| In recent years, pollution of the water environment which characterized by eutrophication is becoming a serious issue. Therefore, it is very important for taking some useful measures to control the water eutrophication. Moreover, the excessive discharge of nitrogen and phosphorus into water promotes the eutrophication of water. Thus, nitrogen removed from the water has attracted more interests recently. Nowadays, the effect of low intensity ultrasound irradiation on improving the activity of biological cells or enzymes has been confirmed by a great number of studies, which leads to opening up a new field of ultrasound application.In this study, low intensity ultrasound is used to enhance the nitrogen removal in the sequencing batch reactor. The optimum frequency, intensity, and irradiation time of ultrasound treatment were determined. The parameters of water (the organic and NH4+-N) were investigated to reveal the ultrasonic effects on nitrogen removal. The denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH) in molecular ecology technology are performed to analyze microbial community structure and their variations during different periods of this study. The main conclusions are as follows:①Ultrasound irradiation with different parameter combination has great influence to performance of nitrogen removal of the wastewater treatment. The results show that low intensity ultrasound irradiation can change the activity of sludge and the reaction system, enhance the nitrogen removal of wastewater treatment with the biological process. The optimum parameters of ultrasound treatment on nitrogen removal in this study are determined to be 35 KHz,0.15 W cm-2 and irradiation time of 10 min.②Ultrasound irradiation has many effects on the sludge in the reactor. The results are showed that ultrasound treatment could break the sludge, which led to the settle ability of the sludge declined. During the study, negligible negative effects of ultrasound irradiation on the sludge concentration are found.③The treatment performance of ultrasound irradiation enhanced SBR under ultrasound conditions of 35 KHz,0.15 W cm-2 and irradiation time of 10 min is studied. An important phenomenon is observed that the efficiency of the organic and NH4+-N removal varied with the different C/N ratio of the influent. At C/N ratio 15:1 and 10:1,compared with the control reactor of the removal of the organic and NH4+-N of the wastewater treatment, the effects of the ultrasound irradiation could be neglected. However, Compared with those of the control reactor (350 mg COD (g MLSS·d)-1,18.2 mg NH4+-N (g MLSS·d)-1),44.6% and 50% increase in organic load NH4+-N removal load were obtained.④With the decrease of C/N ratio of the influent from 15:1 to 5:1, the diversity of microbial community in the reactor enhanced by the ultrasound irradiation decreased at first, and then increased with operating time. At last, the microbial community is mainly composed of uncultured bacterium, and the quantity of the microbial almost keeps stable.⑤With the decrease of C/N ratio of the influent from 15:1 to 5:1, ammonia oxidizing bacteria (AOB) and Nitrite oxidizing bacteria (NOB) account for total bacteria in the reactor enhanced by the ultrasound irradiation changed from 18% to 35% and from 8% to 2%, respectively. At the same period, AOB accounts for total bacteria in the control reactor increased from 20% to 26%, and NOB decreased from 5% to 1%, respectively. It may be that ultrasound irradiation has effects on activity of AOB and NOB and their community distribution.
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