Since aerobic bacteria grows much faster than anaerobic bacteria, the stability of aerobic granules appears to be poorer than that of anaerobic granules developed in upflow anaerobic sludge blanket (UASB) reactor. Obviously, the poor stability of aerobic granules would limit its application in wastewater treatment practice. Existing evidence shows that the stability of biofilms is closely related to the growth rate of bacteria, i.e. a higher growth rate of bacteria resulted in a weaker structure of biofilms (Tijhuis et al., 1995; Kwok et al., 1998; Liu et al., 2003b). Therefore, research attention has been given to microbial selection-based strategy for improving the stability of aerobic granules (de Kreuk and van Loosdrecht, 2004; Liu et al., 2004c). These would be very useful for the development of full-scale aerobic granules-based bioreactor for wastewater treatment.

The growth of aerobic granules after the initial cell-to-cell attachment is the net result of interaction between bacterial growth and detachment, while the balance between growth and detachment processes in turn leads to an equilibrium or stable granule size (Liu and Tay, 2002). Thus, size evolution of the microbial aggregates can be used to describe the growth of granular sludge. The specific growth rate (|d) by size of microbial aggregates can be defined as dD/dt id = —^ (8.4)

in which D is the mean size of microbial aggregates, and t is the operation time. Integrating equation (8.4) gives ln D = id t + constant (8.5)

Thus the size-dependent specific growth rate of microbial aggregate can be determined from the slope of the straight line described by equation (8.5). Figure 8.9 shows the effect of substrate N/COD ratio on |d (Yang et al., 2004b). It is obvious that a higher substrate N/COD ratio had resulted in a lower specific growth rate of aerobic granules with smaller size (Fig. 8.9).

Moreau et al. (1994) reported that the activity distribution of nitrifying population over heterotrophic population in biofilms was proportionally

related to the relative abundance of two populations under given conditions. As discussed earlier, nitrifying population in aerobic granules is enriched significantly with increasing substrate N/COD ratio. The observed growth rate and mean size at steady state aerobic granules are closely related to the substrate N/COD ratio, i.e. higher substrate N/COD ratio results in smaller granules with lower growth rate (Fig. 8.9). As nitrifying population enriched at high substrate N/COD ratio, heterotrophs in aerobic granules became less dominant. It seems that the high substrate N/COD ratio is an important factor in microbial selection with a predominantly nitrifying population.

It has been known that nitrifying bacteria grows much slower than heterotrophs, while the physical structure of nitrifying biofilms was much stronger than that heterotrophic biofilms (Oga et al., 1991). Increasing evidence shows that the observed growth rate of aerobic granules can be significantly lowered by enriching slow-growing nitrifying population, and this can be achieved through proper control of substrate N/COD or P/COD ratio (de Kreuk and van Loosdrecht, 2004; Liu et al., 2004c).

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