Figure 16.41 Methanogenic substrate options.

supplying and scavenging electrons. However, there are several more methanogenic genuses capable of pursuing this third "methyl" substrate option than are involved with acetoclastis, and in some cases they may also use hydrogen gas as a source of reducing power. Spread among these three catabolic options, most methanogens are only able to use one or two substrate forms, although Methanosarcina is far more facile given its ability to metabolize seven different substrates. In addition, there are methanogens whose usable substrate options lie outside the standard norm, including those able to subsist with various alcohol forms, including ethanol, 1- and 2-proponal, and 1-butanol.

Methanogenic metabolism naturally requires a highly reduced environment and is sustained only by microbes whose life-style is strictly anaerobic. Although the reductive metabolism of carbon dioxide practiced by most methanogens can be viewed as an anaerobic respiration pathway (using CO2 as an electron acceptor in lieu of O2), methanogens do not employ the same sort of cytochromes, quinones, and flavoproteins as are involved in a normal respiratory electron transport chain. Instead, methanogens use a number of highly specialized reducing enzymes and coenzymes for their reductive metabolism, sequentially including coenzyme factor F420, methanopterin, methanofuran, coenzyme M (2-mercaptoethanesulfonic acid), coenzyme factor F430, and a terminal coenzyme HS-HTP (7-mercaptoheptanoyl threonine phosphate).

Aside from the important biochemical roles played by these metabolic compounds, two of these enzymatic forms also provide a unique means of selectively identifying the presence of methanogen cells. Methanopterin, which resembles folic acid structurally, exhibits a bright blue fluorescence when illuminated with light at 342 nm, and coenzyme F420 projects a similarly distinct fluorescent blue-green hue when exposed to 420-nm epifluorescent illumination. Coenzyme factor F430 also absorbs light when irradiated at 430 nm, but unlike F420, this coenzyme does not fluoresce. An environmentally important aspect of F430 activity, though, is that this coenzyme requires nickel at a level that gives methanogens a distinct anabolic requirement for this trace element.

The multistep and metabolically complex nature of these anaerobic mechanisms, therefore, introduces a set of concerns when applied to the pragmatic business of sludge digestion, particularly those systems designed for single-stage processing. Indeed, there is an extremely delicate balance that must be achieved in these single-stage anaerobic reactors between the involved acidogenic and methanogenic sequences in terms of their respective production and use of the acidogenic fermentation intermediates. Specifically, two different forms of key intermediates are involved, including both the low-molecular-weight volatile fatty acids (e.g., acetic, butyric, proprionic, etc.) and hydrogen gas.

The first issue with fatty acid intermediates revolves around system pH and the negative impact that acidic pH levels have on these anaerobic reactions. Methanogens are sensitive to pH outside the range 6 to 8 (Figure 16.42). A decrease in pH below 6 reduces the activity of the methanogens more than that of the acidogens. This causes a buildup of organic acids, further reducing pH. Thus, anaerobic digestion is unstable when confronted with pH disturbances. An important part of the inherent instability of the acidogenic-to-methanogenic linkage, therefore, stems from the fact that short-term lags in methanogenic activity (e.g., perhaps triggered by other environmental stress factors) can lead to upset pH transients from which the methanogens cannot readily recover.

The second crucial aspect with the metabolism of an anaerobic digester is that the reductive intermediate production of reduced hydrogen gas via the fermentative acidogens must be similarly balanced by its consumption during methanogenic metabolism. Should hydrogen gas levels rise within the digester much above trace values (i.e., above

Figure 16.42 Approximate pH response by anaerobic methanogens. (Adapted from Speece, 1996.)

Figure 16.42 Approximate pH response by anaerobic methanogens. (Adapted from Speece, 1996.)

^10~3 atm), the metabolic harmony between acidogenic and methanogenic metabolism will again be disrupted, although in this case the negative impact is on the acidogenic cells.

This phenomenon reflects yet another delicate thermodynamic circumstance with the second set of fatty acid fermentation reactions, where the ambient hydrogen gas level must be kept sufficiently low to effect a net release of free energy. The free-energy values given in le 16.12 aptly qualify this dependence. In the absence of a sink (i.e., methano-genic consumption) for hydrogen gas during anaerobic digestion, the free-energy release for the fermentation of either proprionate or butyrate would be positive in value (with standard values of +76.2 and +48.2 kJ for these respective conversions), at which point these reactions would simply not occur.

Here again, a successfully maintained anaerobic digester system must achieve a balance between fermentative and methanogenic reactions. By holding the intermediate hydrogen gas levels at an acceptably low level (just as the fatty acid buildup had to be

TABLE 16.12 Fermentation Free-Energy Changes for Standard vs. Typical Reactor Values (kJ/Reaction)

Fermentation Type

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