Effect of Filtration and Particulate Fouling in Membrane Distillation

Fouling and scaling in membrane distillation (MD) is one of the significant barriers to its continued growth. Fouling in MD blocks the pores, causing a decline in permeate flux, and may eventually lead to wetting of a hydrophobic membrane by the saline feed, contaminating the permeate. Many previous studies on MD have observed that while MD is more fouling resistant than reverse osmosis (RO), inorganic salt precipitation on the membrane surface under supersaturated conditions may readily foul and cause wetting of the membranes. While most studies have assumed that crystal growth occurs directly on the membrane surface, precipitation of particles in the bulk which then migrate to the surface may play a significant or dominant role. In this study, membrane distillation experiments are run at varied supersaturated salinities with and without filtration in equivalent operating conditions to examine the effectiveness of particulate fouling in MD, and thus also the role of bulk precipitation in inorganic fouling. Conditions were designed to favor surface crystal growth over bulk nucleation, to examine the limiting case. The experiment is paired with heat and mass transfer and solubility numerical modeling to analyze the effects of concentration polarization to accurately calculate the saturation index (SI) in the bulk and at the membrane surface. The results show that the removal of particles precipitating in the bulk makes a tremendous difference in reducing flux decline, visible crystals on the membrane surface, and wetting. This implies that bulk nucleation followed by deposition, rather than surface crystal growth, dominates the scaling process in MD when the bulk is supersaturated.