Nanofiltration of Natural Organic Matter: Removal, Fouling and the Influence of Multivalent Ions
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Date
1998Author
Schäfer, Andrea
Fane, Anthony G.
Waite, T D
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Abstract
The presence of calcium and humic substances or natural organic matter (NOM) in surface waters can
cause severe fouling of nanofiltration (NF) membranes. Conditions of fouling were studied using a
stainless steel stirred cell of volume 185 mL and a membrane area of 21.2*10-4m2 at a transmembrane
pressure of 5 bar. Deposition of organic matter was determined by mass balance in feed and
concentrate samples. Electron microscopy and X-ray photoelectron spectroscopy (XPS) were used to
study the morphology and composition of the fouling layer. During permeate recycle experiments,
which were used for fouling studies, it was found that calcium concentration (as a representative of
multivalent ions) and the type of organic play a major role in fouling. The calcium forms complexes
with some of the organics and deposits on the membrane 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 stirring, flux and transmembrane
pressure, influenced the deposition of organic matter significantly. Irreversible fouling occurred with
all membranes at high calcium concentrations, although the cellulose acetate membrane showed an
overall better performance, possibly due to its low salt rejection and smooth surface. IHSS humic acid
is the organic which deposits most easily and comparison of UV absorbance and DOC data showed
that the fraction which absorbs UV strongest, and is more hydrophobic, deposits preferentially on the
membranes. These substances also have the lowest solubility stressing the importance of
concentration polarisation effects.