Transport characteristics of wastewater effluent organic matter in nanofiltration and ultrafiltration membranes

This study demonstrates the transport characteristics of wastewater effluent organic matter (EfOM) through membrane pores using a four-parameter (intrinsic mass transfer coefficient (k_i), solute concentration near the membrane surface (C_m), solute permeability (P_m), and reflection coefficient (σ) model based on thermodynamics, concentration polarization (CP) and hydrodynamic operating conditions represented by a J₀/k ratio (the ratio of initial permeate flux (J₀) to a back diffusional mass transfer coefficient (k)). EfOM transport characteristics through the pores of four different membranes (a nanofiltration (NF)/ultrafiltration (UF) polymeric pair and two ceramic UF membranes with different molecular weight cutoffs (MWCOs)) were different; the NF polymeric membrane exhibited either convection- or diffusion-dominant conditions, while the UF membranes exhibited convection-dominant conditions in terms of EfOM transport through membrane pores. A critical J₀/k ratio (representing a transitional condition between diffusion- and convection-dominant transport of solute) was found for the examined NF membrane with a MWCO of 250 Daltons. Four different parameters (k_i, C_m, P_m, and σ) were determined by the model to be informative to elucidate the various interactions between EfOM and the tested membranes. EfOM characteristics (size, structure, and functionality) and membrane properties (MWCO, surface/pore charge in terms of zeta potential, and module configurations) were revealed to play a major role in EfOM rejection and flux decline under convection-dominant conditions, as compared to diffusion-dominant conditions.