Characterisation and mitigation of the fouling of ceramic microfiltration membranes caused by algal organic matter released from cyanobacteria

Algal organic matter (AOM) released from cyanobacteria such as Microcystis aeruginosa has a high potential to cause severe fouling of low pressure water treatment membranes including ceramic microfiltration (MF) and ultrafiltration (UF) membranes. In this study, the influence of the characteristics of AOM on the fouling of a 7-channel tubular ceramic MF membrane (ZrO2–TiO2, 0.1µm) was investigated at lab scale. The AOM (3 mg DOC/L) extracted from M. aeruginosa culture at three different phases of growth (10, 20 and 35 days) all caused severe flux decline, and its fouling potential increased with increasing growth time. The role of AOM components in the fouling of the ceramic membrane was further investigated to obtain a better understanding of the interaction between the AOM and the membrane. High molecular weight (MW) biopolymers (such as carbohydrates and proteins) were determined as the major foulants determining the severity of the AOM fouling of the ceramic membrane. The majority of the flux decline was attributed to the large amount of organic matter (51% of total DOC of feed, primarily very high MW hydrophobic molecules) deposited on the ceramic membrane surface to form a thick and dense outer layer. As aquatic humic substances are ubiquitous in surface water, the influence of the interaction between aquatic humic substances and the AOM on the fouling of the MF ceramic membrane was also studied. The irreversible fouling resistance resulting from the mixtures was markedly higher than for all the individual organic fractions. An apparent increase in average molecular size for the AOM-humics mixtures was observed, where some UV-absorbing molecules in the humics appeared to participate in the formation of larger molecules with the AOM.

Feedwater pretreatments using chemical coagulation (with aluminium sulphate (alum), aluminium chlorohydrate (ACH), ferric sulphate and ferric chloride) and an advanced oxidation process (AOP) utilising UV/H2O2 were then evaluated as a means to mitigate the membrane fouling. At their optimum dosages (i.e., 5 mg Al3+/L and 10 mg Fe3+/L), all coagulants could significantly mitigate the membrane fouling, with the hydraulically reversible and irreversible fouling resistance reduced by over 90% and 65%, respectively. The reduction in AOM fouling of the membrane was primarily due to the effective removal of the very high MW biopolymers by the coagulants. Both UV/H2O2 and the coagulation with ACH achieved 90% fouling reductions. However, UV/H2O2 treatment of the AOM produced more low MW compounds, resulting in less flux recovery compared with ACH coagulation due to the enhanced entrapment of these small molecules in the membrane pore structure. Nevertheless, UV/H2O2 treatment could completely remove the cyanobacterial toxin produced by M.aeruginosa, Microcystin-LR (15 µg L-1), spiked into the feedwater, whereas ACH was ineffective for removing this compound.