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Low-salt-rejection reverse osmosis (LSRRO) is a novel reverse osmosis (RO)-based technology that can highly concentrate brines using moderate operating pressures. In this study, we investigate the performance of LSRRO membrane modules and systems using module-scale analysis. Specifically, we correlate the observed salt rejection of an LSRRO module with the water and salt permeabilities of the RO membrane. We then elaborate the impact of membrane properties and operating conditions on the performance of a 2-stage LSRRO, providing design guidelines for LSRRO systems. We further compare the performance of 2-stage and 3-stage LSRRO systems, showing that an LSRRO system with more stages is not always favored due to a larger energy consumption. The performance of a 3-stage LSRRO in treating different feed solutions for minimal/zero liquid discharge (MLD/ZLD) applications is then evaluated. Based on our results, when treating feed waters with a relatively low salinity (e.g., 0.1 M or ∼5,800 mg L

Gypsum scaling via crystallization is a major obstacle limiting the applications of membrane-based technologies and heat exchangers in engineered systems. Herein, we perform the first comparative investigation on the impacts of natural organic matter (Suwannee River humic acid, SRHA) and colloidal particles on the gypsum crystallization process in terms of induction time and crystal morphology. Results show that the presence of SRHA significantly increases the induction time of gypsum crystallization. Specifically, at a solution saturation index of 4.92, the induction time increases 6.5-fold in the presence of 6 mg/L SRHA, compared to the case without SRHA. SRHA also alters the morphology of the formed calcium sulfate crystals, resulting in a polygon-like shape, differing from the characteristic needle-like shape of gypsum in the absence of additives. These changes in crystal morphology are attributed to the adsorption of SRHA on the gypsum crystal surface, blocking the active sites for gypsum growth. In contrast, in the presence of colloidal particles, the observed induction time of gypsum crystallization either decreases or increases, depending on the competitive interplay between the enhancement effect in the nucleation step and the inhibition effect in the subsequent crystal growth step. Furthermore, the formed gypsum crystals in the presence of colloidal particles exhibit a needle-like morphology similar to the crystals formed in the absence of any additives. Our study provides fundamental understanding of gypsum crystallization in feedwaters containing natural organic matter and colloidal particles, highlighting the importance of feedwater composition in gypsum scaling.

Organic fouling and inorganic scaling are the main hurdles for efficient operation of reverse osmosis (RO) technology in a wide range of applications. This study demonstrates dual-functionality surface modification of thin-film composite (TFC) RO membranes to simultaneously impart anti-scaling and anti-fouling properties. Two different grafting approaches were adapted to functionalize the membrane surface with sulfonic groups: (i) non-specific grafting of vinyl sulfonic acid (VSA) via redox radical initiation polymerization and (ii) covalent bonding of hydroxylamide-O-sulfonic acid (HOSA) to the native carboxylic groups of the polyamide layer via carbodiimide mediated reaction. Both approaches to graft sulfonic groups were effective in increasing surface wettability and negative charge density of the TFC-RO membranes without significant alteration of water and salt permeabilities. Importantly, we verified through surface elemental analysis that covalently bound HOSA effectively covers the native carboxylic groups of the PA layer. Both the VSA and HOSA membranes exhibited lower flux decline during silica scaling and organic (alginate) fouling relative to the control unmodified membrane, demonstrating the unique versatility of sulfonic groups to endow the TFC-RO membranes with dual functionality to resist scaling and fouling. In particular, the HOSA membrane showed excellent physical cleaning efficiencies with water flux recoveries of 92.5 ± 1.0% and 88.4 ± 6.4% for silica scaling and alginate fouling, respectively. Additional results from silica nucleation experiments and atomic force measurements provided insights into the mechanisms of improved resistance to silica scaling and organic fouling imparted by the surface-functionalized sulfonic groups. Our study highlights the promise of controlled functionalization of sulfonic groups on the polyamide layer of TFC membranes to enhance the applications of RO technology in treatment and reuse of waters with high scaling and fouling potential.

