『Abstract
　This paper investigated the influence of low-intensity ultrasound in biological nitrification and denitrification. The results showed that the nitrification activity of activated sludge could not be promoted significantly by ultrasound in 5-40 min with intensities ranging from 0.1 to 1.2 W cm^-2. It suggested the fact that nitrifying bacteria were insensitive to ultrasound, possibly related with their specific structures of cell membrane and ways of metabolism. Whereas, biological denitrification was enhanced quite remarkably by ultrasound and the optimal results were achieved at the ultrasonic intensity of 0.2 W cm^-2 and the irradiation time of 10 min. Compared with the control without ultrasonic irradiation, it took 5 h for the enhancement of denitrification rates induced by the optimum ultrasound to reach its peak level (16％). Therefore, ultrasound with intensity of 0.2 W cm^-2 could be employed in the biological denitrification system for 10 min every 5 h to obtain the optimal effect theoretically.

Keywords: Low-intensity ultrasound; Biological nitrification; Biological denitrification; Ultrasonic intensity; Irradiation time』

1. Introduction
2. Materials and methods
　2.1. Materials
　2.2. Ultrasonic irradiation facility
　2.3. Ultrasound method for the study of nitrification
　2.4. Ultrasound method for the study of denitrification
　2.5. Analytical and calculating methods
3. Results and discussion
　3.1. Influence of low-intensity ultrasound on biological nitrification
　3.2. Influence of low intensity ultrasound on biological denitrification
　3.3. The variation of biological denitrification ability after ultrasonic irradiation
4. Conclusions
Acknowledgements
References