The Mechanism Of Dissimilatory Nitrate Reduction To Ammonia In Nitrate-SOAD Reactor

The discovery of dissimilatory nitrate reduction to ammonia（DNRA）shows that denitrification is no longer the only way to reduce nitrate.It affects us in various aspects of ecology and engineering,so we need to Studying the DNRA process helps us better understand the nitrogen cycle and provides us with new ideas in wastewater treatment.In this study,nitrate-SOAD processes were started in SBBR and SBR reactors,respectively,to achieve simultaneous removal of nitrogen and sulfur.After the successful start-up of the reactor,the domestication of respiratory DNRA process was attempted under five S/N conditions.The main conclusions are as follows:（1）In SBBR-SOAD reactor,simultaneous removal of nitrogen and sulfur was successfully achieved when S/N=1.3:1.When S/N=1.5:1,the maximum NH₄⁺-N generated in the reactor was 11.62±0.63 mg/L,and the DNRA process accounted for 35.3±2.3%of the reaction in the reactor,realizing the domestication of DNRA process.Continued to increase the sulfide concentration,the concentration of NH₄⁺-N in the reactor did not increase,it may be because the concentration of sulfide was too high and the p H of the system was also high,which will affect the microbial activity.（2）The denitrification was inhibited by the increase in sulfide concentration,which drived some electrons from S^2-to NH₄⁺,producing ammonium through the DNRA process.The phenomenon of instantaneous accumulation of NO₂^--N indicated that the step of reduction of nitrite to ammonium was probably the key link of DNRA process and the branching point of SOAD and DNRA processes.（3）The ORP can indirectly indicate the type and process of reactions within the system.When there was only SOAD process in the system,ORP had two knee points and indicates the complete reaction time of nitrate and nitrite respectively.When both SOAD and DNRA were present in the system,the nitrite knee and nitrite knee were combined.When the system had a DNRA process,the ORP of the system was kept in a relatively low range,which is basically kept below-350m V.The change in p H indicated that the oxidation of the sulfide may be divided into two steps:first,the sulfide was first oxidized to elemental sulfur or other forms of sulfur and then oxidized to sulfate.（4）Analysis of the microbial community showed that in the SBBR-SOAD reactor,the main dominant bacterium was Thiobacillus,and the relative abundances under the five conditions were 6.75%,41.66%,30.49%,64.43%,69.59%,respectively.Under different conditions,the genus with DNRA functions is different.The relative abundance of Ignavibacterium in conditions A（1.28%）,B（0.64%）,and C（0.02%）gradually decreased,and Hydrogenophaga only present in condition B（0.49%）,C（0.96%）,Haliangium（0.27%）and Leptolinea（0.31%）only in condition A.Geoobacter existed only under conditions D（0.68%）and E（0.67%）.The relatively low sulfide concentration in conditions A and B caused the DNRA phenomenon to be insignificant.The presence of these genus bacteria causes a DNRA process in the reactor.（5）With the increase of S/N in SBR-SOAD reactor,the concentration of NH₄⁺-N in the reactor increased,but the concentration of sludge in the reactor decreased all the time.The dominant bacteria under different conditions of the reactor were all bacteria with sulfur autotrophic denitrification functions,such as Thiobacillus,and other bacteria with denitrification functions.Microbial monitoring did not find a genus with DNRA function or nrf A gene,indicating that the NH₄⁺-N produced was likely to be caused by microbial death,rather than DNRA.The biofilm filler in the SBBR-SOAD reactor provided a place for microorganisms to adhere,which can be fully contacted with sewage,and can respond to the impact of increased sulfide concentration.The SBR-SOAD reactor had a weak ability to deal with sulfide impacts,resulting in a large number of microorganism deaths and ammonia nitrogen.