Real-time Observation of Fouling in Membrane Filtration by Non-invasive Ultrasonic Techniques

March 2004



Membrane fouling is universally accepted as one of the most critical problems limiting the wider application of membranes in liquid separations. The development and utilisation of a suitable non-invasive technique for the on-line monitoring of fouling in industrial and laboratory applications may enable the effectiveness of fouling remediation and cleaning strategies to be quantified.

The present research project was born from the words of Dr Irwing Murdoch, editor of Water Desalination and Re-use and chairman of a task group on membranes who, in 1999, stated that there was a need to revisit problems that limit the reverse osmosis (RO) field. In the project reported on her this was approached by using the tremendous recent gains made in available analytical equipment, including high-frequency digital and computer techniques. This was encouraged and supported by Professors Kranz and Greenberg, then of the Separation Centre, University of Colorado, USA, who had already two or three years experience in ultrasonic techniques. In this research project (#1166) we investigated the use of ultrasonics to detect and monitor in different membrane systems and in different configurations.

Through an initial programme funded by the WRC (#930) in which the use of ultrasonic waves was investigated it was proved that, using ultrasonics, fouling on RO membranes could be seen. This opened a number of doors to this investigation and gave rise to many of the aims.


The overall objective of this research is to develop ultrasonic time-domain reflectometry (UTDR) for its use as an analytical tool for the real-time study of inorganic, organic and protein fouling of various types of membranes, including nylon, polysulfone (PSU) and polyethersulfone (PESU), and modules, including flat-sheet and tubular types. Different separation systems including microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), flat-sheet and tubular modules, and suitable ultrasonic probes were used in this study.

Specific objectives were subsequently the following:


The overall aim of this project was to develop ultrasonic time-domain (UTD) and amplitude-domain reflectometry (UTDR) as a real-time visualisation technique for monitoring fouling in various pressure-driven membrane separations.

This was approached in the following way:

Consideration was first to be given to various designs for the development of the UTDR technique and suitable test cells. The latter should allow the simultaneous pressure-driven separation and monitoring of fouling. Systems for UTDR measurements and ultrasonic cleaning were set up and used. Flat-sheet and tubular membrane cells were designed and used. A mathematical model was to be developed and used to describe the process of ultrasonic testing, as related to the deposition of fouling in complex membrane environments.

UTDR was first to be used for the non-invasive, in situ measurement of particle deposition during microfiltration and, pending successful detection, particle removal, using flat-sheet nylon membranes. Scanning electron microscopy was to be used to confirm UTDR results. Further investigations into the cake-layer deposition, its growth on a membrane surface, and its compressibility were to be considered. This was then to be followed by similar investigations with polysulphone (PS) ultrafiltration membranes. Protein fouling of tubular polyethersuphone membranes (PES) was also to be considered. Finally, UTDR was to be used to monitor fouling and cleaning of reverse osmosis membranes

Results and Conclusions

This report describes the developments and successes achieved in both proving and understanding ultrasonic monitoring of membrane fouling. The overall achievement of this investigation was the development of ultrasonic time-domain (UTD) and amplitude-domain reflectometry (UTDR) as a real-time visualisation technique for fouling monitoring in various pressure-driven membrane separations. It was determined that UTDR can be used successfully to monitor membrane fouling in MF, UF and RO with different membranes, foulants and configuration modules.

Specific results and conclusions, in the following areas, were as follows.

Microfiltration (MF)

Ultrafiltration (UF)

Reverse Osmosis (RO)


The research has clearly shown that UTDR can indeed be successfully used to monitor membrane fouling in MF, UF and RO, with different membranes, module configurations and foulants.

An unsatisfied market need for a modified plugging index meter, based on ultrasonics, has been determined.

Hence, the following recommendations for further investigations in this area are made: