Non-invasive in situ visualisation of membrane fouling and cleaning processes in microfiltration by ultrasonic signal reflection
A better understanding of fouling-layer formation and its detection is needed in order to improve on existing cleaning techniques. An ultrasonic time-domain reflectometry (UTDR) technique has been applied to the non-invasive, in situ, continuous visualisation of fouling and defouling in flat-sheet crossflow microfiltration (MF) membranes. Paper mill effluent from a wastewater treatment plant was used as the feed solution. Fouling experiments were carried out at axial velocities of 1.0 and 42 mm/s, so that two modes of fouling-layer growth were observed. Changes in the amplitude of the acoustic signal response corresponded well with flux-decline behavior. The UTDR technique was able to distinguish between the two modes of growth. The ultrasonic technique could effectively detect fouling-layer initiation, its growth on and removal from the membrane in real-time. Data also showed the formation of a second echo in the time domain at lower operating pressure (100 kPa). Therefore, the UTDR technique was used to quantify the thickness of a fouling layer on the membrane surface. Moreover, the UTDR technique was also employed for monitoring membrane cleaning and evaluating the cleaning effectiveness of various cleaning methods. Results of cleaning experiments show that ultrasound associated with flushing is the most effective cleaning method. Scanning electron microscopy (SEM) pictures were taken of the fouling and defouling layers. The UTDR is a more powerful non-invasive technique for the in situ visualisation of fouling and defouling in crossflow MF membranes.