Polyhydroxybutyrate (PHB) production by Paracoccus denitrificans from hydrolysates of microalgal biomass grown in wastewater treatment photobioreactors

Wastewater treatment plants generate large amounts of residual biomass that could potentially serve as substrates for the production of valuable biopolymers. This study evaluated the valorisation of residual microalgae–bacteria biomass generated during piggery wastewater treatment for polyhydroxybutyrate (PHB) production by Paracoccus denitrificans . The effects of hydrolysis method, hydrolysate concentration, nutrient availability and aeration conditions on biomass growth and PHB accumulation were evaluated. Bacterial growth did not occur on thermo-alkaline hydrolysates. In contrast, 10% thermo-acid hydrolysate supported the highest PHB accumulation, yielding 0.75 g/L biomass with 8.3% PHB in closed reactors. Ultrasound-assisted enzymatic hydrolysate diluted to 20% resulted in similar biomass concentrations but lower PHB accumulation (3.6%). Bacteria consumed multiple carbon and nitrogen sources without clear preference. Glucose addition (1 g/L, C/N = 6.5) during cultivation in 10% thermo-acid hydrolysate increased PHB accumulation up to 17.5% while biomass concentration remained stable. Higher aeration levels were generally associated with lower PHB accumulation, as supported by flux balance analysis. Overall, these results demonstrate the potential of thermo-acid hydrolysates from waste microalgal biomass for PHB production while highlighting the operational limitations associated with oxygen availability and nutrient balance.