TweetIntroduction Oceans contain over 97.2% of the planet’s water. Because of the high salinity of ocean water and the significant costs associated with seawater desalination most of the global water supply has traditionally come from fresh water sources – groundwater aquifers, rivers and lakes. Today, however, changing climate patterns combined with population growth pressures and […]
Oceans contain over 97.2% of the planet’s water. Because of the high salinity of ocean water and the significant costs associated with seawater desalination most of the global water supply has traditionally come from fresh water sources – groundwater aquifers, rivers and lakes. Today, however, changing climate patterns combined with population growth pressures and limited availability of new and inexpensive fresh water supplies are shifting the water industry’s attention: in an emerging trend, the world is reaching to the ocean for fresh water.
Until recently, desalination was limited to desert-climate dominated regions. Technological advances and an associated decrease in water production costs over the past decade have expanded its use in coastal areas traditionally supplied with fresh water resources. Today, desalination plants provide approximately 1% of the world’s drinking water supply and the commissioned and installed desalination capacity has been increasing exponentially over the past 10 years.
Higher Productivity Membrane Elements
A key factor that has contributed to the dramatic decrease of seawater desalination costs over the past 10 years is the advancement of the SWRO membrane technology. Today’s high-productivity membrane elements are designed with several features that yield more fresh water per membrane element than any time in the recent history of this technology: higher surface area, enhanced permeability and denser membrane packing. Increasing active membrane leaf surface area and permeability allows it to gain significant productivity using the same size (diameter) membrane element. Active surface area of the membrane elements is typically increased by membrane production process automation, by denser membrane leaf packing and by adding membrane leaves within the same element.
The total active surface area in a membrane element is also be increased by increasing membrane size/diameter. Although 8-inch SWRO membrane elements are still the “standard” size most widely used in full-scale applications, larger 16-inch and 18-inch size SWRO membrane elements have become commercially available over the past three years and have already found full-scale implementation in several SWRO projects worldwide (Bergman & Lozier, 2010).
In the second half of the 1990s, the typical 8-inch SWRO membrane element had a standard productivity of 5,000 to 6,000 gallons per day (gpd) at salt rejection of 99.6%. In 2003, several membrane manufacturers introduced high-productivity seawater membrane elements that are capable of producing 7,500 gpd at salt rejection of 99.75%. Just one year later, even higher productivity (9,000 gpd at 99.7% rejection) seawater membrane elements were released on the market. Over the past three years SWRO membrane elements combining productivity of 10,000 to 12,000 gpd and high-salinity rejection have become commercially available and are now gaining wider project implementation.
The newest membrane elements provide flexibility and choice and allow users to trade productivity and pressure/power costs. The same water product quality goals can be achieved in one of two general approaches: (1) reducing the system footprint/construction costs by designing the system at higher productivity, or (2) reducing the system’s overall power demand by using more membrane elements, designing the system at lower flux and recovery, and taking advantage of newest energy recovery technologies which further minimize energy use if the system is operated at lower (35% to 45%) recoveries.
Innovative hybrid membrane configuration combining SWRO elements of different productivity and rejection within the same vessel which are sequenced to optimize the use of energy introduced with the feed water to the desalination vessels is also finding wider implementation. In addition, a number of novel membrane SWRO train configurations have been developed over the past five years aiming to gain optimum energy use and to reduce capital costs for production of high-quality desalinated water.
Design and Equipment Enhancements for Lower Energy Use
Energy is one of the largest expenditures associated with seawater desalination. Figure 1 shows a distribution of the energy use within a typical seawater desalination plant. As shown on this figure, the SWRO system typically uses over 70% of the total plant energy.
