Research On N₂O Emission During Completely Autotrophic Nitrogen Removal Over Nitrite Process

CANON process based on ANAMMOX technology has the advantages of no need of organic carbon source,saving the amount of aeration and less excess sludge,which is mainly applied to the treatment of high ammonia nitrogen wastewater,such as landfill leachate,sludge digestion liquid and so on.However,further studies have found that the CANON process is effective in removing nitrogen,while releasing a strong greenhouse gas-N₂ O.In addition to CO2 and CH4,N₂ O is the third largest greenhouse gas,whose GWP is much higher than the former two.Not only that,it can also react with oxygen atoms in the stratosphere,resulting in ozonosphere hole,or generate nitric acid through a series of chemical reactions which can be with rain to form acid rain.All of the above would threaten the safety of human life.Therefore,to prevent the transfer of environmental pollution from the water environment to the atmosphere,it is of great significance for ecological sustainable development to study the release characteristics of N₂ O in CANON process and get the optimal operating parameters to reduce N₂ O emissions.In this paper,with inorganic high ammonia nitrogen wastewater as raw water,after successfully starting the CANON process and ANAMMOX process,it is focused on effects of different factors on N₂ O in the process by changing operating parameters,and will analyse the sources and pathways of N₂ O by chemical inhibition method.Firstly,it started the biofilm CANON process and ANAMMOX granular sludge process.The results are as follows:（1）By inoculating CANON sludge,ANAMMOX reactor was started directly under anaerobic conditions.For the first time,it was fed with synthetic wastewater,and was running for 56 d until failure.Secondly,it started using one anaerobic reactor’ effluent as influent,which lasted for 161 d until successful start-up of anammox granular sludge process.Finally,TN removal rate was 71.53%,TN removal loading was 0.70 kg.（m3.d）-1 and granular sludges have strong resistance.（2）CANON reactor with haycite as carrier,which was under anaerobic conditions before,turned to aerobic conditions to change into CANON process when temperature was 30±1℃.It is lasted for 27 d until the successful start-up.NH₄⁺-N and TN removal rates were up to 92.56%,81.53% respectively.TN removal loading was 1.21 kg.（m3.d）-1.Secondly,in CANON process,it was studied the effects of different factors on CANON and N₂ O emissions,through changing operating parameters,including initial substrates（NH₄⁺-N,NO₂^--N,pH）and aeration conditions（continuous or intermittent）when temperature was 30±1℃.The results are as follows:（1）The higher concentrations of NH₄⁺-N and NO₂^--N were,the greater N₂ O emission rates would be,especially the influence of NO₂^--N on N₂ O,and the highest values were 7.15%,11.94% when NH₄⁺-N=500mg·L-1,NO₂^--N=100mg·L-1.But as pH was higher,N₂ O emission rates were lower,and the highest value was 8.75% when pH=6.64.（2）The greater the aeration rates were,the higher N₂ O emission rates were,which was 9.73%.And N₂ O emissions could be reduced 21% shortening the aerobic duration（0.5h）during intermittent aeration.Finally,it was researched sources and pathways of N₂ O emissions according to preliminary test results when temperature was 30±1℃.The study added NH2 OH,ATU,NaClO3 and CH3 OH.The results are as follows:（1）The oxidation of NH2 OH was the main source of N₂ O emissions during initial aeration reaction,and AOB denitrification resulted in the second N₂ O release peak in the middle of reaction due to accumulation of NO₂^--N under aerobic conditions.（2）Under aerobic conditions,Denitrification was mainly responsible for the N₂ O,which was accounted for 65% of the total N₂ O emissions,followed by AOB that was 23%.In addition,if ANAMMOX could’t consume NO2--N in time,it will expand increase N₂ O emissions.（3）Under anoxic conditions,AOB was mainly responsible for the N₂ O,but denitrification and ANAMMOX could consume N₂ O.The former could compete NH₄⁺-N and NO₂^--N to inhibit AOB denitrification,and the latter could reduce N₂ O that was produced by AOB denitrification to N₂,both of which could reduce N₂ O emissions.To sum up,in CANON process,it was found that NO₂^--N and aeration rates（or DO）were principal factors to affect N₂ O emissions,NH₄⁺-N affect N₂ O emissions indirectly by affecting concentrations of NO₂^--N,and low pH may affect reduction of N₂ O to increase N₂ O emissions.On the other hand,NH2 OH oxidation and AOB denitrification were the main reasons for N₂ O emissions which could be reduced indirectly by ANAMMOX and denitrification under suitable conditions.