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Figure 8.7 Response of a completely mixed activated sludge reactor to a 12 h square wav e response as used to determine the concentration of readily biodegradable substrate. (From G. A. Ekama, et al., Procedures for determining influent COD fractions and the maximum specific growth rate of heterotrophs in activated sludge systems. Water Science and Technology 18(6): 91-114, 1986. Copyright © Elsevier Science Ltd.; reprinted with permission.)

as "soluble" material. The COD of the filtrate is the total soluble COD of the wastewater. Subtraction of the inert soluble COD, S„ provides the value of the readily biodegradable COD, Sso. A major advantage of this technique is that more samples can be analyzed, giving a better measure of the long-term average concentration of the readily biodegradable COD in the wastewater than can be obtained with the bioassay.

Determination of Inert Particulate Chemical Oxygen Demand (X,u). The total COD in a wastewater is given by:

The total influent COD can be measured and Ss<) and S,,, can be estimated by the procedures given previously. If either XS() or XIn is determined, the other can be calculated from Eq. 8.23. It is best to estimate X„, as a parameter for fitting the model to data showing the effect of SRT on the concentration of mixed liquor suspended solids (MLSS) in the CSTRs. Xso can then be calculated from the total influent COD using Eq. 8.23. Ignoring autotrophic biomass, which is usually negligible, the MLSS in the CSTRs comes from four major sources: (1) growth of heterotrophs, (2) production of microbial debris, (3) accumulation of inert suspended organic matter from the feed, and (4) accumulation of undegraded slowly biodegradable substrate. If the SRT is greater than 5 days, the values of Xs and Ss in a bioreactor will be negligibly small relative to XSI, and SS(). This means that the MLSS concentration in a CSTR can be approximated by the simple soluble substrate model which includes inert particulate COD (X|()), provided that the measurement of the influent biodegradable COD includes the biodegradable particulate substrate. In other words, even though we are estimating parameters for ASM No. 1, Eq. 5.46 can be rewritten and used to estimate X1():

Everything in Eq. 8.24 except Xlo is either known or can be assumed. The value of b„ can be evaluated independently by the technique in Section 8.3.2. It is important to recognize that the traditional decay coefficient, bM, is used at this point. That is because Eq. 8.24 is from the simple model. The value of Y,, has already been evaluated. The value of f„ can be assumed to be 0.20, just as before with the simple model. Again, the value of f,> for the simple model is used in this computation because Eq. 8.24 is from that model. COD,,, and S„, are measured values. The SRT (0C) and the HRT (t) are experimentally controlled variables. Generally, it is best to maintain a fixed HRT during the operation of the CSTRs with different SRTs, thereby making the MLSS concentration, XM, a function of only the SRT. The value of XIO can then be estimated by using a one-dimensional search routine which chooses XK, to minimize the error sum of squares when the predicted MLSS concentration (XM) is compared to the values measured in the CSTRs operated over a range of SRTs. If it is necessary to change the HRT as well as the SRT during the studies, then t should be moved to the left side of Eq. 8.24 and predicted values of Xn, t should be compared to the measured values during the estimation of X,„.

Characterization of Nitrogen-Containing Material. Activated sludge model No. 1 includes terms for nitrogen as well as COD. Thus, it is also necessary to characterize them during treatability studies. All soluble forms can be determined by appropriate chemical analyses on the wastewater. The concentration of inert soluble organic nitrogen in the wastewater can be determined by performing Kjeldahl nitrogen tests on aliquots of the samples used to determine the inert soluble COD (See Section 8.2.4). Subtraction of the inert soluble organic nitrogen from the soluble organic nitrogen in the influent gives the concentration of readily biodegradable organic nitrogen in the feed, SNSl). Readily and slowly biodegradable organic nitrogen in the wastewater are assumed to be proportioned in the same way as readily and slowly biodegradable COD:

Because everything in Eq. 8.25 except XNS() is known, its value can be calculated. The sum of the biodegradable organic nitrogen species and the inert soluble organic nitrogen in the wastewater should be less than the total organic nitrogen, which can be measured. The difference between the two is the inert particulate organic nitrogen. Although the latter is not used in the model, its value should be calculated as a check on the procedures. A negative value is evidence of an error.

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