Removal of Microcystis Aeruginosa by oxidation-assisted coagulation: Effect of algogenic organic matter fraction changes on algae destabilization with Al hydrates

Abstract

Oxidation-assisted coagulation has been widely applied to destabilize algae and improve algae removal through a flocculation-sedimentation process in drinking water treatment plants (DWTPs). However, oxidation poses a potential threat to drinking water safety with increasing algal organic matter (AOM) in the algae-polluted surface water. This study investigated the effect of sodium hypochlorite (NaOCl) oxidation-assisted polyaluminum chloride (PACl) coagulation on algae removal, along with the identification of the optical properties for AOM fractions. The results have demonstrated that the increased NaOCl oxidant dose coupled with PACl coagulation induced by sweep flocculation is more effective in Microcystis aeruginosa (MA) cells destabilization and removal, where the highest algae removal rate around 99 %. However, NaOCl oxidation coupled with PACl coagulation fails to reduce the dissolved organic carbon (DOC) in MA suspension attributed to the released AOM. Strong free chlorine destroys MA cells in response to the increases in humic acid-like (HAL) and fulvic acid-like (FAL) substances, while it is sufficient to degrade soluble microbial product-like (SMPL) and aromatic protein-like (APL) substances. In oxidation-assisted coagulation, bigger diameter (483–640 μm) with lower fractal dimension (1.31–1.39) of M. aeruginosa flocs are formed at low chlorine dosing ratio (Cl2:DOC = 1:1), while higher fractal dimension (2.39–2.75) with smaller diameter (143–395 μm) is reached at high chlorine dosing ratio (Cl2:DOC = 3:1). It is concluded that the changes in fluorescent organic matter fraction during chlorination dominantly affect coagulation performance for AOM removal in oxidation-coagulation process.