Interactive effect of crude glycerin concentration and C:N ratio on polyhydroxyalkanoates accumulation by mixed...

Response Surface Methodology (RSM) was used to investigate how the crude glycerin concentration and the carbon to nitrogen (C:N) ratio in the culture medium affect four indicators of polyhydroxyalkanoates (PHAs) accumulation by mixed microbial cultures (MMC): the observed coefficient of active-biomass yield (Y_obs,BA), the observed coefficient of PHA yield (Y_obs,PHA), the PHA content in biomass (X_PHA) and the volumetric productivity (Pr_V). The C:N ratio had the largest effect on Y_obs,BA and Y_obs,PHA. When the C:N ratio was increased, Y_obs,BA decreased and Y_obs,PHA increased, regardless of the concentration of crude glycerin in the culture medium. The C:N ratio also had the largest effect on the PHA content, whereas volumetric productivity was strongly affected by both the C:N ratio and the crude glycerin concentration. The optimal conditions for PHA accumulation were a crude glycerin concentration of 8954 mg COD/L with a C:N ratio of 15.9 mg C/mg N-NH₄, which gave a Y_obs,BA of 0.29 mg COD_BA/mg COD, a Y_obs,PHA of 0.28 mg COD_PHA/mg COD, a X_PHA of 55.6% VSS and a Pr_V of 757.3 mg COD_PHA/L⋅d (550.0 mg PHA/L⋅d). The accumulated PHAs consisted mainly of 3-hydroxybutyrate. By using RSM, it was possible to predict crude glycerin concentrations and C:N ratios not tested here that will allow desirable values of PHA content in biomass or PHA productivity, which can be useful for designing PHA production with MMC.