Oxidation-enhanced ferric coagulation for alleviating ultrafiltration membrane fouling by algal organic matter: A comparison of moderate and strong oxidation

Abstract

Ultrafiltration is increasingly used in fields of water treatment and bio-energy for algae removal and algae harvest, respectively, but membrane fouling caused by extracellular organic matter is still a challenge. In this work, oxidation-enhanced ferric coagulation was assessed for alleviating membrane fouling by extracellular organic matter with Fe(II)/permanganate and Fe(II)/persulfate compared. Characterization of extracellular organic matter was performed before and after pretreatments to evaluate changes in molecular-weight distribution and hydrophilicity. Moreover, filtration tests were carried out to explore performance and mechanisms of fouling control by the enhanced ferric coagulation. The results indicated that both Fe(II)/permanganate and Fe(II)/persulfate could improve rejection of extracellular organic matter by 20%–40%, in particularly for humic-like acid and protein-like substances. Fe(II)/persulfate outperformed Fe(II)/permanganate in alleviating the flux reduction by extracellular organic matter. Simultaneous oxidation and coagulation, which were associated with in situ formed sulfate radicals and Fe(III), respectively, were the principal mechanism for fouling control. The enhanced ferric coagulation only slightly improved the fouling reversibility, and iron and manganese oxides deposited in the external cake layer rather than inside the membrane. Fe(II)/permanganate did not significantly changes pattern of fouling development, whereas Fe(II)/persulfate induced a shift of fouling mechanisms from dual pore blocking and cake filtration to single intermediate blocking. Overall, the strong oxidation (Fe(II)/persulfate) was more beneficial to the control of membrane fouling by extracellular organic matter than the moderate oxidation (Fe(II)/permanganate).