Figure 1 – Energy Use Breakdown of Typical SWRO Desalination Plant
Increased High Pressure Pump Efficiency
One approach for reducing total RO system energy use which is widely applied throughout the desalination industry today is to incorporate larger, higher efficiency centrifugal pumps which serve multiple RO trains. This trend stems from the fact that the efficiency of multistage centrifugal pumps increases with their size (pumping capacity). For example, under a typical configuration where individual pump is dedicated to each desalination plant RO train, high pressure pump efficiency is usually in a range of 80% to 83%. However, if the RO system configuration is such that a single high pressure pump is designed to service two RO trains of the same size, the efficiency of the high pressure pumps could be increased to up to 85%.
Proven design that takes this principle to the practical limit of centrifugal pump efficiency (≈ 90%) is implemented at the 86 MGD Ashkelon seawater desalination plant in Israel, where two duty horizontally split high pressure pumps are designed to deliver feed seawater to 16 SWRO trains at guaranteed long term efficiency of 88%. Continuous plant operational track record over the past 5 years shows that the actual efficiency level of these pumps under this configuration is close to 90%.
A current trend for smaller desalination facilities (plants with fresh water production capacity of 250,000 gpd or less) is touse positive displacement (multiple-piston)high-pressure pumps and energy recovery devices, which often are combined into a single unit. These systems are configured to take advantage of the high efficiency of the positive displacement technology which practically can reach 94% to 97%.
Improved Energy Recovery
Advances in the technology and equipment allowing the recovery and reuse of the energy applied for seawater desalination have resulted in a reduction of 80% of the energy used for water production over the last 20 years. Today, the energy needed to produce fresh water from seawater for one household per year (~2,000 kW/yr) is less than that used by the same household’s refrigerator.
While five years ago, the majority of the existing seawater desalination plants used Pelton Wheel-based technology to recover energy from the SWRO concentrate, today the pressure exchanger-based energy recovery systems dominate in most desalination facility designs. The key feature of this technology is that the energy of the SWRO system concentrate is directly applied to pistons that pump intake seawater into the system. Pressure-exchanger technology typically yields 5% to 15% higher energy recovery savings than the Pelton-Wheel-based systems.
Figure 2 depicts the configuration of a typical pressure exchanger-based energy recovery system. After membrane separation, most of the energy applied for desalination is contained in the concentrated stream (brine) that also contains the salts removed from the seawater. This energy-bearing stream (shown with red arrows on Figure 2) is applied to the back side of pistons of cylindrical isobaric chambers, also known as pressure exchangers (shown as yellow cylinders on Figure 2). These pistons pump approximately 45% to 50% of the total volume of seawater fed into the RO membranes for salt separation. Since a small amount of energy (4 to 6%) is lost during the energy transfer from the concentrate to the feed water, this energy is added back to feed flow by small booster pumps to cover for the energy loss. The remainder (45% to 50%) of the feed flow is handled by high-pressure centrifugal pumps. Harnessing, transferring and reusing the energy applied for salt separation at very high efficiency (94% to 96%) by the pressure exchangers allows a dramatic reduction of the overall amount of electric power used for seawater desalination.In most applications, a separate energy recovery system is dedicated to each individual SWRO train. However, some recent designs include configurations where two or more RO trains are serviced by a single energy recovery unit.
Figure 2 – Pressure Exchanger Energy Recovery System
While the quest to lower energy use continues, there are physical limitations to how low the energy demand could go using RO desalination. The main limiting factors are the osmotic pressure that would need to be overcome to separate the salts from the seawater and the amount of water that could be recovered from a cubic meter of seawater before the membrane separation process is hindered by salt scaling on the membrane surface and the service systems. This theoretical limit for the entire seawater desalination plant is approximately 4.5 kWh/kgal.
Seawater Desalination Cost Trends
Advances in seawater RO desalination technology during the past two decades, combined with transition to construction of large capacity plants, and enhanced competition by using the Build-Own-Operate-Transfer (BOOT) method of project delivery have resulted in an overall downward cost trend. While the costs of production of desalinated water have benefited from the most recent advances in desalination technology, the cost spread among individual desalination projects observed over the past three years is fairly significant.
