Denitrification Performance And Bacterial Communities In A Mixotrophic Denitrification System

The high concentration of nitrate in aquatic environment is one of the reasons for the eutrophication and water contamination in rivers and lakes.Biological denitrification,including heterotrophic and autotrophic denitrification,is the most commonly used solution.Yet mixotrophic denitrification gradually attracting the researchers'attention as it combines the characteristics of both autotrophic and autotrophic denitrification.In this study,reactor operation experiment,sludge activity test and high-throughput sequencing were carried out to analyze the denitrification performance,factors and the bacterial communities of mixotrophic denitrification.Two identical reactors were applied as mixotrophic and autotrophic denitrification reactors,with autotrophic denitrification sludge as inoculated sludge.The autotrophic reactor was fed with thiosulfate as sole electron donor,and the mixotrophic one with thiosulfate and sodium acetate as mixed electron donor.By adjusting the hydraulic retention time?HRT?,the influent NO₃^--N concentration,nitrogen load and other running parameters,the denitrification performance of the two reactors were compared.The sludge activity was measured at the different temperature,with different inorganic salt and electron donor addition,to explore their influence on the sludge activity.Sludge samples of two reactors were collected for high-throughput sequencing to analyze the bacterial community structure in the mixotrophic denitrification system.The results of reactor experiment showed that the mixotrophic reactor could remove NO₃^--N successfully when the each of autotrophic and heterotrophic part shared half of the total nitrogen load,with the COD/N?W/W?ratio 5:1 and the S/N?mol/mol?ratio 1:1.During the whole period,the average NO₃^--N removal rate in the mixotrophic denitrification reactor was 81.1%,which was slightly better than the autotrophic reactor?76.9%?.HRT was one of the important factors in the autotrophic and mixotrophic denitrification systems.Shortening the HRT could increase the NO₃^--N removal load of the reactor,but it would also cause the fluctuation of NO₃^--N removal rate and the accumulation of NO₂^--N.When the influent NO₃^--N were 3 mg/L and 12 mg/L,the HRT were 1h and 0.75h respectively,nearly 90%of NO₃^--N could be removed with no NO₂^--N accumulation in the mixotrophic system.The SO₄^2-concentration in the effluent produced by the mixotrophic denitrification reactor was lower than that by autotrophic denitrification.Compared with the heterotrophic denitrification,the mixotrophic one could avoid the potential secondary pollution of excessive organic carbon dosage.Sludge activity tests showed that the mixotrophic denitrification could be affected by temperature and inorganic salt addition.When each of autotrophic and heterotrophic part shared half of the total nitrogen load,the appropriate temperature for mixotrophic reactor is 20?-34?.Within this range higher temperature could improve the process.Adding inorganic phosphorus could not only increase the bacterial biomass,but also increase the activity of both denitrifying bacteria and the denitrification remarkably.Bacterial community analysis showed that genuses of Thiothrix,Thauera,Dok59 and Acinetobacter predominated in the mixotrophic reactor,while Thiobacillus,Sulfurimonas and Gallionella mainly appeared in the autotrophic reactor.The biodiversities in the mixotrophic reactor were generally higher than the autotrophic reactor.In addition to the heterotrophic denitrifying bacteria,there were also large amount of autotrophic denitrifying bacteria and facultative denitrifying bacteria in the mixotrophic reactor,which proved that by adjusting the proportion of mixed electron donor dose,a harmonious symbiosis could be built up among the autotrophic,heterotrophic and facultative denitrifying bacteria.