Development of flotation and nanofiltration technologies to remove cyanobacteria and cyanotoxins in drinking water treatment

Tese dout., Ciências e Tecnologias do Ambiente, Universidade do Algarve, 2006Dissolved air flotation (DAF) and nanofiltration were optimised and integrated to remove cyanobacteria and cyanotoxins from drinking water. The removal mechanisms of the most commonly occurring cyanobacteria (cultured cells and aggregates of Microcystis aeruginosa and Plankthotrix rubescens filaments) and cyanotoxins (hepatotoxic microcystins, and neurotoxic anatoxin-a) were investigated, as well as the impact of the water background organic (NOM) and inorganic matrixes, using both model and natural waters. Results showed that coagulation/flocculation/DAF is the best process for clarifying cyanobacterial-rich waters (93-99% chlorophyll a removal), without toxin release to water and with lower NOM effect, using low recycle (8%), and lower coagulant doses, slower coagulation, stronger but shorter flocculation than the conventional coagulation/flocculation/settling. Studies with a negatively charged tight nanofiltration membrane demonstrated that while nanofiltration fluxes of low NOM and moderately hard water are largely influenced by the background pH and calcium hardness, rather than by the type of NOM, anatoxin-a and microcystins are efficiently removed (anatoxin-a by electrostatic interactions and steric hindrance, microcystins mainly by steric hindrance), producing a final water of superior quality, also in terms of NOM content, regardless of the variations in feed water quality (pH, calcium hardness, NOM, toxins) and water recovery rate (up to 90%)