| Aerobic granular sludge is characterized with layer structure which could provide a various environment for the coexistence of aerobic, anoxic and anaerobic microorganisms in a single particle and thus is efficient for nitrogen removal. However, its instability has limited a large-scaleapplication.This experiment was conducted in two geometrically identical SBR reactors with different influent COD/N conditions. The seed sludge of this experiment is the activated sludge from a wastewater treatment plant. Heterotrophic bacteria and nitrifying bacteria were selectively cultivated in the two reactors. The physical properties, stability, and nitrogen removal efficiency of the heterotrophic bacteria granular sludge and nitrifying granular sludge were compared in this study, and the following conclusions could be drawn:1. When the COD/N was 10:1, the granules composed of heterotrophic bacteria as well as filamentous bacteria. The density was 1.02g/mL at the initial stage of granulation. With the average particle size increased from 404μm to 2500μm, the density decreased to 1.007g/mL. When the no organic carbon was feed in the reactor, autotrophic granules were obtained, which composed of rod-like bacteria, and the initial density was 1.01g/mL. The density in stable period is as high as 1.06 g/mL. The average particle size is between 445～612μm and scanning electron microscope showed that the rod-like bacteria are the dominant microorganisms;2. In the mature stage, the diameter and the density of autotrophic granules were growing in a same fashion. The diameter of the heterotrophic granule, however, grew with a gradually decreasing density. Therefore, the growth of the former was comparatively steady, while the growth of the latter was not;3. According to the strength analysis, it was predicted that in the autotrophic system, the balance between EPS and porosity was main reason for the long term stability; however, the balance could not be obtained in the heterotrophic system and that the operational conditions could not control the overgrowth of filamentous bacteria was the main reason for the instability. Results from this study could help to select stable operational conditions for the long term stability of aerobic granular systems;4. For heterotrophic bacteria granular sludge, ammonia oxidizing bacteria (AOB) was more active r than the nitrite oxidizing bacteria (NOB). On the contrary, for autotrophic bacteria granules, AOB was worse. With heterotrophic bacteria granular sludge, the activity of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) was decreasing as the particle size grows; and since the autotrophic bacteria granular sludge is weak in resisting aeration corrosion, the activity of AOB in the outer layer of particles is decreasing with the growth of the particle size, while the activity of NOB in the inner layer was increasing with the growth of the particle size;5. With the autotrophic bacteria granular sludge, the short settling time in the initial stage of the experiment was the main reason for the AOB enrichment, and the particle formation was not significantly related to the enrichment of AOB. However, the nitrification rate and particle density was significantly related with each other. The formation of the particles was favorable to NOB fixation, and the accumulation of nitrate can contribute to particle stabilization. Therefore, there was a reinforcing effect between the granulation and NOB;6. With the autotrophic bacteria granular sludge, denitrifying bacteria can use the EPS as a carbon source for the process of denitrification. But the inadequate internal anaerobic environment caused by its small particle size in turn affected the denitrification. Generally, this process will be reinforced with the growth of the particle size.
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