Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Victor M. Bright

Second Advisor

Alan R. Greenberg

Third Advisor

Y. C. Lee

Abstract

In many regions of the world, inorganic fouling (scaling) caused by sparingly soluble salts prevents the exploitation of underutilized brackish groundwater and municipal wastewater resources that require desalination. If such resources could be effectively utilized, pressure on existing scarce water supplies would be reduced. Scaling formation is of immense practical importance since it significantly degrades membrane performance. Knowledge of scaling induction time allows for optimized operation of the desalination unit as well as execution of remediation measures. The presence of scaling is usually indicated by ex-situ measurements such as volumetric flux rate. These measurements, however, indicate the presence of scaling only after significant growth has already occurred. Remediation measures often require the use of expensive anti-scaling agents or back-flushing of the system. Both cases necessitate operational downtime, reducing system efficiency and increasing cost. Additionally, overuse of anti-scaling agents can cause significant reductions in membrane lifetime. The availability of real-time, in-situ monitoring of the membrane condition would provide sensing capabilities for determining optimum timing of scaling remediation measures. Such sensors could be incorporated as control elements in smart membrane/module systems, greatly improving the efficiency of large-scale desalination processes. The work described in this thesis demonstrates the use of integrated electrolytic and ultrasonic sensors installed within a cross flow desalination module. Concentration polarization (CP) of the rejected species near the membrane surface is the precursor to scaling deposition and growth, presenting coupled phenomena that should be investigated in tandem. Thin, flexible electrolytic sensors were manufactured using MEMS (Micro-Electro-Mechanical Systems) fabrication techniques, and were installed on the membrane surface to measure concentration within the concentration polarization boundary layer (CPBL), as well as early-stage scaling. The sensors were mounted at three positions along the length of the flow channel in a flat-sheet module, and experimentally demonstrated the expected concentration dependence on axial position as well as cross flow velocity. Scaling was also detected by these sensors as salt precipitated. Ultrasonic transducers present a more simple systems integration problem, and thus demonstrate more immediate potential in commercial situations at the current time. Transducers were installed at three positions within the filtration module, in direct contact with the back-side of the membrane. Several studies in the literature report the use of externally mounted ultrasonic sensors to detect the presence of membrane fouling. However, significant acoustic energy losses can occur in the use of externally mounted transducers, due to unwanted reflections, scattering and beam spread. This thesis compares data from internally mounted transducers with simultaneous data from externally mounted transducers to evaluate the relative efficacy of both configurations. It should be noted that the real-time monitoring techniques could be applied in many filtrations processes beyond desalination. This thesis serves as a case study to provide a basis for additional research in developing smart membranes/modules for municipal and agricultural wastewater treatment, as well as the processing of pharmaceutical and chemical products that rely upon membrane-based liquid separations.

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