Trickle-Bed Bioreactors for Acetogenic H-2/CO2 Conversion

Acetic acid is an essential industrial building block and can be produced by acetogenic bacteria from molecular hydrogen (H-2) and carbon dioxide (CO2). When gasses are supplied as substrates, bioreactor design plays an important role for their availability. Trickle-bed bioreactors (TBs) have an enhanced gas-to-liquid mass transfer and cells remain in the system by forming a biofilm on the carriers. So far, TBs have been investigated extensively for bio-methanation processes, whereas studies for their use in acetic acid production are rare. In this study, we evaluated the reproducibility of two parallel TBs for acetic acid production from H-2:CO2 (= 70:30) by a mixed culture with a gas flow rate of 3.8 mL min(-1) and a medium flow rate of 10 mL min(-1). Additionally, the effect of glucose addition during the starting phase on the resulting products and microbial composition was investigated by setting up a third TB2. Partial medium exchanges to decrease the internal acetic acid concentration (AAC) combined with recycling of withdrawn cells had a positive impact on acetic acid production rates with maxima of around 1 g L-1 d(-1) even at high AACs of 19-25 g L-1. Initial glucose addition resulted in the accumulation of unwanted butyric acid up to concentrations of 2.60 +/- 0.64 g L-1. The maximum AAC of 40.84 g L-1 was obtained without initial glucose addition. The main families identified in the acetogenic TBs were Peptococcaceae, Ruminococcaceae, Planococcaceae, Enterobacteriaceae, Clostridiaceae, Lachnospiraceae, Dysgonomonadaceae and Tannerellaceae. We conclude that a TB is a viable solution for conversion of H-2/CO2 to acetate using an anaerobic enrichment culture.