Controlling anaerobic digestion to produce targeted compounds

Abstract

Anaerobic digestion is a mix-culture microbial-mediated process that has primarily been applied to produce methane and stabilise organic matter. However, the intermediates of the digestion process (volatile fatty acids, alcohols, and hydrogen) have applications as commodity chemicals or as precursors to a range of biobased products. However, one of the main challenges to broaden the application of anaerobic digestion is the difficulty associated with robustly controlling mixed-culture products, such that a suite of products can be repeatedly produced. Therefore, understanding how the change in microbial populations or loss in microbial functionality influence the behaviour of the rest of the community can prove to be a powerful tool for manipulating and controlling processes towards a desired commodity.The impact of the starting inoculum on long-term anaerobic digestion performance, metabolic activity rates and microbial community composition remains unclear. To understand the impact of starting inoculum, active microbial communities from four different full-scale anaerobic digesters were each used to inoculate four continuous anaerobic digesters. Thereafter, the digesters were operated identically at 15 days solid retention time, an organic loading rate of 1 gnCODnLrs‑1nd‑1 (75:25n-ncellulose:casein), and 37noC for 295 days. The digesters performance converged and stabilised in 80 days, while activity rates and microbial communities converged and stabilised after 145 days of operation. After 295 days, 52% of all identified OTUs were common to all digesters, and this core community accounted for 72% of the total microbial community relative abundance defined by various bacterial taxa (Bacteroidales, Ruminoccocaceae, Kosmotoga and Treponema) and archaeal taxa (Methanosaeta, Candidatus Methanoregula and Methanospirillum). This indicates that deterministic factors (process operational conditions) were a stronger driver in controlling the ultimate microbial composition in a digester rather than the initial microbial community composition. Moreover, Pearson correlation coefficients revealed several significant associations between bacterial taxa found in the digesters and activity rate profiles. For instance, the presence of Armatimonadetes was positively correlated with higher cellulolytic rates and bacteria belonging to genus Synthophobacter and Clostridum or families Veillonellaceae and candidate BA008 (phylum Bacteroidetes) were correlated to higher butyrate and propionate degradation rates. Overall, it seems plausible that process operational conditions can be used to tune microbial composition and functionality in an anaerobic digester.To explore the extent that the anaerobic digestion process can be manipulated byn a sole selection pressure, the solid retention time was isolated as a pressure parameter. Without interruption, the same four continuous anaerobic digesters were subjected to a sequential decrease in solid retention time from 15 to 8 to 4 to 2 days while maintaining the organic loading rate at 1 gnCODnLrs‑1nd‑1, the same substrate composition ratio (75:25n-ncellulose:casein) and the same temperature (37noC). Each solid retention time was operated until steady state was achieved. Results showed that acetoclastic methanogenesis carried out by Methanosaeta remained active down to 2 day solid retention time and only minor accumulation of volatile fatty acids was achieved (less than 3.5% of influent COD). Therefore, solid retention time as an individual selection pressure was not an effective parameter to shift the anaerobic digestion product profile. However, lowering solid retention times induced a shift in metabolic activity rates, where ethanol degradation gained dominance over butyrate and propionate degradation. Solid retention time also influenced the microbial dynamics of the digesters, driving changes at family or genus level, although the most noticeable finding was the formation of biofilms containing a high abundance of Methanosaeta at the lowest solid retention time. This suggests that the different microbial communities in all four digesters developed similar survival strategies under non-favourable methanogenic conditions.To contextualise and prove the applicability of imposed conditions to steer the process, a combination of temperature, retention time and oxygen availability were selected to control the fermentation patterns of primary sludge followed by anaerobic digestion to recover biogas, as part of a bio-refinery concept. Primary sludge pre-fermentation was carried out at different temperatures (20, 37, 55, 70oC), treatment times (12, 24, 48, 72h), and oxygen availability (semi-aerobic, anaerobic). pH was not controlled. The anaerobic biodegradability after pre-fermentation was evaluated using biochemical methane potential tests. The results showed that fermentation at 20 and 37noC was optimal for volatile fatty acids production with acetate and propionate being major products. Anaerobic fermentation at 37, 55 and 70 oC resulted in higher solubilisation yield at the expense of reduced methane production by 20%, while semi-aerobic fermentation allowed both volatile fatty acids recovery and improved methane potential. Replication experiments using a different batch of primary sludge showed that the main trends could be reproduced exemplifying that fermentation and anaerobic digestion products can be controlled by operational decisions.n