The electrochemical oxidation of biologically treated citric acid wastewater in a continuous-flow three-dimensional electrode reactor (CTDER)

•A continuous-flow three-dimensional electrode reactor (CTDER) was constructed.•COD from the citric acid wastewater was effectively removed in CTDER.•The COD removal rate in CTDER was 1.7 times that of the continuous 2D reactor.•Aeration in the CTDER had a negative effect on COD removal.•Organics from wastewater were degraded primarily via indirect oxidation reaction.

In this study, we constructed a continuous-flow three-dimensional electrode reactor (CTDER) to remove organic compounds from biologically-treated citric acid wastewater. We analyzed the effects of the residence time, current density, initial pH, and airflow rate on the reactor’s energy consumption (Es), general current efficiency (GCE), and ability to remove the chemical oxygen demand (COD). We determined that the optimal operation conditions for the reactor included a residence time of 5 min, a current density of 40 mA cm−2, and an initial pH of 5. Under these conditions, we determined that 55.7% of COD was removed. The resulting COD of the effluent was within the National Discharge Standard of China (COD < 100 mg L−1), and the Es and GCE were 274.8 kW h kg COD−1 and 16.2%, respectively. We also found that aeration in the CTDER negatively affected COD removal due to the short-circuiting of the fluid. Further investigation revealed that the CTDER could produce a steady effluent quality where the organic compounds were degraded primarily via the indirect electrochemical oxidation of humic acid-like substances in the citric acid wastewater. Our results demonstrate that the CTDER is a highly efficient system for the electrochemical treatment of biologically-treated citric acid wastewater.