Evaluation of a flat sheet woven fabric membrane for use in water treatment processes suited to rural and peri-urban economies

Asquith, Joshua Steven (2017-03)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH SUMMARY: The present study was undertaken with the view of evaluating a woven fabric membrane as an alternative membrane technology for water treatment purposes for rural and peri-urban economies. Current membranes are not economically feasible or suited to the requirements of most rural and peri-urban economies. As a result, a woven fabric membrane material was developed to produce a membrane that was capable of overcoming these shortcomings. The use of the woven fabric membrane as an alternative membrane for water treatment purposes was evaluated by (i) characterising the inherent membrane features, (ii) evaluating the membrane performance in terms of the product quality and fouling effects and (iii) investigating the effects of flux enhancement and cleaning methods. A literature review was performed to specify the separation requirements of the woven fabric. (i) During the membrane characterisation the membrane’s effective pore size range and morphology were evaluated. The bubble gas transport method was used to estimate the effective pore size range of unused and fouled membrane samples which were found to be 10-24 µm and 7-18 µm respectively. SEM imagine evaluation indicated that the complex multilayer membrane material was formed due the Plain Weave structure and the multiple strands used to produce the respective threads. The effects of pore narrowing due to fouling were concluded to be a result of the highly tortuous membrane pores that formed. (ii) During the membrane performance evaluation the effect that the feed suspension’s physical and chemical make-up had on the resultant product quality and fouling characteristics was indicated. From the performance evaluation it was concluded that the woven fabric membrane was able achieve the required 1 NTU product quality for sub-pore size (< 3 µm) feed suspensions with varying physical and chemical properties, through the formation of a porous fouling layer. This concluded the importance of dynamic membrane operation and how the estimated pore size was not an indication of the separation efficiency of the woven fabric. (iii) The effects of continuous coarse air scouring (7.5 L/min and 2.5 L/min) and fine air scouring (5 L/min and 2.5 L/min) as a flux enhancement method resulted in higher fouling layer resistances compared to those of a dead end operation test. This was a result of the defouling potential of this method effectively defouling the membrane surface. This compromised the dynamic operation of the membrane, as higher degrees of pore blocking occurred due to the absence of the surface fouling layer. A cleaning evaluation was performed to investigate chemically-aided (25 PPM sodium hypochlorite) backflush methods for backflush fluxes of 50, 100 and 250 LMH in order to completely reverse the effects of fouling. Minimal recovery was exhibited by all these defouling methods, due to the high degree of irreversible fouling occurring as indicated by SEM evaluation of the membrane samples. The irreversible fouling was the result of the subpore size feed suspension. The only effective cleaning method was a physical scrubbing of the membrane surfaces. This was concluded from the results of a test that was operated with an initial permeate flux of 100 LMH, for 60-minutes, between physical cleaning, over a period of 10-hours, where no apparent effects of irreversible or irrecoverable fouling occurred. Due to its durability and robustness, the woven fabric was able to withstand these physical cleaning methods, which are not applicable to most available industrial membranes. The findings of this study have clearly shown that the woven fabric membrane was capable of achieving the required separation efficiency for water treatment purposes. The membrane was however limited to physical cleaning methods due to the formation of high degrees of irreversible fouling within the membrane pores. Cleaning these membranes using a physical means was indicated as being a reliable means of defouling the woven fabric membrane. The ease of cleaning these membranes using a physical means, is of far more benefit to rural and peri-urban areas than a potentially hazardous chemically aided cleaning solution.

