Modifying gas transfer membranes with nanoscale zero-valent iron: effects on membrane material properties, treatment performance, and biofilm thickness

Excessive membrane biofilm growth on membrane fibers depends on various factors, with membrane properties playing a pivotal role in influencing microbial affinity for the membrane. To investigate the antibacterial impact of nano-sized zero-valent iron (nZVI) on membrane biofilm structure, pristine (polyvinylidene fluoride (PVDF)) only: HF-0 (PVDF:20/nZVI:0 w/w) and four gas transfer membranes (PVDF:nZVI at different concentrations: HF-1 (PVDF:20/nZVI:0.25 w/w), HF-2 (PVDF:20/nZVI:0.50 w/w), HF-3 (PVDF:20/nZVI:0.75 w/w), HF-4 (PVDF:20/nZVI:1.0 w/w)) were produced. These membranes were assessed for surface morphology, porosity, gas permeability, and biofilm thickness, which ultimately affect biochemical reaction rates in membrane biofilm reactors (MBfRs). Various MBfRs utilizing these gas transfer membranes were operated at different hydraulic retention times (HRTs) and oxygen pressures to assess chemical oxygen demand (COD) removal efficiency and nitrification performance. Incorporating nZVI into the PVDF polymer solution increased surface hydrophilicity and porosity but reduced Young's Modulus and oxygen diffusion coefficients. Confocal laser scanning microscopy (CLSM) analysis revealed an average biofilm thickness of 700 μm in HF-0, HF-1, and HF-3, with a 100 μm decrease in HF-2, even though Escherichia coli growth was observed in HF-3 fibers. Regardless of nZVI dosage, a significant decline in COD removal and nitrification rates occurred at low HRTs and gas pressures.