Integrated Sensors for Real-Time Monitoring of Filtration Performance during Membrane-Based Liquid Separations

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

Integrated Sensors for Real-Time Monitoring of Filtration Performance during Membrane-Based Liquid Separations

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

Transparent Glass/SU8-Based Microfluidic Device with on-Channel Electrical Sensors

This paper presents a transparent microfluidic chip designed for continuous-flow photochemistry applications with integrated electrical sensing. The transparent chip design allows for microscale photochemistry, and permits direct, real-time visual/electrical observation. The microchip uses optically transparent indium tin oxide (ITO) electrodes for reagent and phase tracking. High-speed videography was performed to validate the electrical measurement data

Stress-Free Bonding Technology with Pyrex for Highly Integrated 3D Fluidic Microsystems

In this article, a novel Pyrex reflow bonding technology is introduced which bonds two functional units made of silicon via a Pyrex reflow bonding process. The practical application demonstrated here is a precision dosing system that uses a mechanically actuated membrane micropump which includes passive membranes for fluid metering. To enable proper functioning after full integration, a technique for device assembly must be established which does not introduce additional stress into the system, but fulfills all other requirements, like pressure tolerance and chemical stability. This is achieved with a stress-free thermal bonding principle to bond Pyrex to silicon in a five-layer stack: after alignment, the silicon-Pyrex-silicon stack is heated to 730 °C. Above the glass transition temperature of 525 °C Pyrex exhibits viscoelastic behavior. This allows the glass layer to come into close mechanical contact with the upper and lower silicon layers. The high temperature and the close contact promotes the formation of a stable and reliable Si-O-Si bond, without introducing mechanical stress into the system, and without deformation upon cooling due to thermal mismatch

Comprehensive experimental studies of early-stage membrane scaling during nanofiltration

Nanofiltration (NF) membranes have found more frequent use in recent years for the desalination of seawater and other sources of brackish water because they can be used at lower pressures than more traditional reverseosmosis (RO) technologies, and thus provide overall energy savings. However,membrane fouling still presents a common and significant challenge in practical applications. Currently, the performance of membrane-based liquid separation processes ismost often monitored by external, volumetric flow-based techniques that provide delayed information on fouling layer development. The delay between initial growth and the observation of fully established fouling reduces the efficacy of cleaning and remediationmeasures. The focus of this study is the use of ultrasonic time–domain reflectometry (UTDR) as a non-destructive method for real-time, in-situ monitoring of early-stage inorganic scaling layer formation on NF membranes. This work utilizes miniature-scale ultrasonic transducers that are internally integrated into a flat-sheet cross-flow filtration module and in contact with the membrane. Comparisons are made with results obtained from externally mounted UTDR transducers, a more commonly used arrangement. Results showthatwhile the internal sensors can be somewhatmore sensitive, the significance of this improvement can be negated by scaling deposition that is hindered by the presence of the sensor