Mechanism investigation and optimisation of redox-flow desalination of highly saline source waters

Reverse osmosis (RO) membrane technology is widely used for producing high-quality drinking water. Yet RO permeate is by itself acidic (pH = 5.5 to 6.0), unbuffered and has low mineral content, therefore post treatment i.e., remineralization is mostly required. An ion exchange resin – bipolar membrane electrodialysis hybrid process was developed for sustainable RO permeate remineralization. Fundamental phenomena in the recovery of calcium and magnesium by ion exchange to remineralize reverse osmosis permeate were investigated. Sorption equilibrium and mass transfer kinetics were investigated for weakly acidic (Amberlite IRC747, Amberlite IRC748, Lewatit S8227) and strongly acidic (DOWEX Marathon MSC) cation exchange resins. Most suitable resin for the remineralization process should have high selectivity for calcium and magnesium and low selectivity for monovalent ions to avoid adding undesired ions to the remineralised water downstream as well as relatively fast mass transfer kinetics. The isotherms were correlated with the stoichiometric ion exchange isotherm and the Langmuir-Freundlich (Sips) isotherm. All resins showed high selectivity for ions with higher valence, but weakly acidic cation exchange (WAC) resins showed significantly lower selectivity towards monovalent ions than the strongly acidic cation exchange resin. The influence of each resin functional group, charge density and degree of protonation was shown to have a major effect on the resin selectivity. Amberlite IRC748 had the lowest selectivity (KNH4+/Na+ = 0.77 ± 0.19) and removal (46%) for ammonium in a single-component system. The mass transfer rate was found to be controlled by intraparticle diffusion rather than film diffusion. Amberlite IRC748 is recommended for use in a remineralization process where divalent ions are present because of its favourable sorption and higher mass transfer kinetics (Ks = 8.65 ± 0.58 × 10−12, 7.95 ± 0.38 × 10−12 m2/s for calcium and magnesium, respectively).

Implication of zeta potential at different salinities on boron removal by RO membranes

Desalination is probably the only means for fresh water supply to countries in decertified climate. The majority of GCC counties rely on desalinated water for fresh water supply to major cities. Over 70% of the desalinated water in the GCC comes from thermal desalination plants including Multi Stage Flash (MSF) and Multi Effect Distillation (MED). The new trend in the desalination plant in the GCC is 30% Reverse Osmosis (RO) and 70% thermal. However, these percentages vary from one to another country depending on feed water quality and expertise. For example, Oman Sea has lower salinity than the Gulf water and hence Oman uses more RO for desalination than MED and MSF. This decision is also driven by economy as RO process less energy intensive and hence the produced water is less expensive as compared to thermal plants. On the contrary, Qatar and Kuwait use more MSF followed by MED due to the high salinity and low quality feed water. This is also because trials of RO in both Qatar and Kuwait were not successful because of the problems of membrane fouling and restrict pre-treatment requirements due to the quality of the water intake.The advantages of RO over thermal technologies are well known in terms of lower energy consumption and the cost of produced water; but are not yet taken advantage of in the GCC zone. One of the reasons is blamed on high feed water salinity and bad water quality; other reasons such as lack of experience, red tides and reliability are contributed to the dominance of thermal plants. However, field experience showed that good pretreatment and optimized RO design may overcome the problems of high feed salinity and bad water quality. Several RO plants, such as Fujairah in UAE, are good examples of a working RO technology in the harsh water environment. Good RO design includes design and optimization of both pretreatment and post-treatment. Field experience showed that most of RO plants failure was due to inefficient pretreatment which resulted in providing low quality water to the RO membrane that caused fouling. Fouling, including biological and scaling, can be handled once an efficient pretreatment process is available. Recent advances in pre-treatment techniques include the combination of Forward Osmosis (FO) with RO among other methods. Recent studies by the authors including commercial implantations have shown that the combination of FO with RO addresses the most technical challenge of RO process and that is fouling, which results in lower energy consumption and less chemical additives. Experience showed fouling in FO process in reversible, i.e. can be removed by backlashing while fouling in conventional RO process is irreversible.In this study, the feasibility of integrating FO with RO process for the desalting of the Gulf water in Qatar is presented. The results are expressed in terms of specific energy consumption, process recovery, produced water quality, chemical additives and overall process cost.The implementation of RO for desalination is not only reducing the cost of desalination but also the environmental impact. More R&D should be done to provide useful data about RO application and suitability for the Gulf water. The R&D should be focused on laboratory to market development of RO technology using rigorous lab scale and pilot plant testing program.

