Cycle Time

Basically, the SBR cycle time represents the duration of the substrate oxidation reaction; and it is interrelated to the hydraulic retention time. If the SBR is run at an extremely short cycle time, microbial growth can be hindered by an insufficient reaction time for microorganisms to break down substrates.

As a result, the sludge loss due to hydraulic washout from the system cannot be compensated by the growth of bacteria. Research showed that when a complete washout of sludge blanket occurred, a failure of nitrifying granulation resulted at a very short cycle time (Tay et al., 2002b). As showed in Fig. 5.7, only at a cycle time of 3 h, good nitrifying granules could be formed, while cycle time of 6 and 12 h has no granule formation.

Consequently, if the cycle time is kept much longer than that required for a biological reaction, hydrolysis of the biomass would eventually cause a negative effect on microbial aggregation (Tay et al., 2002b; Chen et al., 2003; Pan et al., 2004; Zhou, 2004). The cycle time of SBR should be short to suppress biomass hydrolysis, but long enough for biomass growth and accumulation in the system. Other research also demonstrated that, for SBRs operated at the optimum cycle time, aerobic granules still could not form if the settling time was kept longer than 15 min (Qin et al., 2004a,b).

Fig. 5.7. Nitrifying granulation at different cycle time. (a) cycle time of 12 h; (b) 6h; (c) 3h. Bar: 1 mm (Tay et al., 2002b).

This implies that cycle time would not be decisive in aerobic granulation in SBR.

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