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

摘要

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 prctreatment 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/IrO_2-Ta_2O_5 and Ti/IrO_2-SnO_2-Sb_2O_5) were prepared by the Pechini method, obtaining homogeneous coatings with uniform composition and high roughness. The OH formed at the DSA surface from H_2O oxidation were not detected by electron spin resonance. However, their indirect determination by means of H_2O_2 measurements revealed that TiyTrO_2-SnO_2-Sb_2O_5 is 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/IrO_2-Ta_2O_5 anode was evaluated from the TOC removal and the UV/Vis absorbance decay.The mineralization was very poor in 0.05 M Na_2SO_4, whereas it increased considerably at a greater Cl~ content, meaning that the oxidation mediated by electrogenerated species such as Cl_2, HC1O, and/or C1O~ 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).