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

Li, Jianxin (2002-12)

Thesis (PhD)--Stellenbosch University, 2002.

Some digitised pages may appear illegible due to the condition of the original hard copy.


ENGLISH ABSTRACT: Membrane fouling is universally accepted as one of the most critical problems limiting the wider application of membranes in liquid separations. The development and utilization 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 overall objective of this research is to develop ultrasonic time-domain reflectometry (UTDR) and 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 flatsheet and tubular types. Different separation systems including microfiltration (MF) and ultrafiltration (UF), flat-sheet and tubular modules, and suitable ultrasonic probes were used in this study. Results of this study show a good correlation between the UTDR signal response and the development of a fouling layer on a membrane surface. UTDR effectively detected the appearance, growth and movement of a fouling layer echo as fouling proceeded. Cake (fouling)-layer compressibility was observed by UTDR. The structure and compaction of an asymmetric PSU membrane could be detected by UTDR. UTDR was also successfully used for monitoring membrane cleaning and evaluating the cleaning effectiveness o f various cleaning methods. UTDR results corroborated the flux measurements and SEM analyses. The ultrasonic unit is a programmed microprocessor, and can be used to compare reference and test signals to produce a differential signal (a fouling layer echo). A differential signal indicates the state and progress o f a fouling layer on the membrane surface in actual operations. Both amplitude and arrival time of differential signals as a function of operation time provide useful quantitative information, i.e. changes in thickness and density of a fouling layer, on the fouling processes. A predictive modelling program, ultrasonic reflection modelling (URM), was developed to describe the processes of ultrasonic testing related to the deposition of fouling layers on membrane surfaces. The mathematical model could substantiate changes in the densities of the fouling layer as well as the thickness. This is important as 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 actual UTDR 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.

AFRIKAANSE OPSOMMING: Membraan-aanvuiling of -verstopping is die grootste struikelblok wat die meer algemene aanwending van membrane vir verskillende watersuiweringsprosesse beinvloed. Die ontwikkeling en gebruik van ‘n geskikte nie-inmengende tegniek vir die in-lyn meting van aanvuiling van membrane in laboratorium-en nywerheidstoepassings mag ‘n geleentheid bied vir die kwantifisering van die verwydering van aanvuiling en skoonmaakstrategiee. Die hoofdoel van hierdie studie was die ontwikkeling van ultrasoniese tydgebiedsweerkaatsing (Eng: ultrasonic time-domain reflectometry, UTDR) en die gebruik daarvan as ‘n analitiese metode vir die studie van anorganiese-, organiese- en bio-besoedeling op verskeie tips membrane, insluitend nylon, polisufoon (PSU) en polietersulfoon (PESU), in beide platvel- en buismodules. Verskeie skeidingsisteme, insluitend mikrofiltrasie (MF) en ultrafiltrasie (UF) is ontwerp en gebruik in hierdie studie. Eksperimentele resultate het goeie ooreenstemming tussen die UTDR seinrespons en die ontwikkeling van ‘n aanvuilingslaag op die membraanoppervlakte bewys. Die ultrasoniese tegniek kon die vorming, groei en beweging van ‘n bevuilingslaagterugkaartsing waarneem namate bevuiling vorder. Aanvuilingslaagsamepersing is deur UTDR waargeneem. Die struktuur en samepersing van ‘n asimmetriese PSU membraan is ook deur UTDR gesien. UTDR is verder suksesvol gebruik om die skoonmaak van membrane te monitor en om die skoonmaakgeskiktheid (cleaning effectiveness) van verskeie skoonmaakmetodes te bepaal. UTDR resultate het permeaatvloeimetings en SEM analyses bevestig. Die ultrasoniese eenheid is ‘n geprogrameerde mikroverwerker, en kan gebruik word om verwysings- en toetsseine te vergelyk, en dan ‘n differensiaalsein te gee (‘n aanvuilingslaagweerklank). ‘n Differensiaalsein dui die toestand en vordering van ‘n aanvuilingslaag op die membraanoppervlakte gedurende gebruik aan. Beide amplitude asook aankomstyd van differensiaalseine as funksies van gebruikstyd verskaf bruikbare kwantatiewe inligting, dws. Veranderings in die dikte en digtheid van ‘n aanvuilingslaag, op die aanvuilingsproses. ‘n Voorspellingsmodelleringprogram - ultrasonieseweerkaatsingsmodellering (Eng: ultrasonic reflection modeling, URM) is ontwikkel om die proses van ultrasoniese toetsing by die deponering van aanvuilingslae op membraanoppervlaktes beter te beskryf. Veranderings in die digtheid en dikte van die aanvuilingslaag teenvloei is verwant aan dikte en digtheid (saampersbaarheid). Die voorspelde resultate van aanvuilingslaagdeponering stem goed ooreen met die werklike UTDR-metings wat in MF en UF gemaak is. Bio-aanvuiling is suksesvol waargeneem deur UTDR in buisvormige UF membrane. Ultrasoniese frekwensiespektra kan dus as ‘n bykomende metode gebruik word vir die waarneming van aanvuiling op skeidingsmembrane.

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