Enhancement Of Biological Nitrogen Removal From Wastewater By Lowintensity Ultrasound

The traditional biological denitrification process is easy to be interfered by external environmental factors,resulting in the decrease of microbial activity and biological treatment performance.Ultrasonication is an environmentally friendly physicochemical technology,which is widely used in sludge treatment.In recent years,low-intensity ultrasound have been applied to biological treatment reactors,and it has been found that it can effectively improve biological activity and pollutant treatment efficiency under low temperature,low dissolved oxygen and low C/N.Therefore,low intensity ultrasound is adopted to effectively improve the microbial activity of the target flora and enhance the biological nitrogen removal by properly controlling the ultrasonication conditions.In this study,SBBR reactor was irradiated with lowintensity ultrasound.By controlling different ultrasonic irradiation parameters,the nitrogen removal efficiency of the SBBR was compared with the control group,and the ultrasonic parameters of biological denitrification were optimized.The enhancement effect of ultrasound on biological nitrogen removal under different C/N and different influent ammonia concentrations was studied.The EPS,microbial community structure and functional flora of microorganisms under different working conditions were analyzed to explore the enhancement mechanism of ultrasound on biological denitrification.The main conclusions of this study are as follows:(1)The removal of ammonia can be enhanced by low intensity ultrasound under the conditions of low influent ammonia concentration.The higher the ultrasonic power,the stronger the enhancement of the removal of ammonia.When the power was 45 W,the NAR was the highest.AOB accounted for the highest proportion,and NOB accounted for less than the control group.The higher the ultrasonic power,the lower the TN removal rate.The proportion of denitrifying bacteria in ultrasonic group decreased with the increase of ultrasonic power.The removal rates of ammonia and TN in the reactor were the highest under the ultrasonic power of 180 W,the irradiation time of 10 min and maintaining DO of 2mg/L.The content of EPS in the ultrasound group was higher than that in the control group,and with the increase of ultrasound power,the content of EPS increased gradually.The results of High-throughput sequencing showed that heterotrophic nitrification and aerobic denitrification bacteria existed in the biofilm under higher ultrasonic power.The control group had the highest proportion of NOB bacteria.The higher the ultrasound power,the higher the proportion of Candidatus Kuenenia.Under the operating mode of ultrasound in each cycle,the removal rate of ammonia was the highest when the ultrasonic power was 45 W under the ultrasonic time of 10 min.When the power was 45 W and the ultrasonic time was 5min,the TN removal rate was the highest.The removal rate of TN was the highest when the ultrasonic time was 30 min after one cycle on,and one cycle off.(2)With low ultrasonic power,as C/N increases,NAR decreases and TN removal rate increases.When C/N was 4,the removal rate of ammonia nitrogen was the highest.With the increase of C/N,the proportions of Nitrosomonas decreased gradually,and the proportions of Nitrosomonas increased when C/N was 5.With the different of C/N,the dominant bacteria of denitrifying bacteria changed.When C/N is 3,the proportion of anammox bacteria is the highest.At high ultrasonic power,the removal rates of ammonia and TN in the ultrasonic group at different C/N levels were higher than those in the control group.The increase of carbon source concentration will promote denitrification,while the DNRA reaction occurs when C/N is high.In this condition,the proportion of AOB bacteria was low and heterotrophic nitrificationaerobic denitrifying bacteria exists.Under the same C/N condition,the proportion of NOB bacteria in the ultrasound group was lower than the control group.Ultrasound can inhibit the of filamentous bacteria.When C/N is 1,the ratio of Candidatus Kuenenia is the highest,which is different from the low ultrasonic power phase.(3)Under the conditions of low influent ammonia concentration,TN removal load of ultrasonic group was higher than that of control group.Increasing the influent ammonia concentration too quickly may damage the microorganism in the reactor.When the load of influent ammonia was increased,there was an accumulation of nitrite nitrogen.The NAR of ultrasonic enhanced group was higher than that of the control group.With the increase of ammonia concentration in the ultrasonic enhanced group,EPS content decreased slightly.When the hydraulic retention time was increased,the influent load of ammonia decreased,the removal rate of ammonia and TN increased,and the removal load of TN decreased.EPS content in blank group was significantly increased.The higher the concentration of ammonia was,the higher the content of EPS was.There were heterotrophic nitrification and aerobic denitrification bacteria in both the ultrasound group and the control group.In the ultrasonic enhanced group,the proportion of heterotrophic nitrification-aerobic denitrifying bacteria was lower than the proportion of traditional denitrifying bacteria.The proportion of Anammox bacteria in the control group was higher than that in the ultrasonic enhanced group.(4)Under the condition of high influent ammonia concentration,the removal rate of ammonia and TN in the control group was lower than that in the ultrasonic group.When the concentration of ammonia is high,TN removal load is also high.The ammonia removal rate and TN removal load of the ultrasonic enhanced group were still higher than those of the control group when the influent ammonia concentration of the four groups was increased.When the influent concentration of ammonia is the highest,the proportion of Anammox bacteria is the highest.Under this condition,there are coupling effects of Anammox,shortcut nitrification and denitrification.The higher the concentration of ammonia was,the higher the content of EPS was.Under the condition of high concentration of influent ammonia,ultrasonic effect can promote the proportion of AOB and denitrifying bacteria.With the same concentration of influent ammonia,the proportion of Anammox bacteria in the control group was higher than that in the ultrasonic enhanced group.