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

Abstract

Background Slow degradation kinetics of long-chain fatty acids (LCFA) and their accumulation in anaerobic digesters disrupt methanogenic activity and biogas production at high loads of waste lipids. In this study, we evaluated the effect of effluent solids recirculation on microbial LCFA (oleate) degradation capacity in continuous stirred-tank sludge digesters, with the overall aim of providing operating conditions for efficient co-digestion of waste lipids. Furthermore, the impacts of LCFA feeding frequency and sulfide on process performance and microbial community dynamics were investigated, as parameters that were previously shown to be influential on LCFA conversion to biogas. Results Effluent solids recirculation to municipal sludge digesters enabled biogas production of up to 78% of the theoretical potential from 1.0 g oleate l−1 day−1. In digesters without effluent recirculation, comparable conversion efficiency could only be reached at oleate loading rates up to 0.5 g l−1 day−1. Pulse feeding of oleate (supplementation of 2.0 g oleate l−1 every second day instead of 1.0 g oleate l−1 every day) did not have a substantial impact on the degree of oleate conversion to biogas in the digesters that operated with effluent recirculation, while it marginally enhanced oleate conversion to biogas in the digesters without effluent recirculation. Next-generation sequencing of 16S rRNA gene amplicons of bacteria and archaea revealed that pulse feeding resulted in prevalence of fatty acid-degrading Smithella when effluent recirculation was applied, whereas Candidatus Cloacimonas prevailed after pulse feeding of oleate in the digesters without effluent recirculation. Combined oleate pulse feeding and elevated sulfide level contributed to increased relative abundance of LCFA-degrading Syntrophomonas and enhanced conversion efficiency of oleate, but only in the digesters without effluent recirculation. Conclusions Effluent solids recirculation improves microbial LCFA degradation capacity, providing possibilities for co-digestion of larger amounts of waste lipids with municipal sludge.

Absence of oxygen effect on microbial structure and methane production during drying and rewetting events

“…Since Hydrogenispora and Alkaliphilus are both identified as fermentative bacteria, producing acetate as well as hydrogen ( Hydrogenispora) and formate ( Alkaliphilus) 44 , 45 , our observations implied that their increase in relative abundance could potentially serve as new substrate suppliers for both formate-dependent, hydrogenotrophic, and acetotrophic methanogens. The correlations between Aegiribacteria and Methanolinea have yet to be explored, except for a recent study reporting their cooccurrence in anaerobic digesters with high sulfide content 63 . Moreover, Aegiribacteria and Methanolinea showed a relatively high degree of centrality, closeness and connectance in both the drying-rewetting and recovering groups, while their betweenness drastically decreased after the incubation stage (Table S4 ), suggesting that they might act as keystone species in methanogenic function recovery according to Berry and Widder 22 .…”

Methane yield response to pretreatment is dependent on substrate chemical composition: a meta-analysis on anaerobic digestion systems

“…LCFAs from the lipid-rich substrate are usually stabilized when co-digested with low biodegradability co-substrates 10 , improving overall biogas production. Alternative operational approaches such www.nature.com/scientificreports/ as effluent solid recirculation or pulse feeding has also shown promising results on increasing the capacity of AD for handling high loads of lipids 34,35 .…”

Identifying targets for increased biogas production through chemical and organic matter characterization of digestate from full-scale biogas plants: what remains and why?

“…From a physical/mechanical perspective, the often higher TS resulting from co-digestion with FW compared with, for example, sewage sludge digestion, provides more surface area for the lipids to adhere to, and could thus lead to better mixing with the fats [50] and improved lipid accessibility for the microorganisms. Furthermore, co-digestion plants typically have mandatory pasteurization of their substrate mixtures (as in FW1-FW3 in this study), in which heating, melting, and dissolution of the fat fractions result in better mixing with the remaining substrate and thereby improved degradation.…”