Simultaneous removal of chemical oxygen demand, turbidity and hardness from biologically treated citric acid wastewater by electrochemical oxidation for reuse

In this study, we investigated the use of an electrochemical oxidation process to simultaneously remove chemical oxygen demand (COD), turbidity, and hardness from biologically treated citric acid wastewater that was non-biodegradable and contained high concentrations of inorganic salts. We employed a Ti/RuO2–IrO2 anode system to analyse the effects of electric voltage and initial chloride concentration on COD, turbidity, and hardness removal as well as on energy consumption. Under optimal conditions (electric voltage of 9 V, initial chloride concentration of 3000 mg L−1), organic pollutants were almost completely mineralised after 60 min of electrolysis. The resulting COD (6 mg L−1) and turbidity (3 NTU) of effluent was within the Water Reuse Standard of China (COD < 30 mg L−1, turbidity < 5 NTU) while energy consumption (Es) and general current efficiency (GCE) were 99 kW h kg COD−1 and 30%, respectively. We also found that electric voltage and initial chloride concentrations increased the rate of hardness removal. Further investigation revealed that organics were mainly degraded by electrochemical oxidation of humic and fulvic acid-like substances, and hardness was primarily removed via the production of CaMg(CO3)2 precipitate on the cathode surface.

► We analysed electrochemical oxidation of biologically treated citric acid wastewater.
► This process simultaneously and effectively removed COD, turbidity, and hardness.
► Humic and fulvic acid-like substances were eliminated from wastewater.
► After electrolysis treatment, the effluents meet the requirements of water reuse standard of China.