Transformation of Acetate and Hydrogen into Methane

As described by Fig. 8.4, the hydrogen partial pressure must be reduced down to pH2 <10-4 bar by Methanosarcina and Methanobacterium to enable conversion of propionate to acetate (see Section 8.1.3). The specific growth rate was described using Monod kinetics (Table 8.2). If the rate ofH2 oxidation is measured as COD, the mean specific removal rate follows as:

with mean values of pmax _ 1.3 d-1 and YX/S _ 0.04 g MLSS (g COD)-1. This specific rate is higher than that of propionate degradation (see Section 8.2.3). If there are no toxic effects from heavy metal ions, these bacteria grow rapidly and propionate degradation is not inhibited by higher pH2.

Methane formation from acetate by Methanosarcina and Methanotrix (Eq. 8.4) is a much slower reaction. From three studies (Table 8.2), a mean specific removal rate can be calculated as:

with pmax = 0.18 d-1 and YX/S = 0.037 g MLSS (g COD)-1. This is low compared to acetate formation from propionate. Therefore, it is the most important step of anaerobic digestion and is more frequently studied. All authors mentioned in Table 8.2 considered Haldane kinetics with non-ionized acetate as the substrate:

The frequently observed disruption of digestion processes with sinking pH and enrichment of lower fatty acids can be explained in part by substrate inhibition. As shown in Eq. (8.22), some authors observed an inhibition of methane formation by propionate described as a non-competitive inhibition.

Because we have to distinguish three totally different groups of anaerobic bacteria, we actually have to measure the concentration of these groups X. This is very difficult and often not realized although it is a requirement for correct determinations of pmax values. Normally, the unit g L-1 MLSS or MLVSS (mixed liquid volu-tile suspended solids) is used (Table 2.6).

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