UF membrane fouling by natural organic matter (NOM) : the effect of divalent cations, NOM composition and size

Several studies have been done and shown that natural organic matter (NOM) is a major UFmembrane foulant, and that different components and sizes of NOM cause different forms of fouling.Although several researchers have tried to identify the components of NOM that cause fouling of UFmembranes, the results are confusing and conflicting. Due to these conflicting opinions, it is stillnecessary to study further the causes of UF membrane fouling, and to find a relationship betweenNOM composition, NOM size, divalent cation concentration in feed water and fouling.Trial dialyses (3.5kDa and 10kDa) were carried out for 6 days and 21 days with pre-filtered surfacewater using synthetic surface water as dialysate. The aim of the trials was to optimise the dialysisprocess for NOM fractionation by size. DOC, Ca2+, Mg2+, soluble silica and bacteria were monitoredat intervals during the dialysis process. LCOCD samples were also taken and analysed. In order tostudy the effect of ion exchange (IX) on the NOM, trial IX runs were made with pre-filtered surfacewater and synthetic surface water. Ca2+/Mg2+, Na and DOC were again monitored. LCOCD sampleswere taken before and after IX, and analysed. Half the volumes of each of the NOM fractionsobtained from the dialysis experiments were then softened by ion exchange (IX). The different NOMfractions (with low and high Ca2+) of different composition were characterised and fed to UFmembranes (100kDa) at a flux of 138.5L/m2h, filtration cycle times of 31.5 minutes and backwashduration of 1.75 minutes. Transmembrane pressure was monitored during the filtration. LCOCDanalysis was carried out for the feed and permeate.For a dialysate recirculation of 95L/h, sample to dialysate ratio of 5.2:80L, a dialysate changefrequency of 3 times per 24 hours, the shortest duration of dialysis was about 7 days for both 3.5kDaand 10kDa. Bacterial growth in the dialysis bags did not significantly reduce the DOC, but couldhave been the cause of increased the amount of biopolymers by releasing EPS. 91-93% ofhydrophobic NOM was removed by dialysis. Removal of organic carbon (OC) increased withdialysis duration and MWCO of the bags. The IX resin removed TOC-4%, POC-29%, DOC-2.5%,hydrophobics-98%, Ca2+-98% and Mg2+-100% and only 0.2% hydrophilics. An increase inbiopolymers from 120% in UF5 (6d, 10kDa, 1.1 mgDOC/L, 151 mgCa/L)-120% to 223% inUF9 (21d, 10kDa, 0.82 mgDOC/L, 133 mgCa/L), in the absence of humics and a negligibleincrease in building blocks (4.3% to 5.7%), resulted in almost double the fouling rate from3.5 to 6.6 mBars/min per mgDOC/L. The Ca/DOC ratio only reduced slightly from 146 to141 during UF5 and UF9 respectively. This suggested that biopolymers were the majorcause of total fouling. For a similar NOM composition, an increase in Ca/DOC ratio from1.18 in UF10 (21d, 10kDa, 1.04 mgDOC/L, 1.1 mgCa/L) to 141 in UF9 (21d, 10kDa, 0.82mgDOC/L, 133 mgCa/L) caused an increase in average total fouling rate from 5.8 to 6.6mBars/min per mgDOC/L. There was a generally higher total fouling in the case of highCa2+ concentration with HB NOM than in the case of low Ca2+ concentration and no HBNOM. Comparing results from UF5 (6d, 10kDa, 1.1 mgDOC/L, 151 mgCa/L) and UF9 (21d,10kDa, 0.82 mgDOC/L, 133 mgCa/L), while the other NOM components reduced, anincrease in biopolymers from 120% in UF5 to 223% in UF9 resulted in a large increase Bwfouling from 6.74 to 11.12 mBars per mgDOC/L when the Ca/DOC ratio did notsignificantly change (141 and 146). This suggested that biopolymers were the main cause ofBw fouling. An increase in biopolymers from 120% in UF5 to 223% in UF9, while humics,building blocks and neutrals decreased, resulted in a drastic increase in nBw fouling from8.6 to 31.5 mBar per mg DOC/L per cycle, when the decrease in Ca/DOC ratio was minor(146 to 141). This suggests that biopolymers were also the main cause of nBw fouling.Biopolymers (polysaccharides, proteins and amino sugars) were the major cause of total,backwashable and non-backwashable fouling. Presence of Ca2+/Mg2+ enhanced the fouling of UFmembran