Combined fouling of forward osmosis membranes: Synergistic foulant interaction and direct observation of fouling layer formation

• Published in: Journal of membrane science Vol. 407-408; pp. 136 - 144
• Main Authors: Liu, Yaolin, Mi, Baoxia
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.07.2012; Elsevier
• Subjects: air; artificial membranes; bubbles; Chemistry; Cleaning; Colloidal state and disperse state; Combined fouling; crystallization; crystals; Direct observation; Exact sciences and technology; Forward osmosis; fouling; Fouling reversibility; General and physical chemistry; gypsum; Gypsum scaling; Membrane; Membranes; Organic fouling; osmosis; synergism; Synergistic effect; Direct observation; Gypsum scaling; Forward osmosis; Organic fouling; Membrane; Cleaning; Combined fouling; Synergistic effect; Fouling reversibility; Crystal growth; Water; Nucleation; Growth rate; Membrane surface; Osmosis; Mechanism; Efficiency; Air bubble; Fouling; Membrane interaction; Crystals; Crystallization; Rinsing; Morphology; Reversibility; Models; Kinetics
• ISSN: 0376-7388; 1873-3123
• DOI: 10.1016/j.memsci.2012.03.028

[Display omitted] ► A synergistic effect was observed in the combined gypsum and alginate fouling. ► A direct observation FO system was used to monitor fouling layer formation. ► Alginate increases gypsum crystal size and accelerates crystallization kinetics. ► Heterogeneous crystallization is the dominating scaling mechanism in the presence of alginate. This study investigated the combined fouling by organic and inorganic foulants in forward osmosis (FO) membrane processes. Alginate and gypsum were used as model foulants for organic and inorganic fouling, respectively. A synergistic effect between alginate fouling and gypsum scaling was observed in the combined fouling experiment: the coexistence of foulants caused faster flux decline than the algebraic sum of flux declines due to individual foulants. It was found that the synergistic effect was mainly a result of the aggravated gypsum scaling in the presence of alginate molecules: alginate molecules act as nuclei in gypsum crystal growth, thus significantly increasing the size of gypsum crystal and accelerating crystallization kinetics. Besides, in the presence of alginate, the dominating scaling mechanism switched from bulk crystallization to surface/heterogeneous crystallization. In order to better understand the effect of alginate on the kinetics of gypsum crystal growth, a membrane window cell coupled with a microscope was used to directly observe crystal formation on the FO membrane surface during the fouling experiments. The direct observation results showed that alginate shortens the nucleation time, increases the gypsum growth rate, and changes the morphology of gypsum crystals. Finally, cleaning experiments were performed by rinsing the fouled FO membranes with pure water and continuously introduced air bubbles. It was found that the cleaning efficiency for membranes fouled by combined foulants was lower than that for membranes fouled by individual foulants.