Ultrasonic Reflectometry for Monitoring the Effect of Pressure on Sludge Fouling of MF Membranes

Membrane fouling remains the key limitation for the widespread use of membrane bioreactors (MBR) for wastewater treatment. This constraint has led to an increasing number of studies that examine the influence of various operational parameters and physicochemical properties on fouling layer formation and characteristics. In other membrane applications real-time monitoring has proven to be useful by providing a more quantitative characterization of fouling layer formation [1]. One such technique, ultrasonic reflectometry (UR), has been successfully used to detect fouling formed by a wide range of foulants including calcium sulfate [2], yeast [3], proteins [4] and biofilm [5]. Based upon the well-documented advantages of UR measurement [6], the objectives of this study were to utilize UR for real-time detection of fouling layers formed by municipal activated sludge and for quantification of the effect of pressure on the fouling layer structure.
The ability of UR to detect and monitor sludge fouling was studied in a series of replicated experiments of 15, 30 and 60-min duration that used commercial microfiltration (MF) membranes at a transmembrane pressure of 15 kPa. By analyzing the peak of the ultrasonic signal reflected from the membrane surface, it was observed that ultrasonic amplitude was reduced as a function of fouling intensity. Findings showed that there was a statistically significant correlation between the reduction of ultrasonic amplitude and the degree of fouling. This association was validated by real-time flux measurements as well as post-mortem characterization metrics that included optical image analysis, gravimetric measurement and membrane protein concentration. The reduction of the reflected ultrasonic amplitude signal is due to the presence of a hydrated fouling layer, which serves as an impedance matching layer with the water-swollen membrane.
Furthermore, the effect of pressure on membrane performance and ultrasonic amplitude was studied in 5-h filtration experiments conducted at two pressures: 15 kPa and 25 kPa. Results showed that there is higher filtration resistance but less ultrasonic amplitude reduction by the fouling layer formed at higher pressure. This somewhat counterintuitive finding is explained by compression of the fouling layer, i.e., a more compressed (denser) and less hydrated layer provides less attenuation and consequently a higher signal amplitude as compared to a water-swollen layer at lower pressure.

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