Effect of shear patterns and EPS on fouling in microfiltration

Thesis

Concentration polarisation and fouling reduce performance as well as increase costs. In order to mitigate these effects, understanding the cause and effects of these phenomena is crucial. It has been hypothesized, and to a certain extent shown, that amelioration of fouling can be achieved through the use of time varying shear for example use of air-sparging or sharp changes in crossflow velocity. Nevertheless the effect of shear on membrane fouling, in particular its effect on the foulant deposition and the transmission of small molecules in microfiltration, is not well understood.

The goal of this project was to achieve an understanding of various foulant be- haviour. The work was divided into two parts. Firstly, the observations of fouling by freshwater algae, Chlorella Sorokiniana, were carried out at Nanyang Technolo- gical University, Singapore. Observations using macroscopic parameters were ex- amined with an optical non-invasive observation technique called Direct Observa- tion Through Membrane (DOTM). The result yielded a novel relationship between operating flux, crossflow velocity and transmission of extracellular polysaccharide. Interestingly shear was shown to have positive as well as negative effects on fouling of microfiltration membrane. The analysis of permeate has clearly shown that a maximisation of shear rate was not ideal.

The second part was concerned with observations of the effect of shear patterns on membrane fouling using newly fabricated special membrane filtration cell, Direct Shear Stress Test Cell (DSSTC), designed to fit an Anton Paar rheometer and op- erate at constant flux. Unlike the-constant-shear-filtration cells, one could impose a very wide variety of shear regimes including intermittent sharp changes of direction and sinusoidal oscillations in the DSSTC. The effect of shear patterns on transmis- sion and fouling of a model polysaccharide (Dextran Blue) through microfiltration membranes was carried out at various conditions. Again, the results showed that the maximisation of shear rate was not ideal. The effect of shear patterns and EPS on fouling by yeast suspension was also studied using the DSSTC. The benefit of oscillatory shear is foulant dependent. For example, square wave oscillatory shear led to lower relative fouling for yeast EPS, but it resulted in higher relative fouling for unwashed yeast.

Authors: Pongpairoj, P

• Publication date: 2013
• DOI: 10.5287/ora-z6oazovjx
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: EPS; microfiltration; shear
• Subjects: Chemical and process engineering