Effects of intracellular/dissolved ratios of microcystin-LR on its removal by ultrafiltration

Microcystins (MCs) are dangerous toxins produced by cyanobacteria in eutrophic waters that are increasingly used worldwide for human consumption after potabilization.In this study, we present the results of laboratory and pilot-plant experiments, aimed at deepening the knowledge of the mechanisms governing the equilibrium of dissolved and particulate-bound MCs, and exploring the possible use of ultrafiltration (UF) for their removal from eutrophic water within the potabi- lization treatment. Firstly, we analysed the presence of MCs in filtered water after three months of pure culture of Microcystis aeruginosa and different cycles of freezing and thawing, showing an increase of dissolved MCs due to cell breakage caused by the refrigeration cycles. Secondly, we performed filtration tests in a pilot UF plant, using samples of demineralised water and raw water from a eutrophic lake, both spiked with MCs. The tests demonstrated the possibility to remove MCs by adsorption rather than by mechanical seizing. The tests with demineralised water denoted a tendency to desorption after reaching the adsorption equilibrium, an adsorption isotherm less effective than the one observed in earlier lab-scale studies, and the need for very well controlled chemical washings to clean the membranes. Conversely, the raw lake water tests showed a higher removal efficiency, allowing to reach a final concentration of less than 1 μg/l, and a tendency to maintain the removal efficiency for longer cycles. These results allow us to discuss the role of the adsorption-release process on the efficiency of the UF process, and suggests the exploration of non- conventional operating rules aimed at maximizing the removal of MCs by UF.