Microcystis aeruginosa -laden surface water treatment using ultrafiltration: Membrane fouling, cell integrity and extracellular organic matter rejection

Despite its superb separation performance, ultrafiltration (UF) still faces challenges in treating the Microcystis aeruginosa-laden water of lakes or reservoirs, due to membrane fouling and poor rejection of soluble organics. In this work, to better understand the mechanisms of membrane fouling, cell breakage and organic rejection and their mutual influence, a comparative UF experiment was conducted under a variety of transmembrane pressures (TMPs, 50-250 kPa) with lab-cultured Microcystis aeruginosa. Membrane fouling was characterized with respect to flux decline and fouling reversibility, and cell breakage during UF filtration was evaluated using a flow cytometer. Moreover, the rejection of extracellular organic matter (EOM) by UF was investigated with respect to the dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV254) and microcystin-LR (MCLR). The results indicated that the accumulation of Microcystis cells and EOM on the membrane surface caused serious reversible fouling that substantially aggravated with the increasing TMP and was successively governed by pore blocking and cake filtration. The cell breakage during filtration was less than 5% and mainly occurred in the cake layer due to hydraulic shear, but the breakage did not substantially vary with increasing TMP. EOM removal by UF ranged from 40% to 70% (in terms of DOC removal), and the removal performance increased with the reversible resistance, implying a trade-off between organic removal and permeability. Regarding soluble and small organics such as MCLR, a higher degree of removal was also found at higher TMP, despite of some variations over the duration of the filtration tests, and the cake layer retention proved to be the principle removal mechanism, especially during steady filtration stages.