Ro

"Hi"

where P is the power input in kW, RO is the oxygen requirement in kg/hr. and is the in-process energy efficiency for the mechanical aeration system in kg O,/ (kW-hr). The value of ti,, typically ranges from 0.7 to 1.2 kg 0:/(kW - hr).h~ For diffused air systems, the process air requirement can be calculated from the following dimensional expression:

•Ho where Q is the air flow rate in mVmin, RO is the oxygen requirement in kg/hr, and Ti0 is the field oxygen transfer efficiency expressed as the percent of the oxygen in the air actually transferred to the liquid. The value of T|0 depends on the nature of the diffuser and the depth at which the air is released.K: It typically lies in the range of 6 to 15%, but with values as low as 4% and as high as 80% observed under unusual circumstances. The volumetric power input required to meet the oxygen requirement is obtained by dividing P or Q by the bioreactor volume.

As illustrated in Figure 9.6, the lower limit on the volumetric power input, n,, is determined by the need to maintain solids in suspension. For mechanical aeration systems, this input, Ilr is around 14 kW/1000 nr. The manufacturer of a particular aeration device should be consulted for a more exact value. For spiral roll diffused aeration systems, a minimum air input rate, FIL of 20 m7(min-1000 m') is generally required. For full floor coverage aeration systems the corresponding value is 37 m7(min-1000 m2 of bioreactor floor area).4h717"x~ If these requirements exceed the volumetric power input required to meet the oxygen requirements, the power input required for solids suspension must be provided. This results in increased power usage and higher DO concentrations than required purely for process reasons. Consequently, the minimum volumetric power input can be used with the power required to meet the minimum oxygen requirement to establish the upper feasible bioreactor volume, Vr, in nv :

The volumetric power input must also be less than the value that causes excessive shear of the activated sludge floe, ni:. Excessive shear will disperse a portion of the solids into a poorly settleable form that will not be removed in the clarifier and will, therefore, pass into the effluent. For mechanical aeration systems, the typical maximum volumetric power input, n, ,,, is 60 kW/1000 nr\ while for diffused aeration systems n,:y is approximately 90 m7(min-1,000 m1).""' 1 To avoid excessive floe shear, the reactor volume should be no smaller than the lower feasible bioreactor volume associated with n,,, which has been designated V, , s to emphasize that it is determined by the floe shear criterion. Its value in m1 is given by:

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