Solute transport in a submerged forward osmosis membrane system

Van Wyk, Linda Alida (2019-12)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Wastewater treatment with forward osmosis (FO), an osmotically driven membrane process, has been investigated for the osmotic dilution of seawater prior to desalination in an attempt to lower the energy consumption of seawater reverse osmosis (RO). This hybrid FO-RO process provides a dual-barrier for the effective rejection of wastewater contaminants, thereby potentially producing a high quality permeate. Due to the higher rejection capacity of FO membranes compared to ultrafiltration membranes, the FO process can advance wastewater treatment in submerged membrane bioreactors. The aim of this study was to investigate the transport and rejection of selected weakly-rejected solutes in a submerged FO system with a commercially available FO membrane. The benefit of the dual-barrier rejection mechanism of the FO-RO hybrid could then be investigated by simulation of its final permeate quality with the experimentally determined rejections of the selected model solutes. To this end, a bench-scale FO setup was designed and constructed. The baseline performance of the FO membrane was firstly evaluated by considering the effects of the membrane orientation, hydrodynamic conditions and osmotic pressure gradient on the water flux and reverse draw solute flux. Phenol, as an organic water contaminant, and boron and lithium, as inorganic water contaminants, all with different physicochemical properties and potentially weak membrane rejections, were used to study the solute transport and rejection. With a draw solution of seawater quality, water fluxes of 20 L∙m-2∙h-1 and 32 L∙m-2∙h-1 were obtained when the active layer of the membrane was in contact with the feed solution (AL-FS orientation) and draw solution (AL-DS orientation), respectively. The AL-FS orientation exhibited exceptional flux stability at the expense of dilutive internal concentration polarisation. With no hydrodynamic conditions at the submerged membrane surface, concentrative external concentration polarisation (CECP) of the reverse diffused draw solute resulted in a significant water flux decline to below 8 L∙m-2∙h-1 in both membrane orientations. A Reynolds number of 1 100 at the submerged membrane surface was sufficient to mitigate CECP. It was found that the solute rejection improved with an increasing osmotic pressure gradient. The rejection of the neutrally charged solutes, boron and phenol, was independent of their concentration gradients in both membrane orientations. An increase in the ionic strength and decrease in the pH of the feed solution with increasing concentrations of lithium chloride and boric acid increased the rejection of lithium, most likely due to its reduced electrostatic interactions with the negatively charged membrane surface. As opposed to boron and phenol, the lithium rejection in the AL-DS orientation was higher than in the AL-FS orientation as the electrostatic attraction of lithium to the membrane in the AL-DS orientation was perceived to be insignificant. It is postulated that the electrostatic attraction of lithium to the negatively charged membrane surface significantly compromised its rejection, such that it was approximately 16% lower than that of phenol and boron in the AL-FS orientation at neutral pH conditions. The respective experimental phenol, boron and lithium rejections of 91%, 93% and 81% were implemented in the simulation of the FO-RO hybrid process. By its dual-barrier and intermediate dilution effects, the FO-RO hybrid provided an improved permeate phenol concentration of 1.1 μg∙L-1, compared to 9.0 μg∙L-1 provided by a standalone wastewater RO process. The permeate quality of a standalone seawater RO unit could be improved from 315 μg∙L-1 boron and 149 μg∙L-1 lithium to 32 μg∙L-1 boron and 25 μg∙L-1 lithium with typical influent seawater concentrations.

