Valorization of Fishmeal Wastewater for Polyhydroxyalkanoate (PHA) Production by Bacillus cereus: Process Optimization and Scale-Up

Recently, polyhydroxyalkanoates (PHAs) have gained significant attention as a bioactive material for replacing petrochemical plastics. PHAs can be produced by microorganisms growing on sludge substrates. In this study, fish-processing wastewater was investigated as an alternative substrate for PHA production using Bacillus cereus. Wastewater dilution, carbon-to-nitrogen ratio modification, and the addition of fish oil as a lipidic substrate were examined, and bacterial growth and biopolymer production were optimized. First, wastewater was diluted (25–100%) and examined. The 50% dilution treatment was selected, yielding a CDM of 0.426 g/L and a PHA content of 6.69%. In subsequent steps, the effects of wastewater fermentation and bacterial adaptation prior to the main production processes were investigated. According to the results, the 50% and 100% fermented treatments exhibited higher CDM values (0.970–1.022 g/L) compared to the non-fermented treatments. Cultures inoculated with adapted bacteria showed superior performance (CDM: 1.455 g/L, PHA: 0.499 g/L, PHA content: 34.63%) relative to non-adapted treatments. The effect of the carbon-to-nitrogen (C/N) ratio was also optimized by supplementing two carbon sources: glucose and crude fish oil. The optimal treatment T1 (effluent + 0.6 g/L glucose) had a CDM of 1.32 g/L and a PHA content of 0.215 g/L. Treatment 1, which consisted solely of effluent and fish oil, exhibited higher values (CDM: 1.12 g/L, PHA: 0.65 g/L) and was therefore considered the cost-effective treatment. Subsequently, a scale-up process was conducted in a 4 L bioreactor over 300 h under semi-continuous, long-term cultivation. The optimal harvesting time for the biopolymer was achieved during the fourth cycle (180–240 h). The produced biopolymer was characterized using FTIR, NMR, TGA, DSC, SEM, and XRD analyses, confirming the production of a copolymer, specifically poly(3-hydroxybutyrate-co-3-hydroxyvalerate). This study used wastewater from the fish industry for the production of biodegradable polyhydroxyalkanoates.