The Impact Factors And Mechanisms Of Nitrogen Removal By Anammox In The Presence Of High Concentration Sulfite

The eutrophication of water has been one of the water pollutions which is concerned by people because of its wide influence and directly external expression.The one of the main causes of eutrophication is discharge of nitrogen-containing wastewater.The high concentration（NH₄）₂SO_3,which is translated from SO₂through wet ammonia flue gas desulfurization process in industrial applications,need to be treated through a suit technology urgently.In this study,it’s conducted that anaerobic ammonia oxidation bacteria is domesticated in EGSB through replacing nitrite with sulfite in order to study effect of SO₃^2-and O₂in anaerobic ammonia oxidation（ANAMMOX）and discuss the mechanism about conversion and removal of ammonia nitrogen.（1）In the ANAMMOX system with high concentration of sulfite,the main conversion product of sulfites is sulfate and the conversion ratio is more than 75%.Moreover,with the long-term culture conducting,ratio of conversion is increasing and efficiency is close to 100%.Elemental sulfur and sulfide can hardly be detected.The removal rate of ammonia nitrogen is at 50%for a long time steadily and the removal rate is 500 mg N/（L·d）.The ratio of ammonia nitrogen removal to sulfur loss（difference between sulfite change and sulfate change）-n（△NH₃-N）/n（△S）is much greater than the theoretical value of 2 in sulfate-dependent ammonium oxidation（SAMO）.（2）Due to the high initial EGSB reflux,the peristaltic pump may bring trace oxygen which acts as an electron acceptor in the system.Therefore,the reflux ratio of the reactor was adjusted include three stages:15:1,7.5:1 and 0.Results show that the conversion of ammonia nitrogen and sulfite is gradually reduce with the loss of the reflux ratio.Removal of ammonia nitrogen is 11%and conversion rate of sulfite reaches only 40%at 0 reflux.But the changes of removal efficiency are not sync in the process of adjusting.An obvious lag exists between change of sulfite and one of ammonia nitrogen removal rate.（3）A small amount of low-purity oxygen（5%O₂,95%N₂）was exposed to the reactor and the oxygen aeration flow rate was set to 5 m L/min and 10 m L/min.After exposure to oxygen,most sulfites were converted to sulfate in the reactor and the conversion efficiency was greater than 95%.Average removal efficiency of ammonia nitrogen increased from 10%at 0 aeration to about 42%at 5 m L/min and about 34%at10 m L/min.It’s worth noting that the removal efficiency of ammonia nitrogen decreased at a highest flow rate.Two possible reaction mechanisms are put forward on the basis of the experimental results.First,the reaction mechanism is completely operated by microorganisms that one use oxygen to oxidize ammonia-nitrogen and then nitrogen is generated by anaerobic ammonia-oxidizing bacteria.Second,microorganisms cooperate with free radicals.Under aerobic conditions,the Fe²⁺/SO₃^2-advanced oxidation system generates thiooxide radicals,which can oxidize ammonia nitrogen into nitrite.An AOB use different valence nitrogen to generate nitrogen.（4）According to the operation of the reactor,the conditions of batch experiment were set as NH₄Cl 0.2 g/L,Na₂SO₃0.2 g/L,n Fe²⁺/n SO₃^2-0.06:1,gas 5%O₂+95%N₂and flow rate 10 m L/min.The rate that ammonia nitrogen was transferred into nitrate is 8%,which proved that there was a possibility the ammonia nitrogen is oxidized by radicals under the condition of the reactor.（5）16S r RNA gene sequencing and microbial community analysis were performed on sludge at different stages.Result of microbial community analysis showed that Bacillus benzoevorans,the dominant bacterium in SAMO,was not detected.And the specific functional gene aom A of complete nitrifying bacteria wasn’t detected.The relative abundance of Nitrosomonas was very small（only exceeding 1%to 2.1%at the stage of 10 m L/min）and the relative abundance of nitrifying helicobacter changed within a range about 1-2%.No subspecies of nitrifying helicobacter were found in the horizontal analysis of the genus.Therefore,it is believed that the major effect of oxygen is not directly used by microorganisms to oxidize ammonia nitrogen.The change of relative abundance of Planctomycetes was consistent with the change of ammonia nitrogen removal efficiency in the reactor and the relative abundance was relatively high.So An AOB can be considered to have an important contribution in the reaction.（6）It’s speculated that Fe²⁺/SO₃^2-advanced oxidation system exists in the reactor so that thiooxide radicals(mainly SO₄^·-)are generated to oxidize part of the ammonium for the action of trace oxygen.The oxidation is stopped to nitrite by microorganism or nitrate is reduced by Thiobacilus so that the product can convert into nitrogen with ammonium by the anaerobic ammonia oxidation bacteria.The autotrophic denitrification system coupled with advanced oxidation of SO4·-radical and ANAMMOX is named as sulfite-dependent ammonium oxidation.