Optimization of Granule Size

The concentric layers were typically arranged in sequence as obligate aerobic bacteria, facultative anaerobic bacteria, obligate anaerobic bacteria, and finally a core of dead and lyzed cells. The presence of anaerobic bacteria can potentially diminish the stability of the granules due to the production of acids and gases from fermentation. Another negative effect of anaerobic bacteria on the wastewater treatment process is the occurrence of floating granules, which could occur if anaerobic bacteria are allowed to incubate in medium containing nitrate accumulated due to nitrification (Fig. 6.6). There were anaerobic conditions in the layer of settled granules. Therefore, floating of the granules was probably due to gas production during denitrification, similar to the floatation of denitirifying granules (Etchebehere et al., 2002). This potential floating of the microbial granules in case of high organic or nitrate load leading to the production of gases in anaerobic zone of the granule can deteriorate wastewater treatment.

Fig. 6.6. Floating of the granules after settling. Fig. 6.6a to 6.6e correspond to 2, 24, 25, 28, and 32 min after settling of the granules. The sample was collected at the end of one cycle of cultivation in SBR fed with synthetic wastewater which was composed of 1000 mg L-1 of COD (ethanol), 300 mg L-1 of ammonia nitrogen, 2400 mgL-1 of bicarbonate, and micronutrients. Almost all ammonia was oxidized to nitrate by the end of cycle.

Fig. 6.6. Floating of the granules after settling. Fig. 6.6a to 6.6e correspond to 2, 24, 25, 28, and 32 min after settling of the granules. The sample was collected at the end of one cycle of cultivation in SBR fed with synthetic wastewater which was composed of 1000 mg L-1 of COD (ethanol), 300 mg L-1 of ammonia nitrogen, 2400 mgL-1 of bicarbonate, and micronutrients. Almost all ammonia was oxidized to nitrate by the end of cycle.

To avoid the formation of anaerobic layer and core and possible deterioration of wastewater treatment, the aerobic granules should have a diameter that is less than twice the distance from the granule surface to the anaerobic layer. This minimal distance is 850 |xm (Table 6.2). Therefore, diameter of the granules without anaerobic layer and core of lyzed cells should be less than 1.7 mm.

Another approach of size optimization is based on the assumption that the entire granules should have a porous biomass-filled matrix without a core filled by dead and lyzed cells. Depth and thickness (Hl) of the layer of porous biomass linearly correlated with granule diameter (Dg) by equation (6.1):

The optimal size of the aerobically grown microbial granule (Dc) may be calculated from equation (6.1) using the condition that 2Hl = Dg, which means that whole granule with diameter less than or equal to Dc is consisting entirely of a porous matrix. The value of Dc calculated from equation (6.1) for this condition is 0.5 mm.

Physiological parameters such as specific COD removal or oxygen uptake rate cannot be used for conclusion on optimal diameter of the granules because increase of granule size diminishes the TOC and COD removal rate per 1g of VSS of the granules (Toh et al., 2002). The optimal diameter of the studied aerobic granule is less than 1.7 mm considering absence of the layer of obligate anaerobic bacteria or less than 0.5 mm considering that the whole granule should have a porous biomass-filled matrix. Design of the granulation process and reactor must include the condition to select or retain in the reactor the granules with a diameter smaller than the critical diameter. This critical diameter may be substrate-and process-specific parameter.

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