Effect of the membrane exclusion mechanism on phosphate scaling during synthetic effluent desalination

Calcium phosphate scaling is one of the main limitations in effluent desalination using membranes. This may be overcome by tailoring membranes with lower rejection of the scalant ions. In this study, we systematically examined the use of negatively and positively charged membranes, rejecting ions mainly based on Donnan exclusion, as a low-scaling alternative to dielectric-exclusion-dominated polyamide NF membranes for effluent desalination. The two charged membranes exhibited a lower calcium and especially phosphate rejection than the polyamide membrane. Consequently, the calcium phosphate supersaturation and then the propensity to scaling of the charged membranes were much lower than the polyamide membrane. This also allowed filtering at a much higher recovery ratio with the charged membranes. It was also found that, despite the fact that the charged membranes had an opposite fixed charge, their scaling behavior was similar. Apparently, although these membranes showed opposite selectivity towards scalant ions (phosphate and calcium) in single salt solutions, the rejection pattern in mixed salt solutions resulted in similar saturation indices, much lower than for polyamide membrane. The scale formed on all three membranes was identified as amorphous calcium phosphate (ACP), although its saturation index was lower than its solubility factor. This was explained by concentration polarization which increases the saturation index in the solution adjacent to the membrane surface. Tests in absence of permeate flux showed a much slower precipitation that took a few days compared with filtration conditions (few hours). In addition, under these conditions, the effect of the scaling on the membrane permeability was generally reduced and the scale contained crystalline calcium phosphate products, different from ACP. The results indicate that the ion rejection and resulting polarization next to the membrane surface plays a crucial role in scaling. Thus, tuning ion selectivity of NF membranes towards scalant ions presents a promising alternative for scaling mitigation during effluent desalination.