Continuous electrocoagulation treatment of pulp and paper mill wastewater: operating cost and sludge study

The present research deals with the treatment of agri-based pulp and paper mill wastewater by continuous electrocoagulation (CEC) process using iron (Fe) as an electrode material. Effects of flow rates (dm(3) h(-1)): 0.5-4.0 and residence time (tau): 0.5-4.0 h were investigated on degradation of chemical oxygen demand (COD), color, total solid (TS), turbidity, specific energy consumption (SEC), instantaneous current efficiency (ICE) and electrochemical degradation index (EDI). At flow rates of 1.0 and 0.5 dm(3) h(-1), COD removal efficiency of 78.20 and 82.15%; and color removal efficiency of 79 and 90%, respectively, was achieved. TS concentration of wastewater slurry was also reduced by 65% after 3 h residence time with flow rate of 1 dm(3) h(-1). The specific energy consumption (kW h per kgCOD(removed)) was decreased from 16.3 to 14.3 with decrease in tau from 4 to 1 h. At a supply charge concentration of 0.62 A h dm(-3), the current efficiency (CE) values were 310% and 274% after tau = 2 and 1 h, respectively. Dissolution and consumption of electrodes were also studied with a change in flow rates. Sludge obtained after the CEC process was analyzed for settling and filterability characteristics, morphology and elemental analysis, point of zero charge, physicochemical and elemental characterization, and TS concentration. The operating cost of the process was also calculated based on the electrical energy and electrode consumption and was found to be 61.0 Indian Rupees (0.9 USD) for the treatment of 1 m(3) of wastewater.