Modeling and simulation are useful tools for optimizing

Besides the addition of NaOH for pH control, also the substrate may provoke Na+ inhibition. According to literature, a lower PHB content (56% to 67%) is obtained on waste glycerol ( Cavalheiro et al., 2009, Ibrahim and Steinbuchel, 2009 and Posada et al., 2011) than on Stattic (74% to 76% PHB content) ( Kim et al., 1994 and Mozumder et al., 2014b). Mothes et al. (2007) found a decrease of the PHB content from 70% (pure glycerol) to 48% by C. necator using 5.5% NaCl contained waste glycerol. This was attributed to the accumulation Na+ in the culture medium, which negatively affects the PHB accumulation metabolism of C. necator ( Posada et al., 2011), and thus results in a decreased PHB content. Moreover, using waste glycerol as substrate, the PHB content decreased by delaying the time instant at which nitrogen (N) limitation was imposed because of increasing accumulation of Na+ ( Mozumder et al., 2014a). Seemingly contradictory, a recent study ( Passanha et al., 2014) reported the improvement of PHB production using C. necator by the addition of NaCl medium. They obtained a PHB content of 61% on VFA as carbon source; the PHB content was increased to 80% in the presence of 2.56 g/L Na+ (6.5 g/L NaCl) but decreased to 20% upon further addition of Na+ to 5.90 g/L Na+ (15 g/L NaCl). However, retina used a nutrient medium, not containing any Na+ while in this study mineral medium was used, already containing 1.69 g/L Na+. The maximum PHB content of 78% obtained on the same mineral medium in this study is very close to the maximum PHB content of 80% obtained by Passanha et al. (2014) using a nutrient medium to which Na+ was added to 2.56 g/L. However, since mineral medium is used more often than the more expensive nutrient medium, most authors, report a decreasing PHB production with addition of Na+ ( Cavalheiro et al., 2009 and Mothes et al., 2007).