Treatment of secondary effluent using a three-dimensional electrode system: COD removal, biotoxicity assessment, and disinfection effects

•Secondary effluent of mixed industrial wastewater is treated by 3D electrode system.•Kinetic analysis shows that CODCr degradation follows pseudo-first-order kinetics.•Wastewater toxicity increases after treatment due to active chlorine generation.•Three-dimensional electrode system shows good disinfection effects simultaneously.

A three-dimensional electrode system was used to treat secondary effluent from industrial wastewater. The removal efficiency for chemical oxygen demand (CODcr) in the three-dimensional electrode system was about 50%, while it was 20% in a two-dimensional electrode system. Energy consumption after 60 min of treatment at a current density of 5 mA cm−2 is about 180 kW h kg−1 CODcr, which is much lower than that for the two dimensional electrode system (300 kW h kg−1 CODcr). Kinetic analysis shows that the abatement of organics follows apparent first-order reaction. For the low values of the current density (lower than 7.5 mA cm−2), the abatement of CODcr is mainly influenced by the current density, suggesting an electron oxidation reaction control process. However, a mass transfer limitation occurs at high current density, since the abatement dose not depend on the current density but on the flow dynamic pattern. Acute biotoxicity of the wastewater has been investigated and is found to increase rapidly (more than 95% inhibition) after 30 min of treatment. Further analysis using the three-dimensional fluorescence spectra shows no more toxic organics generation by electrochemical treatment, suggesting that the acute biotoxicity is caused primarily by the free chlorine generated. Furthermore, the system shows good disinfection effects by 3-log reduction of colony-forming units after 30 min of treatment.