Scale-Up performance and high-cell-density operation of a three-stage PHA biorefinery using liquor-making wastewater

Polyhydroxyalkanoates (PHA) are biodegradable biopolyesters considered viable alternatives to petroleum-derived plastics. While laboratory studies and pilot-scale trials have demonstrated the feasibility of producing PHA from waste streams, scale-up performance and process optimization of three-stage systems remain insufficiently understood. In this study, liquor-making wastewater characterized by high organic strength and nitrogen deficiency was used as a representative feedstock to investigate scale-up behavior in a PHA biorefinery process. Distinct scale-up responses were observed across different process units. In the anaerobic stage, acidification performance was suppressed during scale-up, necessitating compensatory interventions such as extending the sludge retention time to reproduce laboratory-level performance (volatile fatty acids/SCOD = 0.84 g COD_VFA g^-1 COD). In contrast, the PHA-enrichment stage exhibited robust scalability. A high-cell-density intensification strategy was subsequently developed by increasing influent organic concentration coupled with nitrogen supplementation during the famine phase, achieving a volatile suspended solids (VSS) concentration of 11.8 g L^-1 while maintaining high PHA accumulation (0.686 g PHA g^-1 VSS). However, excessive filamentous growth emerged under high-cell-density conditions, likely associated with oxygen limitation. Techno-environmental assessment showed that the high-cell-density process reduced global warming potential and operating expenditure by 28.3% and 25.2%, respectively, while identifying electricity consumption as the dominant contributor to overall impacts. These findings highlight that scale-up requires unit-specific strategies tailored to the characteristics of individual process stages, and support high-cell-density operation as an effective strategy for enhancing PHA production from waste streams.