AFRIKAANSE OPSOMMING: Die behandeling van afvalwater met voorwaartse of direkte osmose (FO), wat ‘n osmoties gedrewe membraanproses is, kan die osmotiese verdunning van seewater voor ontsouting fasiliteer ten einde die energieverbruik van seewater tru-osmose (RO) te verlaag. Hierdie gekoppelde FO-RO proses voorsien ‘n dubbele versperring vir die effektiewe verwerping van kontaminerende komponente in afvalwater, met ‘n hoë kwalitieit permeaat as produk. Met die hoër verwerpingskapasitieit van FO membrane in vergelyking met ultrafiltrasiemembrane, kan die FO proses die behandeling van afvalwater bevorder. Hierdie projek het beoog om die oordrag en verwerping van sekere swak verwerpde komponente in ‘n gedompelde FO sisteem te ondersoek met ‘n kommersieël beskikbare FO membraan. Die voordeel van die dubbele verwerpingsmeganisme van die gekoppelde FO-RO proses kon vervolgens ondersoek word deur die simulasie van die finale permeaatkwaliteit met die eksperimenteel bepaalde verwerpings van geselekteerde modelkomponente. ‘n Bank-skaal FO opstelling was ontwerp en opgerig. Die basislyn gedrag van die FO membraan was eerstens ge-evalueer deur die effekte van die membraanoriëntasie, hidrodinamiese kondisies en die osmotiese drukgradiënt op die watervloed en tru-soutvloed (RSF) te oorweeg. Fenol, as ‘n organiese waterkomponent, en boor en litium, as anorganiese waterkomponente, al drie met verskillende fisies-chemiese eienskappe en potensiële swak membraanverwerpings, was gebruik om die oordrag en verwerping van opgeloste stowwe in FO te bestudeer. In die teenwoordigheid van ‘n trekoplossing van seewater kwaliteit was ‘n watervloed van 20 L∙m-2∙h-1 en 32 L∙m-2∙h-1 gelewer deur die membraan met die aktiewe laag na die voeroplossing (AL-FS oriëntasie) en trekoplossing (AL-DS oriëntasie), onderskeidelik. ‘n Merkwaardige vloed-stabiliteit was vertoon in die AL-FS oriëntasie ten koste van interne verdunning konsentrasie polarisasie (DICP). Met geen hidrodinamiese kondisies by die gedompelde membraanoppervlak nie het eksterne konsentrerende konsentrasie polarisasie (CECP) van die tru-gediffundeerde trekoplosmiddel ‘n afname in die water vloed tot onder 8 L∙m-2∙h-1 veroorsaak in albei oriëntasies. ‘n Reynoldsgetal van 1 100 by die gedompelde membraanoppervlak was voldoende om CECP teen te werk. Dit was bevind dat die verwerping van waterkomponente in FO verbeter met ‘n toenemende osmotiese drukgradiënt oor die membraan. Die verwerping van die ongelaaide komponente, boor en fenol, was onafhanklik van hul konsentrasiegradiënt in albei membraanoriëntasies. ‘n Toename in die ioonkonsentrasie en afname in die pH van die voeroplossing met toenemende konsentrasies van litiumchloried en boorsuur het die verwerping van litium verbeter, heel waarskynlik as gevolg van die verminderde elektrostatiese interaksies van litium met die elektronegatiewe membraan. Kontrasterend teenoor boor en fenol was die verwerping van litium in die AL-DS oriëntasie hoër as in die AL-FS oriëntasie, aangesien die elektrostatiese aantrekking van litium na die membraan in die AL-DS oriëntasie moontlik gering was. Dit word gepostuleer dat die verwerping van litium noemenswaardig ly onder elektrostatiese aantrekkings na die elektronegatiewe membraanoppervlak tot so ‘n mate dat eersgenoemde 16% laer was as die verwerping van fenol en boor in die AL-FS oriëntasie onder neutrale pH kondisies. Die onderskeidelike eksperimenteel bepaalde fenol-, boor- en litiumverwerpings van 91%, 93% en 81% was geïmplementeer in die simulasie van die gekoppelde FO-RO proses. As gevolg van die dubbele versperring- en verdunningseffek, kon die proses ‘n verbeterde permeaatkonsentrasie van 1.1 μg∙L-1 fenol lewer, in vergelyking met die alleenstaande afvalwater RO permeaatkonsentrasie van 9.0 μg∙L-1. Die permeaatkwaliteit van ‘n alleenstaande seewater RO eenheid kon verbeter word van 315 μg∙L-1 boor en 149 μg∙L-1 litium na 32 μg∙L-1 boor en 25 μg∙L-1 litium met tipiese konsentrasies in die seewater en afvalwater voerstrome.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107235
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