Enhancing The Deep Nitrogen Removal Of Mainstream Anammox Process By Introducing Organics

Anammox is a resource-efficient and sustainable biological nitrogen removal technology for wastewater treatment.Its application in the treatment of mainstream wastewater from municipal wastewater treatment plants can greatly reduce the aeration energy consumption and external carbon input,and at the same time facilitate the recycling of organic compounds in wastewater and reduce carbon emissions.However,due to the characteristics of low temperature,low ammonia nitrogen and fluctuating water quality of municipal wastewater,the operation of the mainstream anammox process is not stable,and the problem of nitrate accumulation caused by excessive proliferation of Nitrite-oxidizing bacteria（NOB）is easy to occur,failure to meet discharge standards is a major obstacle to mainstream anammox.In the early studies of anammox,organics were generally considered to be a disadvantage and should be removed to the maximum extent possible through pretreatment.Recent studies have shown that appropriate amount of organic matter can reduce effluent nitrate and promote total nitrogen removal through the synergistic effect of denitrification and anammox.However,most of the above studies focused on wastewater with high ammonia nitrogen,and there is rarely the problem of excessive proliferation of NOB.Further study is needed to determine whether nitrogen removal by anammox can be promoted by adding appropriate amount of organic matter under the condition of municipal wastewater quality.Therefore,this study investigated the effect and mechanism of adding appropriate amount of organic matter on nitrogen removal performance of mainstream anammox and the effect of different influent C/N.In this paper,the effect of introduce appropriate amount of organic matter on nitrogen removal performance of mainstream anammox was investigated.The sequencing batch reactor（SBR）was used to start the anammox,and the reactor was operated in two stages.In stage I,the influent was almost free of degradable organic matter,and in stage II,the introduction of synthetic wastewater increased the influent C/N by 0.9.The experimental results showed that in stage I,significant nitrate accumulation occurred in the system,ΔNO₃^-/ΔNH₄⁺was up to 1.17,which was significantly higher than the theoretical value of 0.26 for anammox reaction.The total nitrogen removal rate of the reactor decreased from 83%to 30%,accompanied by an increase in the abundance of NOB and a decrease in the abundance of anammox bacteria.In phase II,by introducing synthetic wastewater,nitrate accumulation disappeared rapidly within 2 days.At stage II,the nitrogen removal performance of the reactor was good,with the total nitrogen removal rate reaching 87%and the effluent nitrogen only 3.4 mg/L.At this stage,the abundance of NOB decreased,and anammox bacteria and partial denitrification microorganisms were enriched.Further analysis showed that the mechanism of enhancing nitrogen removal performance of mainstream anammox by adding appropriate amount of organic matter mainly included three aspects.First,some organic matter was utilized by aerobic heterotrophic bacteria,which consumed dissolved oxygen in water,it provided a more strict anaerobic environment for the reactor,enhanced the inhibition of NOB and reduced the production of nitrate.Second,partial denitrification（NO₃^-→NO₂^-）was realized,and nitrate removal was coordinated with anammox.Third,by adding organic matter,EPS secretion is promoted and sludge granulation in the reactor is promoted,which is conducive to the enrichment of slow-growing anammox bacteria.The effect of influent C/N ratio on nitrogen removal performance of mainstream anammox was investigated.Start up the mainstream anammox in SBR reactor,control other operating conditions are the same,and control the influent C/N of 0.5,1 and 2,respectively.When the influent C/N was 0.5,the anammox reactor was unstable and its nitrogen removal performance deteriorated.The activity and abundance of NOB increased significantly,the abundance of anammox decreased,nitrate accumulated significantly in effluent,and the removal rate of total nitrogen was only 34%.When the influent C/N is 1,denitrification and anammox synergize to achieve nitrogen removal.The removal rate of total nitrogen was up to 93%.Partial denitrifying bacteria and anammox were enriched.Anammox is still the main nitrogen removal pathway,and its contribution to nitrogen removal is up to 97%.When the influent C/N is 2,the anammox activity is inhibited.The abundance of denitrifying bacteria increased significantly,while the activity and abundance of anammox decreased significantly,and the removal rate of total nitrogen decreased to 60%.Based on the above results,the optimal C/N of influent for the mainstream anammox process is 1.To sum up,this study confirmed that the operation of the mainstream anammox was unstable during long-term operation,while the influent C/N of the reactor was about 1,a small amount of organic matter not only enhances the nitrogen removal of anammox,but also inhibits NOB and improves operational stability.Therefore,by setting a reasonable C/N,the organic matter can achieve the aim of enhancing the deep nitrogen removal in the mainstream anammox process.