From waste to bioplastics: bio-based conversion of volatile fatty acids to polyhydroxyalkanoates

Production of polyhydroxyalkanoates (PHAs) as bio-alternative to petroleum-based plastics is an important field in the biorefinery to move forward in the development of the circular economy. PHAs are bioplastics stored inside microbial cells as carbon reservoirs and can be produced from a broad range of renewable resources such as waste streams. One important waste stream is food waste that can be converted into volatile fatty acids (VFAs) by anaerobic digestion. The produced effluent from food waste is not only rich in VFAs but also, other nutrients such as nitrogen and phosphorus that can be used by the microorganisms to produce PHAs. The aim of this research is to convert VFAs produced from food waste into PHAs, in which two approaches have been studied. The first approach was to use microbial mixed cultures (MMCs) while the second used microbial pure cultures.

The MMCs were enriched in sequencing batch bioreactor cultivations, where nitrogen and carbon starvation were combined to enhance the selection phase. PHA accumulation of the selected cultures was studied in nitrogen-limited fed-batch cultivations. The second approach studied five different PHA producing bacteria: Cupriavidus necator, Burkholderia cepacea, Bacillus megaterium, Bacillus cereus and Bacillus cereus. To select the most promising bacteria, synthetic medium with the same VFAs composition as in MMCs study was used for pre-screening experiments. Both, pure and mixed culture studies, resulted in the production of PHAs containing (R)-3-hydroxybutyrate, (R)-3-hydroxyvalerate and (R)-3-hydroxyhexanoate as monomers and VFAs were consumed with a high rate by the microorganisms.