Understanding Of Nitrous Oxide Production From Biological Nitrogen Removal Processes In Aerobic Granular Sludge Systems

Nitrous oxide(N2O)is a potent greenhouse gas.During biological wastewater treatment,N2 O is mainly generated from biological nitrogen removal(BNR).Recently,ammonia oxidizing bacteria(AOB)are identified as the major contributor to N2 O production in wastewater treatment plants.It can produce N2 O through the AOB denitrification pathway and the hydroxylamine(NH2OH)oxidation pathway.At present,the production mechanism of N2 O production in activated sludge system has been extensively studied,but its mechanism in aerobic granular sludge system has not yet been elucidated.This thesis aims to study the N2 O emission and production pathways of the aerobic granular sludge system through experimental study and mathematical modeling.The aerobic granular sludge was cultured in a laboratory-scale Sequencing Batch Reactor(SBR).Dissolved oxygen(DO)is commonly recognized as an important factor influencing N2 O production by AOB.Low DO concentration will hinder the progress of nitrification,while high DO concentration is not conducive to the AOB denitrification pathways.Therefore,the work focused on the effect of DO(0.5-3.0 mg O2/L)on N2 O production in the aerobic granular sludge system.It was demonstrated that the N2 O emission factor(the ratio between N2 O nitrogen emitted and the ammonium nitrogen converted)decreased from 1.66% at DO=0.5 mg O2/L to 0.17%at DO=3.0 mg O2/L.The site preference(SP)measurements indicated that both the AOB denitrification and hydroxylamine oxidation pathways contributed to N2 O production,and DO had an important effect on the relative contributions of the two pathways.As DO increased from 0.5 to 3.0 mg O2/L,the contribution of AOB denitrification decreased from 50.49% to 19.38%,while the contribution of hydroxylamine oxidation pathway increased from 49.51% to 80.62%.The sludge flocs existing in the granular sludge system will affect the performance of the system.This work utilized a mathematical approach to systematically study the influence of the coexisting sludge flocs on total nitrogen(TN)removal and N2 O production in a granular nitritation-anaerobic ammonium oxidation(Anammox)process,based on a 2-pathway N2 O production model concept.The modelling results revealed that the highest TN removal efficiency decreased by about50% as the fraction of sludge flocs in the system increased from 10% to 40%,while the maximum production rate of N2O(N2OR)increases from 1.52 mg N/g VSS/hr to1.96 mg N/g VSS/hr.Meanwhile,both dissolved oxygen(0.05–0.30 mg O2/L)and the size of granule(200–400 μm)could also influence the TN removal efficiency and N2 O production.The N2 OR of the system first rised and then fell with the increase of DO concentration,and decreased with the increase of particle size.In addition,the hydroxylamine oxidation pathway dominated the AOB denitrification pathway in both granules and sludge flocs under various testing conditions.Overall,the research results revealed the influence of DO,particle size of granular sludge,and the proportion of sludge flocs on the N2 O produced by AOB in the aerobic granular denitrification sludge system,which would have potential guidance for the design and operation of wastewater treatment plants with the goal of reducing N2 O emissions.