Characteristics And Mechanisms Of NO And N₂O Accumulation During Nitrite Denitrification

Biological nitrogen removal?BNR?is an effective and economic technology in wastewater treatment.Nitrite denitrification,as a novel BNR developed in recent years,has received increasing attention from domestic and foreign experts.Compared with conventional biotechnology,there are many advantages in nitrite denitrification,suhc as fast reaction rate,carbon source saving,and low sludge production.However,some unexpected intermediates,nitric oxide?NO?and nitrous oxide?N₂O?,are produced in this process due to influence of operating condition and water quality.It is known that NO and N₂O accumulations can induce various environmental problems,including greenhouse effect,acid rain,ozone layer destruction,photochemical smog and so on.In addition,NO can also cause serious effect on microorganisms,animals and human beings.Therefore,it is not only to achieve high pollutant removal but also to control NO and N₂O productions in biological wastewater treatment.There are many researches on N₂O accumulation in nitrite denitrification,however,NO accumulation was rarely studied.Thus,it is important to further investigate the mechanism of NO and N₂O productions in nitrite denitrification.Based on this,some effective improvement measures can be put forward,which is of great significance to promote the wide application of this tecnology.In this study,the effects of different factors?such as COD/N,initial pH,influent concentration,phosphorus concentration,and starvation time?on the characteristics of NO and N₂O productions during nitrite denitrification were investigated,and the mechanism was analyzed in detail by the microbial functional genes,microbial community structure,and activated sludge mathematical model.The main conclusions are as follows:?1?Low COD/N could mitigate NO accumulation,but led to high N₂O accumulation.It is speculated that NO accumulation is caused by rapid denitrification with accumulated electron in anaerobic stage.N₂O accumulation is related to electron competition.?2?Low pH and high influent concentration could cause serious NO and N₂O accumulation.When NO was higher than 3.58 mg L^-1,which could completely inhibit the activities of reductases involved in denitrification.High NO and N₂O accumulation are mainly caused by significant free nitrous acid?FNA?and NO inhibition at low pH and high influent concentration.In addition,the stimulation of nitrite reductase?Nir?by nitrite may also contribute to NO accumulation.?3?The NO and N₂O accumulation at high phosphorus concentration was only 9.85 and11.07%of that at low phosphorus concentration.The strong recovery of microbial activity and the high abundance of denitrifier at high phosphorus concentration can promote NO reduction.There were more Polyhydroxyalkanoates?PHA?can be synthesized at high phosphorus concentration,which is conducive to alleviating the electronic competition among reductases.Meanwhile,the enrichment of denitrifying phosphate-accumulating organisms at high phosphorus concentration also contributes to reduce N₂O production.?4?High NO accumulation occurred at beginning of the test under different starvation time is mainly related to the imbalance of electron distribution caused by feast after famine.Furthermore,the longer the starvation would cause more serious imbalanced electron distribution.While,the imbalance of electron distribution is alleviated due to low microbial activities at long starvation time.The N₂O accumulation is mainly attributed to electron competition,free nitrous acid?FNA?inhibition,and starvation time.In addition,results suggest that the starvation time affects NO accumulation more significantly than that of N₂O.?5?High DO was detected under different conditions during nitrite denitrification,which is partly related to NO dismutation during NO detoxication.Furthermore,the more NO accumulation,the more serious the inhibition of denitrification,and the more obvious the NO dismutation.?6?The comprehensive and accurate mathematical models of the Activated sludge model for nitrogen?ASMN?and Activated Sludge Model with Indirect Coupling of Electrons?ASM-ICE?were established based on the analysis on nitrite denitrification.They can be used not only to predict the pollutants transformation,but also to analyze the mechanism of NO and N₂O accumulations.The establishment of this model can provide a theoretical basis for the actual operation and parameter optimization of the nitrite denitrification.