Understanding the Anaerobic Digestibility of Lignocellulosic Substrates Using Rumen Content as a Cosubstrate and an Inoculum
2021
Fonoll, Xavier | Shrestha, Shilva | Khanal, Samir Kumar | Dosta, Joan | Mata-Alvarez, J. | Raskin, Lutgarde
While rumen microorganisms are known to facilitate the hydrolysis of lignocellulosic substrates in anaerobic digestion (AD), it is unclear how rumen content can be used to maintain rumen microorganisms in continuous-flow AD systems. We used rumen content as either an inoculum or a cosubstrate in two separate AD experiments, and performed microbial and multivariate statistical analyses to study how to best use this resource to enhance AD. Hydrolytic bacteria such as Fibrobacter spp. remained present for two months of reactor operation when the rumen content was used as an inoculum, producing 0.3–0.5 g of short-chain fatty acids (SCFA) as acetic acid per gram of volatile solids (VS)fed. The lack of acetoclastic methanogens in the rumen content initially led to the accumulation of SCFA (10–15 g of SCFA as acetic acid per liter) as well as a low pH (5.9–6.8) and a low methane yield (0.02–0.05 L of CH4 per gram of VSfed). The reactor performed similarly (0.1–0.2 L of CH4 per gram of VSfed) to the control, which was not inoculated with rumen content, toward the end of the experiment, and the microbial analyses showed a washout of the rumen bacteria. Rumen hydrolytic bacteria remained in the reactor during the codigestion experiment. However, the methane yield (0.1–0.2 L CH4 per gram of VSfed) was similar to that of the control reactor, which did not receive the rumen content as a cosubstrate, because the reactor conditions (pH > 6.9) were not favorable for the activity of rumen bacteria. Our analyses suggest that using rumen content as a cosubstrate at a low pH (6.0–6.5) is necessary to maintain rumen hydrolytic bacteria and enhance hydrolysis.
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