Chitosan flocculation of cardboard-mill secondary biological wastewater

Flocculation is a common secondary treatment procedure for the removal of suspended solids, colloids and organic matter present in industrial wastewater. In the present study, the flocculation of cardboard industry wastewater, treated by a biological process in an aerated lagoon, was examined using commercial grade polyaluminium chloride (PAC) and chitosan (CHITO) dissolved in acetic acid as flocculating agents. A series of flocculation jar-tests was conducted under different conditions. The influence of the flocculant dosage and the temperature of the lagoon on the quality of the treated wastewater was investigated. Optimum temperature for PAC was in the range 13–21 °C for a dosage of 0.3–0.4 mL L−1 but the results were highly temperature-dependent; PAC lowered chemical oxygen demand (COD) by 40–45% and turbidity by 55–60%. With CHITO, the process was more efficient than with PAC for an effective dosage of 7 mL L−1 and no influence of temperature was observed. Chitosan lowered the COD by over 80% and turbidity by more than 85%. It generated bigger flocs making settling faster than with PAC. It also removed residual colour and led to a significant decrease in the amount of heavy metals present in the effluent.