Organic nanocolloid fouling in UF membranes

Ultrafiltration (UF) membranes may be used as both a primary water treatment and/or as a pretreatment for reverse osmosis or nanofiltration during desalination. Important operational issues continue to revolve around the choices of materials, nominal molecular mass cutoff (MWCO), and filtration hydrodynamics. To study the former two points, we isolated a fraction of natural organic matter, which we refer to as organic nanocolloids, from a drinking water source using a dialysis membrane with a MWCO of 2000 g/mol. We measured the physico-chemical properties of these nanocolloids and four high-quality UF membranes and performed short-time filtration measurements. The organic nanocolloids had a relatively higher molecular mass compared to typical NOM and an effective diffusion coefficient of ∼3.20 × 10−6 cm2/s. Under the same filtration conditions, the organic nanocolloids were shown to have different transport and theoretical surface interaction characteristics through regenerated cellulose (RC) than through polyethersulfone (PES) membranes. These differences made minimal difference in the filtration figures-of-merit – these were the observed total organic carbon (TOC) rejection and solution flux decline – when the two materials were in membranes with MWCO smaller than the nanocolloids, but not when the membranes’ MWCO was on the same order as that of the nanocolloids. The RC had greater flux but rejected significantly less TOC and experienced more percentage flux decline over time, even though it theoretically would have less favorable interaction potentials with the organic nanocolloids.