§ 3 - Energy considerations

Hybrid membrane system for desalination and wastewater treatment : Integrating forward osmosis and low pressure reverse osmosis

In order to ensure long-term sustainability of the reservoir, the gas industry in Qatar is faced with the challenge of reducing the volume of produced and process water (PPW) sent to disposal wells by 50% [1-3]. Recently, Qatargas initiated a project to recycle process water and thus, reduce disposal volumes using commercial advanced water treatment technologies [4]. One emerging technology, “osmotic concentration” (OC) has been identified that offers a low-energy alternative to conventional thermal or membrane volume reduction methods. Osmotic concentration is a membrane filtration process that mimics first step in a forward osmosis (FO) system. It requires a high salinity draw solution (DS) which passes on one side of a semi-permeable FO membrane while the feed passes on the other side. Water from the feed is drawn through the membrane, via natural osmosis, reducing the feed volume and increasing the volume of the draw solution. This paper summarizes the results of bench-scale volume reduction tests wit...

Systematic Analysis Reveals Thermal Separations Are Not Necessarily Most Energy Intensive

Influence of microbubble two-phase feed flow on reverse osmosis desalination using different salt concentrations

Forward osmosis(FO)is an emerging technology that has attracted numerous attention for its potential applications in many areas,such as desalination,waste water treatment,agriculture,and power generation.FO has shown many advantages over reverse osmosis process in terms of low energy consumption,high rejection of a wide range of contaminants and lower fouling tendency.In this paper the concept and mechanism of FO are presented,and the calculation and determination of the osmotic pressure of solution are discussed.The relationship between concentration polarization and mass transport through the FO membrane on the phenomenological model is analyzed.Some strategies about the enhancement of mass transfer processes of FO operation are suggested.The opportunities and challenges exist side by side in the development of FO technology.

The basis of a novel method for seawater desalination is outlined. In this work, pressure-retarded osmosis (PRO) energy is obtained and used posteriorly for the reverse osmosis (RO) process for seawater desalination. Although PRO process coupled with an RO process has been studied in the past, however, in this work, there is a fundamental difference. Instead of bringing river or wastewaters with low salinity to the coast to be mixed with the seawater to run the PRO process, here is the seawater which is deliberately salinized. This technique has one important consequence, namely, that it is no longer required to be in places where rivers or wastewaters flow into the sea. This important difference eliminates this until now somehow paradoxical requirement if one considers that regions needing desalination are generally poor of water resources. On the other hand, it is not a coincidence that regions needing desalination plants are also regions with rich open salt deposits in the neighborhood; high evaporation, high concentration of salt deposits, and the need for freshwater are all of them directly correlated. Therefore, the idea proposed in the paper is consistent with the problem. The high evaporation in the region which is causing the need for desalination also is creating the solution to do this by using the salt deposits created. The economic feasibility of this method is preliminarily assessed in terms of the thermodynamic limits of extractable energy and then with the cost of the salt required to obtain this energy which is compared with the price from electrical grid. It was found that in order to reduce the amount of salt required for the process, and to make the cost of energy competitive, it is necessary to direct the hypersaline draw solution (draw solution) in a cyclic loop and to have the highest possible volume fraction for the nonsalinized solution (feed solution). Additional R&D is required to explore the possibilities of this concept.

