Application of an Electro-oxidation Treatment Method to Industrial Paper Mill Effluents in the Lab

The study was conducted in 2 stages to explore the effects of effluent variables and reaction mechanism of applying an electro-oxidative system to a typical industrial paper mill effluent. The first stage study aimed mainly to analyze the contributions of raw material (domestic and imported European old corrugated containerboard, OCC) components and chemical additives of the industrial paper process to the electrical conductivity built-up of the effluent. In the second stage, the primary (dissolved air flotation, DAF) treated effluent from an industrial paper mill was treated in our lab’s fixed-bed electro-oxidative reactors. The reactor had a total volume of 400 mL, contained 2 stainless steel (S 304) rods wrapped in permeable synthetic fiber membrane to serve as electrodes, and the cell was filled to 80~95% of its volume with iron beads. The effluent was recirculated in a semi-batchwise fashion. The pH was adjusted to 3.0, 6.8, and 9.0; and reaction time was between 0 and 120 min. At pH 6.8, a 23 factorial design using 2 levels each of 3 variables: hydraulic retention time (HRT) (57 and 180 s), electrode gap (5 and 15 mm), and electric current density (287 and 3454 A m-2) was studied. The effluent parameters investigated were electrical conductivity, COD and true color. The results indicate that both domestic and imported OCC raw materials contributed little to the electrical conductivity built-up of the effluent, while among the chemical additives, alum contributed the most to the conductivity built-up. As for electro-oxidative treatment results, HRT showed significant main effect on COD removal, whereas all main effects and interactions were insignificant on electrical conductivity reduction. As for color removal, both the main effects of HRT and the distance between electrodes had highly significant effects. The post-treatment effluent was analyzed using the cyclic voltammetry (CV) for the presence of redox pair, and Fe2+/Fe3+ redox was shown to exist. At pH 3.0, the treatment had no apparent efficacy, and increase in ferric ion concentration released from reactor medium led to increase in electric conductivity of the wastewater. At pH 9.0, on the other hand, COD removal of 28% and true color removal of 93.7% was achieved. However, in this case, all the main effects and interaction of electrode gap, current density were statistical insignificant. Our reactor design of a bench-top electro-oxidation unit could attain high current density, however, its pollutant removal efficacy was rather poor. Further remodeling and parametrizing of the unit is necessary.