Value-added utilisation of industrial by-products from bioenergy processes for growth of the PHB synthesising bacterium Cupriavidus necator

The utilisation of low-value by-products derived from the production of bioenergy as nutrient media for microbial growth was investigated using the polyhydroxybutyrate (PHB) synthesising bacterium Cupriavidus necator as a case study. The potentially suitable by-products crude glycerol (derived from biodiesel production as carbon source) and digestate (resulting from biogas production as source for nitrogen and other nutrients) contain significant amounts of nutrients but also dissolved and solid impurities. In order to minimise negative effects such as clogging in bioreactor infrastructure and complications during product recovery in a future industrial production scale, the amount of solids was reduced by filtration and centrifugation. In addition, the treatment of the liquid anaerobic digestate (LAD) included dilution and pH stabilisation in order to reduce growth inhibition due to excessive concentrations of compounds in the digestate and an unsuitable pH. Experiments were carried out in a stirred tank bioreactor comparing growth on a medium consisting only of treated LAD and crude glycerol with growth on a reference media containing crude glycerol but mineral salts instead of LAD. Fermentation with crude glycerol and LAD showed slightly reduced specific growth rates (0.1 1/h) but a similar cell dry weight (8.8 g_CDW/L) compared to the reference media with crude glycerol and mineral salts (0.14 1/h and 9.5 g_CDW/L). As no specific conditions were set to promote PHB accumulation, only low levels of PHA of 5,45w_PHB/w_CDW-% ( ± 1,5) were observed for both medium variations at the end of growth.