Many factors have been known to affect the formation of aerobic granules in sequencing batch reactor (SBR) as briefed earlier. Compared with continuous microbial culture, the main feature of SBR is its cycle operation, i.e. each cycle consists of filling, aeration, settling, and discharging. In SBR, the settling time is likely to exert a selection pressure on the sludge particles. Only particles that can settle down within a given settling time would be retained in the reactor, otherwise they would be washed out of the system. Selection pressure in terms of upflow velocity has been recognized as a driving force towards successful anaerobic granulation in upflow anaerobic sludge blanket (UASB) reactors (Hulshoff Pol et al., 1988; Alphenaar et al., 1993). Similarly, in aerobic granulation a selection pressure should be created to promote the formation of aerobic granules in SBR.

It has been observed that biogranulation can occur by different species including methanogens, acidifying bacteria, nitrifying bacteria, denitrifying bacteria, and aerobic activated sludge. Tay et al. (2002) studied nitrifying granulation and found that even for the defined nitrifying bacteria, nitrifying granules formed only at a strong selection pressure. These seem to indicate that aerobic granulation is a microbial phenomenon induced by environmental conditions through changing the microbial surface properties and metabolic behaviors (Tay et al., 2001b; Pan et al., 2004; Qin et al., 2004b; Wang et al., 2005a). Therefore, aerobic granulation should be species-independent and could be inducible instead of constitutive (Liu et al., 2005a).

Aerobic granulation in SBR could fail without the proper control of settling time or exchange ratio during the operation of SBR. The exchange ratio is defined as the liquid volume withdrawn at the end of the given settling time over the total reactor working volume (Wang et al., 2005a). For a column SBR with the same diameter, the exchange ratio is proportionally correlated to the height from the discharging port to the water surface as illustrated in Fig. 4.1. Settling time and exchange ratio in SBR could be the most effective selection pressures for aerobic granulation.

Aeration Exchange ratio: 80%

Aeration 60%

Aeration

Aeration 20%

Fig. 4.1. Schematic interpretation of exchange ratios in column SBRs (Liu et al., 2005 a).

Aeration Exchange ratio: 80%

Aeration 60%

Aeration

Aeration 20%

Fig. 4.1. Schematic interpretation of exchange ratios in column SBRs (Liu et al., 2005 a).

However, the question is how the settling time and exchange ratio determine aerobic granulation in SBR. In the operation of a column SBR for aerobic granulation, the effluent is discharged at a discharge outlet (Fig. 4.1), i.e. the volume of water above the discharge port is withdrawn at the end of the designed settling time. Liu et al. (2005b) proposed the following equation to describe the settling velocity of bioparticles:

where Vs is the settling velocity of bioparticles, dp is diameter of particle, SVI stands for sludge volume index, X is biomass concentration, a and f are two constant coefficients. Equation (4.1) shows that the settling velocity of aerobic granular sludge is determined by the size of granule, SVI and biomass concentration of granules. If the distance for mixed liquor to travel to the discharge port is L (Fig. 4.1), the corresponding traveling time of bioparticles can be computed as follows

Equation (4.2) shows that a higher Vs results in a shorter traveling time for bioparticles to the discharge port. Hence, the bioparticles with a traveling time longer than the designed settling time would be discharged out of the reactor. In this case, Liu et al. (2005a) proposed that there would be a minimum settling velocity, (Vs)min, for bioparticles to be retained in the reactor; and it can be defined as follows:

settling time

Equation (4.3) implies that bioparticles with a settling velocity less than (ys)min could be withdrawn from the reactor, while only those bioparticles with a settling velocity greater than (Vs)min can be retained in the system.

