Technical evaluation and economic optimisation of coagulation-flocculation process for the pre-treatment of over-reused effluent of paper mills in cardboard recycling industry.

The aim of this study is to evaluate the performance of coagulation and flocculation (C/F) process and economic analysis for the advanced primary treatment in order to obtain a semi-closed water circuit in the pulp mills in the cardboard recycling industry. Since the high water quality level is not required in the pulpers, the dosages in this research were determined without pH changes or adjustment to decrease the capital and operational expenditures. The results showed that FeCl3 with the concentration of 2000 mg/L gave the best quality when coagulants were used solely. Moreover, using alum with the concentration of 1500 mg/L provided acceptable quality for the effluent re-circulation and the removal/reduction percentages of 42, 82, 71 and 97 were achieved for chemical oxygen demand (COD), total suspended solids (TSS), turbidity and colour, respectively. It was also found that the combination of coagulants did not improve the process performance significantly. The best combination was ferric chloride (500 mg/L) and alum (2000 mg/L) with the efficiency of 47% and 98% in terms of COD and colour removal/reduction, respectively, while the best couple in TSS removal was FeCl3 (500 mg/L) and CaCO3 (500 mg/L) with the efficiency of 88%. The chemicals coupled with cationic polyacrylamide (C-PAM) did not enhance the quality of pre-treated solutions when the settling time reduced up to 15 minutes. The cost analysis also showed that alum is the most cost-effective coagulant with the costs in the range of 0.06–0.12 $/kg removed TSS, 0.06 to 0.11 $/kg removed COD and 0.06–0.1 $/m3·reduced NTU whereas calcium carbonate is the economic coagulant regarding colour reduction with the cost of 0.003 $/m3·reduced unit and more than the reduction efficiency of 90%. [ABSTRACT FROM AUTHOR]