Multi-effect distillation brine treatment by membrane distillation: Effect of antiscalant and antifoaming agents on membrane performance and scaling control

One of the main advantages of the membrane distillation (MD) process is its ability to treat highly saline feed waters such as thermal desalination brines at moderate temperatures. However, scaling remains one of the major obstacles, causing a significant flux decline and membrane pore wetting. Furthermore, antiscalant and antifoaming agents are commonly utilized in conventional thermal desalination to prevent scale formation, and their effects on MD operation are not yet well understood. This study explores a multi-effect distillation (MED) brine as a potential feed source for MD system with respect to process performance and membrane scaling. The influence of chemicals present in MED brine as well as feed temperature on the scaling process is addressed in terms of vapor flux and salt crystals formation. The scale formation was monitored with the non-invasive optical coherence tomography (OCT) imaging, and results were validated by scanning electron microscopy (SEM). Additionally, the elemental composition of the scale was determined and its effect on membrane contact angle was evaluated. We found that depending on its concentration, the antiscalant prolonged the induction time of salt crystallization whereas antifoaming showed the opposite effect. Scaling mostly occurred due to calcium sulfate crystals formation with the large size needle-shaped crystals favored at higher feed temperature. However, no pore wetting was observed including locations where crystal deposition occurred. Results show that thermal desalination brine, which is already preheated and chemically pretreated, could be an appropriate feed source for MD to further increase the overall water recovery and reduce the marine environmental impact by reducing the brine discharge volume and its temperature.