Thesis (MSc)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: At the Mondi Kraft paper mill in Piet Retief, paper machine effluent is pre-treated by means
of dissolved air flotation (DAF) and a mierostrainer prior to ultrafiltration (UF). Despite the
rigorous pre-treatment of the effluent, severe fouling of the UF membranes still persisted,
resulting in a sharp decrease in operational flux. In an attempt to improve the flux
performance of the UF membranes an investigation was launched into the possible causes of
membrane fouling. The study yielded the following results:
Ultraviolet-visible (UV-Vis) spectrophotometric analyses of varIOUS effluent samples
collected from different locations at the mill indicated the presence of aromatic compounds.
Lignosulphonate appeared to be the main constituent in all the samples analysed.
UV-Vis spectrophotometry was also performed on fouling extracted from membranes In
order to evaluate the different approaches attempted to reduce membrane fouling. Most of
the UV-Vis spectra obtained did not show the absorbance maxima below 210 nm that were
seen for the paper machine effluent, DAF product, lignosulphonate and mierostrainer product.
This indicated that the compounds with absorbance at lower wavelengths did not foul the
membranes to the same extent as the aromatic substances with absorbance maxima between
230 and 400 nm.
The influence of pH on the absorption of the various effluent samples was also investigated.
An increase in pH resulted in (1) a "shift" in the wavelength scans from a lower to a higher
wavelength, suggesting ionisation (deprotonation) with a subsequent delocalization of
electrons and (2) an increase in the turbidity.
The increase in turbidity which accompanied the Increase in pH could be explained by
complex formation between the carboxylate ions, phenolic groups and divalent metal ions
present in the effluent. Inductively coupled plasma analyses of several effluent samples with
pH values 7 and 13 indicated the presence of significant amounts of Ca2+ ions in the effluent.
There was a significant decrease in the Ca2+ levels with an increase in pH, which supported
the hypothesis that Ca2+ might contribute to complex formation. This resulted in a decrease
in solubility and an increase in turbidity. The addition of a chelating agent
(ethylenediaminetetra-acetic acid disodium salt) to an effluent solution at pH 13 redissolved
the precipitate and considerably reduced the turbidity. The subsequent addition of CaCh again induced precipitation and increased turbidity, confirming the role of Ca2+ in complex
formation.
Gel permeation chromatographic analyses of mierostrainer product at pH 13 showed the
formation of high molecular mass organo-calcium complexes. The exact molecular mass of
the complexes present in the mierostrainer product could not be determined by electro spray
mass spectrometry because of their poor ionisation ability.
Atomic force microscopy and scanning electron microscopy (SEM) showed distinct
differences in the membrane surface texture before and after fouling. Furthermore, SEM
images of the UF membranes exposed the limited ability of the 30 urn microstrainer, installed
downstream from the DAF unit, to remove residual fibres from the DAF product.
Static fouling experiments performed on all the flocculants and coagulants used In the
paper-making process at the mill showed that none of these substances fouled the UF
membranes.
Cleaning of the UF membranes with Triton XIOO®, a nOn-IOnIC surfactant, caused a
temporary increase in the operating flux to values higher than that of the initial flux.
Mechanical cleaning of the UF membrane surface with spongebalIs proved to be one of the
most effective and successful methods to prevent flux loss caused by fouling.
Pre-coating of the UF membranes with Plutonic" FI08, another non-ionic surfactant, did not
promote membrane productivity. Evaluation of various types of membranes indicated that
hydrophilic or negatively charged membranes withstood membrane fouling more effectively
than hydrophobic UF membranes under the same operating conditions. / AFRIKAANSE OPSOMMING: By Mondi Kraft se papier meule in Piet Retief word afloopwater vanaf die papiermasjiene vir
hergebruik met behulp van ultrafiltrasie (UF) behandel. Opgeloste lugflotasie (OLF) en
mikrosiwwing word as voorbehandeling vir die UF membraanproses ingespan. Ondanks die
intensiewe voorafbehandeling wat toegepas word, vind daar geweldige aanvuiling van die UF
membrane plaas wat tot die vinnige verlaging in bedryfsfluks aanleiding gee. 'n Ondersoek
na die moontlike oorsake van membraan-aanvuiling het die volgende bevindinge opgelewer:
Ultraviolet-sigbare (UV-Vis) spektroskopie van water monsters wat by die meule versamel is,
het die teenwoordigheid van aromatiese komponente aangetoon, met lignosulfonaat die
hoofkomponent in al die monsters wat ontleed is.
