Scaling vs. biofouling: Challenges in direct-contact membrane distillation for SWRO brine reconcentration

Abstract

Membrane distillation (MD) is a promising technology for reconcentrating the brine from seawater reverse osmosis desalination facilities to achieve minimal or zero-liquid discharge. However, MD is susceptible to both scaling and biofouling, yet their specific impacts on membrane performance under different seawater brine conditions remain poorly understood. This study used a custom-made MD system to determine the impacts imposed by scaling (brine only) and biofouling (brine with native microbes) on membrane performance Distilling hypersaline brine from the Gulf of Aqaba by heating the feedwater to 50 °C resulted in minor precipitation of salts and sparse biofilm on the membrane surface with minimal effects to distillate flux and salt rejection. However, increasing the feedwater temperature by only 8 degrees to 58 °C led to significantly accelerated scaling, which impaired membrane performance, reducing distillate flux by 40 % and salt rejection by 55 % after 58 h. Inoculating the heated brine with microbes collected from the Gulf of Aqaba resulted in even faster reduction in salt rejection by 96 % and distillate flux by 95 %. The critical deficiency in the separation capacity was due to membrane wetting following the development of biofilm captured by optical coherence tomography. Correspondingly, most bacteria attached to the membrane surface appeared distressed and were embedded in organic matter, while a fraction was observed penetrating the pore structure. This bacterial infiltration suggests a risk of irreversible fouling, which could significantly shorten MD lifespan. These findings highlight that even a slight increment in feedwater temperature can lead to significant and detrimental impact on MD performance. Therefore, optimizing feedwater temperature based on site-specific environmental conditions, particularly chemical composition and microbial load is critical to minimizing detrimental effects and ensuring the long-term sustainability of MD for SWRO brine reconcentration.