Waste PET-derived biofilm carriers for aerobic wastewater treatment: Biofilm development, growth kinetics, and simultaneous nutrient and microplastic removal

Increasing demand for wastewater treatment technologies has highlighted the need for biofilm systems capable of removing nutrients and emerging contaminants while promoting resource circularity. This study proposes an aerobic Upcycled PET Biofilm Carrier Reactor (UPBCR) that valorizes waste PET bottles as biofilm media, integrating polymer selection, biofilm development, nutrient transformation, and microplastic retention. A polymer-selection strategy compared biofilm formation on PET, HDPE, and PMMA. PET exhibited the highest biofilm-supporting capacity, with a biofilm thickness of 428.83 μm, biomass accumulation of 1.33 mg/cm 2 , and a biofilm formation rate of 0.0657 mg/cm 2 ·day. The PET carrier was applied in an aerobic UPBCR operated under organic loadings of 200–600 mg/L COD. Microbial growth kinetics were described by the Monod model, yielding a maximum specific growth rate (μ max ) of 2.72 day −1 and a half-saturation constant (Ks) of 78.72 mg/L. The system achieved removal of NH 4 + (76.59%), TKN (77.02%), total phosphorus (86.54%), orthophosphate (88.74%), and microplastics (91.25%). Nitrogen removal was driven by nitrification and microbial assimilation, while phosphorus removal was associated with orthophosphate uptake and polyphosphate accumulation. Microplastic retention occurred through biofilm entrapment and sludge settling. These findings demonstrate that recycled PET-based biofilm systems provide a platform for nutrient and microplastic control in wastewater treatment.