Integrated electrocoagulation and clarifier air flotation (EC-CAF) as a pretreatment method for wastewater treatment in the used oil refinery industry: a review of full-scale applications

This study assessed a full-scale electrocoagulation–chemical air flotation (EC–CAF) system for treating wastewater from used oil refining processes, which are characterised by high non-biodegradable pollutant concentrations. The system showed excellent removal of total suspended solids (TSS), with efficiencies ranging from 86.8% to 90.7% (averaging 88.6%). This performance resulted from the effective formation and flotation of iron hydroxide flocs. However, the removal of dissolved pollutants was limited, with average efficiencies of 12.5%, 7.6%, and 13.3% for chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), and 13.3% for sulphate (SO₄²⁻). These low rates are due to recalcitrant organic compounds and dissolved ionic species that were not effectively removed through coagulation–flotation mechanisms. Increasing the electrical input (2.06–3.60 kWh/m³) and current density enhanced floc formation and TSS removal but minimally affected dissolved contaminants. The Faraday efficiency ranged from 53% to 66%, indicating nonideal electrochemical conditions resulting from side reactions and electrode passivation. Sludge characterisation revealed amorphous iron-based flocs with a high adsorption capacity for PM. Although the EC–CAF system is effective as a pretreatment, it requires integration with advanced oxidation or biological processes to achieve comprehensive removal of dissolved pollutants in nonbiodegradable wastewater.