Abstract
This study aims to elucidate the role of sulfide and its precursors in anaerobic digestion (i.e., cysteine, representing sulfur-containing amino acids, and sulfate) on microbial oleate conversion to methane. Serine, with a similar structure to cysteine but with a hydroxyl group instead of a thiol, was included as a control to assess potential effects on methane formation that were not related to sulfur functionalities. The results showed that copresence of sulfide and oleate in anaerobic batch assays accelerated the methane formation compared to assays with only oleate and mitigated negative effect on methane formation caused by increased sulfide level. Nuclear magnetic resonance spectroscopy of sulfide-exposed oleate suggested that sulfide reaction with oleate double bonds likely contributed to negation of the negative effect on the methanogenic activity. Methane formation from oleate was also accelerated in the presence of cysteine or serine, while sulfate decreased the cumulative methane formation from oleate. Neither cysteine nor serine was converted to methane, and their accelerating effects was associated to different mechanisms due to establishment of microbial communities with different structures, as evidenced by high-throughput sequencing of 16S rRNA gene. These outcomes contribute with new knowledge to develop strategies for optimum use of sulfur- and lipid-rich wastes in anaerobic digestion processes.
Highlights
Waste lipids are attractive substrates for use in anaerobic digestion (AD) processes due to their high methane potential and energy density (Alves et al, 2009)
Methane formation in the oleateamended assays were faster in the presence of cysteine and serine compared to the Journal Pre-proof corresponding assays without the amino acids (Fig. 1c), where the maximum methane formation rate was achieved 2-3 days after the incubation start followed by gradual decline (Fig. 1d)
Methane formation pattern in sulfideamended assays was different from the assays that were supplied with cysteine, serine, or sulfate, implying that different mechanisms may lie behind their effect on oleate conversion to methane
Summary
Waste lipids are attractive substrates for use in anaerobic digestion (AD) processes due to their high methane potential and energy density (Alves et al, 2009). LCFA encompass surface active properties, foaming is another complication observed at high loads of lipids (Kougias et al, 2013). In this context, kinetics of LCFA degradation sets a limit on the capacity of anaerobic digesters for handling lipid loads without encountering process disturbances. Production of hydrogen by this pathway is endergonic and proceeds only if the hydrogen partial pressure is maintained at low levels This in turn necessitates syntrophic partnership of the LCFA-degrading bacteria with hydrogen-utilizing microorganisms, such as methanogens, for obtaining an efficient LCFA conversion to methane (Duarte et al, 2018). Sulfate reduction in AD processes is often associated with a declined methane production from organic wastes, including waste lipids, due to utilization of the organic acids (and/or hydrogen) by sulfate-reducing bacteria and a declined availability of substrates for methanogenesis (Raskin et al, 1996; Wu et al, 2021)
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