Competition of Sulfate Reducers with Methanogens in Methane Reactors

Municipal wastewater or wastewater from sugar production, slaughterhouses, breweries, etc., normally contains less than 200 mg L-1 of sulfate. If sulfuric acid is used, e.g., to clean stainless steel containers and pipes in the dairy industry or to maintain an acid pH in bioreactors for bakers' yeast or citric acid production, or if ammonium sulfate is used to inhibit metabolic routes in bakers' yeast for the production of biochemicals, the wastewater contains large amounts of sulfate. Sulfite-containing wastewater is also generated by the starch and cellulose industry during bleaching of the raw products.

If these wastewaters are subjected to anaerobic treatment, the methanogenic bacteria must compete with sulfate reducers for the hydrogen equivalents from COD degradation (Omil et al., 1998). In anaerobic digesters sulfate reduction is the favored reaction, due to the higher affinity of sulfate reducers for reducing equivalents or hydrogen, leading to sulfide production at the expense of reduced biogas formation. Sulfide subsequently forms heavy metal precipitates and, if still present in larger amounts, remains in solution and may be toxic to acetogens and methanogens. Some hydrogen sulfide leaves the reactor with the biogas, which may contain up to a few percent of H2S. Then gas purification is necessary before the gas can be used as fuel for gas engines. Hydrogen sulfide in biogas causes not only odor problems but also corrosion of fermenters and pipes. Methanogenesis of dissolved and particulate organic material in sulfate-rich wastewater is possible, if acidogenesis and sulfate reduction (stripping of H2S together with CO2) is separated from methanogenesis in a phased or a staged process (Lens et al., 2002). The higher affinity of sulfate reducers than methanogens for reducing equivalents can also be used for sulfate and heavy metal ion removal from wastewater in the first stage of a two-stage anaerobic process (Elferink et al., 1994).

In comparison to methanogens, which have a rather restricted substrate spectrum (Table 1.3), sulfate reducers are metabolically more versatile. Sulfate reducers can utilize polymers such as starch, monomers such as sugars, fatty acids, formate, aliphatic and aromatic compounds, as well as molecular hydrogen (Widdel, 1988) to generate reducing equivalents for sulfate reduction (Eq. 10):

Undissociated hydrogen sulfide is toxic to both methanogens and sulfate reducers. A 50% inhibition of methanogenesis was observed at a total sulfide concentration of 270 mg L-1 (Oleszkiewicz et al., 1989), whereas 85 mg L-1 sulfide inhibited sulfate reducers (McCartney and Oleszkiewicz, 1991). For stable methanogenesis, no more than 150 mg L-1 sulfide should be accumulated (Speece et al., 1986). H2S toxicity can be avoided by intensive flushing with biogas to strip the H2S for adsorption onto Fe2(OH)3. If large amounts of H2S are formed during anaerobic treatment, essential heavy metal ions for methanogens (Fe, Ni, Mo, Co, etc.) may be precipitated as sulfides, which may lead to deficiencies in heavy metal bioavailability for the wastewater population.

The metabolites of the fermentative and acetogenic phase of anaerobic wastewa-ter treatment systems, mainly acetate and CO2 + H2, are substrates for methanogens and for sulfate reducers. If wastewater with a high sulfate concentration is treated in a methane reactor, the population may gradually shift from hydrogeno-trophic methanogens toward hydrogenotrophic sulfate reducers, due to a more favorable Ks value for hydrogen of the sulfate reducers. For pure cultures of Desulfo-vibrio sp. the Ks value for hydrogen is 1 ^M, but for hydrogenotrophic methanogens it is only 6 ^M (Kristjansson et al., 1982). For acetate, such a dramatic disadvantage of methanogens was not observed. The affinities of sulfate reducers and of methan-ogenic bacteria (valid at least for Methanosaeta sp.) for hydrogen are in the same range, 0.2-0.4 ^M (Elferink et al., 1994).

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