Most recently commissioned large seawater desalination projects worldwide produce desalinated water at an all-inclusive cost of US$3.0 to US$5.5/kgal. However, the traditionally active desalination markets in Israel and Northern Africa (i.e., Algeria) have yielded desalination projects with exceptionally low water production costs (110 MGD SWRO Plant in Sorek, Israel – US$2.00/kgal; 87 MGD Hadera Desalination Plant, Israel – US$2.27/kgal; 132 MGD Magtaa SWRO Plant in Algeria – US$2.12/kgal).
On the other end of the cost spectrum, some of the most recent seawater desalination projects in Australia had been associated with the highest desalination costs observed over the past 10 years – i.e., the Gold Coast SWRO Plant in Queensland at US$10.95/kgal; the Sydney Water Desalination Plant at US$8.67/kgal; and the Melbourne’s Victorian Desalination Plant at US$9.54/kgal.
While this extreme cost disparity has a number of site-specific reasons, the key differences associated with the lowest and highest-cost projects are related to five main factors: (1) desalination site location; (2) environmental considerations; (3) phasing strategy; (4) labor market pressures; (5) method of project delivery and risk allocation between owner and private contractor responsible for project implementation.
The desalination projects with highest and lowest costs have a very distinctive difference in terms of project phasing strategy. While the large high-cost projects incorporate single intake and discharge tunnel structures built for the ultimate desalination plant capacity (which often equals two times the capacity of the first project phase), the desalination projects on the low end of the cost spectrum use multi-pipe intake systems constructed mainly from high density polyethylene (HDPE) that have capacity commensurate with the production capacity of the desalination plant. Additional multiple intake pipes and structures are installed as needed at the time of plant expansion for these facilities.
While the single-phase construction of desalination plant intake and outfall structures dramatically reduces the environmental and public controversy associated with the plant capacity expansion at a later date, this “ease-of-implementation” benefit typically comes with an overall cost penalty. The notion that the larger costs associated with building complex intake and outfall concrete tunnels in one phase will somehow be offset by economies of scale usually does not yield the expected overall project cost savings. The main reason is the fact that the cost of 100 linear feet of deep concrete intake or discharge tunnel is over four times higher than the cost of the same capacity intake or discharge constructed from multiple HDPE pipes located on the ocean bottom, while the economy of scale from one-stage construction is usually less than 30%.
Labor market differences can have a profound impact on the cost of construction of desalination projects. The overlapping schedules of the series of large desalination projects in Australia have created temporary shortage of skilled labor, which in turn has resulted in a significant increase in unit labor costs. Since labor expenditures are usually 30% to 50% of the total desalination plant construction costs, a unit labor rate increase of 20% to as high as up to 100%, could trigger sometimes unexpected and not frequently observed project cost increases.
Without exception, the lowest cost desalination projects to date have been delivered under turnkey BOOT contracts where private sector developers share risks with the public sector based to their ability to control and mitigate the respective project related risks.
On the other hand, the most costly desalination projects worldwide have been completed under an “alliance”[(a type of design-build-operate (DBO)] model where the public utility retains the ownership over the project assets but expects the DBO team to take practically all project-related risks. In this case, DBO contractors take upon project risks over which they have limited or no control, by delivering very conservative designs, incorporating high contingency margins in the price of their construction, operation and maintenance services, and by insuring these project risks at very high premiums. As a result, the projects delivered under such structure carry very high contingencies and upfront insurance and performance security payments which ultimately reflect on the overall increase of the cost of water production.
While under a typical BOOT project, the insurance and contingency costs are usually well below 20% of the total capital costs, projects with disproportionate transfer of risk to the private contractor result in built-in insurance and contingency premiums which exceed well over 30% of the total project capital costs. As a result, most often benefits gained from using state-of-the-art technologies, equipment and design, are negated by overly burdensome insurance and contingency expenditures and high cost of project funding.
