Polyhydroxyalkanoates (PHAs) are completely biodegradable polymers, that can be synthesized from renewable carbon resources and with important thermo-plastic properties. During last years, an uncoupled C and N-feeding strategy has been applied in sequencing batch reactors (SBRs) in order to stimulate the PHA-production and the growth of PHA-accumulating microorganisms. Despite of it was demonstrated that the applied organic load rate (OLR) significantly impacts on the PHA-storage response, information on the effect of OLR on the PHA-storing bacteria selection/enrichment remains limited. For this purpose, the main objective of the present study was to evaluate the effect of the applied OLR on the microbial selection of a SBR operated with an uncoupled C and N feeding strategy. The SBR cycle length was set at 12 h and four OLR values (4.25, 8.50, 12.75 and 18 g COD/L d) were tested by changing the concentration of the feeding solution, made of a synthetic mixture of acetic and propionic acids (85% and 15% of the overall COD, respectively). The PHA-storage yield increased by increasing the OLR (up to 0.64 COD/COD at 12.75 g COD/L d) but significantly decreased at 18 g COD/L d (0.1 COD/COD) concomitantly with a longer feast phase and a lower PHA content in the biomass at the end of the feast phase (Fig. 1). The selective pressure induced by the applied OLRs strongly influenced the microbiome composition revealing a high content of putative PHA-storing bacteria, mainly affiliated with Paracoccus, Rhodobacter and Thauera, in the SBR operated at OLRs 4.25, 8.50 and 12.75 g COD/L d (up to 97.4% of total reads) and a low content (5.4%) in the SBR at 18 g COD/L d where the predominance of genus Nitrinicola was instead observed.
