Enhancement Of Denitrification And Antibiotics Removal From Simulated Municipal Wastewater By A Novel 3D-BER System

Previous studies on both conventional bioelectrochemical reactor（BER）and three-dimensional bioelectrochemical reactor（3D-BER）systems mainly focused on reactor design and the optimization of operating factors.The novel 3D-BER system is still in its early development stage,with only a few denitrification studies being reported in the literature,and even fewer studies have been found for antibiotic removal from wastewater treatment plants（WWTPs）effluent.However,antibiotics are used to prevent diseases and promote growth in the livestock industry.In China,the total quantity of antibiotics used was estimated to be approximately 162,000?tons,of which animals consumed 52%.In the United States,approximately 11,200?tons of 22700 antibiotics are prescribed for non-therapeutic purposes,primarily to promote the growth of swine,poultry,and cattle.According to statistics,sulfamethoxazole（SMX）is one of the most common antibiotics in urban domestic sewage,and ciprofloxacin（CIP）is widely found in wastewater from hospitals and pharmaceutical factories.Therefore,the removal of antibiotics from the wastewater treatment process has attracted worldwide attention.Bioelectrochemical technology is an effective approach to removing antibiotics through microbial metabolism and electrochemical redox reactions,which are being studied and optimized for boosting antibiotic removal and simultaneous denitrification in this thesis.The main research contents are summarized as follows:A 3D-BER system was successfully constructed with granular activated carbon（GAC）to eliminate antibiotic SMX from simulated municipal wastewater.It is coupled with heterotrophic and autotrophic denitrification processes to simultaneously remove nitrate and nitrite.The experimental results demonstrated the variation of microbial communities by screening electric current from 0 to 90 m A at p H 6.5±0.12 to 7.9±0.04.The p H variation in the effluent was used to assess the denitrification process first.The optimum p H of this reactor was in the range of 7.0±0.20 to 7.5±0.31,a lower or higher p H condition could affect the autotrophic and heterotrophic denitrification process.The CO₂ produced from the denitrification process at the anode could serve as a good p H buffer to automatically control the p H in the reaction zone.The effect of current intensity was also investigated.It was found that the low current intensity had little effect on degrading by-products but significantly decreased the SMX removal efficacy.The low current intensity also increased the rate of nitrate or nitrite elimination.At the current intensity of 0 m A,the nitrate removal efficiency and the SMX removal efficiency could only reach 59.1%and 43.1%,respectively.However,when the electric current intensity was raised to 60 m A,more than 90.50±4.76%of SMX was degraded.The strengthened bacterial metabolism and the continued electrical stimulation contributed to the simultaneously enhanced SMX and nitrate removal efficiency from simulated wastewater in the novel 3D-BER system.When the current intensity of 3D-BER was 60 m A,extending the accommodation period of the system,the NO₃^--N,NO₂^--N,NH₄⁺-N,total nitrogen（TN）and the SMX removal efficiency reached 96.5±2.4%,99.5±1.5%,88.5±1.4%,78.6±1.0%and90.5±4.8%,which was 69.5±2.5%,94.5±2.3%,72.5±1.6%,58.1±1.2%,and 45.5±2.2%significantly higher than the control system,respectively（p<0.05）.Moreover,the intermediate evolved by SMX biodegradation was proposed based on the analysis of HPLC and LC-MS/MS results.Besides,bacterial community analysis was also performed to reveal substantial changes in microbial communities at the phylum,class,and genus levels after SMX acclimatization.Gammaproteobacteria,Bacilli,Proteobacteria,and Thauera-a well-known aromatic-degrading dominant bacterium present in acclimatized conditions of 60 m A.In the 3D-BER system,when the influent COD/NO₃^--N ratio was in the range of 1.5 to3.5,both autotrophic and heterotrophic denitrifying microorganisms played an important role in denitrification.The experimental results demonstrated that the highest nitrate-N removal efficiency of 98.62%was achieved under the optimum COD/NO₃^--N ratio of 1.5 with an initial p H of 7.5±0.4.Moreover,the nitrate removal efficiency of 81.12%could also be obtained at the COD/NO₃^--N ratio of 3.5 with an initial p H of 8.2±0.3.And the batch denitrification processes always followed zero-order kinetics at all the obtained NO₃^--N concentrations.Subsequently,the removal of another antibiotic,CIP,was studied in the lab-scale 3D-BER system as well.More than 90%of the CIP was removed from the low C/N ratio wastewater.Applied current and the C/N ratio significantly affected the CIP and nitrate removal efficiency.Optimum CIP and nitrate removal efficiency were obtained at the current of 60 m A with a C/N ratio ranging from 1.5 to 3（p H 7.0-7.5）.Under these conditions,the highest CIP,NO₃^--N and TN removal efficiency were achieved at 94.2%,95.5%and 84.3%,respectively.In this novel system,the autotrophic-heterotrophic denitrifying bacteria played a vital role in the removal of CIP and the enhancement of the denitrification process.The autotrophic denitrifying bacteria use CO₂ and H₂as sources to reduce nitrates to N₂.The system had a rich and diverse community at a current intensity of 60 m A,which fluctuated under different C/N ratios.The intermediates were proposed as well,based on the results of LC-MS/MS analysis after efficient biodegradation of CIP.The microbial community of biodegradion was mostly found at the phylum level,and the class level was dominantly responsible for the NO₃^--N and biodegradation of CIP.In summary,this study sheds new insight on the effects of high electric currents on the bacterial community and activated sludge,both of which play an important role in the biodegradation of SMX,CIP,and NO₃^--N removal from simulated municipal low C/N wastewater in a novel 3D-BER system.In summary,this study investigated the influence of influent p H,C/N ratio,and current intensity on the removal efficiency and degradation effects of nitrate,SMX,and CIP in simulated urban sewage,and optimized the reactor operating conditions.At the same time,the microbial community changes are analyzed,and the degradation mechanism of CIP and SMX in a three-dimensional cathode bioelectrochemical reactor is proposed.It is innovative in the research of synergistically enhanced removal of nitrate and antibiotics in the three dimensional bioelectrochemical reactor system.