Capacitive deionization (CDI) is an emerging membraneless water desalination technology based on storing ions in charged electrodes by electrosorption. Due to unique selectivity mechanisms, CDI has been investigated towards ion-selective separations such as water softening, nutrient recovery, and production of irrigation water. Especially promising is the use of activated microporous carbon electrodes due to their low cost and wide availability at commercial scales. We show here, both theoretically and experimentally, that sulfonated activated carbon electrodes enable the first demonstration of perfect divalent cation selectivity in CDI, where we define "perfect" as significant removal of the divalent cation with zero removal of the competing monovalent cation. For example, for a feedwater of 15 mM NaCl and 3 mM CaCl

Technologies capable of selective removal of target contaminants from water are highly desirable to achieve “fit-for-purpose” treatment. In this study, we developed a simple yet highly effective method to achieve calcium-selective removal in an electrosorption process by coating the cathode with a calcium-selective nanocomposite (CSN) layer using an aqueous phase process. The CSN coating consisted of nano-sized calcium chelating resins with aminophosphonic groups in a sulfonated polyvinyl alcohol hydrogel matrix, which accomplished a Ca2+-over-Na+ selectivity of 3.5–5.4 at Na+:Ca2+ equivalent concentration ratio from 10:1 to 1:1, 94 – 184% greater than the uncoated electrode. The CSN coated electrode exhibited complete reversibility in repeated operation. Mechanistic studies suggested that the CSN coating did not contribute to the adsorption capacity, but rather allowed preferential permeation of Ca2+ and hence increased Ca2+ adsorption on the carbon cathode. The CSN-coated electrode was very stable, showing reproducible performance in 60 repeated cycles.

Wastewater nutrient recovery holds promise for more sustainable water and agricultural industries. We critically review three emerging membrane processes - forward osmosis (FO), membrane distillation (MD) and electrodialysis (ED) - that can advance wastewater nutrient recovery. Challenges associated with wastewater nutrient recovery were identified. The advantages and challenges of applying FO, MD, and ED technologies to wastewater nutrient recovery are discussed, and directions for future research and development are identified. Emphasis is given to exploration of the unique mass transfer properties of these membrane processes in the context of wastewater nutrient recovery. We highlight that hybridising these membrane processes with existing nutrient precipitation process will lead to better management of and more diverse pathways for near complete nutrient recovery in wastewater treatment facilities.

Minimal and zero liquid discharge (MLD/ZLD) are wastewater management strategies that are attracting heightened attention worldwide. While conventional reverse osmosis (RO) has been proposed as a promising technology in desalination and MLD/ZLD processes, its application is limited by the maximum hydraulic pressures that current RO membranes and modules can withstand. In this study, we develop low-salt-rejection RO (LSRRO), a novel staged RO process, that employs low-salt-rejection membranes to desalinate or concentrate highly saline feed streams, requiring only moderate hydraulic pressures. Based on process modeling, we demonstrate that LSRRO can overcome the hydraulic pressure limitations of conventional RO, achieving hypersaline brine salinities (>4.0 M NaCl or 234 g L−1 NaCl) that are required for MLD/ZLD applications, without using excessively high hydraulic pressures ( ≤ 70 bar ). In addition, we show that the energy efficiency of LSSRO is substantially higher than traditional thermally-driven phase-change-based technologies, such as mechanical vapor compressor (MVC). For example, to concentrate a saline feed stream from 0.1 to 1.0 M NaCl, the specific energy consumption ( S E C ) of 4-stage LSRRO ranges from 2.4 to 8.0 kWh of electrical energy per m3 of feedwater treated, around four times less than that of MVC, which requires 20–25 kWhe m−3. Furthermore, compared to osmotically mediated RO technologies that require bilateral countercurrent stages to treat hypersaline brines, LSRRO is eminently more practical as it can be readily implemented by using ‘loose’ RO or nanofiltration membranes in conventional RO. Our study highlights LSRRO’s potential for energy efficient brine concentration using moderate hydraulic pressures, which would drastically improve the energetic and economic performance of MLD/ZLD processes.

