A kind of amphoteric chitosan-based flocculant (quaternized carboxymethyl chitosan, denoted as QCMC) has been prepared. QCMC presented significant improvement of water solubility in the whole pH range. The effects of pH, dosage, temperature and original turbidity of algal water on the flocculation performance were investigated. The optimal dosages of QCMC at pH 5, 9 and 12 with original turbidity of 20 NTU at 20 degrees C were 0.1, 0.6 and 2.0 mg/L, respectively, which were much less than that of chitosan, PAM, Al-2(SO4)(3) and FeCl3. The floc properties during grow, breakage and regrow period were also evaluated at different pH values in terms of floc size, strength and density. It was demonstrated that QCMC produced larger, stronger and denser flocs than Al-2(SO4)(3). There is every indication that QCMC is more suitable for algal harvesting than other traditional coagulants or flocculants. (C) 2014 Elsevier Ltd. All rights reserved.