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Comparison of fouling behaviours between alginate and xanthan in reverse osmosis system: Final report

Yip, Albert (2012) Comparison of fouling behaviours between alginate and xanthan in reverse osmosis system: Final report. Other thesis, Murdoch University.

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Fouling behaviours of alginate and xanthan, at two different concentrations, 0.2 g/L and 0.02 g/L during reverse osmosis (RO) were compared. Alginate is an acidic polymer obtained from brown algae and two species of bacterial, Azotobacter and Pseudomonas. Xanthan gum is an anionic bacterial polysaccharide which is produced by Xanthomonas campestris. The structures and composition of those polysaccharides are different, which might lead to difference physical properties, giving different fouling results. For a meaningful comparison of fouling behaviour, system operating conditions were kept constant between experiments (cross flow velocity, pressure, and feed water chemistries). The soluble salts, sodium chloride (NaCl) and calcium chloride (CaCl2), were introduced into the feed solution. The high foulant test conditions applied were 1 mM calcium concentration and total ionic strength of 50 mM by varying NaCl concentration; whereas in the low foulant test conditions a 0.5 mM calcium concentration and 10 mM total ionic strength were applied. Alginate fouling rates were greatly affected by concentration of polysaccharides, with more fouling in high concentration tests; whereas, xanthan fouling was similar in high and low concentration tests.

In general, almost all of salt rejections in foulant fouling tests were above 95%, but in two experiments of low alginate fouling they were around 76% and 77%. The lower salt rejection results might have been resulted from imperfections or damage to the membrane or the contamination of permeate container. From the pH analysis, Xanthan was more acidic than alginate under the same concentration of salts in feed solution.

In addition to foulants, sodium nitroprusside (SNP) was added as it is known to reduce polymer viscosity to compare whether fouling rates were improved. SNP is an oxidizing agent and may cause depolymerisation of polysaccharides. Adding 0.1 mM SNP into high concentration of xanthan or alginate feed solution improved the membrane fouling by slightly increasing normalized flux compared to the same concentration of foulant without SNP. To compare the effect of ionic strength and polysaccharide oxidation, a second compound was included, potassium ferricyanide instead of SNP, which is a similar in structure to SNP, but varies in charge and oxidation potential. The result of membrane fouling at 0.1 mM potassium ferricyanide was similar to the 0.2 mM SNP test, the normalized flux was 0.98.

Item Type: Thesis (Other)
Murdoch Affiliation(s): School of Engineering and Energy
Supervisor(s): Li, Linda
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