the recovery ratio, RR), as these metrics directly impact the levelized cost of water (LCOW) produced. Specifically, a multi-pass AGDD system achieves an overall water recovery of ~70 % and GOR of 7 (88 % latent heat recovery) with an initial feed salinity of 70 g/kg. A comparison is made to its thermodynamically similar counterpart, air gap membrane distillation (AGMD), which reveals that AGDD outperforms AGMD by reducing thermal and mass transport resistances associated with the membrane. The corresponding LCOW is 1.6× lower than AGMD, making it promising as a brine concentrator to achieve salinities >200 g/kg for minimal liquid discharge.