Life Cycle of the Granule and Determination of Retention Time for the Granules in SBR

Probably, addition of disrupted, previously formed granules can also be a method for facilitation of granulation. It can be useful in case the life cycle of the granule is short and new granules are produced not from the existing granules but from the cells or particles of disrupted granules.

Attachment and removal of granule biomass was evaluated from the curves showing the content of the fluorescent lipophilic tracer in biomass of granules (Fg) and flocs (Fs) for period of cultivation between 2 and 6 days after labeling (Fig. 10.1). Concentration of granular biomass (VSS) during that period was stable, at 6.5 ± 0.2 g L-1. Concentration of floc biomass (VSS) was 0.15 ± 0.02 g L-1. The hydraulic residence time was 0.33 d, which corresponded to a daily exchange of three reactor volumes. Therefore, the ratio of produced granular biomass to produced floccular biomass was 14.5. This ratio was close to 18.3, the initial ratio of granular labeled biomass to the flocculent labeled biomass after 4 h of labeling (one growth cycle in SBR is 4 h). The granules were retained in the SBR while the flocs were washed out with the effluent. Therefore, stable concentration of granular biomass can be due to the balanced attachment and detachment of the flocs to granules. Content of lipophilic tracer in granular biomass was stable for 6 days of study (Fig. 10.1) and was thought to be because of balanced attachment and detachment of the flocs to granules or balanced growth and destruction of the granules. It cannot be the result of negligible degradation of granules because the labeled biomass permanently released as the labeled flocs. The tracer content could decrease if the rate of granule growth is higher than the rate of granule degradation.

Time, d

Fig. 10.1. Changes in fluorescence of the lypophilic label in the column sequencing batch reactor (curve 1, label of fast settling granules) and in the effluent (curve 2, fluorescence of sludge x 10). Dashed lines show the trend of fluorescence of the granules and the flocs.

Time, d

Fig. 10.1. Changes in fluorescence of the lypophilic label in the column sequencing batch reactor (curve 1, label of fast settling granules) and in the effluent (curve 2, fluorescence of sludge x 10). Dashed lines show the trend of fluorescence of the granules and the flocs.

The duration of the life cycle of the granule was evaluated by determining the intersection of the extrapolated line of Fs with the time axis assuming that this is the time (tg) taken for all labeled material to be detached from the granules and removed from the SBR. The value of tg was 11.8 days for detachment of 100% of labeled material (or degradation of 100% of labeled granules). This means that labeled matter could disappear from the granules completely after approximately 12 days, i.e. that lifetime for the studied granules or for the labeled matter of these granules was 12 days. A similar duration is required to transform flocs into granules. If the lifetime of the granules (not just lifetime of their labeled matter) is 12 days, the retention time for the granules must be probably less than their lifetime to avoid degradation of the granules. Therefore, it would be better for stable wastewater treatment process to use particles from disrupted granules as the seeds to ensure their further growth to the matured and stable granules, however.

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