A novel annular gap reactor was designed to create a controlled shear environment in which aerobic granular sludge could be developed. The bacterial and eukaryal community structures during two aerobic granular sludge experiments were tracked using denaturing gradient gel electrophoresis (DGGE). The first granule cultivation experiment, using an organic loading rate of 1.6 kg/m3d COD, resulted in biomass that was dominated by filamentous bacteria and Zoogloea ramigera colonies. A second experiment with a higher organic loading rate of 6 kg/m3d COD developed a granule-like morphology but was ultimately dominated by filamentous fungi. Species identification via DGGE band purification and DNA sequencing closely matched the observed sludge morphology and behavior.

Granules were observed after more than two years of operation in two semi-continuously fed intermittently aerated reactors treating swine wastewater with aerobic:anoxic cycles of 1:1 h and 1:4 h. Subsequently, the granules and flocs were compared with respect to physical characteristics, activity, and microbial community structure. Granules exhibited higher specific nitrification and denitrification rates than flocs. However, once granule structural integrity was disrupted, the rates decreased to levels similar to those of flocs. Membrane hybridizations using 16S rRNA-targeted probes showed that ammonia oxidizing bacteria populations in flocs and granules were dominated by Nitrosomonas and Nitrosococcus mobilis. Granules provided better conditions for Nitrospira compared to flocs. The diversities of the dominant bacterial populations in granules and flocs were not significantly different. Our findings highlight the importance of structural integrity of granules to their nitrogen removing activity.

Annular gap bioreactors or Couette-Taylor bioreactors (CTB) were used to apply varying shear forces to the biomass in this study. The shear forces in these bioreactors were controlled by adjusting the rotational speeds, allowing the shear to be independent of aeration rates. Both the inner-cylinder rotating CTB (i-CTB) and outer-cylinder rotating CTB (o-CTB) showed decreasing mean floc diameter as the shear increased. Denaturing gradient gel electrophoresis (DGGE) revealed that microbial communities from the activated sludge from both i-CTB and o-CTB were changing over time and indicated the potential contribution of protozoa to the development of aerobic granules.

No abstract

No abstract