Influence of the properties and characteristics of sugar-beet pulp extract on its fouling and rejection behaviour during membrane filtration

In the present work, some of the factors that influence the rejection and fouling patterns observed during sugar-beet pulp extract ultrafiltration were examined. This material contained pectins, exhibiting a wide molecular weight distribution and a high tendency for aggregation. A dual pattern in the variation of the observed rejection coefficient with permeate flux was observed depending on the molecular weight: decreasing with increasing permeate flux for molecular weight fractions higher than 10 kD and the opposite for fractions less than 3 kD. Pectins were mainly responsible for the fouling properties of the pulp extract as ultrafiltration of purified beet-pectin solutions demonstrated. Efforts to alter the aggregation patterns in the extract both by changing the solutions’ pH and by adding a reducing agent showed that the interaction between different components of the extract, rather than between pectins only, had a positive effect on the fouling behaviour.

UF of sugar-beet pulp extract caused severe membrane fouling, mainly due to the pectin presence in the material. Isolation of the pectin fraction was possible by diafiltration. The fractionation is adversely affected by increasing flux. This occurs through both decreasing rejection of high molecular weight components and increasing rejection of low molecular weight components. Changes in the properties of the extract (pH, effect of pectin alone and addition of a reducing-depolymerising agent) revealed that aggregation — particularly among different components of the extract — and generally colloidal stability of the material lead to an improved fouling behaviour during UF. It does not affect, however, the fractionation selectivity, indicating that the low-MW carbohydrate fractions present in the extract are more likely to be the major cause of fouling.