Sequential electrochemical treatment of dairy wastewater using aluminum and DSA-type anodes

Dairy wastewater is characterized by a high content of hardly biodegradable dissolved, colloidal, and suspended organic matter. This work firstly investigates the performance of two individual electrochemical treatments, namely electrocoagulation (EC) and electro-oxidation (EO), in order to finally assess the mineralization ability of a sequential EC/EO process. EC with an Al anode was employed as a primary pretreatment for the conditioning of 800 mL of wastewater. A complete reduction of turbidity, as well as 90 and 81 % of chemical oxygen demand (COD) and total organic carbon (TOC) removal, respectively, were achieved after 120 min of EC at 9.09 mA cm⁻². For EO, two kinds of dimensionally stable anodes (DSA) electrodes (Ti/IrO₂-Ta₂O₅and Ti/IrO₂-SnO₂–Sb₂O₅) were prepared by the Pechini method, obtaining homogeneous coatings with uniform composition and high roughness. The·OH formed at the DSA surface from H₂O oxidation were not detected by electron spin resonance. However, their indirect determination by means of H₂O₂measurements revealed that Ti/IrO₂-SnO₂–Sb₂O₅is able to produce partially physisorbed radicals. Since the characterization of the wastewater revealed the presence of indole derivatives, preliminary bulk electrolyses were done in ultrapure water containing 1 mM indole in sulfate and/or chloride media. The performance of EO with the Ti/IrO₂-Ta₂O₅anode was evaluated from the TOC removal and the UV/Vis absorbance decay. The mineralization was very poor in 0.05 M Na₂SO₄, whereas it increased considerably at a greater Cl⁻content, meaning that the oxidation mediated by electrogenerated species such as Cl₂, HClO, and/or ClO⁻competes and even predominates over the·OH-mediated oxidation. The EO treatment of EC-pretreated dairy wastewater allowed obtaining a global 98 % TOC removal, decreasing from 1,062 to <30 mg L⁻¹.