Effects of chemical preservation on flux and solute rejection by reverse osmosis membranes

This study investigates the impacts of chemical preservation on the performance of polyamide reverse osmosis membranes with respect to water permeability and solute rejection. Three preservative chemicals, namely formaldehyde, sodium metabisulfite, and 2,2-Dibromo-3-Nitrilopropionamide, were evaluated for membrane preservation at pH 3 and 7. Experimental data show that chemical preservation may change the membrane surface properties, and consequently water permeability and solute rejection efficiency of the membrane are negatively impacted. The impacts of preservation on boron rejection and sodium rejection are similar in magnitude and more significant than those on water permeability. The results indicate that the impact of chemical preservation on the membrane depends on both the preserving chemicals used and the solution pH value. More importantly, the undesirable impacts of chemical preservation can be minimised by appropriate selection of the preservatives and by preserving the membrane in a reducing condition. A near-neutral pH (i.e., pH 7) is necessary to avoid any significant negative impacts on membrane performance due to chemical preservation using either formaldehyde or sodium metabisulfite. Results reported here suggest that the previously recommended minimum pH value of 3 of the preservative solution may be inadequate

Chemical cleaning effects on properties and separation efficiency of an RO membrane

This study aims to investigate the impacts of chemical cleaning on the performance of a reverse osmosis membrane. Chemicals used for simulating membrane cleaning include a surfactant (sodium dodecyl sulfate, SDS), a chelating agent (ethylenediaminetetraacetic acid, EDTA), and two proprietary cleaning formulations namely MC3 and MC11. The impact of sequential exposure to multiple membrane cleaning solutions was also examined. Water permeability and the rejection of boron and sodium were investigated under various water fluxes, temperatures and feedwater pH. Changes in the membrane performance were systematically explained based on the changes in the charge density, hydrophobicity and chemical structure of the membrane surface. The experimental results show that membrane cleaning can significantly alter the hydrophobicity and water permeability of the membrane; however, its impacts on the rejections of boron and sodium are marginal. Although the presence of surfactant or chelating agent may cause decreases in the rejection, solution pH is the key factor responsible for the loss of membrane separation and changes in the surface properties. The impact of solution pH on the water permeability can be reversed by applying a subsequent cleaning with the opposite pH condition. Nevertheless, the impacts of solution pH on boron and sodium rejections are irreversible in most cases

Enhanced boron rejection by NF/RO membranes by complexation with polyols: measurement and mechanisms

Boron rejection by nanofiltration (NF) and reverse osmosis (RO) membranes in the presence of glycerol, mannitol and sorbitol was investigated as a function of feed solution pH and boron:polyol molar ratio. In the presence of polyols, significant boron rejection improvement was obtained and the extent of the impact was directly related to the stability constant of the boron-polyol complex. Polyols could complex with boron in either the boric acid or borate anion forms; however the complexation between polyol and boric acid appeared to be incomplete. With and without the presence of polyols, boron rejection was strongly pH dependent. The increase in boron rejection due to polyol addition was higher for the NF membrane compared to the RO membrane. A boron:polyol molar ratio of 1:1 appeared to be adequate. The presence of polyols did not cause any observable membrane fouling. Results reported here suggest that the addition of polyols could allow NF membranes to be effectively used for boron removal

Boron removal by reverse osmosis membranes in seawater desalination applications

Rapid development of seawater desalination applications using membrane technology in recent years has reignited the issue of boron in drinking water. This is reflected by an increasing number of scientific investigations on the removal of boron by reverse osmosis (RO) and to a lesser extent by nanofiltration (NF) membranes over the last decade. This review provides a summary of the current knowledge relevant to the removal of boron by NF/RO membranes in seawater desalination applications. In seawater, boron exists almost exclusively in the form of boric acid. The speciation of boric acid and other physicochemical properties are delineated and systematically related to the rejection of boron by NF/RO membranes. Studies investigating the rejection of boron by NF/RO membranes reported to date are summarised and reviewed to comprehensively elucidate key factors governing the transport of boron in a typical NF/RO filtration process. The speciation of boric acid as a function of the feed solution pH appears to be the single most important factor governing the rejection of boron by NF/RO membranes. In addition, results reported in the literature reveal an intricate relationship between the speciation of boric acid and several other operating parameters. Some of these operating parameters such as feed solution temperature and ionic strength can directly influence the rejection of boron and at the same time alter the intrinsic dissociation constant of boric acid, hence, indirectly affecting boron rejection by NF/RO membranes. This review also delineates key mathematical modelling approaches, capable of describing the transport of boron in NF/RO filtration processes. The studies included in this review highlight the potential for further process optimisation to enhance the removal of boron in seawater desalination applications. However, the discussions provided also emphasize the need for more fundamental research to improve the scientific understanding of boron rejection by NF/RO membranes. In particular, there is a pressing need to evaluate the impact of membrane fouling and chemical cleaning on the long-term performance of NF/RO systems with respect to boron removal

