Insights into real industrial wastewater treatment by Fenton's oxidation in gas bubbling reactors

In the present study, bubbling reactors (BRs) were chosen to design a new procedure for real industrial wastewater (WW) treatment by Fenton's oxidation. The process was carried out in BRs under batch mode for the treatment of a WW with a high organic load (chemical oxygen demand (COD) above 7000 mgO₂/L), being the efficient mixing of the liquid phase ensured by the gas bubbling. The parameters that influenced the WW treatment (i.e., H₂O₂ and Fe²⁺ concentration, and initial pH) were optimized in a smaller BR (0.5 L volumetric capacity); the maximum oxidation efficiency (dissolved organic carbon (DOC) removal = 52% and COD removal = 83% after 60 min) was reached under the following conditions: Q_air = 1.0 L/min (measured at room temperature and atmospheric pressure), [H₂O₂] = 22.5 g/L, [Fe²⁺] = 0.75 g/L, and pH = 4.6 - original WW pH. It was not detected any significant effect in the process efficiency of the air flow rate and gas phase composition (i.e., N_2, and air), but when the process was performed with continuous O₂ bubbling an increase in the DOC removal (from 43% to 53%) was observed after 5 min of oxidation. Even so, the high costs discourage the use of pure oxygen streams in real WWTPs. To understand the dynamics of the process, the continuous air bubbling was compared to another mixing mode (mechanical stirring), and similar mineralization was achieved, proving the feasibility of Fenton's process in a BR. In addition, the gas bubbling proved to be more efficient in terms of heat dissipation during the treatment, decreasing temperature profiles along the oxidation of heavily charged real effluents. An effective scale-up with a bubble column reactor with a higher volumetric capacity by a factor of almost one order of magnitude was also proved, providing similar mineralization. The final effluent was non-toxic and more biodegradable.