Sulfate Reduction Relative to Methane Production in High-Rate Anaerobic Digestion: Microbiological Aspects

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Abstract
In the high-rate anaerobic reactors studied (ca. 10 g of chemical oxygen demand [COD] removed per liter of reactor per day), the sulfate-reducing bacteria (SRB) were poor competitors of methane-producing bacteria (MPB), scavenging only on the order of 10 to 20% of the total electron flow. The relatively noncompetitive nature of the SRB in this type of reactor is in sharp contrast to the tendency of the SRB to dominate in natural environments and in other types of anaerobic digesters. Various factors such as the feedback inhibition of H2S on the SRB, iron limitation, the origin of the SRB inocula, biokinetics, and thermodynamics were investigated. The outcome of the SRB-MPB competition under the reactor conditions studied appeared to be particularly determined by two factors. The SRB, as predicted by the Vmax-Km kinetics, competed most effectively at low substrate levels (<0.5 g of COD per liter). The MPB, however, appeared to colonize and adhere much more effectively to the polyurethane carrier matrix present in the reactor, thus compensating for the apparent lower growth rates. Even if the reactor was initially allowed to be predominantly colonized by SRB, the MPB could regain dominance.