Influence of the C/N ratio on the performance of polyhydroxybutyrate (PHB) producing sequencing batch reactors at short SRTs

Many waste streams that are suitable substrates for mixed culture bioplastic (polyhydroxyalkanoate, PHA) production are nutrient limited and may need to be supplemented to allow sufficient growth of PHA accumulating bacteria. The scope of this study was to investigate the necessity of nutrient supplementation for the enrichment of an efficient PHA producing mixed culture. We studied the influence of different degrees of carbon and nitrogen limitation on the performance of an acetate-fed feast-famine sequencing batch reactor (SBR) employed to enrich PHA storing bacteria. The microbial reaction rates in the SBR showed a shift with a change in the limiting substrate: high acetate uptake rates were found in carbon-limited SBRs (medium C/N ratios 6-13.2 Cmol/Nmol), while nitrogen-limited SBRs (medium C/N ratios 15-24 Cmol/Nmol) were characterized by high ammonia uptake rates. Biomass in strongly nitrogen-limited SBRs had higher baseline PHA contents in the SBR, but carbon-limited SBRs resulted usually in biomass with higher maximal PHA storage capacities. The PHA storage capacity in a nitrogen-limited SBR operated at 0.5 d SRT decreased significantly over less than 5 months operation. For the microbial selection and biomass production stage of a PHA production process carbon limitation seems thus favourable and nutrient deficient wastewaters may consequently require supplementation with nutrients for the selection of a stable PHA storing biomass with a high storage capacity.