Polyhydroxyalkanoates production by mixed microbial cultures in sequencing batch reactors operated under different feeding conditions

In recent years, the scientific and industrial attention is being focused on the possibility to replace plastic materials with bio-alternatives, having identical or similar properties to conventional ones, but with a lower environmental impact. Among the others, polyhydroxyalkanoates (PHAs) are particularly interesting since they are biologically produced, bio-based, and completely biodegradable in the environment under either aerobic or anaerobic conditions. More in detail, PHA are a family of polyesters with a wide range of thermal and mechanical properties, which depend on the length and composition of the side chain. Even though industrial PHA production processes use pure microbial cultures, mixed microbial cultures (MMC)-based processes are under investigation. The latter are advantageous from an economical and management point of view. The MMC approach implies a multistage process, whereby the microbial selection of PHA-storing microorganisms plays a pivotal role on the overall performance. A common strategy to favour the microbial selection consists in the establishment of dynamic feeding conditions, such as the alternance of excess (Feast Phase) and absence (Famine Phase) of external carbon substrates. Under Feast and Famine (FF) conditions, only those microorganisms which are able to quickly remove the substrate and storing it as PHA during the feast phase, can survive in the following famine phase using the stored PHA as a carbon and energy reserve. The FF conditions can be easily established in Sequencing Bach Reactors (SBR). In this work, a lab-scale SBR was operated under aerobic/anoxic dynamic conditions (AE/ANOX), with an uncoupled supply of carbon and nitrogen sources. The SBR was inoculated with an activated sludge and the cycle length was maintained at 12 hours. The carbon source, consisting of a synthetic mixture of acetic (85% on COD basis) and propionic (15%) acid, was supplied at the beginning of the aerobic feast phase at an overall organic load rate (OLR) of 4.25 gCOD/Ld. The nitrogen required for the microbial growth was supplied as ammonium sulphate in correspondence to the beginning of the famine phase. Nitrite was also provided along with ammonium sulphate, as an electron acceptor for the consumption of the PHA previously stored in the aerobic feast phase. During the SBR operation, the process was unstable and the intracellular polymer content reached a maximum value of 15% (wt/wt). A poly(hydroxybutyrate/valerate) (PHBV) copolymer was stored with an HV content of 23±1% (wt/wt). In a previous study, with the SBR being operated under similar conditions but under fully aerobic dynamic feeding (ADF), the intracellular polymer content reached a value of 40±2% (wt/wt), with a HV content of 25±1% (wt/wt). Despite a substantial reduction in the intracellular polymer content, the AE/ANOX process did not significantly affect the polymer composition and, in turn, the thermo-mechanical properties of stored PHA. Overall, the AE/ANOX strategy needs to be further investigated and optimized in terms of PHA production. This also because it presents several advantages, such as the possibility to integrate the production of PHA with the nitrification/denitrification process for the treatment of wastewater, as well as the possibility to save, during the anoxic famine phase, approximately 36% of the oxygen requirement if compared to ADF processes.