Fertilizer drawn hollow fiber forward osmosis for desalination
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Continuous increase in fresh water demand has underscored the importance of developing a low cost water desalination process. Fertilizer drawn forward osmosis (FDFO) presents a promising step forward for low cost desalination using the natural osmotic pressure of the fertilizer draw solution (DS) as a driving force. FDFO carries a distinct advantage over other FO processes because the final diluted draw solution requires minimal to no treatment processes and it can be easily used for any useful fertigation application. This helps to eliminate the energy intensive permeate recovery step for the FO process and represents an economical desalination option. However, the performance ratio outcome for the earlier FO studies has highlighted a number of areas that can be improved in relation to FO performance. This study evaluated FDFO using eight commercial fertilizers as DS for the flat sheet FO membrane using sea water (SW) quality feed (35 g/L NaC1) and targeting the NPK fertilizer and water requirements for tomato crops. Diverse results were achieved as some of the fertilizers showed significant flux while others showed negative or very low flux outcomes. This indicated that all commercial fertilizers may not be effectively used as DS for the SW quality feed. The results with various quality feed solutions (FS) and DS concentrations indicated that the flux performance does not vary in a linear sense with the changes in Δπ. Varying flux outcome for various individual or mixed fertilizer DS’s carrying similar Δπ values reflects the involvement of some unknown interactions between the DS and membrane surfaces, both at the active layer (AL) and the support layer (SL), for these specific results. These results further highlighted the fact that the osmotic pressure of the DS alone may not be used as the main criteria for the DS selection but rather the association between the DS solutes and the active and support layers of the membrane are also vital in terms of understanding the FO flux performances. In addition, these outcomes revealed a number of limitations in relation to the FDFO e.g. reverse solute flux issues, higher nutrients concentration in the final DS and low recovery for osmotic equilibrium issues. These fertilizer DS’s were further assessed and their performance was compared for cellulose triacetate (CTA) flatsheet and polyamide (PA) hollow fiber FO (HFFO) membranes to understand the association between the DS properties and the membrane characteristics for the FO outcome. It was observed that at similar operating parameters, the PA hollow fiber showed a comparatively better outcome in terms of flux and reverse solute flux (RSF). HFFO was also evaluated for the effects of various operating conditions and markedly enhanced performances were found. It was observed that for 2 M NaC1 as DS and DI water as FS, the HFFO successfully delivered water flux of 62.9 LMH at DS/FS Reynolds number (Re) of 3750/1500 whereas the same membrane in AL-FS orientation showed a flux of 9.67 LMH at DS/FS Re of 200/500. This indicated a flux increase of about 511% for a set of two operating conditions for the same FO membrane which further suggested that the changes in the operating conditions induce some indistinct changes in the membrane structure that can affect the water transport phenomenon through the membrane. It is therefore recommended that further studies be undertaken to investigate the real mechanism for the water transport through the membrane as this could contribute to the development of a higher performing membrane for the FO process. Results also indicate that cationic and anionic parts of the DS seriously affect the RSF outcomes. Further evaluation in this regard may contribute towards the creation of a better DS for the FO process with reduced RSF consequences. The HFFO membrane was further evaluated for inorganic scaling and organic fouling issues using brackish ground water quality FS loaded with various model organic foulants such as humic acid, alginate and bovine serum albumine (BSA). During these FO fouling studies, it was noted that the commonly used FO fouling protocol which is similar to the RO fouling protocol may not be successfully used to evaluate FO fouling. The RO fouling was evaluated against a fixed driving force (hydraulic pressure) and any changes in the flux performance were referred to the fouling impact. However, in FO, as the driving force (net osmotic pressure difference between the FS and DS) kept changing constantly, it was really difficult to predict any flux change which was particularly associated with the scaling or fouling. For any two tests, at any particular time, the FO did not show the same driving force and hence for the evaluation of the fouling, the flux comparison for two different curves was not always useful. Accordingly, a new protocol is suggested for the FO fouling studies. The fouling results indicated that FO, like the RO membrane, also posed potential operational risks in terms of scaling and fouling. The HFFO membrane indicated varying degrees of fouling potential for the membrane used in the AL-FS and AL-DS orientation and these were not related to membrane properties. Instead the hydrodynamic conditions employed for the process affected the fouling potential of the membranes used. Results indicated that the higher crossflow rate helped to keep the membrane clean from inorganic scale and the turbulence shear force did not allow scale build-up at the high Re. It was also observed that the inorganic scaling was not fully reversed for the HFFO membrane used in the AL-FS and AL-DS orientations which employed hydraulic cleaning practices because the cleaning totally depended on how the flow shear forces using various cross flowrates were applied on the membrane surface. For the organic foulants, the turbulence shear force could not overcome the membrane–foulant interactions and foulants layer deposited on the membrane surfaces and reduce the FO performance which was not recovered by hydraulic flushing. The chemical cleaning which used HC1, NaOH and EDTA was evaluated and it was found that the EDTA (pH 11) showed a better outcome for FO membrane cleaning.
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