Coupling Of Anode And Cathode Reactions In(Bio)Electrochemical Systems For Simultaneous Removal Of Carbon And Nitrogen From Coking Wastewater

Coking wastewater is considered to be one of the most complicated and difficult-to-treat wastewaters,characterized by high levels of chemical oxygen demand（COD）,elevated concentrations of ammonium and severe toxicity.The commonly used biological methods for coking wastewater treatment faces the outstanding challenge of the low efficiency of carbon and nitrogen removal,because of the low biodegradability that is caused by the presence of high-concentration phenols,cyanides,ammonia,and biorefractory substances in the coking wastewater.To meet the discharge standard,the traditional process for coking wastewater treatment often comprises a series of biological reactors that sequentially remove organic compounds and nitrogen,and requires considerable consumption of alkali reagents used to neutralize the excess protons produced during nitrification and other aerobic oxidation processes;this substantially increases the construction and operation costs.In recent years,electrochemical wastewater treatment technology has attracted much attention because of its advantages including high reaction rate and efficiency,easy handling and operation,and avoidance of secondary pollution.The electrochemical method is usually composed of bioelectrochemical wastewater treatment and electrochemical catalytic wastewater treatment.To overcome the bottlenecks of the traditional process,in this study,we attempt to propose concepts of using novel electrochemical strategies including microbial fuel cell（MFC）technology（a typical bioelectrochemical method）and alternatively the combination of electro-oxidation（EO）and electrocoagulation（EC）processes,to achieve simultaneous carbon and nitrogen removal without the amendment of extra alkali reagents.First,we report that the utilization of a single MFC reactor consisting of an anaerobic anode and an aerobic cathode enables simultaneous nitrification and denitrification,and accordingly allows efficient carbon and nitrogen removal with zero alkaline consumption.The MFC exhibited faster reaction kinetics for COD and total nitrogen（TN）removal than the same configured reactor analogous to the traditional aerobic biological reactor（ABR）.At a hydraulic retention time（HRT）of 125 h,the efficiencies of COD and TN removal in the MFC reached 83.8±3.6 and 97.9±2.1,respectively,much higher than the values of 73.8±2.9 and50.2±5.0 obtained in the ABR.Furthermore,the degradation in the MFC of the main organic components,including phenolic compounds（such as phenol,2-methylphenol,3-methylphenol,4-methylphenol,and 2,4-dimethlyphenol）and nitrogenous heterocyclic compounds（such as quinolone,pyridine,indole,and isoquinolone）was greater than that in the ABR.The enhancing effect was attributed to the self-pH adjustment capability of the MFC system.It was also manifested by the increased abundances of heterotrophs,nitrifiers,and denitrifiers enriched in the MFC.The correlations between the current density and the rates of COD and TN removal suggested that the extent of the current from the anode to the cathode was a critical parameter for the overall performance of MFCs in the coking wastewater treatment.Secondly,we demonstrate that the integrated EO-EC system outperforms the single EO or EC reactor in terms of higher efficiencies of COD and nitrogen removal from the artificial wastewater.The results showed that the EO-EC system enabled 68.4±2.7%of COD and92.0±0.7%of total nitrogen removal(operation conditions:PbO₂ anode,2 mA cm^-2;Fe anode,1 mA cm^-2;reaction time:6 h),substantially larger than those obtained from the single EO reactor（43.4±3.1%%;64.3±1.4%）and EC reactor（19.6±2.9%;19.6±2.1%）.The preliminary mechanistic study indicated that phenol and thiocyanate（the main COD contributors）and ammonium were removed primarily via direct oxidization at the PbO₂ electrode.The indirect oxidation promoted by the chlorine active compounds generated in the system also makes a partial contribution to the conversion of ammonia nitrogen.The roles of the Fe anode in the integrated system may include the generation of coagulants for contaminant removal,the adjustment of electrolyte pH favorable for the oxidation reactions,and the generation of Fe（II）reacting with hypochlorite to produce radicals.The energy consumption analysis suggested that the integrated EO-EC system required less energy for carbon and nitrogen removal as compared to the individual EO or EC reactor.It was also revealed that the specific energy consumption was closely linked with the extent of currents applied to the Fe and PbO₂anodes.