A comparison of silver‐ and copper‐charged polypropylene feed spacers for biofouling control

Both brackish water desalination and seawater desalination processes are well established and in common use around the globe to create new water supply sources. The farther the location of the source water from the ocean or seashore, the lower the salinity (TDS) of the water and the lower the osmotic pressure that needs to be overcome when desalinated water is produced. This is one of the major reasons that brackish desalination is often considered less costly than seawater desalination. A number of project considerations, however, indicate that seawater desalination can be beneficial and more cost-effective than brackish water desalination. To make a fair comparison, we need to properly compare all major aspects of both types of projects to define the best and most appropriate desalination technology. While brackish water has less feed water TDS, it is more challenging to dispose of the produced concentrate. Also, although brackish water desalination needs less energy to overcome osmotic pressure, it usually requires more energy to draw the water from the well than it takes to pump seawater from the open ocean intake. Another factor is that the temperature of the brackish well water may be lower than the temperature of ocean water, giving seawater desalination an advantage in energy demand. In comparing brackish to seawater desalination, these major aspects should be evaluated: (1) Locations of seawater and brackish water plants, relative to the major consumers of the desalinated water, (2) Transportation (pumping and disposal) costs of the feed water and produced water, (3) Potential colocation of a seawater plant with a large industrial user (e.g., power plant) of the seawater for cooling or other purposes, (4) Produced quality of brackish water and seawater desalination in terms of major minerals and emerging contaminants, (5) Sustainability of the water source: capacity and depth of the brackish water wells, as well as the type of soil. (6) Technical and economic aspects of produced concentrate disposal, (7) Permitting process costs for brackish and seawater desalination, and (8) The economics of both brackish and seawater desalination treatment processes: capital costs, operational and maintenance (O&M) costs, lifetime water cost, and total water cost (TWC). This paper discusses the major evaluation criteria and considerations involved in properly comparing the economic and technical aspects of brackish and seawater desalination to determine the more favorable desalination technology for a given desalination project.

Fouling of high pressure-driven NF and RO membranes in desalination processes: Mechanisms and implications on salt rejection

Development of copper-charged polypropylene feedspacers for biofouling control

One Step Membrane Filtration: A fundamental study

The role of desalination in bridging the water gap in Jordan

Water quality parameters such as ATP, total direct cell counts, AOC, biofilm formation rate and destructive membrane studies are not suitable for biofouling monitoring and prediction. Therefore, a monitor named membrane fouling simulator was developed. In a comparison study, the same feed channel pressure drop development in time and the same fouling accumulation was observed in spiral wound membrane elements and membrane fouling simulators. Chemical dosing to the membrane fouling simulator feed water showed that a biofouling inhibitor was not inhibiting biofouling, but was even contributing to biofouling. It is shown that other chemicals such as acid and antiscalants may contribute to biofouling as well. It was found that the feed spacer presence strongly influences the feed spacer channel pressure drop increase caused by biofilm accumulation: in nanofiltration and reverse osmosis systems biofouling is a feed spacer problem. A new set of monitors for membrane fouling studies and methods for biofouling monitoring are described. A state of the art on global membrane fouling simulator use is given.

