Effects of High Salinity on Alginate Fouling during Ultrafiltration of High-Salinity Organic Synthetic Wastewater

Ultrafiltration is widely employed in treating high-salinity organic wastewater for the purpose of retaining particulates, microbes and macromolecules etc. In general, high-salinity wastewater contains diverse types of saline ions at fairly high concentration, which may significantly change foulant properties and subsequent fouling propensity during ultrafiltration. This study filled a knowledge gap by investigating polysaccharide fouling formation affected by various high saline environments, where 2 mol/L Na(+) and 0.5–1.0 mol/L Ca(2+)/Al(3+) were employed and the synergistic influences of Na(+)-Ca(2+) and Na(+)-Al(3+) were further unveiled. The results demonstrated that the synergistic influence of Na(+)-Ca(2+) strikingly enlarged the alginate size due to the bridging effects of Ca(2+) via binding with carboxyl groups in alginate chains. As compared with pure alginate, the involvement of Na(+) aggravated alginate fouling formation, while the subsequent addition of Ca(2+) or Al(3+) on the basis of Na(+) mitigated fouling development. The coexistence of Na(+)-Ca(2+) led to alginate fouling formed mostly in a loose and reversible pattern, accompanied by significant cracks appearing on the cake layer. In contrast, the fouling layer formed by alginate-Na(+)-Al(3+) seemed to be much denser, leading to severer irreversible fouling formation. Notably, the membrane rejection under various high salinity conditions was seriously weakened. Consequently, the current study offered in-depth insights into the development of polysaccharide-associated fouling during ultrafiltration of high-salinity organic wastewater.