Impact of microcystis aeruginosa on membrane fouling in the treatment of a biologically treated effluent
Creators: name: Goh, Yong Tee
Publisher: name: RMIT University
Year: 2024

description: Membrane fouling by wastewater effluent organic matter remains a major limitation for water recycling, especially during an algal bloom. <i>Microcystis aeruginosa</i> is the dominant cyanobacterium in wastewater treatment plants around the world, particularly in the wastewater lagoons. The aim of this study was to determine the factors affecting the performance of the microfiltration of a biologically treated lagoon effluent containing <i>M. aeruginosa</i>, and to investigate the interactions between the foulants and the membrane with or without pre-treatments throughout the algal growth cycle. <br><br>For the lag growth phase, the presence of <i>M. aeruginosa</i> in the effluent had negligible impact on flux profile; full flux recovery was achieved with a low alum dose (1 mg Al<sup>3+</sup> L<sup>-1</sup>) and comparable results were achieved for aluminium chlorohydrate (ACH) treatment at the same Al<sup>3+</sup> dose. <br><br>For the exponential growth phase, the poorer flux profiles were attributed to the change in the type of organic matter and the increased presence of algal cells, rather than increase in DOC. At 5 mg Al<sup>3+</sup> L<sup>-1</sup>, ACH treatment gave better DOC reduction compared with alum. These coagulants removed different types of organic compounds as exemplified by the greater removal of fluorescent organic matter by alum, whereas ACH gave greater removal of non-fluorescent organic matter. Alum treatment gave a better flux profile compared with ACH treatment. Hydraulic cleaning of the membranes used for the alum-treated samples gave higher clean water fluxes compared with ACH-treated samples, indicating that the alum coagulum protected the membrane from internal fouling. For alum-treated samples, the majority of the foulants and aluminium hydroxide coagulum were removed by hydraulic cleaning, giving high flux recovery (98%). However, for the ACH-treated samples the foulants and coagulum were strongly attached to the membrane due to the polymerized nature of the ACH precipitate, causing difficulties in hydraulic cleaning, leading to lower flux recovery (63%) compared with alum.<br><br>For the stationary growth phase, the large increase in the cell and EOM content, and the change in the type of organic matter had a detrimental impact on the flux profiles compared with the earlier growth phases. Although coagulation (alum or ACH) improved the flux profiles during the stationary phase, a higher coagulant concentration was necessary to achieve a satisfactory amount of permeate, but this led to a severe drop in flux recovery. Increasing the alum dosage to 20 mg Al<sup>3+</sup> L<sup>-1</sup> improved the flux profiles due to the development of a protective coagulum on the membrane surface. In contrast to the exponential phase, surface analysis of the cleaned membranes for the alum-treated samples showed that the foulants and coagulum were recalcitrant, especially for samples with high EOM content, leading to the large reduction in flux recovery. This was shown to be due to excreted phosphorus-containing materials in the EOM-rich fraction reacting with the aluminium cations, impeding the hydrolysis of alum and causing strong affinity of foulant and the coagulum to the membrane.<br>