AFRIKAANS OPSOMMING: Die huidige studie is onderneem met die doel om 'n geweefde tekstielmembraan as 'n alternatiewe membraantegnologie vir waterbehandeling in landelike en buitestedelike ekonomieë, te evalueer. Huidige membrane is òf nie ekonomies lewensvatbaar nie, òf dit is nie geskik vir die vereistes van die meeste landelike en buitestedelike ekonomieë nie. Gevolglik is 'n geweefde tekstielmembraanmateriaal ontwikkel om 'n membraan op te lewer wat in staat is om hierdie tekortkominge die hoof te bied. Die aanwending van die geweefde tekstielmembraan as 'n alternatiewe membraan vir die doeleindes van waterbehandeling is geëvalueer deur (i) die inherente membraaneienskappe te karakteriseer, (ii) die membraan se prestasie in terme van produkgehalte en bekorstingseffekte te evalueer, en (iii) die gevolge van vloeibevordering en skoonmaakmetodes te ondersoek. 'n Literatuuroorsig is uitgevoer om die skeidingsvereistes van die geweefde tekstiel te spesifiseer. (i) Die karakterisering van die membraan het die effektiewe bestek in die grootte van die membraan se porieë, en die morfologie, geëvalueer. Die borrelgasvervoermetode het die effektiewe speling van die grootte van porieë van ongebruikte en vuil membraanmonsters as 10-24 µm en 7-18 µm onderskeidelik geskat. SEM skattingsevaluering het aangedui dat die komplekse membraanmateriaal met sy veelvuldige lae gevorm is deur die Plain Weave struktuur, en die veelvoud stringe gebruik is om die onderskeie drade te skep. Die gevolgtrekking was dat die vernouing van porieë weens bekorsting die gevolg was van die uiters gewronge membraanporieë wat gevorm is. (ii) Evaluering van membraanprestasie het die effek wat die toevoersuspensie se fisiese en chemiese samestelling op die gevolglike produkgehalte en bekorstingseienskappe gehad het, bepaal. Die prestasie-evaluering het tot die slotsom gekom dat die geweefde tekstielmembraan in staat was om die nodige 1 NTU produkgehalte van toevoersuspensies met sub-poriegrootte (<3 µm) met wisselende fisiese en chemiese eienskappe te bereik deur die vorming van 'n poreuse bekorstingslaag. Dit het die vereiste dinamiese werking van die membraan afgesluit. (iii) Die gevolge van voortdurende ruwe lugspoeling (7.5 L/min. en 2.5 L/min.) en fyn (5 L/min. and 2.5 L/min.) lugspoeling as metode om vloei te bevorder, het aanleiding gegee tot hoër bekorstingslaagweerstande in vergelyking met 'n doodloopbedryfstoets. Dit was die gevolg van die ontkorstingspotensiaal van hierdie metode, wat die membraan se oppervlak doeltreffend ontkors het. Dit het die dinamiese werking van die membraan gekompromitteer, aangesien meer porieblokkering plaasgevind het weens die afwesigheid van die bekorstingslaag op die oppervlak. 'n Skoonmaakevaluasie is uitgevoer om terugspoelmetodes wat chemies aangehelp is (25 DPM natrium hipochloriet) vir terugspoelvloeiing van 20, 100 en 250 LMH om die gevolge van bekorsting heeltemal om te keer. Minimale herstel is deur al hierdie ontkorstingsmetodes getoon, weens die voorkoms van uiterste grade van onherstelbare bekorsting, soos getoon deur die SEM evaluering van die membraanmonsters. Die onherstelbare bekorsting was die gevolg van die toevoersuspensie wat kleiner was as die poriegrootte. Die enigste doeltreffende skoonmaakmetode was om die membraanoppervlak fisies te skrop. Dit is afgesluit deur die resultate van 'n toets wat uitgevoer is met 'n aanvanklike deurlaatvloeiing van 100 LMH, vir 60 minute, tussen fisiese skoonmaak, oor 'n tydperk van 10 uur, met geen ooglopende gevolge van onherstelbare of onherkrygbare bekorsting wat plaasgevind het nie. Weens die duursaamheid en stewige aard van die geweefde tekstiel kon dit hierdie fisiese skoonmaakmetodes, wat nie op die meeste beskikbare nywerheidstekstiele toegepas word nie, weerstaan. Ter opsomming het hierdie studie duidelik getoon dat die gevolgtrekking bereik kon word dat die geweefde tekstielmembraan in staat is om doeltreffend die skeidingseffektiwiteit wat vir waterbehandelingsdoeleindes vereis word, te bereik. Net fisiese skoonmaakmetodes kon egter vir die membraan gebruik word, weens die vorming, binne die membraanporieë, van uiterste grade van onherstelbare bekorsting. Die gemak waarmee hierdie membrane fisies skoongemaak kan word is van veel groter waarde vir landelike en buitestedelike gebiede as wat potensieel skadelike chemiese skoonmaakoplossings sou wees.

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