Seawater Desalination Challenges in US
Water Production Costs
Currently, the cost of desalinating seawater in the US is relatively high compared to that of traditional low-cost water sources (groundwater and river water) and to production costs for water reclamation and reuse for irrigation and industrial use. Indeed, the cost of traditional local groundwater water supplies in some parts of the US are as low as US$0.50/kgal to US$0.90/kgal. However, the quantity of such low-cost sources in coastal urban centers of California, Texas, Florida, South Carolina and other parts of the US exposed to recent long-term drought pressures is very limited.
The generally lower costs for production of reclaimed water and for implementation of water conservation measures have often been used as an argument against the wider use of seawater desalination. This argument however, is fatally flawed by the fact that water conservation and reuse do not create new sources of drinking water – they are merely a rational tool to maximize the beneficial use of the available water supply resources. Under conditions of prolonged drought when the available water resources cannot be replenished at the rate of their use, aggressive reuse and conservation can help but may not completely alleviate the need for new water resources and water rationing.
Typically, seawater desalination cost benefits extend beyond the production of new water supplies. If seawater desalination is replacing the use of over-pumped coastal or inland groundwater aquifers, or is eliminating further stress on environmentally sensitive estuary and river habitats, than the higher costs of this water supply alternative would also be offset by its environmental benefits. Similarly, seawater desalination provides additional benefits in the time of drought where traditional water supplies may not be reliable and their scarcity may increase their otherwise relatively low costs.
Salt separation from seawater requires a significant amount of energy to overcome the naturally occurring osmotic pressure exerted on the reverse osmosis membranes. This in turns makes seawater desalination several times more energy intensive than conventional treatment of fresh water resources. Table 1 presents the energy use associated with various water supply alternatives. The table does not incorporate the costs associated with raw water treatment and product water delivery.
Table 1 – Energy Use of Various Water Supply Alternatives
While energy use for seawater desalination is projected to decrease by 10 to 20 % in the next 5 years as a result of technological advances discussed previously, the total energy demand for conventional water treatment would likely increase by 15 to 20 % in the same time frame because of the energy demand associated with the additional treatment (such as micro- or ultra-filtration, ozonation, UV disinfection, etc.) which would be needed in order to meet the most recent regulatory requirements for production of safe drinking water in the USA.
A number of the seawater desalination projects under consideration in California and Florida are proposed to be collocated with power generation plants which currently use seawater for production of electricity. Under the collocation configuration the desalination plant does not have a separate intake and discharge to the ocean and both the desalination plant intake and desalination plant discharge are connected to the exiting power plant discharge outfall or canal.
Collocation yields a number of benefits mainly because it avoids construction and permits for new intake and concentrate discharge facilities, and because of the energy cost savings associated with the desalination of warmer source water. However this intake configuration alternative has been considered undesirable by some environmental groups due to the potential loss of marine organisms caused by the impingement of marine organisms against the screens of the power plant intake and their entrainment inside the power plant conveyance and cooling system and subsequently inside the desalination plant.
Based on recently introduced regulatory requirements, the 21 once-through cooling plants along the California coast are required to prepare comprehensive plans for discontinuation of their use of open intakes and switching to air-cooling towers or to water close-circulation cooling towers in order to reduce impingement and entrainment of marine organisms.
Opponents of collocated seawater desalination plants have often present the argument that if the power plant changes its cooling system in the future, seawater desalination under collocated configuration at the particular location would no longer be available. This argument however, is unfounded in reality, because even if the host power plants abandon once-through cooling in the future, the desalination projects will still retain the main cost-benefits of collocation – avoidance of the need to construct a new intake and outfall. The cost savings from the use of the existing power plant intake and outfall facilities would be over 25%, resulting in a significant net benefit with or without the power plant in operation.
Recent studies of wedge-wire screens in Santa Cruz, California indicate that this type of open intake may prove to be a viable alternative for dramatic reduction in impingement and entrainment of marine organisms. Typically, wedge-wire screens are designed to be placed in a water body where significant prevailing ambient cross flow current velocities (³ 1 ft/s) exist. This cross high flow velocity allows organisms that would otherwise be impinged on the wedge-wire intake to be carried away with the flow.
