Comparison of Various Reducing Agents for Methane Production by Methanothermobacter marburgensis .

Biological methanation is driven by anaerobic methanogenic archaea, cultivated in different media, which consist of multiple macro and micro nutrients. In addition, a reducing agent is needed to lower the oxidation-reduction potential (ORP) and enable the growth of oxygen-sensitive organisms. Until now, sodium sulfide (Na 2 S) has been used mainly for this purpose based on earlier published articles at the beginning of anaerobic microbiology research. In a continuation of earlier investigations, in this study, the usage of alternative reducing agents like sodium dithionite (Na 2 S 2 O 4 ) and L-Cysteine-HCl shows that similar results can be obtained with fewer environmental and hazardous impacts. Therefore, a newly developed comparison method was used for the cultivation of Methanothermobacter marburgensis . The median methane evolution rate (MER) for the alternatives was similar compared to Na 2 S at different concentrations (0.5, 0.25 and 0.1 g/L). However, the use of 0.25 g/L Na 2 S 2 O 4 or 0.1 g/L L-Cys-HCl led to stable MER values over consecutive batches compared to Na 2 S. It was also shown that a lower concentration of reducing agent leads to a higher MER. In conclusion, Na 2 S 2 O 4 or L-Cys-HCl can be used as a non-corrosive and non-toxic reducing agent for ex situ biological methanation. Economically, Na 2 S 2 O 4 is cheaper, which is particularly interesting for scale-up purposes.