Interactive effect of crude glycerin concentration and C:N ratio on polyhydroxyalkanoates accumulation by mixed microbial cultures modelled with Response Surface Methodology

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 (Yobs,BA), the observed coefficient of PHA yield (Yobs,PHA), the PHA content in biomass (XPHA) and the volumetric productivity (PrV). The C:N ratio had the largest effect on Yobs,BA and Yobs,PHA. When the C:N ratio was increased, Yobs,BA decreased and Yobs,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 Yobs,BA of 0.29 mg CODBA/mg COD, a Yobs,PHA of 0.28 mg CODPHA/mg COD, a XPHA of 55.6% VSS and a PrV of 757.3 mg CODPHA/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. [Display omitted] •C:N ratio markedly affects crude-glycerin conversion to active biomass and PHAs.•High C:N ratio and moderate crude glycerin content improve volumetric productivity.•Volumetric productivity was limited by low active biomass yield.•Accumulated PHAs mainly consisted of 3-hydroxybutyrate.