Abstract
This research assessed the microbiological suitability of oleate degradation coupled to sulfidogenesis by enriching communities from anaerobic sludge treating dairy products with S0, , , and S2 as electron acceptors. The limiting factor hampering highly efficient oleate degradation was investigated in batch reactors. The best sulfidogenic performance coupled to specialization of the enriched bacterial community was obtained for S0- and S2-reducing enrichments, with 15.6 (± 0.2) and 9.0 (± 0.0) mM of sulfide production, respectively. Microbial community analyses revealed predominance of Enterobacteraceae (50.6 ± 5.7%), Sulfurospirillum (23.1 ± 0.1%), Bacteroides (7.5 ± 1.5%) and Seleniivibrio (6.9 ± 1.1%) in S0-reducing cultures. In S2-reducing enrichments, the genus Desulfurella predominated (49.2 ± 1.2%), followed by the Enterobacterales order (20.9 ± 2.3%). S0-reducing cultures were not affected by oleate concentrations up to 5 mM, while S2-reducing cultures could degrade oleate in concentrations up to 10 mM, with no significant impact on sulfidogenesis. In sequencing batch reactors operated with sulfide stripping, the S0-reducing enrichment produced 145.8 mM sulfide, precipitating Zn as ZnS in a separate tank. The S2 fed bioreactor only produced 23.4 mM of sulfide precipitated as ZnS. The lower sulfide production likely happened due to sulfite toxicity, an intermediate of thiosulfate reduction. Therefore, elemental sulfur reduction represents an excellent alternative to the currently adopted approaches for LCFA degradation. To the best of our knowledge, this is the first report of oleate degradation with the flux of electrons totally diverted toward sulfide production for metal precipitation, showing great efficiency of LCFA degradation coupled to high levels of metals precipitated as metal sulfide.
Highlights
Anaerobic reactors have been widely used for the treatment of lipid-rich wastewater, and, due to the high energy content of lipids, they are generally coupled to the production of biogas (Alves et al, 2009; Dasa et al, 2016)
Oleate (C18:1), stearate (C18:0), and palmitate (C16:0) are long-chain fatty acids (LCFAs) commonly present in anaerobic digesters fed with lipids, from which oleate shows the highest abundance in wastewater systems (Komatsu et al, 1991), reasonably good solubility and high toxicity to anaerobic digestion communities (Lalman and Bagley, 2002; Zhang et al, 2011)
The archaeal community, is more resilient to increased LCFA concentrations compared to the bacterial community (Lalman and Bagley, 2002; Zhang et al, 2011; Ma et al, 2015), with partial inhibition of archaeal activity due to a reversible mineralization of LCFA (Pereira et al, 2005)
Summary
Anaerobic reactors have been widely used for the treatment of lipid-rich wastewater, and, due to the high energy content of lipids, they are generally coupled to the production of biogas (Alves et al, 2009; Dasa et al, 2016). When major products of β-oxidation accumulate to thermodynamicallylimiting levels, further oxidation of LCFA and propionate is hampered, inhibiting the digestion to proceed (Labatut et al, 2014). In this scenario, the presence of LCFA has been reported to impact the activity of hydrolytic, acidogenic, acetogenic bacteria, and methanogenic archaea (Lalman and Bagley, 2002; Pereira et al, 2005). Despite the reversibility of inhibition, the biogas production from the degradation of lipidic matter has yet several trammels on its way
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