Sulfide level in municipal sludge digesters affects microbial community response to long-chain fatty acid loads

Abstract

BackgroundWaste lipids are attractive substrates for co-digestion with primary and activated sewage sludge (PASS) to improve biogas production at wastewater treatment plants. However, slow conversion rates of long-chain fatty acids (LCFA), produced during anaerobic digestion (AD), limit the applicability of waste lipids as co-substrates for PASS. Previous observations indicate that the sulfide level in PASS digesters affects the capacity of microbial communities to convert LCFA to biogas. This study assessed the microbial community response to LCFA loads in relation to sulfide level during AD of PASS by investigating process performance and microbial community dynamics upon addition of oleate (C18:1) and stearate (C18:0) to PASS digesters at ambient and elevated sulfide levels.ResultsConversion of LCFA to biogas was limited (30% of theoretical biogas potential) during continuous co-digestion with PASS, which resulted in further LCFA accumulation. However, the accumulated LCFA were converted to biogas (up to 66% of theoretical biogas potential) during subsequent batch-mode digestion, performed without additional substrate load. Elevated sulfide level stimulated oleate (but not stearate) conversion to acetate, but oleate and sulfide imposed a synergistic limiting effect on acetoclastic methanogenesis and biogas formation. Next-generation sequencing of 16S rRNA gene amplicons of bacteria and archaea showed that differences in sulfide level and LCFA type resulted in microbial community alterations with distinctly different patterns. Taxonomic profiling of the sequencing data revealed that the phylum Cloacimonetes is likely a key group during LCFA degradation in PASS digesters, where different members take part in degradation of saturated and unsaturated LCFA; genus W5 (family Cloacimonadaceae) and family W27 (order Cloacimonadales), respectively. In addition, LCFA-degrading Syntrophomonas, which is commonly present in lipid-fed digesters, increased in relative abundance after addition of oleate at elevated sulfide level, but not without sulfide or after stearate addition. Stearate conversion to biogas was instead associated with increasing abundance of hydrogen-producing Smithella and hydrogenotrophic Methanobacterium.ConclusionsLong-chain fatty acid chain saturation and sulfide level are selective drivers for establishment of LCFA-degrading microbial communities in municipal sludge digesters.

Effluent solids recirculation to municipal sludge digesters enhances long-chain fatty acids degradation capacity

“…The syntrophic lifestyle of the species of Candidatus Cloacimonas was previously suggested based on identification of genes related to syntrophic propionate oxidation in the presence of hydrogen-utilizing microorganisms [ 49 ]. In addition, the occurrence of members of phylum Cloacimonetes (e.g., Cloacimonadales W27, Cloacidonadaceae W5 , and Candidatus Cloacimonas ) during anaerobic lipids and LCFA degradation in different anaerobic environments has been used as an argument for their potential involvement in lipid degradation [ 16 , 50 , 51 ], which is in line with our observation on the prevalence of Candidatus Cloacimonas in R2 following oleate pulse feeding. Nevertheless, Candidatus Cloacimonas had the lowest degree, betweenness, and closeness centrality among the bacteria in the co-occurrence network, suggesting that this genus had limited interactions with other microorganisms [ 35 ] and, therefore, its contribution to the integrity and functions of the microbial community is possibly marginal.…”

“…Bacterial community response to the oleate pulse feeding was different in R5, with effluent recirculation, where Smithella prevailed instead of Candidatus Cloacimonas as compared to R2. Smithella is able to oxidize fatty acids (e.g., propionate and butyrate), n -alkanes (e.g., C 9 –C 12 ), and an association of Smithella to LCFA conversion in PASS digesters has been reported [ 16 , 52 , 53 ]. Smithella is also able to dismutate propionate to butyrate and acetate, and it has low sensitivity to high hydrogen partial pressure, e.g., during syntrophic propionate oxidation [ 52 , 54 ].…”

“…In this respect, retention time of lipid-rich substrates in digesters has a profound effect on degradation efficiency of LCFA and process stability [ 14 , 15 ]. Prolonged retention time of LCFA-amended sludge (e.g., by applying a batch-mode operation after a period of continuous feeding of lipids), where the accumulated LCFA are mainly confined to the solid fraction, results in higher conversion efficiency of LCFA [ 6 , 16 , 17 ]. LCFA feeding frequency has also been identified as an important parameter for kinetics and microbial stability during anaerobic degradation of LCFA, where pulse feeding (instead of continuous feeding) may trigger the activity of β-oxidizing bacteria and improve LCFA degradation [ 18 ].…”

“…In a previous study of the sulfide-induced changes in methanogenic activity and turnover of anaerobic digestion intermediates, including LCFA, we observed that the microbial community in municipal sludge digesters that were exposed to sulfide was able to convert oleate to methane with a faster rate compared to those without sulfide exposure [ 19 ]. We further observed that sulfide is a selective driver for the establishment and growth of LCFA-degrading bacteria in municipal sludge digesters [ 16 , 19 ].…”

Diversity and Abundance of Microbial Communities in UASB Reactors during Methane Production from Hydrolyzed Wheat Straw and Lucerne

“…These two primer sets have previously been shown to adapt well to Illumina sequencing. Moreover, the archaeal community generated from these two primer sets is generally consistent, but with higher resolution in the results for primer set 516F and 915R [44,61,108,109]. However, the results of these two primer sets were somewhat contradictory for sample 1A_204.…”