Refractory oil wastewater treatment by dissolved air flotation, electrochemical advanced oxidation process, and magnetic biochar integrated system

Abstract

Petrochemical and food processing industries generate environmentally hazardous wastewaters that contain oil, volatile organic compounds (VOCs), and heavy metals. Such non-biodegradable contaminants require a complicated treatment process before disposal owing to their heterogeneous components. However, the conventional treatment process has relatively low removal efficiency and produces secondary pollutants. Here, we developed a hybrid treatment system with a high removal efficiency of suspended solids (SS), VOCs, and heavy metals during the pre-, main-, and post-treatment steps, respectively. In the pretreatment, dissolved air flotation with a modified slit nozzle, which generates size-tunable micro/nano-sized bubbles, removed 93.5% of the chemical oxygen demand (COD). The increase in the size of the bubbles in the range of 2–20 μm led to the effective removal of oil particles with a size comparable to that of the bubbles even with a low amount of coagulant. In the main treatment process, VOCs were degraded by an electrochemical advanced oxidation process (EAOP). Boron-doped diamond (BDD) electrodes were capable of degrading the non-degradable compounds 1, 4-dioxane, dichloromethane and tetrachloroethylene without generating sludge, compared to the low removal efficiency of the single and composite catalyst-doped dimensionally stable anodes. Due to the wide potential window, BDD was capable of generating hydroxyl radical. Such strong oxidant was the main factor in the high degradation efficiency. The environmentally friendly hybrid system was applied to the food and laundry wastewater treatment. In the pretreatment process, 99.7% of the oil was removed, and 95% of the COD was removed by the dual BDD–DSA electrodes in the EAOP. In the post-treatment process, more than 90% of heavy metals, including Cu, As, and Pb, were removed by a magnetic biochar adsorption process, and 90% of the biochar was recovered after the process.