Use of low frequency and density ultrasound to stimulate partial nitrification and simultaneous nitrification and denitrification
Introduction
Recently, more stringent effluent nitrogen standards (ammonia nitrogen ⩽1.0 mg/L; total nitrogen (TN) ⩽10 mg/L) have been imposed upon municipal wastewater treatment plants (WWTPs) in Beijing (DB11/890-2012), China. Consequently, technologies that perform partial nitrification and simultaneous nitrification and denitrification (SND) have attracted attention because they offer several advantages over conventional biological nitrogen removal, such as oxygen consumption, carbon resource, and land savings (Third et al., 2003, Wang and Yang, 2004, Peng and Zhu, 2006, Chen et al., 2009, Daniel et al., 2009, Wang et al., 2010). Extensive, research has been focused on partial nitrification and SND achieved by low dissolved oxygen (DO). However, low DO is difficult to maintain stable operation of bioreactors, especially in the sludge bulking control. Hence, studies on partial nitrification and SND achieved at full aerobic (DO above 3.0 mg/L) are still essential.
Ultrasound is a cyclic sound pressure with a frequency greater than 20 kHz. In recent years, several studies have indicated that low frequency and density/intensity ultrasound can be used to enhance the performance of wastewater treatment (Liu et al., 2005, Sears et al., 2005, Xie et al., 2008, Xie and Liu, 2010), particularly for nitrogen removal (Biradar et al., 2010, Duan et al., 2011, Zhang et al., 2011). Low frequency and density ultrasound may improve the growth of microbial cells by increasing the amount of transported oxygen and nutrients transported to, and waste products away from the cells, whereas high frequency/density ultrasound could kill microbial cells in the sludge and shatter the sludge granular (Pitt and Ross, 2003).
Ultrasonic treatment is an emerging and highly effective mechanical pretreatment method to enhance the biodegradability of sludge. Thus, it is very useful to all WWTPs in treating and disposing sewage sludge (Wei et al., 2003, Pilli et al., 2011). Previous works have found that ultrasonic lysis–cryptic growth process showed promising results ranging from 29.0% to 100.0% reduction of excess sludge in activated sludge systems (Rai et al., 2004, Yoon et al., 2004, Zhang et al., 2007, Zhang et al., 2009, Mohammadi et al., 2011).
For the practicability of using ultrasound to enhance nitrogen removal, the present study aims to stimulate partial nitrification and SND processes by using low frequency and density ultrasound, as well as to optimize the ultrasound irradiation mode and clarify the behavior of ultrasonic treatment in a sequencing batch reactor (SBR). Two SBRs, with and without ultrasound, were operated in parallel and triplicated at different irradiation times. Nitrite accumulation and SND efficiency were measured and compared. Changes in system properties resulting from ultrasound irradiation, e.g., removal load, biological activities, temperature, pH, microbial community, carbon source denitrification, and sludge properties were further discussed.
Section snippets
Experimental setup
Two SBRs with the same specifications and operating conditions were used. One was coupled with an ultrasound generator (ZJS-1000-500, Hangzhou Success Ultrasonic Equipment Co. Ltd.), known as the ultrasound enhanced reactor (UER), while the other was operated in parallel without an ultrasound as the control reactor. Each reactor had a working volume of 2.7 L. The operational cycle of the SBR was 8 h, during which 5 min was spent for water inflow, 3 h for anoxic, 4 h for aeration, 50 min for sludge
The performance of partial nitrification and SND stimulated by ultrasound
The changes in effluent concentrations in the UER and control reactor at triplicate are shown in Fig. 1. Significant nitrite accumulation in the UER effluent occurred with the decrease in effluent concentration in Run 4. The results indicated that the optimal prolonged ultrasonic time of 2.0 h could inhibit the conversion of , and promote partial nitrification.
Fig. 2 shows that accumulation and TN removal were both obtained during the aerobic period in
Conclusion
Low frequency and density ultrasound with the irradiation time of 2.0 h was found to effectively stimulate partial nitrification and SND processes at full aerobic in the SBR. It improved the nitrite accumulation ratio and SND efficiency to 73.9% and 72.8%, respectively. Ultrasound irradiation could improve AOB activity for removal, inhibit NOB activity for nitrite accumulation, and offer an additional carbon source for denitrification. The shock wave produced by ultrasound could increase
Acknowledgement
The authors wish to thank the National Environmental Protection Research of China (2011467042) for their support of this study.
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