A 2-mm cylindrical wedge wire screen intake is also planned to be tested for one year at the West Basin Municipal Water District’s Ocean Water Desalination Demonstration Facility in Redondo Beach. This demonstration facility is currently under operation.
Various sub-surface intake technologies (i.e., beach wells, horizontally directionally drilled and slant wells and innovative infiltration gallery configurations) have been heavily promoted by the California Coastal Commission and local environmental groups as a viable alternative to power plant collocation and construction of new open intakes along the California coast. Ongoing long-term studies of subsurface intakes in Long Beach and Dana Point, California are expected to provide comprehensive data that would allow completing a scientifically-based analysis of the viability and performance benefits of alternative subsurface intakes.
Seawater desalination plants along the US coastline would produce concentrate of salinity that is approximately 1.5 to 2 times higher than the salinity of the ambient seawater (i.e., in a range of 52 ppm to 67 ppm). While most marine organisms can adapt to this increase in salinity, some aquatic species such as abalone, sea urchins, sand dollars, sea bass and top smelt, are less tolerant to high salinity concentrations. Therefore, thorough assessment of the environmental impact of the discharge of concentrate and of any other byproducts of the seawater treatment process is a critical part of the evaluation of project viability.
At seawater desalination projects that are proposed to be collocated with power plants, the desalination plant discharge is planned to be diluted with the cooling water of the power plant to salinity levels that typically do not have significant impact on aquatic life. The magnitude and significance of impact, however, mainly depend on the type of marine organisms inhabiting the area of the discharge and on the hydrodynamic conditions of the ocean in this area, such as currents, tide, wind and wave action, which determine the time of exposure of the marine organisms to various salinity conditions.
Extensive salinity tolerance studies completed over the past several years at the Carlsbad seawater desalination demonstration facility in California indicate that after concentrate dilution with power plant cooling seawater down to 40 ppm or less, the combined discharge does not exhibit chronic toxicity on sensitive test marine species. Recent acute toxicity studies at this facility further show that sensitive marine species can event tolerate salinity of 50 ppm or more over a short period of time (2 days or less).
Some seawater desalination projects are planning to use deep injection wells to discharge the high-salinity seawater concentrate generated during the reverse osmosis separation process. However, the full-scale experience with this concentrate disposal method to date is very limited.
A third disposal alternative, besides injection wells and co-disposal with power plant cooling water, currently under consideration for implementation at a number of seawater desalination projects in the US, is the discharge of the concentrate through existing wastewater treatment plant ocean outfall. International experience with such co-located discharges is fairly limited. However, this technology may have a number of merits similar to these derived from the collocation of desalination and power generation plants.
Proving that concentrate discharge from a seawater desalination plant is environmentally safe requires thorough engineering analysis including: hydrodynamic modeling of the discharge; whole effluent toxicity testing; salinity tolerance analysis of the marine species endogenous to the area of discharge; and reliable intake water quality characterization that provides basis for assessment of concentrate’s make up and compliance with the numeric effluent quality standards applicable to the point of discharge. Comprehensive pilot testing of the proposed seawater desalination system is very beneficial for the project environmental impact analysis.
Summary and Conclusions
Over the past decade seawater desalination has experienced an accelerated growth driven by advances in membrane technology and environmental science. While conventional technologies, such as sedimentation and filtration have seen modest advancement since their initial use for potable water treatment several centuries ago, new more efficient seawater desalination membranes and membrane technologies, and equipment improvements are released every several years. Although, no major technology breakthroughs are expected to bring the cost of seawater desalination further down dramatically in the next several years, the steady trend of reduction of desalinated water production costs coupled with increasing costs of water treatment driven by more stringent regulatory requirements, are expected to accelerate the current trend of increased reliance on the ocean as an attractive and competitive water source. This trend is forecasted to continue in the future and to further establish ocean water desalination as a reliable drought-proof alternative for many coastal communities in the United States and worldwide.