§ 1 - Pretreatment

BackgroundAs an appropriate tool, membrane process is used for desalination of brackish water, in the production of drinking water. The present study aims to investigate desalination processes of brackish water of Qom Province in Iran.MethodsThis study was carried out at the central laboratory of Water and Wastewater Company of the studied area. To this aim, membrane processes, including nanofiltration (NF) and reverse osmosis (RO), separately and also their hybrid process were applied. Moreover, water physical and chemical parameters, including salinity, total dissolved solids (TDS), electric conductivity (EC), Na+1 and Cl−1 were also measured. Afterward, the rejection percent of each parameter was investigated and compared using nanofiltration and reverse osmosis separately and also by their hybrid process. The treatment process was performed by Luna domestic desalination device, which its membrane was replaced by two NF90 and TW30 membranes for nanofiltration and reverse osmosis processes, respectively. All collected brackish water samples were fed through membranes NF90-2540, TW30-1821-100(RO) and Hybrid (NF/RO) which were installed on desalination household scale pilot (Luna water 100GPD). Then, to study the effects of pressure on permeable quality of membranes, the simulation software model ROSA was applied.ResultsResults showed that percent of the salinity rejection was recorded as 50.21%; 72.82 and 78.56% in NF, RO and hybrid processes, respectively. During the study, in order to simulate the performance of nanofiltartion, reverse osmosis and hybrid by pressure drive, reverse osmosis system analysis (ROSA) model was applied. The experiments were conducted at performance three methods of desalination to remove physic-chemical parameters as percentage of rejections in the pilot plant are: in the NF system the salinity 50.21, TDS 43.41, EC 43.62, Cl 21.1, Na 36.15, and in the RO membrane the salinity 72.02, TDS 60.26, EC 60.33, Cl 43.08, Na 54.41. Also in case of the rejection in hybrid system of those parameters and ions included salinity 78.65, TDS 76.52, EC 76.42, Cl 63.95, and Na 70.91.ConclusionsComparing rejection percent in three above-mentioned methods, it could be concluded that, in reverse osmosis process, ions and non-ion parameters rejection ability were rather better than nanofiltration process, and also better in hybrid compared to reverse osmosis process.The results reported in this paper indicate that the integration of membrane nanofiltration with reverse osmosis (hybrid NF/RO) can be completed by each other probably to remove salinity, TDS, EC, Cl, and Na.

In recent years, the increasing threat to groundwater quality due to human activities has become a matter of great concern. The groundwater quality problems present today are caused by contamination and by overexploitation, or by combination of both, which are faced by many Indian states. Today, reverse osmosis (RO) membranes are the leading technology for desalination of groundwater because of their strong separation capabilities and exhibiting a great potential for treatment of waters worldwide. However, the RO process had some problems due to the formation of polarization films because high pressure operation and by-products which may generate bacteria and fouling. Also, high energy consumption and brine disposal problem is faced in RO process due to the limited recovery of water. These problems may be overcome by other membrane thermal process such as a membrane distillation (MD). This paper addresses the outline of RO and MD process for desalination. RO has developed over the past 40 years and MD is an emerging technology for brackish water desalination and yet is not fully implemented in industry. The MD is the better alternative to RO for desalination theoretically found in the literature.

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.

Polarization phenomena in integrated reverse osmosis and membrane distillation for seawater desalination and waste water treatment

Experimental separation of toxic metal in water has been done using reverse osmosis membrane made from composite material. Experiment was done by simulation in which metals that will be observed solved with water in different concentration and then used as feed solution in reverse osmosis process. Metals observed were Cr 6+ , Mn 2+ and Pb 2+ and reverse osmosis process was done at pressure of 40 Bar for all metals. Experiment result showed that value of feed solution concentration would affect flux and coefficient rejection of membrane. Composite membrane with polyacrylamide as active layer of membrane can reject metals observed with value of rejection coefficient more than 90%, except for Mn 2+ metal that have concentration 250 ppm and 500 ppm. Keywords: Composite membrane, Reverse osmosis, Toxic metal, Rejection, Flux.

