Ammonium Formation Mechanism And Affecting Factors Of A Nitrite-based Simultaneous Sulfur Autotrophic And Heterotrophic Denitrification Process

High strength nitrogen and sulfur containing wasterwater was severely harmful to the environment and human body.For remove nitrogen and sulfur pollutants efficiently,sulfide-oxidizing autotrophic denitrification（SOAD）technology had been widely and deeply studied,such as SANI（sulfate reduction,autotrophic denitrification and nitrification grated）.Since high strength nitrogen and sulfur wastewater usually containing organic matter,simultaneous autotrophic and heterotrophic（mixotrophic）denitrification process was also highly concerned by researchers.This study focused on the characteristics and key influencing factors of nitrite-based simultaneous sulfur autotrophic and heterotrophic mixed denitrification process in a sequencing batch biofilm reactor（SBBR）fed with nitrite,sulfide and sodium acetate supplementation.Six different COD/N ratios and five different S/N ratios were studied for the effect of decarburization,denitrification and desulfurization,as well as the accumulation characteristics of dissolved N₂O and NO.In addition,the phenomenon of dissimilatory nitrite reduction to ammonia was discovered in the system and then the effecting factors was discussed under different COD/N and S/N.Based on this,the mechanism of ammonia production was explored.This results provided further theoretic support for the high strength sulfur and nitrogen laden wastewater treatment.The major conclusions were summarized as below:（1）The results under different operating conditions showed that in the sulphur mixotrophic nitrite denitrification reactor,simultaneous removal of carbon,nitrogen and sulfur achieved with the proper ratio of COD/N and S/N.Under long-term operation in the influent conditions of COD,nitrite and sulfide loading of 180 mg·L^-1,60 mg·L^-11 and 60mg·L^-1,respectively,removal efficiencies of nitrite and sulfide could achieved at 100%,and COD removal rate up to 86%.（2）The online monitoring results of dissolved N₂O and NO in the reactor under different influent conditions showed that the influent water quality with low S/N and high COD/N was benefit to reduce the accumulation of dissolved N₂O,meanwhile,the presence of DO in the system would decrease the production rate of N₂O.Moreover,the peak of NO accumulation and the rate of NO decreased with increasing influent S/N,while COD/N had little effect on the NO of the system,and the instantaneous influent brought NO accumulation in the denitrification system..（3）Simultaneously,dissimilatory nitrite reduction to ammonia was found in this reactor.The reduction of dissimilatory nitrite to ammonium required a certain concentration of organic matter and sulfide in the influent.If the concentration was too low,the growth of DNRA cells cannot be supported.However,if the concentration was too high,the capacity of ammonium would be inhibited.It was found that when the influent COD/N>2,S/N>1 and the oxidation-reduction potential（ORP）<-400 mV,the ammonium concentration increased significantly.In addition,the effect of ammonia production would be enhanced with higher S/N ratio in invariant COD/N ratio of influent.And when S/N ratio remained invariable,the yield of ammonia is significant if the COD/N ratio was 3,with 29.49 mg·L^-11 NH₄⁺-N produced.What’s more,based on the experimental results of controlling the influent COD of180 mg·L^-11 and S^2--S of 60 mg·L^-11 and then exposing N₂O gas to the reactor without adding nitrite,it is proposed that it is possible to reduce nitrous oxide to ammonium.（4）The microbial analysis showed that sulfur autotrophic denitrification,heterotrophic denitrification as well as dissimilatory nitrite reduction to ammonia were coexisted in this nitrite-type carbon-nitrogen-sulfur synchronous removal system.Furthermore,the mechanism of ammonia production may be the process of dissimilatory nitrite reduction to ammonia in the presence of low redox potential and excess electron donor.