Enhancing integrated removal of Microcystis aeruginosa and adsorption of microcystins using chitosan-aluminum chloride combined coagulants: Effect of chemical dosing orders and coagulation mechanisms

• Two dual-coagulants and a novel composite were prepared using CTS and AC.
• CTS combined AC dual/composite coagulants caused no damage to M. aeruginosa cells.
• CTSAC obtained best cell removal for stronger entrapment and bridging ability.
• The first addition of AC aids the MCs adsorption of CTS.
• CTSAC (2.6 mg/L CTS and 7.5 mg/L AC) has desired EOM removal ability.

The effects of two chitosan (CTS)-aluminum chloride (AC) dual-coagulants with different dosing orders and a composite coagulant CTSAC (CTS and AC premixed) on removal of Microcystis aeruginosa (M. aeruginosa) cells and second metabolites in cyanobacteria-laden drinking water treatment were investigated, and in comparison with AC and CTS. Response surface methodology (RSM) was used to model and optimize the coagulation processes. The coagulation mechanisms were investigated via zeta potential, scanning electron microscope (SEM), and three-dimensional excitation-emission matrix spectra (EEM) analyses. The investigated dual/composite coagulants caused no evident damage to M. aeruginosa cells, and achieved better removal effects than coagulant individually applied. The CTSAC removed cells was the most effective for stronger entrapment and bridging ability. While the highest adsorption of extracellular microcystins (MCs) was obtained through AC–CTS (AC added firstly). CTS–AC (CTS added firstly) provided slight higher extracellular organic matter (EOM) removal. The optimal coagulation performance was obtained when CTSAC was set as 2.6 mg/L CTS plus 7.5 mg/L AC, under which 97.8% of intact cells, 53.08% of extracellular MCs and almost all EOM were simultaneously removed. Overall, the combination of CTS and AC coagulation process has proven to be a better substitute for conventional coagulation in cyanobacteria-laden drinking water treatment.

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