Valorization of Fishmeal Wastewater for Polyhydroxyalkanoate (PHA) Production by Bacillus cereus: Process...

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.