Exogenous inhibition for remediation and prevention of sludge bulking in mixed culture polyhydroxyalkanoates systems: Performance, microbial response and life cycle assessment

As a biodegradable substitute for fossil-based plastics, polyhydroxyalkanoate (PHA) exhibits broad commercial prospects owing to its capability of biosynthesis from waste biomass. But its industrial scalability is severely restricted by sludge bulking in mixed microbial culture PHA systems, and efficient, rapid-response remediation strategies remain scarce. This study evaluated three inhibition strategies for sludge bulking control during PHA-producer enrichment: direct quorum-sensing inhibition (QSI) strategy with N-acetylcysteine (NAC), indirect QSI strategy with γ-caprolactam (GCL), and stress strategy with tetracycline (TC). Results verified that NAC and TC effectively stabilized sludge settleability. Specifically, NAC controlled sludge bulking by maintaining a low polysaccharide-to-protein (PS/PN) ratio (0.18–0.27) of extracellular polymeric substances (EPS), while enhancing PHA production capacity significantly (9.79% increase, reaching PHA content of 0.684 ± 0.008 g PHA/g VSS). In contrast, TC stimulated EPS synthesis (maximum: 275.5 ± 5.0 mg/g VSS) and regulated the PS/PN ratio at 0.15–0.25 to optimize microbial hydrophilic-hydrophobic balance. The regulation of key nodal genes (e.g., lsrF, cdgJ, eps, exo ) and inhibition of Thauera (primary bulking-causing genus) were the core mechanisms for the improved sludge settleability. According to Life cycle assessment (LCA) and economic evaluation, NAC and TC strategies cut environmental loads by 30%–45% and generated extra revenue of $1.5–$3 per m 3 substrate during PHA production relative to the bulking group. This study pioneered the use of the inhibition effects of NAC and TC to regulate PHA system stability, guaranteeing PHA manufacturing and providing critical impetus for the high-value resource recovery of waste biomass.