Operation of a sequencing batch reactor for cultivating autotrophic nitrifying granules
Introduction
Nitrogen compounds like ammonia and nitrate can be found in many wastewaters and need to be removed in order to prevent oxygen depletion and eutrophication of surface waters. Nitrification, the biological oxidation of ammonia, was described already a century ago. Extensive reviews on autotrophic nitrification and nitrogen removal from wastewater are available (van Benthum et al., 1996). The rate-determining step in this process is nitrification, which is accomplished by autotrophic nitrifying bacteria under aerobic conductions (Ruiz et al., 2003, Ni et al., 2008). It is difficult to obtain and maintain sufficient nitrifying bacteria in wastewater treatment plants due to their very low growth rates.
In order to solve the problem, various techniques for retaining nitrifying bacteria with high density in a reactor have been recently proposed, e.g., entrapment in a hydrogel matrix of polyvinyl alcohol (Myoga et al., 1991) or polyethylene glycol (Sumino et al., 1992, Isaka et al., 2007). However, development of a simpler and more effective immobilization method for nitrifying bacteria is still demanded.
Aerobic granulation represents an innovative cell immobilization strategy in biological wastewater treatment and it is attracting increasing interests (Beun et al., 1999, Zheng et al., 2005, Su and Yu, 2005, Wang et al., 2007, Liu et al., 2009). Aerobic granules are self-immobilized microbial aggregates that are usually cultivated in sequencing batch reactors (SBR) without adding a carrier material. Many researchers reported aerobic granulation for efficient treatment of organic wastewater (Zheng et al., 2005, Su and Yu, 2005, Wang et al., 2007). However, the information on the nitrifying bacteria granulation with inorganic wastewater rich in ammonium is limited (Tsuneda et al., 2003, Liu et al., 2008, Ni and Yu, 2008). Tsuneda et al. proved that nitrifying bacteria could be self-immobilized in an aerobic upflow fluidized reactor. Spherical, pseudocubic and elliptical granules with a diameter of 0.35 mm were produced at the bottom of the reactor after 300 days of operation. The reactor was operated continuously at a hydraulic retention (HRT) of 7.6 h and influent -N of 500 mg/l. Liu et al. also cultivated nitrifying granules in an SBR (Tsuneda et al., 2003). Although autotrophic nitrifying granules for nitrification have been developed, the formation of nitrifying granules and the distribution of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in the nitrifying granules are still not clear up to now yet.
Therefore, the main objective of this work was to cultivate nitrifying granules in an SBR and elucidate the distribution of AOB and NOB in the granules. The physical properties of granules, such as hydrophobicity of cell surface, settling velocity and specific gravity, were also investigated. It is expected that the work would be useful to better understand the mechanisms responsible for the granulation of nitrifying cultures and apply them for the treatment of -N-laden inorganic wastewaters.
Section snippets
Reactor set-up and operation
The SBR had a working volume of 3.4 l with an internal diameter 6.0 cm and a height of 130.0 cm. The reactor was operated for 6 h each circle with a HRT of 12 h. Effluent was drawn at 60.0 cm from the bottom, resulting in 1.7 l of mixed liquor left in the reactor after effluent withdrawal. The filling and withdraw time were 2 and 5 min, respectively. The settling time was varied from 20 to 5 min and the remainder was the reaction time. The seeding sludge, taken from an aeration tank in Wangxiaoying
Formation of nitrifying granules
The seeding sludge with a mean floc size of 0.10 mm had a fluffy, irregular and loose-structured morphology. After 120-day operation, spherical and elliptical granules were formed. These granules increased in size and their average diameters reached 0.32 mm. The nitrifying granules had a compact and round-shaped structure with a clear outer shape. No filamentous bacteria were observed on the granule surfaces.
The aerobic granulation, i.e., from dispersed sludge to mature granules, is a gradual and
Conclusions
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After 120-day of operation, compact nitrifying granules were formed. Their surface hydrophobicity, settling velocity and specific gravity increased with the sludge granulation.
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The AOB were close to the granule surface, while the NOB were found in the deeper layer of granules. The total number of Nitrobacter was much smaller than that of the AOB.
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The Michaelis–Menten equation could appropriately describe the -N utilization rate of the granules with a vm of 18.0 mg/g-VSS/h and Km of 36.7 mg/l.
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An
Acknowledgements
The authors wish to thank the Natural Science Foundation (50625825, 50738006 and 50828802), the Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (2008ZX07316-002 and 2008ZX07316-003) for the partial support of this study.
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