Aerobic granulation draws increasing interest in the quest for innovative techniques in biological wastewater treatment. Compared to conventional activated sludge flocs, aerobic granulation has several advantages, such as excellent settleability, ability to withstand high organic load, dense and strong microbial structure, and tolerance to toxicity.

In the nitrifying aerobic granular sludge, the N removal mechanism of simultaneous nitrification and denitrification (SND) is carried out by the autotrophic ammonia oxidizing bacteria (AOB) in the surface where oxygen is rich, and the anoxic denitrifiers inside the granule where oxygen is rare.

However, in the heterotrophic aerobic granular sludge cultivated at high COD and N loading rates, the relative abundance of heterotrophic microorganisms increase considerably with the raise of influent COD, while AOB decrease remarkably. The SND mechanism is not adequate to explain the N removal when heterotrophic microorganisms are dominant. Explorations of the N removal mechanism need to be done at a high COD and N loading rates in the heterotrophic aerobic granular sludge.

Prof. Huang Jun's group of Chengdu Institute of Biology cultivated a heterotrophic aerobic granular sludge with modified piggery wastewater, the granular sludge was capable of simultaneously removing COD and N at high COD and N loading rates. Further studies show that isolate heterotrophic Thauera strain TN9 was the most dominant microorganism in the granular sludge and played an important role in granular sludge formation, stability, N and COD removal. No ammonia oxidizing archaea (AOA) or anammox bacterium was detected in the granular sludge. Although AOB were detected using amoA clone libraries, they were not the dominant microorganisms in the granular sludge. Therefore, it can be conclude that the heterotrophic nitrification and autotrophic nitrification coexist in the granules. The heterotrophic nitrification might contribute more to the N removal at high COD and N loading rates. More results have been published in Bioresource Technology, 2013, 143:439-446.

This research got supports from the National High-Tech Research and Development Program of China and the Open Fund from Key Laboratory of Environmental and Applied Microbiology, Chinese Academy of Sciences  

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