Upcycled PET-Based Anoxic Biofilm Reactor (UPABR) for simultaneous removal of organic, nutrients, and microplastics from wastewater: Performance, kinetics, and mechanistic insights

The development of sustainable biofilm carriers for simultaneous removal of organic matter, nutrients, and microplastics (MPs) remains a critical challenge in wastewater treatment. This study proposed a novel upcycling strategy by converting waste polyethylene terephthalate (PET) bottles into structured biofilm carriers and integrating them into an Upcycled PET-Based Anoxic Biofilm Reactor (UPABR). The engineered PET media provided a high specific surface area (444 m²/m³) and favorable surface properties for biofilm attachment and MPs retention. A 75-day seeding period enabled stable biofilm formation, with volatile attached solids (VAS) and volatile suspended solids (VSS) reaching 2,284 and 1,922 mg/L, respectively. Scanning electron microscopy confirmed dense and heterogeneous biofilm structures, indicating strong microbial colonization and stable biomass attachment. The reactor achieved 93.13% COD removal following first-order kinetics (k₁ = 0.1518 h⁻¹). Monod kinetics yielded a maximum specific growth rate (μ max ) of 1.87 d⁻¹ and a half-saturation constant (Ks) of 80.28 mg/L, indicating favorable substrate utilization and active biomass growth. Nutrient removal efficiencies reached 75.59% for NH₄⁺-N, 74.27% for NO₂⁻-N, 84.02% for NO₃⁻-N, 70.20% for TKN, 81,02% for orthophosphate, and 77.37% for TP, confirming effective anoxic nutrient transformation and stable biological conversion pathways. The UPABR PET–biofilm coupling enhanced MPs retention through physical entrapment and sedimentation, with FTIR analysis identifying polymer types. This study demonstrates that upcycled PET functions as a dual-purpose biofilm carrier and MPs retention matrix, enabling UPABR to couple wastewater treatment and waste PET valorization in a single unit for integrated and sustainable multi-pollutant removal in circular wastewater management.