Ekstrakte afkomstig van aangevuilde membrane is ook met behulp van UV-Vis-spektroskopie
geanaliseer om verskeie benaderings te evalueer om 'n afname in membraan-aanvuiling te
bewerkstellig. Die oorgrootte meerderheid spektra het nie die absorpsie maksima onder
210 nm aangetoon wat teenwoordig was in monsters van die papier masjien afloopwater,
OLF uitvloeisel, lignosulfonaat en mikrosif produkwater nie. Dit het aangedui dat die
komponente wat by laer golflengte absorbeer nie die UF membrane in dieselfde mate aanvuil
as daardie komponente wat by hoër golflengtes (tussen 230 en 400 nm) absorbeer nie.
Die invloed wat pH op die absorpsie van komponente teenwoordig in die onderskeie
afloopwatermonsters het, is ook ondersoek. 'n Toename in pH het bygedra tot (1) 'n
verskuiwing in die spektra vanaf 'n lae na 'n hoër golflengte vanweë ionisasie
(deprotonering) met gevolglike delokalisasie van elektrone en (2) 'n toename in turbiditeit.
Die toename in turbiditeit wat verband hou met die toename in pH was verduidelik aan die
hand van kompleksvorming tussen die karboksilaat ione, fenoliese groepe en divalente metaal
ione in die afloopwater. Induktief gekoppelde plasma analise van verskeie water monsters by
pH 7 en 13 het die teenwoordigheid van 'n groot hoeveelheid Ca2+ aangetoon. 'n Verlaging
in die vlakke van opgeloste Ca2+ het met die toename in pH verband gehou. Dit het die
moontlike verbintenis tussen Ca2+ en kompleksvorming ondersteun wat bygedra het tot die
afname in oplosbaarheid en toename in turbiditeit. Die byvoeging van
etileendiamientetra-asynsuur-dinatriumsout, 'n kelerings reagens by afloopwater (pH 13) het
die presipitaat weer in oplossing gebring en die turbiditeit merkwaardig verlaag. Die byvoeging van CaCh het weer presipitasie geïnduseer, met 'n gevolglike toename in
turbiditeit. Hiermee is Ca2+ se rol in kompleksvorming bevestig.
Gelpermeasie-chromatografiese analise van die mikrosif produk (pH 13) het die vorming van
hoë molekulêre massa organo-kalsium komplekse bevestig. Dit was egter nie moontlik om
met behulp van massaspektrometrie die korrekte molekulêre massa van die komplekse te
bepaal nie vanweë hul onvermoë om te ioniseer.
Atomiese krag mikroskopie en skandeer elektron mikroskopie (SEM) het duidelik die voor en
na verskil getoon wat aanvuiling op die membraantekstuur gehad het. 'n SEM foto van die
aangevuilde UF membraan het die onvermoë van die mikrosif blootgelê om oorblywende
vesels vanuit die OLF produkwater te verwyder.
Resultate bekom gedurende passiewe aanvuilingseksperimente het aangetoon dat al die
in-proses flokkulante en koagulante wat gebruik word by die papier meule geen bydrae tot die
aanvuiling van die UF membrane maak nie.
Skoonmaak van die UF membrane met Triton XIOO® bring 'n verhoging in bedryfsvloed
teweeg, maar die verhoging, wat hoër as die oorspronklike vloed is, is kortstondig.
Meganiese skoonmaak van die buismembrane met behulp van sponsballe blyk die mees
effektiewe skoonmaakmetode te wees.
Voorafbehandeling van die UF membrane met Plutonic" F 108 het nie die
membraanproduktiwiteit verhoog nie. Daar is ook bevind dat hidrofiliese of negatief gelaaide
membrane groter weerstand bied teen aanvuiling in vergelyking met hidrofobiese UF
membrane onder dieselfde bedryfstoestande.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52595 |
| Date | 12 1900 |
| Creators | Domingo, Garth Selby |
| Contributors | Swart, P, Jacobs, E. P., Stellenbosch University. Faculty of Science. Dept. of Biochemistry. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 101 p. : ill. |
| Rights | Stellenbosch University |
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