Optimization of electrokinetic pretreatment for enhanced methane production and toxicity reduction from petroleum refinery sludge

Abstract

This study examined the effect of electrokinetic pretreatment on petroleum sludge (PS) released from the wastewater treatment plants of petrochemical industries for enhanced biodegradation and contaminant removal. The application of electric field on PS through direct current is optimized with the combined variation of applied voltage (40–80 V), exposure duration (20–120 min) and distance between graphite electrodes (8–16 cm) using central composite design-response surface methodology (CCD-RSM). The optimization study revealed significant interaction among the response variables to obtain an optimum condition (60 V, 83.5 min, 11.6 spacing) for maximization of solubilization in terms of soluble chemical oxygen demand (230% increment against untreated) and volatile fatty acids (172% increment against untreated) concentrations for accelerated hydrolysis of complex PS. BMP batch assays were performed at different inoculum and sludge ratios (0.3, 0.4, 0.5 and 0.7) based on volatile solids content after pretreatment at the optimized condition which resulted in accumulated methane ranging from 5.16 to 6.61 L/gVSadded (untreated - 3.9 L/gVSadded). The mixing ratio of 0.4 showed the maximum methane enhancement of 69.2% compared to untreated. The maximum removal of organic content (62.8%), oil and grease (71.74%), and total petroleum hydrocarbon (52.9%) were also observed for the mixing ratio of 0.4. The FTIR study showed the efficacy in hydrocarbon dissociation and decomposition after pretreatment of PS. The net energy gain (3508 kJ) and phytotoxicity reduction of batch digestate after the anaerobic digestion suggest the economic feasibility and decontamination efficiency of the electrokinetic pretreatment technique respectively. Further research could be performed to evaluate the viability of this pretreatment for enhanced methane recovery at field-scale levels to relate to these lab-scale postulations.