Effect Of C/N/S On Biological Nitrate Reduction

The rapid development of society and the continuous improvement of living standards have led to the improvement of man-made nitrogen input.This process is continuously leading to an imbalance in the global nitrogen cycle,which has caused serious impacts on the ecological environment.The variation of NO₃^--N in the ecosystem is the subject of many domestic and foreign scholars’researches.At present,the main pathways are heterotrophic denitrification,autotrophic denitrification,anammox,and dissimilatory nitrate reduction to ammonia（DNRA）.In order to understand these processes and more clearly kown their relationships,this series of experiments were carried out simultaneously in an attached biofilm reactor and a suspended activated sludge reactor,with CH₃COONa and Na₂S as electron donors at the same time,and NO₃^--N as an electron acceptor,we set six different conditions:Phase a（C/N/S=8:2:1）,Phase b（C/N/S=4:2:1）,Phase c（C/N/S=2:2:1）,Phase d（C/N/S=2:2:2）,Phase e（C/N/S=2:2:3）,and Phase f（C/N/S=2:2:4）.Under the condition that the electron acceptor is unchanged,the activity of heterotrophic and autotrophic denitrifying bacteria is continuously change by reducing C/S,to study the effect of electron donor species on nitrate transfer in a mixotrophic culture.The main conclusions were summarized as below:（1）Electron donors are an essential part of biological nitrogen transfer,and different types of electron donors have different effects on biological nitrogen transfer pathways.Under each reaction condition in the two reactors,the denitrification pathway is major the nitrogen transfer pathway,and both were higher than 72%.Through different C/N/S changes,the proportion of various pathways in the nitrogen cycle in the two systems can be changed,so that these processes coexist such as heterotrophic denitrification,autotrophic denitrification,and DNRA,but their proportions are different.（2）When organic matter is used as the main electron donor,heterotrophic denitrification is major the nitrogen transfer pathway;as the proportion of organic matter decreases and the proportion of sulfides gradually increase,the nitrogen transfer pathway gradually turns to autotrophic denitrification.At the same time,in the case of limited electron acceptors（NO₃^--N）,high electron donors may cause the denitrification process to change to DNRA process.（3）In the two reactors,when sulfide is used as the main electron donor,the autotrophic denitrification process gradually becomes dominant;when the sulfide is excessive,the sulfate produced will act as an electron donor and participate in sulfate reduction,resulting in a large amount of S^2-in the effluent;at the same time,a white suspended layer was found on the surface of the reactor,which was S⁰.At the same time,with the increase of electron donors,the sulfur in the state of white suspension gradually increased.（4）In the attached biofilm reactor,a small amount of NH₄⁺-N appeared in the effluent in some stages,which were 0.17（a stage）,1.7（b stage）,3.09（d stage）,4.9（e stage）,2.5（f stage）mg/L.In the suspended activated sludge reactor,the NH₄⁺-N in the effluent was 2.69（e-phase）and 8.9（f-phase）mg/L.The optimal stage of the DNRA process of the biofilm system is C/N/S=2:2:3,and the stage with the highest proportion of DNRA in the activated sludge system is C/N/S=2:2:4.It is worth noting that NH₄⁺-N accumulation did not occur in the environment of high organic electron donors（C/N/S=8:2:1）in the activated sludge system.With limited nitrate as an electron acceptor,excess electron donors are a key factor in the DNRA process.（5）The oxidation-reduction potential（ORP）and p H can comprehensively determine the approximate existence of remaining substances in the system.ORP can reflect the amount of electron donors in the system and the reducibility of the system.At the e and f phases of excess sulfides as the main electron donor,the reduction conditions are extremely strong（ORP<-400m V）,The ammonium yield of activated sludge system is better,while the biofilm system under the same conditions accounted for a relatively high proportion of denitrification.p H can provide an indicator for the denitrification process in a mixotrophic culture.（6）Identification and analysis of microbial community structure,at the phylum level,Proteobacteria is the main functional bacteria in this system,accounting for more than 50%.At the genus level,Thauera is the most abundant genus.When organic matter is used as the main electron donor,heterotrophic bacteria predominate;when sulfide is dominant,the proportion of autotrophic microorganisms increases significantly.At the same time,it is also found that DNRA functional bacteria also change,and the change of microbial structure can be provide evidence in various pathways in the nitrogen transfer paths.