Recovering biomethane from fats, oils, and greases: Examining the impacts of microbial ecology on anaerobic codigester stability and bioconversion kinetics

Abstract

Recovering biomethane with anaerobic digestion is of global interest to reduce carbon footprints and improve process economics for the treatment of organic wastes. Fats, oils, and greases (FOG) are desirable co-substrates for biomethane recovery because they have a substantially higher energy density than wastewater treatment solids or livestock manure. Yet, biomethane recovery from FOG codigestion at wastewater treatment plants or agricultural digesters can be limited due to process inhibition caused by long-chain fatty acids (LCFA) accumulation. Currently, there is a lack of understanding regarding the role of anaerobic digester microbial communities in maintaining efficient conversion of LCFA into biomethane. The ultimate goal of this research was to improve the reliability of biomethane recovery during FOG codigestion by elucidating relationships between microbial community composition and LCFA bioconversion kinetics. The ability to accurately monitor LCFA-degrading populations in anaerobic digesters was obtained by developing and validating quantitative PCR (qPCR) assays for the syntrophic LCFA β-oxidizing genera of Syntrophomonas and Syntrophus. These qPCR assays were then utilized to measure population changes in a codigester treating FOG and municipal wastewater treatment solids at increasing FOG loadings for over 150 days. A relationship was developed that correlated higher effluent LCFA concentrations with higher influent FOG loading rates normalized to digester Syntrophomonas 16S rRNA gene concentrations. Subsequently, the impacts of LCFA feeding strategy on LCFA bioconversion kinetics were investigated using bench-scale codigesters that were either pulse-fed every two days or continuously-fed daily with oleate. The results showed that Bacteria and Archaea community compositions in the codigesters diverged based on LCFA feeding frequency and LCFA loading. Predictive models for LCFA bioconversion kinetics were developed as a function of absolute concentrations of selected Syntrophomonas taxa. DNA-stable isotope probing (SIP) based metagenomics confirmed that different LCFA-degrading syntrophic bacteria were selected with different codigester LCFA feeding frequencies. Taken together, the results of this study demonstrate that higher codigester FOG loadings can be achieved by developing a higher biomass concentration of LCFA-degrading syntrophic consortia, and that the codigester feeding strategy can be adjusted to biologically select for LCFA-degrading populations with higher LCFA bioconversion kinetics at high FOG loadings.