| Due to anthropogenic activities such as the input of non-point nitrogen-containing pollution from agricultural,potential industrial wastewater and domestic sewage discharge,nitrogen surplus in coastal and nearshore waters has become a global problem and one of the prominent water environment problems in China.So,it is urgent to carry out relevant treatment technology research.The nitrogen surplus in coastal waters is mainly manifested in the high content of total nitrogen,in which nitrate accounts for more than 90% of total nitrogen.In view of the effective control of COD,BOD and other “carbon source” pollution indicators in coastal waters such as surface water and municipal sewage treatment plant effluent in recent years,these water bodies are characterized by high nitrate content,low organic carbon and high concentration of sulfate.It is difficult to treat such coastal waters by traditional activated sludge water treatment process,and the high cost associated with “additional carbon source” further restricts the nitrogen removal efficiency.Since the cathode of bioelectrochemical reactor has the advantage of producing clean electron donors,the three-dimensional biofilm electrode reactors(3D-BERs)can be constructed to effectively remove total nitrogen and nitrate in coastal low carbon water bodies under the interference of sulfate.In this study,a three-dimensional biofilm electrode denitrification device(D-3DBER)was successfully constructed for efficient nitrogen removal based on the theoretical and technical research in laboratory,and it achieved stable denitrification performance in the treatment of municipal sewage treatment plant effluent with a capacity of 1 m3/d.The major contents and findings are summarized as follows:(1)Removal of nitrate and sulfate from low carbon water by a three-dimensional electrode biofilm reactor: An up-flow three-dimensional biofilm electrode reactor(3D-BER)with graphite rod as anode,stainless steel mesh as cathode and granular activated carbon as granular electrode was established to remove nitrate and sulfate from low organic carbon water.The effects of influent sulphate concentration,applied current and hydraulic retention time(HRT)on the removal of nitrate and sulphate in the 3D-BER were investigated,and the removal process of nitrate and sulphate was inferred in conjunction with the distribution of functional bacteria.Results indicated that the nitrate removal efficiency decreased with the increase of influent sulfate.Moreover,high electric current(> 240 m A)and short HRT(< 18 h)deteriorated the performance of nitrate and sulfate removal.When the influent of SO42-was 150 mg/L,applied current was 240 m A and HRT was 18 h,the removal efficiencies of NO3--N and SO42-were 88.49 ± 4.50% and 29.35 ± 5.50%,respectively.The high-throughput sequencing revealed that denitrifying bacteria dominated in the lower part of the reactor while sulfate reducing bacteria dominated in the upper part of the reactor.According to the experimental results,it was speculated that denitrification in 3D-BER mainly occurred in the lower part of the reactor while sulfate reduction process mainly carried out in the upper part.Besides,it was also conjectured that oxidation products of sulfide could serve as supplementary electron donors to enhance nitrate removal in the 3D-BER.Corrosion peeling of graphite rod was present during the experiment.(2)Effects of anode material on electrochemical reduction of nitrate in the presence of sulfate: Aiming at the problem that graphite rod was easy to be corroded and peeled,the anode materials were optimized and selected.The electrochemical reduction performances of nitrate and sulfate in three-dimensional electrode reactors with TA1 titanium rod(3D-ER),titanium suboxide coated TA1 titanium rod(3D-ER-T)and titanium suboxide coated graphite rod(3D-ER-G)as anodes were analyzed and compared,and the anode materials suitable for subsequent bioelectrochemical experiments were determined according to the performances of the three reactors for nitrate and sulphate removal at different currents and the durability of anode materials.Results indicated that 3D-ER-T exhibited stable and efficient NO3--N removal ability(46%-95%)under the current of 160-320 m A.3D-ER and 3D-ER-G achieved high NO3--N removal efficiency(> 88%)at