Biofouling potential of various NF membranes with respect to bacteria and their soluble microbial products (SMP): Characterizations, flux decline, and transport parameters

A problem of considerable concern in membrane filtration processes is biofouling with microorganisms and their soluble microbial products (SMP). This work focused on NF membrane biofouling by either bacteria (biotic filtration/biofouling) or SMP (abiotic filtration/biofouling); the former was analyzed using NF membrane filtration, the concept of concentration polarization (CP), and the convection-diffusion-electrophoretic model (CDE), using Flavobacterium lutescens bacteria, with the latter investigated through NF membrane filtrations using SMP extracted from the bacteria. Four different NF membranes were tested: piperazine versus meta-phenylene diamine (MPD)-based polyamide thin-film composite (TFC) membranes. Through membrane characterizations, including contact angle, surface charge, SEM, and AFM, the MPD-based NF membranes exhibited relatively hydrophobic and higher roughness compared to the piperazine-based NF membranes. The convection-diffusion-electrophoretic migration model, in conjunction with the CP concept, could demonstrate the electrostatic interaction of bacteria against trans-membrane pressure during filtration. With increasing operating pressure, the CP thickness of the bacteria boundary layer decreased due to compaction of the CP layer. The CP layers of bacteria for the MPD-based membranes having relatively high negative surface charges, were less compacted in thickness due to electrostatic repulsion between the membrane surfaces and bacteria. All membranes exhibited high repulsive forces with the extended DLVO (XDLVO) approach, due to the high AB interaction energy in an aquatic system. To evaluate the interactions between the membrane surface and bacteria during filtration, the Hamaker constants of various NF membranes were also determined by measuring the contact angle between the membrane surface and bacteria. NF membranes with greater hydrophobicity and roughness exhibited higher biofouling potential in terms of membrane-bacteria interactions identified from the Hamaker constants estimation. From SMP (extracted from F. lutescens at a decay condition) characterizations, some portions of colloidal NOM (i.e., with higher molecular weights) were revealed to be comprised of SMP through the IR spectra analysis. Also, they were found to have high hydrophilicity and biodegradability from XAD resins and BDOC measurements, respectively. When the SMP were compared to other macromolecules (bovine serum albumin, colloidal NOM, and Suwannee River NOM) in terms of fouling through membrane fouling experiments, they provided high membrane fouling potential. Various transport parameters (mass transfer coefficient, membrane selectivity, and solute permeability) of the SMP were determined by irreversible thermodynamic models combined with the film model to quantitatively investigate the SMP behaviors in NF membranes [S. Lee, G. Amy, J. Cho, Sherwood correlations for natural organic matter (NOM) in nanofiltration (NF) membranes and its applicability, J. Membr. Sci., 240 (1-2) (2004) 49-65]. Relatively hydrophobic and rougher membranes (i.e., MPD-based NF membrane) exhibited higher selectivity, in terms of SMP, compared to relatively hydrophilic membranes.