The Enhanced Denitrification Technologies And Mechanism Based On Sulfur Cycle

The effluent of wastewater treatment plants(WWTPs)was strictly limited according to the discharge standard with the decrease of water environmental capacity and the increasingly serious shortage of water resources.However,at present,total nitrogen(TN)in effluent from many WWTPs was difficult to stably reach the up-to-standard discharge.There are many reasons of this phenomenon,and one of the main reasons is that the denitrification was not complete due to insufficient carbon source in the WWTPs.Dosing organic carbons would not only cause an increase of the treatment cost,but also might cause secondary pollution if not disposed well.Moreover,large amounts of biomass production led to the subsequent sludge disposal problem.Sulfate commonly exists in wastewater.And it was inevitable that sulfide or hydrogen sulfide,would produce by sulfate reducing bacteria that reduce sulfate to sulfide under anaerobic conditions.The endogenous sulfide produced by sulfate reduction can be used as electron donors for autotrophic denitrification,which can effectively reduce the TN in effluent,so this technology can make better use of those waste generated in situ.Moreover,it can also solve the problem of excessive sludge produced in sewage treatment process,and has the potential for engineering application.Against this background,the enhanced denitrification technologies based on sulfur cycle was systematical investigated in this study.Firstly,the potential applications of endogenous sulfide for enhanced denitrification in anodic mixotrophic denitrification MFC were revealed.Then,the performance of bio-cathode denitrification based on sulfate reduction and its response to COD/SO₄^2-ratios were elucidated.In addition,the influence of nitrate concentrations on sulfate reduction in the MFC reactor were clarified.Besides,the influence mechanism was revealed from the perspective of electron balance,microbial community and functional genes,and it can provide the theories foundation for further exploring the potential of sulfide autotrophic denitrification in WWTPs.The main conclusions are as follows:(1)The removal efficiency of acetate and sulfide achieved up to 100%in all the anodic mixotrophic denitrification MFCs.The major intermediates and end products in the process of sulfide oxidation were thiosulfate and sulfate,respectively,and the sulfate generation ratio was as high as 65.70%to 76.63%.Those electrons produced during acetate and sulfide oxidation process were mainly used for denitrification and electricity generation,with the maximum power densities of 40.60 m W/m² and nitrate removal efficiency of more than 92.37%.The removal of nitrate was thought to be determined by the combined effects of sulfide-based autotrophic and heterotrophic denitrification.Specifically,the sulfide-based denitrification contributed 6.35%-73.25%to nitrate removal,while the heterotrophic denitrification contributed 20.54%-86.02%to nitrate removal.Electron balance showed that the electrons produced during acetate and sulfide oxidation process redistributed between denitrification and electricity generation.More electrons flowed to electrode for electricity generation with the increased influent sulfide concentrations,while more electrons were used for nitrate reduction with the increased influent acetate concentrations.Therefore,adjusting the ratio of sulfide and acetate in the influent could help us to make better use of the sulfide-based autotrophic denitrification.(2)Part of the electrons generated during the oxidation of sulfide,reduced from sulfate reduction,were directly transferred to the electrode for electricity generation,but not for the denitrification.However,the microbes on the cathode in the bio-cathode MFC could use those electrons.Therefore,those electrons transferred to the electrode during the sulfide oxidation process have the potential to be further explored.(3)The anodic influent COD/SO₄^2-would affect the sulfate reduction efficiency and the bio-cathode denitrification performance.Increasing the influent COD concentration was beneficial to improve the competition of the methanogens for carbon sources.As the influent COD/SO₄^2-ratio increased from 0.44 to 1.11,the electron flow that was utilized by methanogens increased,while the electron flow that was utilized by sulfate reducing bacteria decreased.Thus,the activity of methanogens increased,and the activity of sulfate reducing bacteria decreased.Electron balance analysis showed that with the influent COD/SO₄^2-ratio increased from 0.44 to 1.11,the total amount of electrons generated by the external circuit of the MFC system increased,which improved the performance of the bio-cathode denitrification.However,the removal efficiency of nitrate in the bio-cathode chamber only increased from 22.46%to 26.47%.This was largely due to the fact that nitrate was not achieved complete denitrification,but accumulated in the influent with the form of nitrite and nitrous oxide.(4)Obvious inhibition of nitrate on the sulfate reduction in the MFC reactor was observed,as the average removal rate of sulfate decreased from 35.24%to 3.82%when the influent nitrate concentration increased from 0mg/L to 84mg/L.After adding nitrate,on one hand,sulfate reducing bacteria was always at an unfavourable status in the process of competing with the denitrifying bacteria for the electron donors.On the other hand,the intermediates of denitrification,including nitrite and nitrous oxide,would also inhibit the activity of sulfate reducing bacteria.After a long period of competition,those sulfate reducing bacteria would be eliminated gradually,and eventually the nitrate inhibited the sulfate reduction significantly.However,the addition of nitrate would not permanently inhibit the activity of sulfate reducing bacteria,and the activity of sulfate reducing bacteria could be restored after stopping the addition of nitrate.(5)Those functional microorganisms played a key role in the removal and conversion of pollutants in the MFC reactors.The strict autotrophic denitrifying bacteria Thiobacillus was a dominant genus at high sulfide concentration,while the heterotrophic denitrifying bacteria Arenimonas was significantly enriched at low sulfide concentration.Whether in the low or high sulfide concentration,those facultative denitrifying bacteria,including Xanthomonadaceae,Pseudomonas and Azoarcus,both with a high relative abundance,which were the main functional microorganisms responsible for accomplishing sulfide complete oxidation to sulfate.In addition,the microbial functional prediction based on KEGG revealed that the sulfide complete oxidation functional gene of Sox was more sensitive to sulfide than the functional gene of Sqr,as the Sox abundance increased with the increase of sulfide concentration,while the Sqr gene abundance was not affected by sulfide concentration.Higher COD/SO₄^2-did harm to the sulfate reducing bacteria,i.e.Desulfobacter and Desulfococcus,but contributed to the enrichment of the methanogens,i.e.Methanosarcina and Methanosaeta.On the bio-cathode biofilm,the electrophic denitrifier Thiobacillus,by utilizing current as the sole electron donor,was successfully enriched,and its abundance increased from 67.30%to 79.70%.q-PCR analysis results showed that denitrifying functional genes nar K is significantly abundant than nos Z,which was might be the cause of N₂O accumulation in the system.The addition of nitrate promoted the heterotrophic denitrifying bacteria such as Arenimonas,Paracoccus and Thauera to become the dominant genus.The sulfate reducing bacteria Desulfococcus was inhibited by nitrate as its abundance decreased remarkably from 5.97%to 0.17%,when the influent nitrate concentration increased from 0mg/L to 84mg/L.