This study elucidates the relationship between membrane properties and the rejection of trace organic contaminants (TrOCs) in forward osmosis (FO). An asymmetric cellulose triacetate (CTA) and a thin-film composite (TFC) polyamide FO membrane were used for this investigation. The effective average pore radius (rp), selective barrier thickness over porosity parameter (l/ε), surface charge, support layer structural parameter (S), pure water permeability coefficient (A) and salt (NaCl) permeability coefficient (B) of the two membranes were systematically characterised. Results show that measured rejection of TrOCs as a function of permeate water flux can be well described by the pore hindrance transport model. This observation represents the first successful application of this model, which was developed for pressure-driven nanofiltration, to an osmotically-driven membrane process. The rejection of charged TrOCs by the CTA and TFC membranes was high and was governed by both electrostatic repulsion and steric hindrance. The TFC membrane exhibited higher rejection of neutral TrOCs with low molecular weight than the CTA membrane, although the estimated pore size of the TFC membrane (0.42 nm) was slightly larger than that of the CTA membrane (0.37 nm). This higher rejection of neutral TrOCs by the TFC membrane is likely attributed to its active layer properties, namely a more effective active layer structure, as indicated by a larger l/ε parameter, and pore hydration induced by the negative surface charge.

The impact of humic acid fouling on the membrane transport of two pharmaceutically active compounds (PhACs) – namely carbamazepine and sulfamethoxazole – in forward osmosis (FO) was investigated. Deposition of humic acid onto the membrane surface was promoted by the complexation with calcium ions in the feed solution and the increase in ionic strength at the membrane surface due to the reverse transport of NaCl draw solute. The increase in the humic acid deposition on the membrane surface led to a substantial decrease in the membrane salt (NaCl) permeability coefficient but did not result in a significant decrease in the membrane pure water permeability coefficient. As the deposition of humic acid increased, the permeation of carbamazepine and sulfamethoxazole decreased, which correlated well with the decrease in the membrane salt (NaCl) permeability coefficient. It is hypothesized that the hydrated humic acid fouling layer hindered solute diffusion through the membrane pore and enhanced solute rejection by steric hindrance, but not the permeation of water molecules. The membrane water and salt (NaCl) permeability coefficients were fully restored by physical cleaning of the membrane, suggesting that humic acid did not penetrate into the membrane pores.

This study systematically compares the performance of osmotic membrane bioreactor – reverse osmosis (OMBR-RO) and conventional membrane bioreactor – reverse osmosis (MBR-RO) for advanced wastewater treatment and water reuse. Both systems achieved effective removal of bulk organic matter and nutrients, and almost complete removal of all 31 trace organic contaminants investigated. They both could produce high quality water suitable for recycling applications. During OMBR-RO operation, salinity build-up in the bioreactor reduced the water flux and negatively impacted the system biological treatment by altering biomass characteristics and microbial community structure. In addition, the elevated salinity also increased soluble microbial products and extracellular polymeric substances in the mixed liquor, which induced fouling of the forward osmosis (FO) membrane. Nevertheless, microbial analysis indicated that salinity stress resulted in the development of halotolerant bacteria, consequently sustaining biodegradation in the OMBR system. By contrast, biological performance was relatively stable throughout conventional MBR-RO operation. Compared to conventional MBR-RO, the FO process effectively prevented foulants from permeating into the draw solution, thereby significantly reducing fouling of the downstream RO membrane in OMBR-RO operation. Accumulation of organic matter, including humic- and protein-like substances, as well as inorganic salts in the MBR effluent resulted in severe RO membrane fouling in conventional MBR-RO operation.

This study investigates the effectiveness of a single-walled carbon nanotube (SWNT) filter for removal of viruses from water. MS2 bacteriophage viral removal was examined over a range of environmentally relevant solution chemistries, spanning various ionic strengths, monovalent and divalent salts, pH, and natural organic matter (NOM) concentrations. Viral removal by the SWNT filter was governed by physicochemical (depth) filtration. The removal of viruses increased at higher ionic strengths (NaCl) due to suppression of repulsive electrostatic interactions between viruses and SWNTs. Addition of divalent salts, however, had varying impacts. While CaCl(2) increased virus removal, likely due to complexation of calcium ions to viral surfaces, addition of MgCl(2) reduced viral removal by the SWNT filter. Solution pH also had significant impact on viral removal as the interactions between viral particles and SWNTs changed from attractive below the virus isoelectric point (about pH 3.9) to repulsive at higher pH. Suwannee River NOM was shown to be detrimental to filter viral removal. Reduction of viral removal by NOM was attributed to adsorption of NOM macromolecules to viruses and SWNTs, thereby resulting in steric repulsive forces. Modifications of the filter to incorporate thicker SWNT layers mitigate the negative impacts of NOM on filter performance. This study has shown that while it is possible to attain high levels of viral removal over a broad range of solution chemistries, the extent of viral removal will be highly dependent on the specific solution chemistry of the treated water.

