Abstract
Nanofiltration (NF) and tight ultrafiltration (tight UF) membranes are a viable treatment option for high quality drinking water production from sources with high concentrations of contaminants. To date, there is limited knowledge regarding the contribution of concentration polarization (CP) and fouling to the increase in resistance during filtration of natural organic matter (NOM) with NF and tight UF. Filtration tests were conducted with NF and tight UF membranes with molecular weight cut offs (MWCOs) of 300, 2000 and 8000 Da, and model raw waters containing different constituents of NOM. When filtering model raw waters containing high concentrations of polysaccharides (i.e., higher molecular weight NOM), the increase in resistance was dominated by fouling. When filtering model raw waters containing humic substances (i.e., lower molecular weight NOM), the increase in filtration resistance was dominated by CP. The results indicate that low MWCO membranes are better suited for NOM removal, because most of the NOM in surface waters consist mainly of humic substances, which were only effectively rejected by the lower MWCO membranes. However, when humic substances are effectively rejected, CP can become extensive, leading to a significant increase in filtration resistance by the formation of a cake/gel layer at the membrane surface. For this reason, cross-flow operation, which reduces CP, is recommended.
Highlights
Over the past 20 years, membrane filtration has been increasingly implemented in water purification processes [1]
The ratio of 4:1 in the mixtures of Suwannee River NOM (SRNOM) and alginate was selected based on humic substances to biopolymers ratios obtained from size exclusion chromatography (SEC) analyses of local surface water (Jericho Pond, Vancouver, BC, Canada) and is consistent with the ratios reported in literature, which indicates that humic substances account for
The extent of the rejection increased as the molecular weight cut offs (MWCOs) of the membranes decreased, with all of the organic material being rejected by the membrane with a MWCO of 300 Da
Summary
Over the past 20 years, membrane filtration has been increasingly implemented in water purification processes [1]. This increase has mainly been driven by the decreasing costs of membrane systems, and increasingly stringent drinking water quality regulations. Effective removal of natural organic matter (NOM) and viruses is generally limited [2,3]. This is because the molecular weight cut off (MWCO) of UF membranes typically used in drinking water treatment applications is relatively large (i.e., greater than 100,000 Da). Membranes with a MWCO of less than 10,000 Da are required to effectively remove
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