Real-time investigation of fouling phenomena in membrane filtrations by a non-invasive ultrasonic technique

[摘要] ENGLISH ABSTRACT:Membrane fouling is universally accepted as one of the most critical problemslimiting the wider application of membranes in liquid separations. The developmentand utilization of a suitable non-invasive technique for the on-line monitoring offouling in industrial and laboratory applications may enable the effectiveness offouling remediation and cleaning strategies to be quantified.The overall objective of this research is to develop ultrasonic time-domainreflectometry (UTDR) and its use as an analytical tool for the real-time study ofinorganic-, organic- and protein- fouling of various types of membranes includingnylon, polysulfone (PSU) and polyethersulfone (PESU) and modules, including flatsheetand tubular types. Different separation systems including microfiltration (MF)and ultrafiltration (UF), flat-sheet and tubular modules, and suitable ultrasonic probeswere used in this study.Results of this study show a good correlation between the UTDR signal response andthe development of a fouling layer on a membrane surface. UTDR effectively detectedthe appearance, growth and movement of a fouling layer echo as fouling proceeded.Cake (fouling)-layer compressibility was observed by UTDR. The structure andcompaction of an asymmetric PSU membrane could be detected by UTDR. UTDRwas also successfully used for monitoring membrane cleaning and evaluating thecleaning effectiveness o f various cleaning methods. UTDR results corroborated theflux measurements and SEM analyses.The ultrasonic unit is a programmed microprocessor, and can be used to comparereference and test signals to produce a differential signal (a fouling layer echo). Adifferential signal indicates the state and progress o f a fouling layer on the membranesurface in actual operations. Both amplitude and arrival time of differential signals asa function of operation time provide useful quantitative information, i.e. changes inthickness and density of a fouling layer, on the fouling processes.A predictive modelling program, ultrasonic reflection modelling (URM), wasdeveloped to describe the processes of ultrasonic testing related to the deposition offouling layers on membrane surfaces. The mathematical model could substantiatechanges in the densities of the fouling layer as well as the thickness. This is importantas deposit resistance to flow is related to both thickness and density (compressibility).The predicted results of cake layer deposition are in good agreement with the actualUTDR measurements obtained in MF and UF.Furthermore, protein fouling was successfully detected in tubular UF by UTDR.Ultrasonic frequency spectra could be used as an additional tool for fouling detection.

[发布机构] Stellenbosch University