Boron as a surrogate for N‑nitrosodimethylamine rejection by reverse osmosis membranes in potable water reuse applications

The results of this study reveal a strong linear correlation (R2 = 0.95) between the rejections of boron and N-nitrosodimethylamine (NDMA) by six different reverse osmosis (RO) membranes, suggesting that boron can be used as a surrogate for NDMA rejection. This proposal is based on the premise that the rejection of both boric acid and NDMA is governed by steric hindrance and that they have similar molecular dimensions. The concept proposed here is shown to be valid at pH 8 or below where boron exists as the neutral boric acid species and NDMA is also a neutral solute. Observed changes in the rejections of these two species, as a function of permeate fluxes and feed solution temperatures, were also almost identical. Boron rejection increased from 21 to 79%, and the correlation coefficient of the linear regression between boron and NDMA rejections was 0.99 as the permeate flux increased from 5 to 60 L m−2 h−1. Similarly, a linear correlation between boron and NDMA rejections was observed as the feed solution temperature increased from 10 to 40 °C. This linear correlation was also validated in a tertiary treated effluent matrix

Rejection of small solutes by reverse osmosis membranes for water reuse applications: a pilot-scale study

N-nitrosamines and boron are small solutes of particular concern during water recycling applications. Here, we evaluated the rejection of seven N-nitrosamines and boron under a range of operating conditions and feed solution characteristics. The evaluationwas conducted using a pilot-scale reverse osmosis (RO) system to appropriately simulate hydrodynamic conditions of full-scale RO installations. The rejection of seven N-nitrosamines by the pilot RO system varied significantly in the range from 31 to 94%, and rejection increased in the increasing order of their molecular weight. Rejection values obtained from this pilot-scale study were lower than those previously reported in laboratory-scale studies. These discrepancieswere attributed to a difference in RO system operating condition (i.e. recovery) between the pilot-scale study (25%) and laboratory-scale study (b0.1%). Nevertheless, rejection data reported here validate the recent findings fromlaboratory-scale studies with respect to the impact of permeate flux, feed temperature and feed pH on separation efficiencies of N-nitrosamines. Data obtained from this pilot-scale study also validate the strong correlation between boron and NDMA rejection at or below pH 8 regardless of operating conditions and feed solution characteristics. The results suggest that boron rejection can be used as a surrogate for NDMA rejection in full-scale RO installations

The rejection of mono- and di-valent ions from aquatic environment by MWNT/chitosan buckypaper composite membranes: Influences of chitosan concentrations

Owing to the scarcity of proper drinking water is an urgent problem, MWNT/Chitosan membrane is greeting to reject mono- and di-valent ions from water. MWNT/Chitosan membrane was fabricated through the dispersion of Multi-walled carbon nanotubes (MWNTs) in an aqueous solution containing different concentrations of chitosan. The influence of solution concentration on membrane salt rejection properties, as well as contact angle, electrical conductivity, water permeability, mechanical properties, zeta potential, surface area and internal pores morphologies has been investigated. The resulting buckypaper demonstrate that the contact angle (91° ± 4° to 124° ± 3°), electrical conductivity (17 ± 1 to 83 ± 3 S/cm), water permeability (0.59 ± 0.04 to 5.73 ± 0.3 L/m2 h bar), surface area and internal pores morphologies of the buckypaper membranes were decreased by increasing the concentration of chitosan. While, the mechanical properties (tensile strengths varied between 35 ± 2 and 75 ± 3 MPa) and zeta potential of these buckypaper membranes were found to increase with increasing the amounts of chitosan. A buckypaper fabricated from MWNTs and a high concentration of chitosan (0.4% w/v) showed a higher rejection efficiency for these salts, possibly due to their smaller internal pore volumes and lower specific surface area