Although seawater desalination projects in the US face a number of environmental challenges, these challenges can be successfully addressed by carefully selecting the project site, by implementing state-of-the art intake and concentrate discharge technologies and by incorporating energy efficient and environmentally sound equipment and systems.
Nikolay Voutchkov, PE, BCEE Water Globe Consulting, LLC, Stamford, CT. Editor of Desalination Technology: Health and Environmental Impacts (2010) IWA Publishing.
Water Desalination and European Research
Nanofiltration for Brackish Desalination
GWI – Global Water Intelligence: Market-leading Analysis of the International Water Industry, 10, pp.9, 2009.
Bergman R.A. and J.C. Lozier.“Large-Diameter Membrane Elements and Their Increasing Global Use”. IDA Journal. pp. 16, First Quarter 2010.
Global Water Intelligence
International Desalination Ascociation
Joseph Cotruvo, Nikolay Voutchkov, John Fawell, Pierre Payment, David Cunliffe, Sabine Lattemann Desalination Technology: Health and Environmental Impacts (2010) IWA Publishing.
IWA WATER WIKI BY HTTP://WWW.IWAWATERWIKI.ORG IS LICENSED UNDER A CREATIVE COMMONS ATTRIBUTION-SHARE ALIKE 3.0 UNPORTED LICENSE.
BASED ON A WORK AT WWW.IWAWATERWIKI.ORG
Tweet Microporous Silica Based Membranes for Desalination Abstract: This review provides a global overview of microporous silica based membranes for desalination via pervaporation with a focus on membrane synthesis and processing, transport mechanisms and current state of the art membrane performance. Most importantly, the recent development and novel concepts for improving the hydro-stability and separating […]
Microporous Silica Based Membranes for Desalination
Abstract: This review provides a global overview of microporous silica based membranes for desalination via pervaporation with a focus on membrane synthesis and processing, transport mechanisms and current state of the art membrane performance. Most importantly, the recent development and novel concepts for improving the hydro-stability and separating performance of silica membranes for desalination are critically examined. Research into silica based membranes for desalination has focussed on three primary methods for improving the hydro-stability. These include incorporating carbon templates into the microporous silica both as surfactants and hybrid organic-inorganic structures and incorporation of metal oxide nanoparticles into the silica matrix. The literature examined identified that only metal oxide silica membranes have demonstrated high salt rejections under a variety of feed concentrations, reasonable fluxes and unaltered performance over long-term operation. As this is an embryonic field of research several target areas for researchers were discussed including further improvement of the membrane materials, but also regarding the necessity of integrating waste or solar heat sources into the final process design to ensure cost competitiveness with conventional reverse osmosis processes.
Keywords: desalination; pervaporation; microporous silica; metal oxide silica; hybrid silica; carbon template silica
- 1. Introduction
Water is essential for life and the rapid increase in the global population, and corresponding urbanization has seen the demand for both the quantity and quality of fresh water increase dramatically. One of the major challenges of the 21st century, if not the most important of all, is water scarcity, with the security of social and economic development of a country closed linked to its water resources. Nearly every industrial sector is dependent upon the availability of water, and water shortages have a resounding impact on all levels of society from the general public to health and politics. Indeed, the major problems encountered by water shortages include drought and famine, loss of production in primary industries, loss of job opportunities, poor health and hygiene as well as an increase in the cost of fresh water. This situation is made more complex by the fact that, according to the World Health Organization, more than 15% of the world’s population have no access to potable water and more than 37% have no access to sanitation . Against this backdrop, desalination is becoming an increasingly important tool in the fight to the global demand for clean water.