Cleaning of organic-fouled reverse osmosis membranes with concentrated salt solutions has been investigated. Polysaccharides (alginate and pectin) and Suwannee River natural organic matter were used as model organic foulants. By systematically varying the chemical and physical factors affecting salt cleaning efficiency, we were able to elucidate the processes and mechanisms involved during salt cleaning. Chemical factors investigated included salt dose, salt type, and organic foulant composition, while physical factors included cleaning contact time, crossflow velocity, cleaning solution temperature, and permeation rate. Atomic force microscopy (AFM) was utilized to quantify the reduction in intermolecular foulant-foulant adhesion upon salt cleaning. Our results showed that salt cleaning was quite effective in cleaning reverse osmosis membranes fouled by gel-forming hydrophilic organic foulants, such as alginate and pectin. The proposed mechanism for salt cleaning involves structural changes of the cross-linked gel layer on the membrane surface upon exposure to the salt solution followed by an ion exchange reaction that induces the breakup of calcium-foulant bonds as well as calcium bridging (cross-linking) between foulant molecules. The results obtained from AFM force measurements as well as foulant release experiments confirmed that these chemical reactions were the major mechanisms of salt cleaning. Salt cleaning appears to be an effective cleaning method, and may prove useful for membrane-based advanced wastewater reclamation, where a large fraction of the organic foulants is hydrophilic.

The potential to incorporate silver nanoparticles (Ag-NPs) as biocides in membranes for water purification has gained much interest in recent years. However, a viable strategy for loading the Ag-NPs on the membrane remains challenging. This paper presents a novel, facile procedure for loading Ag-NPs on thin-film composite (TFC) reverse osmosis membranes. Reaction of silver salt with a reducing agent on the membrane surface resulted in uniform coverage of Ag-NPs, irreversibly bound to the membrane, as confirmed by XPS, TEM, and SEM analyses. Salt selectivity of the membrane as well its surface roughness, hydrophilicity, and zeta potential were not impacted by Ag-NP functionalization, while a slight reduction (up to 17%) in water permeability was observed. The formed Ag-NPs imparted strong antibacterial activity to the membrane, leading to reduction of more than 75% in the number of live bacteria attached to the membrane for three model bacteria strains. In addition, confocal microscopy analyses revealed that Ag-NPs significantly suppressed biofilm formation, with 41% reduction in total biovolume and significant reduction in EPS, dead, and live bacteria on the functionalized membrane. The simplicity of the method, the short reaction time, the ability to load the Ag-NPs on site, and the strong imparted antibacterial activity highlight the potential of this method in real-world RO membrane applications.

Fertiliser-drawn forward osmosis (FDFO) desalination has been recently studied as one feasible application of forward osmosis (FO) for irrigation. In this study, the potential of membrane scaling in the FDFO process has been investigated during the desalination of brackish groundwater (BGW). While most fertilisers containing monovalent ions did not result in any scaling when used as an FO draw solution (DS), diammonium phosphate (DAP or (NH4)2HPO4) resulted in significant scaling, which contributed to severe flux decline. Membrane autopsy using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD) analysis indicated that the reverse diffusion of DAP from the DS to the feed solution was primarily responsible for scale formation during the FDFO process. Physical cleaning of the membrane with deionised water at varying crossflow velocities was employed to evaluate the reversibility of membrane scaling and the extent of flux recovery. For the membrane scaled using DAP as DS, 80-90% of the original flux was recovered when the crossflow velocity for physical cleaning was the same as the crossflow velocity during FDFO desalination. However, when a higher crossflow velocity or Reynolds number was used, the flux was recovered almost completely, irrespective of the DS concentration used. This study underscores the importance of selecting a suitable fertiliser for FDFO desalination of brackish groundwater to avoid membrane scaling and severe flux decline. © 2014 Elsevier Ltd.

An optical monitoring technique is used to investigate the dynamics of coagulation of kaolin suspensions with ferric chloride. Particular attention is given to the effects of coagulant dose, solution pH and mixing intensity on coagulation dynamics. Results show that this monitoring method provides valuable information on the dynamics of aggregates in coagulation with Fe(III) salts.

