Development of sensors for real-time characterization of concentration polarization and membrane fouling

Concentration polarization (CP) and fouling are two primary factors affecting membrane lifetime and operating performance. This study involves the development and use of two real-time technologies to characterize CP and fouling of sparingly soluble inorganic salts (calcium sulfate) for a better understanding of these phenomena during membrane filtration processes.; Ultrasonic time-domain reflectometry (UTDR) is an in situ, noninvasive real-time technique that has been successfully utilized to quantify fouling and cleaning of flat sheet membranes. This study describes the extension of UTDR for the measurement of fouling and cleaning in commercial membrane modules employing spiral wound elements. A new signal analysis protocol was developed such that systematic changes in the entire acoustic spectrum as a function of module operation time could be represented in terms of shift factors. It provided a simplified quantitative indication of the extent of membrane fouling and cleaning. Results indicate the effectiveness of the methodology and the potential development of UTDR technique with an ultimate goal of providing on-line sensors for the timely detection of fouling and cleaning of commercial liquid-separation membrane systems.; However, testing also indicated that UTDR was not capable of detecting CP, which acts as a precursor to the onset of fouling. Therefore, we developed a design, fabrication and attachment strategy to use microfabricated interdigitated capacitors integrating different vertical spans for both qualitative and quantitative characterization of CP. Comprehensive experiments were conducted in a flat-sheet nanofiltration (NF) membrane module to determine the detection capability and reliability of the sensors. The microcapacitors show the desired capability to detect the initiation and gradual development of the CP boundary layer in real-time under different operating conditions. The steady-state near-surface concentrations measured by the capacitors with small vertical spans were compared with the theoretical predictions from a CP model by Weber (2000). Results indicate that the trend of the in-situ near-surface concentration measurements provided by the microcapacitors agrees with that predicted by the CP model.; Moreover, the combined measurements using microcapacitors and UTDR transducers provide the first-ever demonstration of real-time continuous monitoring from the inception of CP through fouling in a flat-sheet membrane module. Overall, this work enables the real-time detection of the CP and fouling phenomena which occur near the membrane surface and are difficult to observe using conventional approaches. The direct experimental observation obtained can provide important insights concerning CP behavior and fouling mechanism.