Selection of Pure Cultures for Facilitated Granule Formation

The production of compact aerobic granules is favored by a short sludge settling time, which selects for bacterial aggregates with a high settling velocity (Beun et al., 1999, 2000). The formation of a stable granular structure has also been positively correlated with the strength of the hydro-dynamic turbulence caused by the upflow aeration in a sequential batch reactor (Tay et al., 2001a).

The enrichment culture, with the ability of accelerated granulation, can be obtained by repeating 10 min settling and batch cultivation of fast settling microbial aggregates isolated from the aerobic granular sludge. Aggregation index (AI) of cells in the enrichment culture increased from 4 to 34% after 13 cycles of selection, and is transferred to a liquid medium (Fig. 10.4).

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Fig. 10.4. Aggregation index and supernatant turbidity change during enrichment culture transfers. (A): Aggregation Index; (■): Turbidity of supernatant after 10min settling.

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Fig. 10.4. Aggregation index and supernatant turbidity change during enrichment culture transfers. (A): Aggregation Index; (■): Turbidity of supernatant after 10min settling.

Enrichment culture, produced after 13 cycles of selection and transfers, was used because there were no significant changes of cell aggregation ability after 13 cycles (Fig. 10.4). During batch cultivation of enrichment culture after 13 transfers, cell aggregation increased in stationary phase

Both the increased AI and decreased supernatant turbidity after settling of biomass indicated the good aggregation ability of enrichment culture.

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