Biodegradability enhancement of waste lubricating oil regeneration wastewater using electrocoagulation pretreatment

As a sustainable management of fossil fuel resources and ecological environment protection, recycling used lubricating oil has received widespread attention. However, large amounts of waste lubricating-oil regeneration wastewater (WLORW) are inevitably produced in the recycling process, and challenges are faced by traditional biological treatment of WLORW. Thus, this study investigated the effectiveness of electrocoagulation (EC) as pretreatment and its removal mechanism. The electrolysis parameters (current density, initial pH, and inter-electrode distance) were considered, and maximal 60.06% of oil removal was achieved at a current density of 15 mA/cm², initial pH of 7, and an inter-electrode distance of 2 cm. The dispersed oil of WLORW was relatively easily removed, and most of the oil removal was contributed by emulsified oil within 5-10 μm. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that effective removal of the biorefractory organic compounds could contribute to the improvement of biodegradability of WLORW. Thus, the 5-day biochemical oxygen demand/chemical oxygen demand ratio (BOD₅/COD) was significantly enhanced by 4.31 times, which highly benefits future biological treatment. The routes of WLORW removal could be concluded as charge neutralization, adsorption bridging, sweep flocculation, and air flotation. The results demonstrate that EC has potential as an effective pretreatment technology for WLORW biological treatment.