Study On Nitrous Oxide Emission And Variation Of Enzymes Activity In Activated Sludge During Biological Nitrogen Removal

With the rapid development of society and economy,the pollution caused by the discharge of nitrogen compounds into the water body is becoming more and more serious.Because of the advantages of convenience in applying,good economy and wide adaptability to wastewater,biological nitrogen removal technology has become the primary choice to solve the problem of pollution caused by wastewater containing ammonia-nitrogen.Biological nitrogen removal is playing an irreplaceable role in reducing nitrogen discharged.However,a large amount of N₂O which has a strong warming effect will be generated during the biological nitrogen removal in the wastewater treatment,which potentially threats the earth environment,so that it can not be ignored.Therefore,how to maximize the positive effect of biological denitrification and reduce N₂O production as much as possible is a hot issue and a focus of current research.Catalysis of enzyme is a key factor to control the production of N₂O during the biological treatment of wastewater.It is known that the activity of the enzyme could determine the production path of N₂O as well as its emission.If we can insight into the change rule of the external environment condition and the N₂O producing enzyme activity,the emission of N₂O may be controlled and consequently it will be achieved to reduce the emissions.Therefore,facing the increasingly severe greenhouse gas emission reduction pressure,it is very necessary to study the mechanism of N₂O production during the biological denitrification process,and further to reveal its influencing factors and control strategies.In this work,anoxic-aerobic SBR-biological nitrogen removal systems were applied.Extraction of key enzymes such as HAO and NOR closely related to the production of N₂O were investigated firstly.The measurement method of the enzyme's activity was optimized.Based on these,the effects of the aeration and C/N ratio on the release of N₂O and the change of HAO activity during the biological nitrogen removal process were studied.Finally,relationship between the activities of HAO and NOR and the N₂O emission were verified.Results drawn from the research are as follows:1.The effects of the high pressure cell disruption method,disruption pressure,the disruption times and the amount of lysate on the DNA content in the crude extract of activated sludge were studied.An enzyme extraction and activity determination method was established.The results suggest that:?1?the effect of pressure crushing is better than that of ultrasonic disruption.Disruption pressure,disruption times as well as the amount of lysate dosage had a significant effect on the activity of DNA and HAO in the crude extract from denitrification activated sludge?P<0.01?.?2?A higher enzyme activity obtained when the 2 mol/L HCl solution was used as the enzyme reaction terminator when the electron acceptor and donor ratio of the enzyme reaction solution was 1:1?P<0.01?.?3?Taken the DNA content,activity and specific activity of the enzyme into overall consideration,the test condition with 110 MPa,5 mL lysis solution,and 2 crushing times was recommended to study the HAO activity for the biological sewage treatment,with the purpose of saving time and keeping enzyme activity.?4?During the determination of reduction activity for NOR,the enzyme activity of the heated sample was higher than that of unheated?P<0.05?.2.The effects of the aeration rates on the produce of N₂O in nitrifying process as well as the activity of HAO were researched.Based on these,the main production processes of N₂O under different aeration conditions were investigated and the inter-relationship between HAO activity and N₂O emission was studied.The results show that:?1?the aeration rate has different effects on the activity of nitrification and denitrification,which led to different N₂O emissions.?2?Under a low aeration rate?0.6L/min?,N₂O produced by the nitrification and denitrification stage was related to the activity of HAO.However,N₂O produced in the denitrification stage was not related to the activity of HAO with a higher aeration rate?0.8 and 1.0 L/min?.?3?When the aeration rate was 0.6 L/min,the N₂O emission was much larger than that with other aeration rates,which is due to the occurrence of aerobic denitrification,and the release of N₂O during the aerobic stage was closely related to the HAO activity.?4?During the aerobic stage,the lower the aeration rate was,the higher the removal rate of TN achieved,at the same time the N₂O emission increased.3.The main formation processes of N₂O in denitrification with different C/N ratio in A/O SBR were also studied,and the influence rule of C/N ratio on the main formation processes of N₂O,as well as the activity of key enzymes such as HAO and NOR,which could be determine the N₂O emission to a great extent,were further investigated.The results show that:?1?a higher removal rate of TN will be obtained with a suitable C/N ratio,otherwise the relevant removal rate of TN was low and more N₂O will be released.?2?Although the activity of HAO in anoxic stage was higher than that in aerobic stage,N₂O produced in this stage was independent with HAO.N₂O produced in the aerobic stage was related to the activity of HAO.?3?The activity of HAO with the C/N ratio of 3.5 and 9.3 was significantly higher than that with the C/N ratio of 6.5,and the corresponding N₂O emission was also relatively high.The high release of N₂O in the aerobic stage with a high C/N ratio?9.3?was possible ascribed to the high activity of HAO or the nitrification and denitrification of heterotrophic nitrifying bacteria.?4?The reduction activity of NOR with the C/N ratio of 3.5 and 9.3was significantly higher than that with the C/N ratio of 6.5.?5?The abundance of bacteria belonged to genus Nitrospira decreased under C/N ratio of 9.3,which might resulted in the lower oxidation of NO₂^-and the more N₂O production via AOB denitrification under low DO.And the abundance of denitrifying bacteria increased under C/N ratio of 3.5?e.g.Rhodobacter and Flavobacterium?and 9.3?e.g.Pseudomonas and Acinetobacter?,which might be closely related to N₂O production and HAO,NOR activity changes.From this study,some issues on N₂O production concerned with the enzymes can be drawn as the followings.Regardless of C/N ratio,if the DO level is high in the aerobic period,HAO is not involved in the production of N₂O in the anoxic period.In aerobic period,it is liable to accumulate NO₂^-because of high HAO activity,relatively weak NOR activity and NH₂OH fastly oxided.HAO catalyzed reactors and non-biological chemical reactions are considered to be important pathways of N₂O production.When the DO is insufficient,HAO of the microbes in both anoxic period and aerobic period involve in the production of N₂O.However,the NOB will be inhibited at the beginning of the aerobic period leading to lower the NOR activity,NO₂^-is considered to produce a large amount of N₂O mainly through AOB denitrifying process.Therefore,it is suggested that a reasonable control strategy of C/N ratio and DO in biological nitrogen removal system can not only reduce the release of N₂O but also save energy.