We compared the rejection behaviours of three hydrophobic trace organic contaminants, bisphenol A, triclosan and diclofenac, in forward osmosis (FO) and reverse osmosis (RO). Using erythritol, xylose and glucose as inert reference organic solutes and the membrane pore transport model, the mean effective pore size of a commercial cellulose-based FO membrane was estimated to be 0.74 nm. When NaCl was used as the draw solute, at the same water permeate flux of 5.4 L/m 2 h (or 1.5 μm/s), the adsorption of all three compounds to the membrane in the FO mode was consistently lower than that in the RO mode. Rejection of bisphenol A and diclofenac were higher in the FO mode compared to that in the RO mode. Because the molecular width of triclosan was larger than the estimated mean effective membrane pore size, triclosan was completely rejected by the membrane and negligent difference between the FO and RO modes could be observed. The difference in the separation behaviour of these hydrophobic trace organics in the FO (using NaCl the draw solute) and RO modes could be explained by the phenomenon of retarded forward diffusion of solutes. The reverse salt flux of NaCl hinders the pore diffusion and subsequent adsorption of the trace organic compounds within the membrane. The retarded forward diffusion effect was not observed when MgSO 4 and glucose were used as the draw solutes. The reverse flux of both MgSO 4 and glucose was negligible and thus both adsorption and rejection of BPA in the FO mode were identical to those in the RO mode.

A semi-empirical approach for predicting the collision (attachment) efficiency of particle deposition in porous media has been developed. Based on dimensional analysis, a quantitative relationship between the collision efficiency and a set of parameters characterizing the chemical-colloidal properties of the system has been derived. The results of this analysis show that the collision efficiency can be characterized by a one dimensionless parameter. Experimental collision efficiencies determined from controlled deposition experiments exhibited a strong correlation with this dimensionless parameter.

Shear is used to control fouling in membrane bioreactor (MBR) systems. However, shear also influences the physicochemical and biological properties of MBR biomass. The current study examines the relationship between the level of shear and extracellular polymeric substance (EPS) production in MBRs. Two identical MBRs were operated in parallel where the biomass in one reactor was exposed to seven times greater shear forces. The concentrations of floc-associated and soluble EPS were monitored for the duration of the experiment. The stickiness of extracted floc-associated EPS from each reactor was also characterized using atomic force microscopy. A mathematical model of floc-associated and soluble EPS production was applied to quantitatively describe changes in EPS production with shear. Biomass grown in a high shear environment has lower floc-associated EPS production compared to biomass grown in a lower shear environment. This decrease in floc-associated EPS production also corresponds to a decrease in soluble EPS production, which can be explained by both the lower concentration of floc-associated EPS and the production of stickier floc-associated EPS that is more erosion resistant in the high shear reactor. This research suggests that mechanical stresses can have a significant impact on the production rates of floc-associated and soluble EPS-key parameters governing membrane fouling in MBRs.

Effluent organic matter (EfOM) contributes significantly to organic fouling of reverse osmosis (RO) membranes in advanced wastewater reclamation. In this study, the effect of feed solution chemistry (solution pH and Ca(2+) concentration) on the fouling of RO membranes by octanoic acid-selected to represent fatty acids in EfOM-is investigated. Crossflow fouling experiments demonstrate that RO membrane fouling is much more significant at solution pH below the pK(a) of the octanoic acid (pK(a)=4.9) than at an elevated pH. Octanoic acid permeates across the membranes more readily at solution pH below its pK(a) than at elevated pH. At pH below the octanoic acid pK(a), fouling behavior is not affected by calcium ions, whereas at elevated pH, the rate of flux decline decreases with higher calcium ion concentration. The effect of calcium on the fouling behavior was further verified from foulant-foulant adhesion forces, determined by atomic force microscopy (AFM) force measurements under solution chemistries identical to those of the crossflow fouling experiments. To investigate the implications of octanoic acid fouling for wastewater reclamation, the effect of octanoic acid on membrane fouling by a combination of organic foulants in the presence of calcium ions is studied. At a solution chemistry simulating that of typical wastewater effluents, the addition of octanoic acid to a feed solution containing alginate, bovine serum albumin, and Suwannee River natural organic matter, does not enhance membrane fouling behavior. This observation could be attributed to the significant contribution of the alginate-calcium complexes within the fouling layer to the total membrane resistance.