Submerged hollow fibre membranes in bubbling systems

Abstract

This study focuses on the optimisation of submerged hollow fibre membrane performance by analysing the role of air sparging on the reduction of membrane fouling. In submerged hollow fibre membranes, rising bubbles have been shown to induce shear, liquid movement and fibre displacement. The interaction between fibre movement induced by bubbling and the microfiltration performance was assessed for various parameters (fibre tightness, fibre length, fibre diameter, air flowrate, nozzle size, and feed concentration). A model feed of yeast suspension and a series of isolated fibres were used. The fibre movement was assessed by monitoring the displacement using video recording. Bubble population parameters were also measured. The results suggest that bubbleinduced fibre movement plays an important role in controlling membrane fouling. Investigations of the critical flux at various operating conditions also supported these conclusions. Since energy consumption for aeration is a major contributor to the cost in submerged membranes, the potential to minimise the aeration cost has been tested by implementing intermittent aeration and different nozzle sizes. It was found that an optimum condition associated with a low fouling rate could be reached by combining various aeration intermittencies and nozzle sizes. An attempt to suppress fouling without aeration was made by incorporating vibrations into a submerged hollow fibre membrane system. The effects of vibration frequency, type of yeast (washed and unwashed) on the filtration performance were observed. The impact of coagulant addition on filtration enhancement was also analysed. The performance of microfiltration was evaluated based on its critical flux value. The findings in this preliminary study indicated potential fouling control by applying vibrations to submerged membranes. A semi-empirical model was developed to predict the filtration behaviour by taking into account the bubble-induced shear and fibre movement. The predicted critical flux values suggested that membrane fouling appears to be more prominent at low air flowrate, with tight fibres, and higher feed concentrations. The model fits the experimental data with discrepancies from approximately 0.3% to 20%. The predicted filtration profiles at different operating modes demonstrate the importance of bubble-induced shear and fibre movement in the improvement of filtration performance.