Water. Desalination + reuse
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RESEARCH August-September 2015 | Desalination & Water Reuse | 33 | both sources changed the degree to which the polyamide would swell due its uptake of water – a characteristic that has been defined as pivotal in its capacity to transport water in RO. The researchers measured swelling of the polyamide in its pristine state and after combining it with EPS or UFP using a method commonly used to measure changes in organic films in fluids (see box, The measure of the matter). EPS and UFP both were adsorbed onto the polyamide at pH 8 and formed a fouling layer. Yet, adsorption of EPS was fivefold greater than UFP. Repeating the measurements with gold replacing the polyamide – thereby eliminating water absorption from the test – demonstrated the swelling mass increase due to the foulants alone and showed EPS producing a much greater mass increase. Exposing the polyamide membrane sample to EPS or UFP at pH 6 produced negligible difference in swelling. But on shifting to pH 8 the swelling increase in pristine polyamide was significant and still more so with EPS. Yet there was no increase in swelling in UFP-treated polyamide. On returning to pH 6 swelling returned to its previous level in the EPS-coated and pristine polyamide. The natural conclusion that UFP prevents swelling of the polyamide suggests strongly that it could have a damaging effect on flux. So Herzberg's team looked at the impact of EPS and UFP fouling on RO flux in a flat sheet RO unit. Fouling with UFP or EPS as might be expected reduced flux. And on repeating the transitions from pH 6 to pH 8 and back to pH 6 used in the swelling experiment, the implications from those findings were born out. Fouling with EPS caused a greater decrease in RO flux than UFP fouling at pH 6. The flux decrease was still greater on increasing pH to 8 with EPS maintaining its greater effect. But the EPS flux shift was reversed on returning to pH 6, while the flux decline caused by UFP fouling was irreversible on returning the pH to 6, whilst a further decline was observed. "The reason for this further flux decline is unknown and to our knowledge, this is the first study providing such detrimental unexpected RO fouling behaviour of tertiary effluents," Herzberg writes. In a subsequent study[7], Herzberg's team investigated the effects of fouling on a cross-linked aromatic oligoamide film that mimicked the polyamide layer of a conventional RO membrane and "shows more accurately the mechanism of abolished swelling due to fouling," says Herzberg. That study – using the same QCM-D method deployed in the earlier work to measure film swelling – found again that EPS and UFP each arrested the swelling of the oligoamide film with EPS having only a slight effect while UFP impaired swelling significantly. He concludes: "A novel RO fouling mechanism is proposed, in which foulant-membrane interactions hinder membrane swelling and thus increase hydraulic resistance." Conditioning film And foulAnt bACtERiA Having established that a conditioning film of UFP constituents plays an early role in arresting the performance of RO membranes used in wastewater desalination, Herzberg and his colleagues looked at the role it played in the pathway to biofouling. They investigated UFP's effect on the attachment of microorganisms using a known biofilm-forming bacterium, Pseudomonas aeruginosa. This study revealed that UFP amplified dramatically the capacity of the bacterium to bind to RO membrane material. The researchers point out that the bacterium is not typically found in wastewater reclamation systems but the significant effects they recorded suggest further analysis using more representative bacteria would be worthwhile. Looking again at mass changes on polyamide membrane material, the study found that there were no changes when the pristine material was presented with P. aeruginosa. On conditioning the polyamide with UPF the bacteria produced a significant increase in mass beyond the gains arising solely from the conditioning film. And those increases were not reversible by washing with the background solution. On exposing EPS to the conditioned polyamide surface instead of bacteria, Herzberg found that adherence of EPS to the UFP- conditioned membrane material was not appreciably greater than its adsorption to pristine polyamide. CHEmiCAl CulpRit? Herzberg says findings from his polyamide membrane study suggest that: "interactions between the polyamide layer and UFP species are affecting polyamide molecular properties that are important for water transport." More specifically, he proposes that the permanent damage inflicted by UF permeate arises from its observed inhibition of the swelling in the membrane polyamide. These proposals, he says, have been borne out from the work on oligoamide film. Herzberg's team speculates that the marked UFP-induced reduction in swelling of the oligoamide arose from "strong interactions between aromatic groups in the humic acids and the cross-linked aromatic oligoamide-coated surface," that shield polar groups in the oligoamide film from binding water molecules. Should that be the case, Herzberg says, the permeability of RO membranes carrying adsorbed organic foulants from UFP too would be diminished by arrested swelling of the polyamide membrane. He draws attention to the faster fouling of the oligoamide film and more acute decline in flux caused by UFP organics compared to EPS fouling despite the concentration and adsorption of EPS being greater than that of UFP foulants. "Even a small amount of adsorbed UFP organics was detrimental to membrane permeability," he says. The researchers conclude that humic acids in the permeate, which originate from decayed organic matter and are abundant among the effluent organic matter (EfOM) in UFP, bind with the membrane surface and possibly with other foulants to arrest irreversibly the membrane's capacity to swell through water uptake. Such swelling is an acknowledged important feature of water transport across the membrane which suggests the UFP conditioning has an early and significant role in the detrimental effect of biofouling in wastewater recycling. The chemical moieties in play in the humic component of UFP include phenol, hydroxyl, amine, carboxyl and aldehyde groups which could combine with other foulants including proteins and sugars. Herzberg says this is an area ripe for further study. Humic species were negligible in the EPS which includes a higher proportion of large biological polymers. Herzberg postulates that the irreversible nature of the pH-dependent reduction in flux caused by UFP was a product of the size of the humic substances causing the decrease in water passage. He suggests that on increasing pH the swelling response in the polyamide membrane – despite being arrested strongly by UFP constituents – could pull the damaging elements deeper into the membrane surface to further their effect. He cites work by others that showed a significantly weaker deposition of EPS on humic acid compared to silica and alginate substrates [8] which reflected his team's observation that UFP conditioning did not enhance EPS binding to polyamide. The cited work suggested that obstacles created by molecular shape and electrostatic charge caused EPS and humics to repel each