Sequential Combination of Electro-Fenton and Electrochemical Chlorination Processes for Treatment of Anaerobically-Digested Food Wastewater.

A two-stage sequential electro-Fenton (E-Fenton) oxidation followed by electrochemical chlorination (EC) was demonstrated to concomitantly treat high concentrations of organic carbon and ammonium nitrogen (NH4+-N) in real anaerobically-digested food wastewater (ADFW). The anodic Fenton process caused the rapid mineralization of phenol as a model substrate through the production of hydroxyl radical as the main oxidant. The electrochemical oxidation of NH4+ by a dimensionally stable anode (DSA) resulted in temporal concentration profiles of combined and free chlorine species that were analogous to those during the conventional breakpoint chlorination of NH4+. Together with the minimal production of nitrate, this confirmed that the conversion of NH4+ to nitrogen gas was electrochemically achievable. The monitoring of treatment performance with varying key parameters (e.g., current density, H2O2 feeding rate, pH, NaCl loading, DSA type) led to the optimization of two component systems. The comparative evaluation of two sequentially-combined systems (i.e., the E-Fenton/EC system versus the EC/E-Fenton system) using the mixture of phenol and NH4+ under the predetermined optimal conditions suggested the superiority of the E-Fenton/EC system in terms of treatment efficiency and energy consumption. Finally, the sequential E-Fenton/EC process effectively mineralized organic carbon and decomposed NH4+-N in the real ADFW without external supply of NaCl.