NANOFILTRATION OF NATURAL ORGANIC-MATTER - REMOVAL, FOULING AND THE INFLUENCE OF MULTIVALENT IONS

The presence of calcium and humic substances or natural organic matter (NOM) in surface waters can cause severe fouling of nanofiltration (N F) membranes. Conditions of fouling were studied using a stainless ste el stirred cell of volume 185 mi and a membrane area of 21.2x10(-4) m( 2) at a transmembrane pressure of 5 bar. Deposition of organic matter was determined by mass balance In feed and concentrate samples. Electr on microscopy and X-ray photoelectron spectroscopy (XPS) were used to study the morphology and composition of the fouling layer. During perm eate recycle experiments, which were used for fouling studies, it was found that calcium concentration (as a representative of multivalent i ons) and the type of organic play a major role in fouling. The calcium forms complexes with some of the organics and deposits on the membran e surface. Depending on the solution conditions the organic or calcite (on which organics adsorb) precipitate. Factors, which influence the concentration of organics and ions at the membrane surface such as sti rring, flux and transmembrane pressure, influenced the deposition of o rganic matter significantly, irreversible fouling occurred with all me mbranes at high calcium concentrations, although the cellulose acetate membrane showed an overall better performance, possibly due to its lo w salt rejection and smooth surface. IHSS humic acid is the organic wh ich deposits most easily and comparison of UV absorbance and DOC data showed that the fraction which absorbs UV strongest, and is more hydro phobic, deposits preferentially on the membranes. These substances als o have the lowest solubility stressing the importance of concentration polarisation effects.