Fluid mechanism in an air-lift draft tube type sewage treatment reactor, which is operated under the conditions of low gas flow rate based on cross sectional area of the reactor (U
go) and low ratio of draft tube diameter to reactor diameter (D
i/D
o), was studied theoretically and experimentally. Empirical equations of gas hold up, pressure losses and liquid velocity in the draft tube were developed. It was found that liquid flow rate (Q
L) is almost proportional to the ratio of cross sectional area of the draft tube to that of the reactor (A
i/A
o). It was also found that under the conditions of U
go from 0.0005 to 0.005m
3/(m
2·s) and of A
i/A
o from 0.02 to 0.14 circulating time in the annulus is almost proportional to the ratio of the cross sectional area of the annulus to that of the draft tube (A
a/A
i).
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