Enzymatic control of alginate fouling of dead-end MF and UF ceramic membranes

Membrane filtration plays a critical role in advanced wastewater treatment and reuse. However, alginate, one typical polysaccharide secreted by microorganisms in wastewater, may cause significant membrane fouling. This study explored the use of an enzyme, alginate lyase, to catalytically degrade alginate in order to decrease its fouling potential for 0.2 and 0.02 μm γ-Al2O3 ceramic membranes. The results indicated that the enzyme significantly reduced membrane fouling and made the fouling easy to clean (i.e., reversible fouling). The enzymatic degradation of alginate followed Michaelis–Menten kinetics through the β-elimination mechanism. When 20 mg L−1 alginate lyase was added to 325 mg L−1 alginate solution at pH 6.7 ± 0.2, the weight-averaged molecular weight decreased from 35,500 to 2200 Da over a 2 h period. Meanwhile, polydispersity declined from 9.10 to 1.47. Membrane filtrations were compared between the original and the enzyme-reacted alginate at pH 6.7 ± 0.2 and an ionic strength of 0.075 M. At the end of 50-min filtration, alginate lyase reduced the foulant resistance by 82% and 85% for the 0.2 and 0.02 μm membrane, respectively, because the enzyme-reacted alginate had a higher diffusivity and less affinity/attachment to the membrane pores and surfaces. In addition, the enzyme improved the efficiency of backwash as well. Compared to the 0.2 μm membrane, 0.02 μm membrane had a more significant enhancement of backwashing with a 100% recovery rate obtained, because diffusivity is more important to clean tight membrane pores. A decline in the rejection rate of the membranes after enzymatic reactions was observed, which may be explained by decreased steric exclusion as evidenced by the size exclusion chromatography.