Mechanisms of Polysaccharide Foulant Removal from Nanofiltration Membranes: Interplay between Cleaning Efficiency, Calcium Sequestration, and Foulant Mechanics

Membrane cleaning is essential due to the inevitable occurrence of fouling in membrane-based water treatment systems. However, the interactions between hydrogel-like organic foulants and cleaning agents─and how these interactions influence foulant cohesion, rheology, and, ultimately, cleaning efficiency─remain poorly understood. Here, we elucidate the cleaning mechanisms of Ca2+ -alginate-fouled cellulose acetate nanofiltration membranes using citric acid as a model sequestrant. We determine foulant composition (calcium content) and characterize rheological (elastic and viscous moduli) and mechanical (Young’s modulus) properties under 2.3 ≤ pH ≤ 7 (consistent with cellulose acetate pH tolerance) and varying sequestrant concentrations, to rationalize cleaning efficiency observed in cross-flow experiments. At neutral pH and ≤1 mM citric acid, calcium extraction softened the gel, inducing liquid-like rheology and reducing the Young’s modulus 10-fold, consistent with a nondissolutive cleaning mechanism determined by cohesive failure. At higher citric acid concentrations (which lower Ca2+ content from ∼100 to ∼30 mg per gram of alginate), cleaning proceeded through a dissolutive mechanism, though permeance recovery plateaued at ∼80% of that of the pristine membrane. Similar recovery limits were observed at rising cross-flow velocity (450 ≤ Re ≤ 1080) during cleaning and sonication, suggesting that strong foulant-membrane adhesive forces prevent full restoration of membrane permeance. Cleaning at pH 2.3 was ineffective due to alginic acid’s structural integrity, with subsequent neutralization to pH 7 leading to ∼80% permeance recovery.