current of 320 m A.The SO42-removal efficiency of 3D-ER and 3D-ER-T were higher(~ 48%)than that of 3D-ER-G.When the current was 240 m A and HRT was 12 h,the removal efficiencies of NO3--N in 3D-ER,3D-ER-T and 3D-ER-G were 0,59.76 ± 8.28% and 80.97 ± 16.01%,respectively,and the removal efficiencies of SO42-were 5.95 ± 4.35%,3.04 ± 1.87% and 2.46 ± 2.15%,respectively.The concentration variations of NO3--N and SO42-didn’t have obvious stratification with the height of 3D-ER-T.Titanium suboxide coating can effectively slow down the corrosion of anode matrix materials.Titanium suboxide coated TA1 titanium rod and coated graphite rod were selected as the anode materials for subsequent bioelectrochemical experiments.(3)Effects of anode material on bioelectrochemical reduction of nitrate in the presence of sulfate: Titanium suboxide coated TA1 titanium rod(3D-BER-T)and titanium suboxide coated graphite rod(3D-BER-G)selected in the above chapter were used as the anodes of 3D-BERs.The microbial community structures and the performances of nitrate and sulfate removal at different currents and HRTs of the two reactors were explored and compared.The influence of influent dissolved oxygen(DO)on the performance of 3D-BER-T was investigated,and the removal mechanism of nitrate and sulfate in 3D-BER-T was analyzed.Results indicated that the removal efficiencies of NO3--N and SO42-as well as nitrogen selectivity in 3D-BER-T were higher than that in 3D-BER-G.Under the condition of I = 320 m A and HRT = 12 h,the removal efficiencies of NO3--N and SO42-as well as nitrogen selectivity in 3D-BER-T were 91.61 ± 0.42%,43.12 ± 2.57% and 85.69 ± 0.49%,respectively.Besides,3D-BER-T displayed good adaptability to the influent DO.Anode materials affected the microbial community structure and diversity in 3D-BERs.Hydrogenophaga,Dethiobacter,Acetobacterium and Thauera were the dominant bacteria of 3D-ER-T while the dominant bacteria of 3D-BER-G mainly included OPB41,Sulfurospirillum,Hydrogenophaga and Azoarcus.Under the synergistic effect of electrochemical reduction,bioelectrochemical reduction and indirect electrochemical reduction of anode,nitrate and sulfate were effectively removed in 3D-BER-T.The electron donor reserve/resupply mode with sulfur cycle improved the impact load resistance of 3D-BER-T.Titanium suboxide coated TA1 titanium rod was more durable than titanium suboxide coated graphite rod.(4)Efficiency analysis of three-dimensional electrode biofilm reactor for deep denitrification of wastewater treatment plant effluent: In order to verify the practical application effect of 3D-BERs studied in the laboratory,the 3D-BER-T with higher nitrate and sulfate removal efficiency and higher anode durability was selected for scale-up application in a representative municipal sewage treatment plant in coastal areas.A D-3DBER(diameter of 0.7 m,height of 1.65 m)with a treatment capacity of 1 m3/d was constructed for deep denitrification of municipal sewage treatment plant effluent.The denitrification efficiency and energy consumption under different currents were investigated,and the effluent quality and microbial community structure at different heights were analyzed.Results indicated that with activated carbon and light ceramsite(volume ratio was 1:1)as particle electrodes and 5 A current applied,the removal efficiency of NO3--N in D-3DBER was 78.99 ± 4.10%,the selectivity of nitrogen was 83.48 ± 4.20%,and the energy consumption was 1.20 ± 0.13 k Wh/m3.Nitrate in the influent decreased rapidly within the height of 47.5 cm,sulfate began to decrease significantly from the height of 47.5 cm,and both decreased to the lowest at 75 cm,indicating that nitrate reduction took place preferentially in D-3DBER.Proteobacteria and Campilobacterota were selectively enriched in D-3DBER compared with the inoculated sludge,and the microbial community composition of D-3DBER at 75 cm was significantly different from that at other heights.Sulfurimonas(20.96%),Sulfuricurvum(5.32%),Thauera(4.40%)and Denitratisoma(3.87%)were the dominant bacteria at 75 cm of D-3DBER,and the relative abundance of bacteria with denitrifying function was the highest at 75 cm.In addition,the relative abundance of denitrifying bacteria was higher in the lower part of D-3DBER. |
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