Membrane fouling is inevitable during the membrane separation process. The difference in the driving force of reverse osmosis (RO) and forward osmosis (FO) affects the behavior of foulants. Thus, in this work, we examined the behavior of different foulants during the FO or RO process, including before and after physical cleaning of the membrane. The foulants used were alginate (Alg-Na), humic acid (HA), bovine serum albumin (BSA), and colloidal silica. The commercial cellulose triacetate membrane was used for both FO and RO processes to investigate the behavior of foulants fairly. During the RO process, the formation of the gel network between alginate and calcium ions tends to accumulate on the surface of the membrane, leading to the formation of a dense layer of the foulant, consequently decreasing the flux. Having HA in the feed, RO and FO processes had a similar flux decline, whereas having alginate and BSA, the flux decline during the RO process was higher than the FO process. When colloidal silica was presented in the feed, the membrane in the RO process had constant flux throughout the testing, whereas the membrane in the FO process had a remarkable decrease in flux. Silicas were adhered more on the membrane tested in FO. It was presumed that the reverse salt diffusion facilitates the aggregation of the silica on the membrane surface, leading to a reduction of flux by cake-enhanced concentration polarization in the foulant layer of silica. Therefore, the foulant properties, type of draw solution, the structure of the foulant layer, and the interaction between the foulant and membrane are important to consider in the fouling behavior in RO and FO processes. This understanding of the fouling behavior in the FO process will lead to the development of the optimum FO process.

Permeate flux is a key design and operating parameter for the reverse osmosis (RO) process. It has been widely observed from laboratory studies that permeate flux is linearly related to transmembrane pressure, but it has been noted that such a simple linear relationship may not be strictly applicable to a typical full-scale RO process where a long membrane channel is employed. A pilot-scale RO system with a 4-m-long membrane channel was used in this study to investigate the behavior of permeate flux under various operating conditions. It was noted that the linear relationship between the average permeate flux and transmembrane pressure was only true when the RO system was operating at low transmembrane pressure or recovery. The average permeate flux deviated substantially from the linear relationship as the transmembrane pressure increased. It was speculated that thermodynamic equilibrium between the osmotic pressure in the membrane channel and the transmembrane pressure of the RO process might become the limiting factor of RO processes under certain circumstances.

The existing body of experimental data in the open literature clearly indicate that reverse osmosis (RO) processes reject ions of identical valence (i.e., homovalent ions) to different degrees. For example, rejections (or relative permeations) of monovalent anions (such as Cl-, NO3-, Br-, CH2ClCOO-, ClO4-, etc.) during RO processes are different under otherwise identical conditions. The same is true for divalent anions (namely, SO42-, HPO42-, SeO42-) or monovalent cations (namely, Li+, Na+, K+, and NH4+). The solution diffusion model with the Nernst−Planck equation is unable to predict the differential permeation behaviors of homovalent ions. It is recognized that hydrated ionic radii data, if available, could be used to compute interdiffusion coefficients (or salt permeability coefficients) of permeating electrolytes. However, a careful scrutiny of the existing body of hydrated ionic radii data in the open literature provide clear evidence that they are unreliable for polyatomic ions such as nitrate, nitrite, selenate, phosphate, chloroacetate, sulfate, etc. Also, ionic diffusivities computed from equivalent conductance data fail to predict the hierarchy of relative permeations of homovalent ions in RO processes. Central to this study is the underlying scientific premise that ion-exchange selectivity can be used as an effective parameter to predict the relative permeability of homovalent ions in a multicomponent system. For two ions of identical valence, ion-exchange selectivity based on Coulombic interaction is dependent only on their relative hydrated ionic radii, which in turn govern the interdiffusion coefficients of permeating electrolytes. The higher the ion-exchange selectivity of a specific ion, the lower is its hydrated ionic radius and, hence, more permeable is the ion. The theoretical relationship between ion-exchange selectivity and permeability can be well explained with the aid of the Stokes−Einstein equation. Experimental results presented in this study with both monovalent and divalent ions show a strong characteristic correlation between ion-exchange selectivity and relative permeation, i.e., a higher ion-exchange selectivity always leads to a greater permeability. One major advantage of this approach is the ease with which ion-exchange selectivity can be determined by ion chromatography and/or batch isotherm technique. A large body of existing experimental data for RO and nanofiltration processes in the open literature, when carefully reviewed, also validate this scientific premise. Type of membrane, solvent dielectric constant, and pH influence the overall solvent and salt permeation fluxes, but the relative permeation of homovalent ions still follows the ion-exchange selectivity sequence.