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.
Highlights
Waste lipids are attractive substrates for co-digestion with primary and activated sewage sludge (PASS) to improve biogas production at wastewater treatment plants
There were no apparent differences in biogas production that could be attributed to addition of sulfide to F4, F5, and F6 in this period (Table 1)
Following addition of oleate to F2 and stearate to F3, daily biogas production gradually increased by approximately 2.0 l d −1 compared with the control when the long-chain fatty acids (LCFA) loading rate reached 0.67 g volatile solids (VS) l−1 d−1 on day 208 (Fig. 2c, d)
Summary
Waste lipids are attractive substrates for co-digestion with primary and activated sewage sludge (PASS) to improve biogas production at wastewater treatment plants. 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. Introduction of lipids into anaerobic digesters may perturb process performance due to accumulation of long-chain fatty acids (LCFA) formed as intermediate degradation products of lipids [5]. Continuous supplementation of lipid-rich substrates to anaerobic digesters may cause LCFA accumulation, which in turn can lead to operational disturbances such as sludge foaming, inhibition of microbial activities, acidification, and process failure [5].
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