Coagulation–Flocculation for Polyhydroxyalkanoate Downstream Processing

Polyhydroxyalkanoates (PHA), a family of natural microbial biobased and biodegradable polyesters, are considered potential substitutes for traditional fossil fuel-based polymers. However, high operating costs during production and processing hinder commercial competitiveness. Separation of PHA-containing cells from the fermentation broth via centrifugation is both capital- and energy-intensive, especially at the industrial scale. In this study, an alternative method for bioseparation of the PHA-containing cells via coagulation is demonstrated for the first time. At a 15 mL scale screening, the best candidates for coagulating agents were found to be cationic polymer (at 2.67 mg/mL) and cationic polymer combined with aluminum sulfate (at 1 mg/mL). A novel dilution method was also developed to improve bioseparation in fermentation broth at higher cell concentrations (optical density above 30). Mechanical and chemical characterization of polymer derived from the coagulation-based process was also demonstrated to be similar to commercial polyhydroxybutyrate (PHB), a member of the PHA family. Therefore, the proof-of-concept of coagulation-based processing as a low-energy alternative was demonstrated, thus paving the way for future development for improved cost-competitiveness of large-scale PHA downstream processing. The LCA evaluation reveals that using only cationic polymer results in higher global warming potential and nonrenewable energy use compared to centrifugation. However, combining a cationic polymer with aluminum sulfate shows a 25% lower global warming potential. The chemical cost analysis indicates that this combination is more cost-effective.