Enrichment of a solventogenic anaerobic sludge converting carbon monoxide and syngas into acids and alcohols

An anaerobic granular sludge was acclimatized to utilise CO in a continuously gas-fed stirred tank bioreactor by applying operating conditions expected to stimulate solventogenesis, i.e. the production of alcohols, and allowing to enrich for solventogenic populations. A cycle of high (6.2) and low (4.9) pH was applied in order to produce volatile fatty acids first at high pH, followed by their bioconversion into alcohols at low pH. The addition of yeast extract stimulated biomass growth, but not necessarily solventogenesis. The highest concentrations of metabolites achieved were 6.18 g/L acetic acid (30th day), 1.18 g/L butyric acid (28th day), and 0.423 g/L hexanoic acid (32nd day). Subsequently, acids were metabolized at lower pH, producing alcohols at concentrations of 11.1 g/L ethanol (43rd day), 1.8 g/L butanol (41st day) and 1.46 g/L hexanol (42nd day), confirming the successful enrichment strategy. Similarly, the enriched sludge could also convert syngas into acids and alcohols.