Membrane Filtration technique is being accepted worldwide as an environment friendly and energy efficient technique in Desalination Industry as compared to Thermal Desalination techniques. However, the performance of membranes which include permeate flux and rejection is affected by the membrane fouling. The properties of membrane and surface features such as porous structure, hydrophilicity/hydrophobicity charge, polymer characteristics, surface roughness determine the fouling potential of the membrane. The hydrophilic and smooth membrane surface is usually considered desirable in tackling membrane fouling issues. Therefore, many studies have focused on to enhance surface characteristics of membranes by surface coating with polymers and nanomaterials. Since, membrane coating is not done during fabrication of the most commercially available membranes, therefore, it is also important to determine the surface features of the commercially available membranes to investigate their membrane fouling potential. Thus, the objectives of this study were (1) to perform membrane surface characterization of commercial Reverse Osmosis (RO) and Nanofiltration (NF) membranes using techniques such as SEM, AFM, FTIR and XPS; (2) to measure hydrophilicity/hydrophobicity of commercial RO and NF membranes through water contact angle measurement using sessile drop method and (3) to measure the flux and percentage rejection of NF and RO membranes using Dead end filtration technique. Here, the characterization of membrane surface in terms of surface roughness, using SEM and AFM, showed that the commercial RO membrane had more ridge and valley structures and higher average surface roughness i.e. 71.24 nm as compared to NF membranes (6.63 nm). In addition, water contact angle measurements showed that the NF membrane was more hydrophilic as compared to RO membrane. The average contact angle found for RO membrane was 59.94°. On the other hand, it was observed that NF membrane is extremely hydrophilic in nature. Due to which, contact angle value was not obtained for most of the runs. The droplet could diffuse in less than 5 seconds. In addition, the dead-end filtration experiments showed that the RO membrane had much lower flux as compared to NF membrane. This can be associated with the pore structure of these membranes. Since, the NF membrane has porous structure, in oppose to RO membrane, the flux of the NF membrane is usually higher than the RO membranes. As the membrane surface roughness and hydrophobicity makes it more susceptible to the fouling leading to reduction in membrane flux and performance, it can be concluded from this study that there is a need for surface coating of RO membrane with suitable nanomaterials such as graphene oxide to improve its hydrophilicity and surface smoothness. This will eventually make the membrane more resistant to membrane fouling and will establish the use of membrane filtration technique in desalination industry in Qatar in the future. Microorganisms have been isolated from Gulf sea water, identified and differentiated and are being used to study the biofouling of RO and NF membranes, that would be coated to limit the fouling problems. Acknowledgement: This research was made possible by NPRP grant # [9-318-1-064] from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the author[s].

Analysis of desalination alternates for phosphoric acid plant in Tunisia

Effect of feed spacer arrangement on flow dynamics through spacer filled membranes

Reverse osmosis (RO) membrane technology is considered to be the premier process used for the purpose of seawater and brackish water desalination and water treatment of municipal and industrial wastewater for water reclamation and reuse. Membrane biofouling is a significant challenge in RO processes due to the interference of biofilm formed on the membrane surface on membrane performance. Thus, diverse areas of research are geared towards the understanding, prevention, and control of biofouling. Diagnosis of biofouling is difficult since no single microbial assay on the source water can accurately predict biofouling during the RO process. Biofouling evaluation methods of fouled membranes and collected biofoulants from the treatment processes are counterproductive when biofouling prevention is warranted. It is therefore important for the detection tests to be predictive enough taking into consideration the water quality characteristics of the source feed water, the properties of the RO membrane used for the water treatment, and the hydrodynamic properties during the RO process. This chapter provides an overview of biofouling tests most commonly used for detection of biofouling in the source feed water, and in the foulants and fouled membranes. It has a brief section on the use of flow cell units that can simulate hydrodynamic conditions in the RO plant with the ability to predict biofouling.

Removal of silica from brackish water by electrocoagulation pretreatment to prevent fouling of reverse osmosis membranes

Brackish water desalination using reverse osmosis and capacitive deionization at the water-energy nexus

