Selection of Microbial Seeds from Granules

The efficiency of biological wastewater treatment depends on the growth of metabolically capable microorganisms and efficient separation of those organisms from the treated effluent. Bacterial cells used in conventional wastewater treatment aggregate and form flocs. To separate these flocs in conventional activated sludge system, a big secondary sedimentation tank is required because of relatively slow settling velocity of sludge flocs. In contrast, microbial granules settle significantly faster. The average settling velocity of microbial granules with a diameter of 3.2 mm was 0.97 cm s-1 (Etterer and Wilderer, 2001). This good settleability of the granules makes settling tanks superfluous (de Bruin, 2004; de Kreuk and van Loosdrecht, 2004). The benefits expected from aerobic granulation are compact treatment plants and simple reactor design (de Kreuk et al., 2005).

The purpose of this research was to select aggregate-associated bacterial cultures from microbial granules and to examine their ability to accelerate formation of granules during wastewater treatment. One way to achieve this goal was to isolate small aggregates, to disperse them, and then to study reaggregation. However, it was found by Snidaro et al. (1997) that it would be unlikely to disrupt totally the microcolonies of activated sludge flocs without significant cell lyses because cells are tightly bound together by a gel matrix. These microcolonies had a medium diameter of 13 |xm and were linked by polymers (Li and Ganczarczyk, 1990; Jorand et al., 1995; Snidaro et al., 1997). Therefore, the idea of our experiments was to select self-formed microcolonies after destruction of granules, to separate microcolonies/microaggregates by fast settling, and then to grow them in fresh medium. By repeating this selection procedure, aggregates-forming microbial culture was enriched, and microbial strains with high aggregation ability have been isolated. Cell aggregation in enrichment culture appeared during stationary phase of batch cultivation. It was suggested, the depletion of nutrients could stimulate cell aggregation. It is known that under starvation, bacterial cell surface become more hydrophobic and it might facilitate cell aggregation (Bossier and Verstraete, 1996).

Microbial granulation is an autoselection process, a priori causing accumulation of cells with high aggregation ability in formed granules. Therefore, these cells could be selected, isolated, selected, and used to start up a facilitated granulation process.

Microbial cells with high cell surface hydrophobicity and high set-tleability were selected from the disrupted granules. The granules were taken from a reactor, disrupted in a beater for 2 min, and then the disrupted granules were filtered through a 25-^m pore membrane. Two kinds of microaggregates produced were studied after 30 min of settling. One type of microaggregates, with high hydrophobicity, was accumulated in the biofilm attached to the water-air interphase. Another type of microaggregates, with high settling velocity, settled down and accumulated on the bottom of the tube. The size distributions of these microaggregates were different (Fig. 10.2). Microaggregates with high hydrophobicity had narrow size distribution with mean diameter of particles 3 |xm, while diameter of particles without any selection (cells from the bulk of suspension) was 2 |xm. Particles with high settleability had wider size distribution with mean diameter of particles 6 |xm. Fast formation of two types of cell aggregates from microbial granules was used for selection of microbial seeds facilitating formation of microbial granules.

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