Investigating the microstructures and surface features of seawater RO membranes and the dependencies of fouling resistance performances

Membrane application in reverse osmosis (RO) membrane is getting more attention especially in producing drinking water. However, RO membrane faces challenges that reduces its performance such as its permeation flux, salt rejection, additional energy demand, lifetime decrease, extra pre-treatment process, cleaning and maintenance. The challenge is the formation of fouling. RO membrane fouling can happen inside or outside the membrane and the characteristics of membrane fouling differs from one type to other types, depending on the nature and location of membrane fouling. There are several types of RO fouling, which are Biofouling, Organic Fouling, Inorganic Fouling and Colloidal Fouling. The causes of RO membrane are different from one to another. The properties and materials of the solution entering RO membrane are important as it affects the type of fouling of RO membrane fouling. All of the RO membrane foulings need to be considered during membrane usage and demand solution to be controlled. In order to control the fouling in Reverse Osmosis membrane, there have been several control solutions discovered to the membrane fouling challenges. The control solutions are specified to each one of the fouling, in spite of wide applications for some of it. The control solutions are pre-treatment, which has many methods such as photo oxidation, coagulation, scale inhibitor, ion exchange resins, granular media and membrane treatment, membrane monitoring, membrane cleaning, surface modification, and material addition to membrane or novel membrane material. With various control solutions discovered, the RO membrane still faces fouling issue and is still demanding some more advanced applicable control solutions.

Integration of MBR with NF/RO processes for industrial wastewater reclamation and water reuse-effect of membrane type on product water quality

Performance evaluation of a brackish water reverse osmosis pilot-plant desalination process under different operating conditions: Experimental study

Development of novel backwash cleaning technique for reverse osmosis in reclamation of secondary effluent

A membrane system was applied for ultrapure water production from the treatment of saline effluent from the canned food industry. The industrial effluent presented a high saline concentration, including sodium chloride, calcium carbonate, calcium sulfates, and magnesium. The effluent was treated using a system of reverse osmosis (RO) and a post-treatment process consisting of ion exchange resins (IEXRs). The RO was accompanied by the addition of a hexametaphosphate dose (2, 6, and 10 mg/L) as an antiscalant to avoid the RO membrane scaling by minerals. In turn, IEXRs were used for water deionization to produce ultrapure water with a reduced concentration of monovalent ions. The antiscalant dose was 6 mg/L, producing clean water from RO permeates with an efficiency of 65-70%. The brine from RO was projected for its reuse in food industry processes. The clean water quality from RO showed 20% total dissolved solids (TDS) removal (equivalent to salts). The antiscalant inhibited the formation of calcium salt incrustation > 200 mg/L, showing low fouling. In turn, anionic resins removed 99.8% of chloride ions, whereas the monovalent salts were removed by a mix of cationic-anionic resin, producing ultrapure water with electrical conductivity < 3.3 µS/cm. The cost of ultrapure water production was 2.62 USD/m3.

