Thesis (MEng.)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: The aim of the thesis was to investigate the use of flow destabilization methods, combined with
permeate backflushing (BIF) or on their own, on flux recovery and maintenance in capillary UF
membrane systems under cross-flow (XF) and dead-end (DE) operating conditions. Various
hydraulic and mechanical methods have been used to remove the accumulated cake layer and
improve steady state process flux. Permeate backflushing (B/F) is the most widely used but the
drawbacks are loss of product and extensive down-time. In a pilot plant study for ultrafiltration
of surface waters containing high NOM, turbidity and cation loads, the use of flow
destabilization, or feed flow reversal (FFR) combined with cross-flow B/F was able to improve
the normalised flux by 10.7 ± 3.4 %, compared with 3.2 ± 1.6 % improvement for BIF without
FFR. When a second B/F included FFR, the flux improvement was 7.0 ± 2.0 % compared with
4.3 ± 2.5 % for a B/F without FFR. The hypothesis was proposed that the flow destabilization
caused slight lifting of the oriented cake layer, while the cross-flow B/F was able to sweep the
lifted cake out of the lumen. If the flow destabilization may be effected by a simple but effective
and low-cost method, and if this flow destabilization may be combined with reverse flow for
short durations, the "lift-and-sweep" approach will be the ideal method of maintaining process
flux and increasing membrane life. Such a flow destabilization method, now named "reversepressure
pulsing" (RIP), was developed. The method involves circulation of feed water in a
recycle loop for 2 s to gain momentum, followed by closure of a fast-action valve upstream of
the modules. The momentum of the water in the concentrate loop carries it into an air-filled feed
accumulator, while concentrate and reverse-flow permeate (which also lifts the fouling layer) are
discharged to the atmosphere using the recycle pump for 15 s. When the valve opens again, the
air in the accumulator forces the water under pressure through the membrane lumens, causing a
pressure pulse and flow perturbations that lift, shift and break up the fouling layer. During 3 such
"lift-and-sweep" events, the cake is lifted and the debris is swept out of the lumen. Experimental
results for uninterrupted dead-end filtration at a UF pilot plant using RIP only on a severely
fouled membrane, indicated that the RIP increased the flux by 18.4 % and decreased the dP by
8.2 % over a 7.2 h period. The method is effective in removing the cake layer intermittently and
no long-term flux decline occurred for a period of 555 h since the previous chemical cleaning. / AFRIKAANSE OPSOMMING: Die doel van die tesis was om die gebruik van vloei-destabiliserings metodes, alleen of
gekombineer met permeaat-terugwas, op vloed-herwinning en instandhouding in kapillêre UF
membraan-stelsels tydens kruisvloei en doodloop bedryf, te ondersoek. Verskeie meganiese en
hidrouliese metodes word gebruik in membraan stelsels om die koeklaag op die membraan se
oppervlak te verwyder en die gestadigde-toestand vloed te verbeter. Vanhierdie metodes word
permeaat-terugwas die meeste gebruik, maar het sy nadele insluitend verlies van produk en
produksietyd. In 'n loodsstudie vir die ultrafiltrasie van oppervlakwaters wat hoë beladings
NOM, turbiditeit en katione bevat, is die waarneming gemaak dat kruisvloei terugwas met vloeidestabilisering
(voerrigting-verandering) die genormaliseerde vloed met 10.7 ± 3.4 % kon
verbeter, vergeleke met 'n 3.2 ± l.6 % verbetering sonder voerrigting-verandering. Vir 'n tweede
terugwas was die verbetering 7.0 ± 2.0 % vergeleke met 4.3 ± 2.5 % sonder voerrigtingverandering.
Die hipotese was voorgestel dat die vloei-destabilisering die geoiënteerde koeklaag
van die oppervlak gelig het, en die kruisvloei terugwas die geligde koeklaag uit die lumen kon
vee. Indien hierdie vloei-destabilisering bewerk kan word deur 'n eenvoudige maar effektiewe
manier, en indien dit gekombineer kan word met terugvloei van produk vir kort tydperke, sal
hierdie "lig-en-vee" benadering die ideale metode wees om die membrane se vloed te verbeter en
leeftyd te verleng. So 'n vloei-destabiliseringsmetode, nou genoem "terugdruk-pulsering", is
ontwikkel. Die metode behels die sirkuiering van voer-water vir 2 s in 'n hersirkulasielus om
momentum op te bou, gevolg deur die toemaak van 'n snel-aksie klep stroom-op van die
modules. Die water in die konsentraat-lus se momentum dra dit vorentoe tot in In lug-gevulde
voer-akkumulator, terwyl konsentraat en terug-vloei permeaat (wat ook tot 'n mate die koeklaag
lig) ook na die atmosfeer gewend word vir 15 s deur die hersirkulasiepomp. As die klep weer
oopgaan, ontspan die lug in die akkumulator, en forseer die water daarin onder druk deur die
membraan-lumens. Die druk-puls en vloei-perturbasies lig, skuif en breek die koeklaag op.
Tydens 3 agtereenvolgende "lig-en-vee" aksies word die koeklaag effektief opgebreek en uit die
lumen gevee. Eksperimentele uitslae vir ononderbroke doodloop bedryf op uitermate
aangevuilde membrane van 'n ultrafiltrasie loodsaanleg toegerus met terugdruk-pulsering, het
getoon dat die vloed met 18.4 % verbeter kon word en die dP met 8.2 % verminder kon word in
slegs 7.2 h. Die metode breek die koeklaag effektief op, en geen langtermyn vloed-afname is
waargeneem vir meer as 555 h sedert die vorige chemiese was-prosedure nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/51763 |
| Date | 12 1900 |
| Creators | Botes, Jacobus Petrus |
| Contributors | Bradshaw, S. M., Jacobs, E. P., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various pagings) : ill. |
| Rights | Stellenbosch University |
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