Cationic xylan- (2-methacryloyloxyethyl trimethyl ammonium chloride) polymer as a flocculant for pulping wastewater

•Cationic xylan-DMC was prepared in an aqueous solution and used as a flocculant.•Significant reductions in COD, BOD, TOC and organics of effluent were observed.•The changes in hydrodynamic size and chord length of formed flocs were determined.•The sedimentation of formed flocs was analyzed quantitatively. In this work, pulp mill wastewater was treated with a novel polymer flocculant, which was synthesized through polymerizing (2-methacryloyloxyethyl) trimethyl ammonium chloride (DMC) and xylan. Xylan-DMC polymer removed 94.5% of turbidity, 61.7% of chemical oxygen demand (COD), 45.0% of lignin, 65.7% of sugar and 73.5% of biological oxygen demand (BOD) from the wastewater at the polymer concentration of 500 mg/L. The flocculation mechanism was fundamentally assessed with determining hydrodynamic size and chord length of flocs in the wastewater at different dosages of xylan-DMC polymer in the system. By adding polymers, the number of flocs with a small chord length (10–50 μm) decreased, while that with a large chord length (150–300 μm) increased, indicating the small particles agglomerated via bridging induced by the polymer. The sedimentation of formed flocs was quantitatively investigated by a vertical scan analyzer and the results depicted that the flocs could settle readily from the system.