Abstract
Substrates with high sulfate levels pose problems for biogas production as they allow sulfate reducing bacteria to compete with syntrophic and methanogenic members of the community. In addition, the end product of sulfate reduction, hydrogen sulfide, is toxic and corrosive. Here we show how sulfate addition affects physiological processes in a thermophilic methanogenic system by analyzing the carbon flow and the microbial community with quantitative PCR and amplicon sequencing of the 16s rRNA gene. A sulfate addition of 0.5 to 3 g/L caused a decline in methane production by 73–92%, while higher sulfate concentrations had no additional inhibitory effect. Generally, sulfate addition induced a shift in the composition of the microbial community towards a higher dominance of Firmicutes and decreasing abundances of Bacteroidetes and Euryarchaeota. The abundance of methanogens (e.g., Methanoculleus and Methanosarcina) was reduced, while sulfate reducing bacteria (especially Candidatus Desulforudis and Desulfotomaculum) increased significantly in presence of sulfate. The sulfate addition had a significant impact on the carbon flow within the system, shifting the end product from methane and carbon dioxide to acetate and carbon dioxide. Interestingly, methane production quickly resumed, when sulfate was no longer present in the system. Despite the strong impact of sulfate addition on the carbon flow and the microbial community structure during thermophilic biogas production, short-term process disturbances caused by unexpected introduction of sulfate may be overcome due to the high resilience of the engaged microorganisms.
Highlights
The anaerobic digestion of organic waste of changing composition and quantity for biogas production involves the risk of introducing undesirable substances to the system, which endanger optimal process performance (Illmer and Gstraunthaler 2009; Wagner et al 2014)
We examined the effects of various SO42− levels on the applied methanogenic system regarding CH4 production, SO42− reduction, microbial community composition, and abundance of the relevant physiological groups
The viability test of methanogens and SO42− reducing bacteria (SRB), in which microorganisms from a 3 g SO42−/L sample at day 28 were re-cultivated, was positive for both groups as 10.4% ± 0.6 CH4 and 4.2% ±
Summary
The anaerobic digestion of organic waste of changing composition and quantity for biogas production involves the risk of introducing undesirable substances to the system, which endanger optimal process performance (Illmer and Gstraunthaler 2009; Wagner et al 2014). One of these substances is sulfate (SO42−), which can be introduced in the reactor when digesting wastes from the food industry, from the production of alcohol, yeast, citric acid and edible oils, and the paper industry (Colleran et al 1995). The inhibitory levels for dissolved sulfides in the literature range from 100 to 800 mg/L (Chen et al 2008)
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