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Impact of hydrodynamic conditions and membrane configuration on the permeate flux in submerged membrane systems for drinking water treatment Lei, Xiaoling E.

Abstract

Submerged membrane systems are increasingly being used in drinking water treatment applications. One of the major reasons driving this increase is the relatively low operating cost associated with submerged membrane systems, compared to their external counterparts. However, the operating cost for submerged membrane systems is still relatively high when compared to that associated with conventional treatment technologies (such as sand filtration). The need to maintain a high permeate flux is one of the most significant factors affecting the capital and operating cost associated with submerged membrane systems. Air sparging is often needed to maintain a high permeate flux in submerged membrane systems. However, the air sparging mechanism that enhance the permeate flux is poorly understood. As a result, the infrastructure and control of air sparging in submerged membrane systems,are typically designed based on a time and capital extensive trial-and-error approach. This research was undertaken to investigate the air sparging mechanisms that promote a high permeate flux in a submerged membrane system. The results indicate that the decline of the permeate flux over filtration time could be characterized by an initial short period of fast permeate flux decline, followed by a longer period of slower permeate flux decline, according to an exponential equation. The hydrodynamic conditions and the system configuration had a significant impact on permeate flux. The crossflow of water along the membrane surface, which occurs as a result of the rising sparged air bubbles, significantly enhanced the permeate flux. The contact between the sparged air bubbles and the membrane surface had a great impact on enhancing the permeate flux. In addition, the physical contact between the membrane fiber in the submerged system, which varied with the tension of the fiber and the intensity of the air sparging, had a significant impact on the permeate flux. The hydrodynamic conditions and the system configuration had a significant impact on the pseudo-steady-state permeate flux. The maximum pseudo-steady state permeate flux was achieved when the membrane system was operated under dual phase crossflow, with physical contact between the fiber in the membrane module. However, there was no significant benefit of providing a crossflow velocity in excess of 0.2m/s, in terms of maintaining a high, pseudosteady- state, permeate flux. The hydrodynamic conditions and the membrane configuration did not have a consistently significant impact on the initial permeate flux decline coefficient. The hydrodynamic conditions and the system configuration had a significant impact on the pseudo-steady-state permeate flux decline coefficient. The minimum, pseudo-steady-state, permeate flux decline coefficient was achieved when the membrane system was operated under dual phase crossflow, with the physical contact between the membrane fiber in the membrane module. Significant physical contact only occured when fibers in the membrane module were in a loose configuration. The pseudo-steady-state permeate flux decline coefficient was proportional to the inverse of pseudo-steady-state permeate flux for all of the experimental conditions investigated in this present study.

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