Treatment of soluble coffee industrial effluent by electro-coagulation–electro-oxidation process: multiobjective optimization and kinetic study.

This study examines the integrated electro-coagulation–electro-oxidation (EC–EO) process as an alternative for the treatment of an industrial effluent originating from a soluble coffee production. Experiments were carried out in a laboratory-scale batch cell reactor. Monopolar configuration of electrodes was employed (EC: anode: iron and cathode: stainless steel; EO: anode: boron-doped diamond-Si and cathode: stainless steel), operating in galvanostatic mode. The effect of main operation parameters on decolorization, chemical oxygen demand (COD) and total organic carbon (TOC) degradation and total operation costs was studied. The experimental design, analysis of variance, response surface methodology and constrained optimization algorithms let to establish the most effective operation conditions as follows: for EC (support electrolyte = 4.32 g of Na₂SO₄/L; current density (J) = 150 A/m²; gap between electrodes = 3 mm; stirring velocity = 350 RPM; electrolysis time (t) = 60 min; and EC's initial pH (pH_{i, EC}) = 4.71 (the pH of raw wastewater)); and for EO (J = 300 A/m²; t = 100 min; gap between electrodes = 3 mm; stirring velocity = 125 RPM; and pH_{i, EO} = 7.0). The integrated EC–EO process let to accomplish ca. 98% decolorization, ca. 73% of both COD and TOC removal, more oxidized (Average Oxidation State = 0.149), biocompatible (BOD₅/COD ratio = 0.404) and less toxic effluent (median lethal concentration of an analyte which causes 50% population death, LC₅₀ lower than 126.43 ppm of raw wastewater), generating 3.782 g/L of sludge, with total operation costs of 8.43 USD/m³. During EC, contaminants with molecular weight (MW) higher than 30 kDa were effectively decomposed. The integrated EO process additionally let to break down pollutants with MW in the range of 5 to 10 kDa. The EC–EO total operation time of 160 min. was required to meet Colombian permissible discharge limits. [ABSTRACT FROM AUTHOR]