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Modeling irreversible fouling in submerged hollow fiber membrane systems for drinking water treatment Lin, Hong

Abstract

The use of low pressure membrane filtration processes in water and wastewater treatment fields has been increasing rapidly due to evolving health concerns and the development of new and lower-cost membranes. Among diverse types of operating mode and membrane modules, submerged hollow fiber membrane systems are very competitive because of their simpler operation, greater ability to resist against fouling, lower maintenance cost and smaller footprint. However, membrane fouling still remains the main disadvantage and limitation of these systems. Membrane fouling decreases the permeate flux which in turn increased the capital and operating costs of membrane systems, and also affects pretreatment needs, cleaning requirements and operating conditions. Therefore, fouling control is an important consideration in the design and operation of a membrane filtration system. A lot of research has been conducted to seek effective methods to control fouling. Based on the understanding of fouling mechanisms and the influence of operating parameters on membrane fouling, numerical models have been developed to quantitatively predict fouling. Nevertheless, most of the studies and models developed to date combine reversible and irreversible fouling together. Irreversible fouling, the main cause of the long term fouling has received limited attention. As a result, there remains a knowledge gap in terms of the mechanisms that govern irreversible fouling as well as the fouling behavior. This research was undertaken to investigate irreversible fouling, and moreover, attempt to develop a simple and reliable model to accurately predict irreversible fouling in submerged hollow fiber membrane filtration systems for drinking water treatment. The study results revealed that even though all experiments were performed with an operating flux that was less than the critical flux, a substantial amount of fouling was observed when filtering over extended periods of time. The extent of fouling was observed to be related to both the operating permeate flux and the system hydrodynamic conditions. Irreversible fouling observed in this study was due to both extensive internal/pore fouling and surface/cake fouling. Internal fouling was the predominant mechanism that governed irreversible fouling. A semi-empirical relationship was developed to model the extent of fouling when filtering over extended periods of time for conditions where the operating permeate flux was less than the critical flux. The relationship was based on the membrane characteristics, the extent of surface/cake fouling and the extent of internal/pore fouling, respectively. The extent of surface/cake fouling was determined to be governed by the operating permeate flux and the system hydrodynamic conditions (i.e. the cross-flow velocity). The extent of internal/pore fouling was determined to be governed only by the operating permeate flux. In addition, the results from the present study indicated that when operating the membrane filtration system below critical flux conditions, for a given volume of permeate filtered, the extent of overall fouling increased as the operating permeate flux increased and decreased as the cross-flow velocity increased.

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