Anaerobic Co-digestion of FOG and Municipal Solids Enhances Volatile Solids Destruction at Full-Scale Facility: How Adding FOG Improved Digester Performance and the Microbial Community that Facilitated FOG Destruction

Due to the possibility of increasing methane yields, anaerobic co-digestion of fats, oils, and grease (FOG) and municipal solids has become an attractive option to employ at water resource recovery facilities (WRRFs). For many practitioners in the industry, however, the risks associated with anaerobic co-digestion of FOG have overshadowed the many benefits. In recent years, much effort has been given to understanding the chemical and microbial aspects of achieving stable co-digestion of FOG. To date, no full-scale studies have been conducted to understand how current lab-scale research on the topic correlates to FOG co-digestion at scale. To fill this gap within the literature, two 1.36-million-gallon digesters (AD#1 and AD#3) receiving primary solids, thickened waste activated sludge, and FOG were sampled for 10 months. COD and volatile solids (VS) concentrations were measured weekly for influent and effluent streams. Concentrations of short-chain fatty acids were measured weekly within each digester, and concentrations of long-chain fatty acids were measured weekly within each digester and in the FOG holding tank. The methane content of biogas was measured once per week. qPCR targeting the genus Syntrophomonas and 16S rRNA gene sequencing were performed on digester biomass twice per month and once per month, respectively. FOG loading averaged 10.3 and 13.8 % of VS and COD loading, respectively, and reached as high as 24.5 and 35 % of VS and COD loading, respectively. The concentration of short-chain fatty acids, pH, alkalinity, and methane content of the biogas all stayed within ranges that indicate stable and healthy anaerobic digestion. LCFA concentrations within the digester were below inhibitory levels throughout the study despite fluctuations within the mass rate of LCFAs delivered to the digesters. A strong positive correlation was found between % VS destroyed and kg of FOG VS fed per day (Figure 1), indicating that FOG was a relatively easily degradable substrate and enhanced overall % VS destruction. This trend suggests that the co-digestion of FOG could help municipalities meet 503c VS reduction requirements on top of added methane production. Microbial analysis revealed that specific bacterial and archaeal genera shifted based on the concentration of LCFA within the digesters and the mass rate of LCFA fed to the digesters. Relative abundance of Methanospirillum and Smithella were enriched when LCFA concentrations within the digesters increased but remained low during high LCFA loading (Figure 2). Previous studies have identified these genera as important in facilitating stable FOG co-digestion, but this study suggests they are more important in lowering elevated intermediate product concentrations. Increases in the concentration of LCFA within the digesters caused sharp changes to 16S rRNA gene copies of Syntrophomonas (Figure 3). In contrast, increases in LCFA mass rate resulted in slower response times for Syntrophomonas abundance. Fully characterizing major intermediates within the anaerobic food web, especially LCFAs, was pivotal in interpreting microbial ecology data.