Membrane technologies have long been an attractive approach to separation in industry, because they are fast and relatively energy efficient processes. In addition, they frequently offer high operational stability, low operating costs and are simple to integrate and control within larger industrial process trains. Indeed, they have been successfully applied to the desalination industry with such vigor that they have long overtaken traditional thermal processes to become the gold standard . In general, there are three main types of membrane processes that are currently applied including reverse osmosis (RO), membrane distillation (MD) and pervaporation (PV) . RO depends on the ability of the ‘dense’ membrane to repel salt ions whilst allowing the passage of water molecules. The transport is governed by a solution-diffusion mechanism with the driving force being an external pressure difference large enough to overcome the osmotic pressure of the salt water. On the other hand, MD is a thermal process that requires a porous, hydrophobic membrane wherein the passage of water vapour only is permissible. PV, by contrast, uses molecular sieve type of membranes that allows only passage to water molecules but relies on a water vapour pressure difference. Both of these desalination processes require very different types of membranes with vastly different properties and configurations. Currently, there are two main types of membranes for water desalination, namely polymeric (e.g., polyamide-, polysulfone-, polyfurane- and cellulose-based for RO and polytetrafluoroethylene for MD) and inorganic composite or ceramic membranes (alumina-, zirconia-, titania-, zeolite-, silica- and carbon-based). Between these two classes of membranes, polymeric membranes are the most mature and well-established in the desalination industry due to their low cost, manufacturability, simple module design and improved permeability and selectivity [4,5]. However, these membranes suffer from swelling phenomenon, a short life-span due to biofouling as well as poor thermal and chemical resistance .
Inorganic membranes, on the other hand, are more resistant to process conditions. In addition, they are by their very nature, porous and hence desalinate via different transport mechanisms to polymeric membranes, based primarily on their pore size. In particular, zeolites and amorphous silica based membranes are attractive candidates for water desalination due to the advantages of their tunable pore sizes and morphology thereby offering higher selectivity. Furthermore, interest in amorphous silica based membranes is gaining momentum because of their simple fabrication techniques, relatively low cost and excellent molecular sieving properties as demonstrated in studies where they are utilized to separate gas molecules [6–9]. In these cases, microporous silica membranes have molecular-sieving structures with pore sizes on the order of the kinetic diameter of the species to be separated (dp = 3–5 Å) and therefore the membrane acts via PV as selective barrier between the water molecule (dk = 2.6 Å) and the hydrated salt ions (e.g., Na+: dk = 7.2 Å and Cl−: dk = 6.6 Å) [10,11], thus allowing the separation of water and salt. However, due to the amorphous nature of the silica material, when exposed to water the silica matrix may undergo dissolution and/or densification . This is a major problem for using silica based membranes in desalination as the effect decreases the overall separation performance and ultimately the quality of the desalinated water. Therefore, a concerted effort has been devoted to improving the hydro-stability of these membranes for various industrial applications.
Many recent reviews have been published for membrane desalination and desalination technologies which are both exhaustive and comprehensive [2,4,5,13–16]. Amongst them, polymeric membranes and zeolites have played a major role. Thus, the contribution of this review is to cover recent studies of non-crystalline microporous silica based membranes for desalination and the new strategies focusing on improving hydro-stability and membrane properties for potential water desalination applications.
2. Membrane Processing and Transport Mechanisms for Water Desalination
Water desalination is a process in which fresh water is extracted from aqueous solutions such as seawater, brackish water and brine, which contain dissolved salts and other minerals. For water molecules to diffuse through a membrane, a driving force must be established, otherwise water molecules will remain mixed in the aqueous salt solution. The driving force is associated with concentration, pressure and temperature difference between the feed and permeate sides of the membrane. In the case of RO processes, the water molecules must overcome the osmotic pressure to diffuse through dense polymeric membranes. As the osmotic pressure of typical saline solutions ranges from 0.2 MPa to 3 MPa for brackish water to seawater respectively, RO desalination processes are generally pressure intensive with pressures of between 6 MPa and 8 MPa commonly used for seawater applications . In contrast, MD does not attempt to overcome the osmotic pressure and so does not require a pressurised feed, although being a thermal process the water flux is proportional to the vapour pressure difference across the membrane. MD generally uses porous hydrophobic membranes, where pore size ranges between 1 µm and 100 Å, and the water vapour permeating via the pores is subsequently condensed downstream to produce fresh water . MD operates at lower temperatures (up to 70 °C) when compared to conventional thermal process such as multi-stage flash or multi-effect distillation. Finally, the PV process, when applied to desalination, employs molecular-sieving (dp = 3–5 Å) ceramic membranes with a narrow pore distribution smaller than the diameter of the hydrated salt ions (>6 Å). Therefore they have the potential to completely reject salt ions while permitting water molecules to permeate. MD and PV are similar processes that can be chiefly identified by the way in which the membrane functions. If the membrane is simply a support structure that allows a meniscus to form on the feed side and plays no role in separation then the process is MD. If however, the membrane actively participates in the separation process then the process is PV. To further provide clarity between these three membrane processes, Figure 1 shows a diagram as comparison of RO, MD and PV in desalination processes.