As shown in equation (4.3), (Vs)min is a function of the settling time and L, which is proportionally related to the exchange ratio (Fig. 4.1). Therefore, the fastest settling bioparticles are heavy, spherical aggregates, while the slowest settling particles, which sometimes cannot be settled properly, are tiny, light, irregularly shaped aggregates. It appears that bioparticles can be selected according to their settling velocity. Equation (4.3) provides the explanation why the settling time and exchange ratio in SBR can serve as the effective selection pressures that allow the selection of good settling bioparticles, leading towards successful aerobic granulation. It has been proposed that SBR should have a high HID ratio to improve the selection of granules by the difference in settling velocity (Beun et al., 2002). In fact, it seems that the HID ratio of SBR is not a selection pressure for aerobic granulation, but a larger H/D ratio is desirable in the design of full-scale SBR because it may allow more space for engineers to manipulate L and subsequently (Vs)min according to actual needs. In addition, there is strong evidence that selection pressures have a profound effect on the surface properties of aerobic granules in terms of cell surface hydrophobicity and extracellular polysaccharides, which in turn favor the formation of aerobic granules in SBR. A similar phenomenon was also observed in anaerobic granulation in UASB reactors (Mahoney et al., 1987; Schmidt and Ahring, 1996). As equation (4.1) shows, the settling velocity of a bioparticle is closely related to the diameter of the aggregate. Thus, it is likely that microbial granulation induced by selection pressures is an effective microbial survival strategy that enables the bacteria to aggregate into big granules and consequently avoid being discharged.

Qin et al. (2004a) studied the effects of various settling times of 5 to 20min on aerobic granulation at a fixed L, while Wang et al. (2005a) looked into the effects of different exchange ratios on aerobic granulation at a given settling time. Thus, using these data, Liu et al. (2005a) further calculated the minimum settling velocity required for bioparticles to be retained in SBR operated under various settling times and exchange ratios. Figure 4.2 shows the relationship between (Vs)min and the fraction of aerobic granules to total biomass by weight in SBRs. It can be seen that the fraction of aerobic granules to total biomass in the reactor almost linearly increases with the increase in (Vs)min, indicated by a correlation coefficient of 0.96.

Figure 4.2 may imply that the effects of settling time and exchange ratio on aerobic granulation can be unified to and interpreted very well by (Vs)min, through which good settling bioparticles would be selected and retained in the reactor. When (Vs)min is smaller than 3.8 mh-1, the suspended bioflocs are dominant in the system (Fig. 4.2). In fact, the typical settling velocity of suspended activated sludge is generally less than 4 to 5mh-1 as reviewed by Giokas et al. (2003). Thus, if the SBR is operated at a (Vs)min below 3.8mh 1, suspended sludge could not be effectively withdrawn from the reactor. Successful aerobic granulation

Fig. 4.2. Relationship between the fraction of aerobic granules to total biomass and (Vs)min: filled circles at different settling times and a constant L of 0.63 m and hollow circles at various L and a constant settling time of 5 min (Liu et al., 2005 a).

Fig. 4.2. Relationship between the fraction of aerobic granules to total biomass and (Vs)min: filled circles at different settling times and a constant L of 0.63 m and hollow circles at various L and a constant settling time of 5 min (Liu et al., 2005 a).

was also reported at the respective settling velocities of 10.0 m h-1 and 16.2 m h-1 (Beun et al., 2000, 2002). The growth rates of aerobic granules were found to be much lower than that of suspended activated sludge (Yang et al., 2004; Liu et al., 2005c). It should be a reasonable consideration that suspended sludge could easily outcompete aerobic granules due to its faster growth. Such outcompetition in turn would repress aerobic granulation and eventually leads to the disappearance of the aerobic granular sludge blanket in SBR if suspended sludge is not effectively withdrawn. Therefore, Liu et al. (2005a) recommended that (Vs)min must be controlled at a level higher than the settling velocity of suspended sludge, otherwise a rapid and successful aerobic granulation could not be achieved and maintained stably in SBR. Therefore, enhanced selection of bioparticles for rapid aerobic granulation can be realized through properly controlling and adjusting the settling time or the exchange ratio in SBR. However, compared with the exchange ratio, control of the settling time is a more flexible manipulation during full-scale SBR operation (Liu et al., 2005a).

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