Electrochemical Degradation of Pulp and Paper Mill Wastewater. Part 1. COD and Color Removal

The electrochemical degradation of agri-based paper mill wastewater (black liquor) was investigated in a 2 dm^3 electrolytic batch reactor using iron plate electrodes. Of the four-, six-, and eight-plate configurations, a current density of 55.56 A/m^2 at neutral pH with a six-plate arrangement was found to be optimal, achieving a maximum chemical oxygen demand (COD) and color removal of 80% and 90% (175 platinum-cobalt units (PCU)), respectively. The chemical dissolution of iron was strongly influenced by pH0. Electrochemical treatment at higher pH0 (pH0 >=9) increases the dissolution of iron electrodes by an order of magnitude. At the optimal current density, the iron electrode consumed is 31.27 g/m^2.h, achieving maximum COD removal. An increase in salinity reduces the treatment time significantly, and the sludge settling characteristics also improve. The addition of polyacrylamide (10 mg/dm^3) to the electrochemical reactor enhances the COD removal rate with a very short treatment time with excellent sludge settleability. Specific energy consumption (SEC) reduces from 6.64 to 5.73 kWh/kg of COD removed with the addition of NaCl (625 mg/dm^3). The posttreatment of electrochemically treated wastewater by chemical coagulation using alum (360 mg/dm^3) along with 20 mg/dm^3 polyacrylamide (PAA) further reduced COD values to <180 mg/dm^3 and a near 100% (<5 PCU) color removal. An overall COD removal of 91% and color removal of near 100% could be achieved by electrochemical treatment followed by coagulation/flocculation.