Sequential "electrochemical peroxidation - Electro-Fenton" process for anaerobic sludge treatment

In this study, we present a sequential electrochemical process for integral treatment of anaerobic sludge, combining for the first time electrochemical peroxidation (ECP) and electro-Fenton (EF). In the first step, ECP (consisting of H₂O₂-assisted electrocoagulation with Fe electrodes) was applied as a conditioning and stabilizing method, whose synergistic electrocoagulation/Fenton oxidation effects considerably reduced the COD, TOC and total suspended solids (TSS) by 89.3%, 75.4% and 85.6%, respectively, under optimized conditions (initial pH of 5, [H₂O₂]/[Fe²⁺] dose ratio of 5, 15.38 mA cm^-2 and 2 h treatment). Furthermore, total coliforms were completely killed within the first hour of treatment. In the second step, EF was successfully applied to mineralize the remaining organic fraction in the liquid effluent after dewatering, achieving 91.6% and 87.2% of COD and TOC removal, respectively, after 4 h of treatment under optimal conditions (pH 3 and 25 mA cm^-2), while almost total COD and TOC removal was attained in 8 h. The Fe sludge generated at the end of the ECP treatment was easily dewatered by filtration and 20.9 g of nutrient-rich dry sludge were produced. The overall cost of the ECP-EF treatment was S$ 0.05 L^-1 sludge. The combined effects of coagulation and Fenton oxidation during ECP revealed that the treatment efficiency is strongly dependent on the rheological properties of the sludge sample.