Reverse osmosis (RO) membranes have been widely applied in membrane filtration processes for water purification, since the high selective RO membranes are designed to reject all materials with particle diameter larger than 10 angstrom (A) [1]. However, this optimal selectivity leads to fouling that can greatly affect the performance and productivity of RO membranes. Biofouling remains as one of the major operational problems in RO processes and is caused by unwanted deposit and growth of microorganisms on the membrane. Numerous biofouling control strategies have been developed to restore the performance of RO membranes, but none of them are able to prevent or remove biofouling completely. A novel cleaning technique using a weak and monobasic acid (pKa=3.34, 25℃)named free nitrous acid (FNA) in combined with hydrogen dioxide (H2O2) was proposed. The effects of FNA with or without H2O2 on biofouling of RO membranes were investigated in Chapter 4, five RO membranes with different degree of biofouling were cleaned using FNA solutions (10, 35 and 47 mg HNO2-N/L) at pH 2.0, 3.0 and 4.0 under cross-flow conditions for 24 hours. The cleaning efficiency of FNA solutions was compared with conventional cleaning solution sodium hydroxide (NaOH, pH 11). The cleaning tests demonstrated that FNA cleaning solutions were more efficient than NaOH at biomass removal and inactivation. At the optimum cleaning conditions (35 mg HNO2-N/L at pH 3.0),FNA has achieved higher biomass removal than NaOH for both heavily fouled (86-96% versus 41-83%) and moderately fouled (92-95% against 89-92%) membranes, respectively.In accordance to the biomass removal, 6-32% of viable cells remained on the moderately fouled RO membranes under the impact of FNA cleaning (pH 3), whereas 38-58% of viable cells stayed on the heavily fouled RO membranes. These results revealed that FNA cleaning is more effective for moderately fouled membranes, implying that early cleaning for biofouling is preferable. Although applying FNA alone, or combining it with H2O2 have shown better efficiency at biofouling removal than NaOH, the cleaning efficiency has not been significantly improved (<1% of enhancement) by adding H2O2 to FNA cleaning solutions. The effects of FNA on scaling of RO membranes were also studied using the same cleaning protocol developed for biofouling control. The results showed that FNA solutions at pH 2.0 and 3.0 were as efficient as conventional cleaning acids (hydrochloric acid and citric acid). The scaling layers which contain 32.4±1.7 g/cm2 of calcium were completely removed by all acidic cleaning solutions. Based on the results, FNA is shown to be a promising cleaning agent for RO membrane biofouling and scaling removal. Further investigation focused on the effectiveness of FNA for biofouling prevention in RO processes (Chapter 5). The results showed that weekly FNA cleanings were unable to prevent fouling in the RO filtration systems, as the hydraulic performances (permeability and salt rejection) of RO membranes have gradually declined over two to three weeks filtration period. Although FNA cleaning was able to restore the permeability of RO membranes for one to two days, continuing declined permeability implied that the fouling rate was greater than the inhibition rate of FNA. The results of prevention tests also showed that FNA was more efficient at biomass inactivation and removal. The biomass contents and viable cells of the fouling layers formed in the experiment filtration unit (with FNA weekly cleaning) were less than half of that in the control filtration unit (without FNA weekly cleaning). Moreover, the results of live/dead cell staining revealed the abundance of viable cells in the control unit(57±5%) was four times higher than that in the experiment unit (13±2%). However, there was no significant difference in the concentration of macromolecules such as proteins and polysaccharides between control and experiment filtration units.

Side effects of antiscalants on biofouling of reverse osmosis membranes in brackish water desalination

This review aims to succinctly summarize recent advances of four key membrane processes (e.g., reverse osmosis (RO), forward osmosis (FO), electrodialysis (ED), and membrane distillation (MD)) in membrane materials and process designs, to elucidate the contributions of these advances to the steadfast growth of brackish water membrane desalination processes. With detailed analyses and discussions, the ultimate purpose of the review is to shed light on the future direction of brackish water desalination using membrane processes. Brackish water has widely varying particulate matter and boron contents, posing great risks of membrane fouling and excessive boron levels to the membrane desalination processes. Recent advances in these four membrane processes largely focus on improving fouling resistance, boron rejection, water flux, and energy efficiency. Aquaporin membranes and thin-film composite polyamide membranes incorporated with nanoparticles exhibit excellent performances for RO and FO, whereas super-hydrophobic membranes prove their great potentials for MD. While recent advances in RO and ED process designs are orientated towards membrane fouling prevention by exploring respectively novel energy-saving membrane-based pre-treatment and reversal operation, recent studies on FO and MD are centered on reducing the energy costs by advancing the fertilizer-drawn concept and utilizing waste heat. Membrane processes are dominating brackish water desalination, and this trend is hardly to change. Membranes based on nanoparticles and other novel materials are deemed the next membrane generation, and innovative membrane process designs have demonstrated great potentials for brackish water desalination. Nevertheless, further works are needed to scale up these novel membrane materials and designs.