Effects of heterotrophic and autotrophic bacteria on the stability of aerobic granular sludge

The characteristics of granules of heterotrophic and autotrophic bacteria were studied. The autotrophic granules were characterized by smaller granule size, higher density, higher concentration of extracellular polymeric substances (EPS) and weaker strength compared with the heterotrophic ones. The diameter and density of autotrophic granules were changed in the same fashion, resulting in stable granules, and the heterotrophic granule, however, grew up with the increase of diameter and the decrease of density. According to the strength analysis, it is proposed that in the autotrophic system, the balance between EPS and porosity is the 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 reason for the instability.