3

Xh I

The equation for the active fraction, Eq. 5.61 is unchanged. As usual, the term F in Eqs. 5.64-6.68 (either directly or through the HRT, t) is the flow rate entering the bioreactor. It should be noted, however, that for this situation, the flow often arises from another bioreactor, e.g., as its waste solids stream. Furthermore, bioreactors receiving only biomass in their feed often do not employ a biomass separator, but discharge their entire effluent stream to a solids dewatering device. In that case the SRT and HRT are equal, allowing further simplification of Eqs. 5.64-5.68.

Example 5.2.3.2

Continue with the problem begun in Example 5.1.3.1. If the waste solids stream from the bioreactor receiving only the soluble substrate is directed to a CSTR with an SRT of 480 hr and an HRT of 24 hr, what will be the MLSS concentration, the active fraction, and the oxygen requirement in the CSTR? In addition, how many mg/hr as COD of solids must be sent to ultimate disposal from it?

a. What is the MLSS concentration?

This may be determined from Eq. 5.66. The value of Xun<> is the same as the value of XM M calculated in Example 5.1.3.2, which was 523 mg/L as COD. The value of X,,,, is the same as the biomass debris concentration in the bioreactor in Example 5.1.3.2. This value was not calculated in the example, but is the difference between X, and XI1H, or 167 mg/L as COD. These values must be substituted into Eq. 5.66, along with the other appropriate values x _ f4^ flfc7 | t' + (0.20)(0.01)(480))523l M V 24 / 1 1 + (0.01 )(480) J

b. What is the active fraction?

This may be determined in either of two ways: from its definition or from Eq.

5.61, which is applicable in this case also. By definition, the active fraction is the concentration of active biomass divided by the MLSS concentration. The active biomass concentration may be calculated with Eq. 5.64.

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