Remediation of per- and polyfluoroalkyls (PFAS) via electrochemical methods

Lithium-air battery (LAB) is one of promising next-generation batteries because of its theoretical energy density as high as 3500 Wh kg-1. However, the battery still has some difficulties to be challenged, among which large overpotential during charging is the most important one for the air electrode (AE). Application of redox mediator (RM) is a common strategy to lower the overpotential. Although RM is usually dissolved into electrolyte solution to make it react uniformly on the surface of the AE, the oxidized product of RM, RM+, diffuses to the Li electrode to be reduced into RM, which is a kind of short-circuit reaction, shuttle effect [1]. Therefore, we fixed LiBr as the RM on the surface of the AE by coating it as a mixture with carbon powder (Ketjen black, KB) and a binder (PVDF) not only to suppress the shuttle effect but also to concentrate the RM on the surface of the AE where it works [2].Three types of LAB cells were used in this study. The first one is “ no RM ” cell as a reference consisting of an air electrode coated on a carbon paper with a slurry of KB and PVDF, an electrolyte solution of 0.2 M lithium bis(trifuluoromethanesulfonyl)imide/diethyleneglycol dimehtyl ether, 0.2 M LiTFSI/G2, and a Li metal negative electrode. The other two contain LiBr as a RM in different ways. The second one is “ RM in EL (electrolyte)” cell which contains 50 mM of LiBr in addition to 0.2 M LiTFSI/G2. The third one is “ RM on AE” cell which contains LiBr in the slurry of KB-PVDF coated on the carbon paper with the same amount as that in the electrolyte solution of the RM in EL cell. Discharge/charge cycle tests were carried out at a current density of 200 mA (g-KB)−1 with a constant capacity of 500 mAh (g-KB)−1 in the range of 2.0 – 4.5 V at 30oC.Figure 1 shows the cycle performance of three types of LAB cells, which presents a better cyclability of the RM on AE cell than the others while that for the RM in EL cell is almost the same as the no RM cell. Figure 2 shows the charge and discharge voltage evolution during cycles as the voltages at midpoint, 250 mAh (g-KB)−1, for each discharge/charge cycle. The RM on AE cell shows the highest discharge voltages, the lowest charge voltages and the longest cycle life between the voltage range of 4.5 to 2.0 V among the three cells. However, the RM in EL cell shows slightly suppressed charge voltage merely early stage of the cycle test, and lower discharge voltages than the others probably due to the shuttle effect of RM, i.e. Br3 -. Therefore, coating of LiBr on AE, RM on AE , is proved to be a better strategy than dissolving LiBr into the electrolyte solution.At the presentation, we will discuss the detail of LiBr reactions as RM and compare the effect of LiBr between RM in EL and RM on AE cells with some analytical data.This study was supported by JST Projest ALCA-SPRING (JPMJAL1301) and NIMS Joint Research Hub Program, Japan.[1] X. G. Wang et al., ChemElectroChem, 4, 2145 (2017).[2] Y. Hayashi et al., J. Electrochem. Soc., 167, 020542 (2020). Figure 1

Coal mine waters often have high salinity, hardness and alkalinity. The treatment of coal mine water requires careful management of multi-stage reverse osmosis (RO) systems to achieve effective recovery of water for domestic reuse, as well as zero liquid discharge to minimise the impact to the local environment. Design of RO systems for coal mine water treatment has been limited to the use of commercial design packages provided by membrane manufacturers, which do not provide insights into the impact of operating parameters such as feedwater salinity, concentrations of sparingly soluble salts, feed pressure and their interactions with different RO modules on the fouling/scaling potential of RO membranes. This also restricts the use of novel RO products and the delivery of an optimum design based on real needs. In this work, a mathematical model was developed to simulate a standard brackish water RO pressure vessel consisting six full-size RO membrane elements, using computational fluid dynamics (CFD). The model can be used to predict the permeate flowrate, water recovery levels, as well as the spatial information of the accumulation and scaling potential of sparingly soluble salts on the membrane surface. The results obtained from the model showed good agreement with the results obtained from the commercial RO design software WAVE. The CFD model was then used to predict the scaling threshold on various positions of a full-scale RO element, at different operating conditions, using parametric simulations based on Central Composite Designs. Outputs from this work not only provide insights into the microscopic flow characteristics of multiple full-scale elements in the RO pressure vessel, but also predicts the position where scaling would occur, at different feed conditions, for any RO products.

Inorganic fouling mitigation by salinity cycling in batch reverse osmosis

Boron removal in new generation reverse osmosis (RO) membranes using two-pass RO without pH adjustment

This research aims to provide an overview of the seawater encroachment threat on agriculture in lowland areas and potential solutions for better practices. It was found that the Mekong river delta experiences severe impacts from climate change with more than 75% of provinces affected by seawater intrusion, of which Kien Giang, Ca Mau and Ben Tre provinces are the most influenced with 70% affected areas. The salinity of river water was observed in the range of 15–30 g/L in 2015; meanwhile, the strongest tolerated rice species reached ceiling values of 3–4 g/L. Emerging challenges were identified due to the uncertain upstream hydrological regime coupled with high levels of tide, field evaporation and water withdrawal. The development strategies of affected provinces are given on the modification of rice tolerant capacity, and modification to aquaculture in areas with high salinity, in which water purification is in urgent demand. Desalination technologies have been proposed with various innovations which are still not practical on a large scale. The desalination of seawater and brackish water by reverse osmosis, nano-filtration, electro-dialysis, ion-exchange resins, electrochemical processes and thermal distillation has been applied to agriculture. The advance reverse osmosis shows most potential because of its advances in treating performance, cost effectiveness and effective rejection of brine.