A bioflocculant, quaternized carboxymethyl chitosan (QCMC), was developed by the quaternization of N,O-carboxymethyl chitosan (N,O-CMC) and characterized by FUR, H-1-NMR, GPC, and potentiometry. The efficiency of the removal of chemical oxygen demand (COD) in printing wastewater by this flocculant was further reported. Results indicated that the capacity of QCMC to remove the COD from tested wastewater was the best one among the investigated flocculants. The pH had great influence on this capacity and the suitable pH for QCMC to treat the tested wastewater was about 5.0. The utilization of aid-flocculant, especially bentonite, could improve this capacity obviously, and the increase of mass ratio of bentonite to QCMC resulted in the increase of the capacities of complex flocculant to remove the COD from the tested wastewater. When the mass ratio of bentonite to QCMC was 40, pH of wastewater was 5.0 and amount of complex flocculant in the wastewater was from 2500 to 3142 mg L-1, the removal ratio of COD was more than 80%. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 299-305, 2010