Figure 1. Schematic representation of transport mechanism through a membrane via
(A) reverse osmosis, (B) membrane distillation and (C) pervaporation for seawater desalination .
PV is a well-established water separation technique particularly in alcohol dehydration, although under those circumstances dense polymeric membranes are typically employed . In PV separation processes, the transport resistance is governed by the sorption equilibrium and mobility of water molecules in the silica membrane based on a molecular sieving mechanism [3,15,16,19,20]. Therefore, the transport of the larger hydrated salt ions is excluded through the membrane . In a typical PV process, the membrane acts as a molecular scale selective barrier between the two phases which consist of the liquid phase in the feed and the vapour phase in the permeate side. In order to create a driving force, vacuum is applied on the permeate side of the membrane while the feed side is kept at atmospheric pressure and temperature. The water molecules permeate through the membrane to the exclusion of the salt ions, evaporate on the permeate side and are then convectively transported to the condenser. Fundamentally, the condenser functions to reduce the water vapour pressure on the permeate side by changing the water phase from vapour to liquid. This function allows for a steady state driving force to be maintained throughout the PV operation.
Similar to MD, PV can operate in several different arrangements. The most common MD operational arrangements have been well reviewed elsewhere . The most common PV arrangements are shown in Figure 2 to provide context and include: (i) vacuum; (ii) air gap and (iii) sweep flow. PV can operate using any setup that allows a vapour pressure gradient to form but does not allow the permeate to flow back into the feed.
Figure 2. Pervaporation (PV) processes in various operational arrangements.
The PV process variables that are commonly investigated include temperature, pressure, total dissolved solids concentration and the ionic strength of the feed solution. The effect of these variables on water transport through the membrane is measured by two important factors which determine the overall membrane performance: (1) flux of the water and (2) selectivity or rejection of the salt ions. The permeate water is captured in a condenser and the flux (kg m−2 h−1), F, of water during a given period of time is calculated using Equation (1):
where M is the permeate mass (kg), S is the membrane surface area (m2) and t is the testing time (h). The salt rejection (%), R, of the membrane is determined by using Equation (2):
R = (Cf - Cp/ Cf) ×100%
where Cf and Cp are the salt concentrations in the feed and permeate solutions, respectively, measured from solution conductivity. Both of the equations are used prolifically in the literature to provide comparison measure for the overall membrane performance in both MD and PV experiments for water desalination. Based on the theory of MD and PV, the salt rejection should equate to 100% since the salt ions will not vapourise under the typical testing conditions. Instead they will crystallize on the inner surface of the membrane on the permeate side if they also find passage across the membrane. There are several reasons that this could occur, but for silica-based membranes this is primarily the result of imperfections in the top layer as a result of poor membrane preparation or silica disintegration in the aqueous environment. Therefore, several research groups have taken this into account by flushing the permeate salt when determining the overall salt rejection [19,22].
As previously alluded to, amorphous silica membranes present an interesting classification problem for membrane desalination technologies, because despite being porous, the water transport through the membrane cannot be described as a conventional MD. One of the major reasons is that in PV using silica based membranes, the pore sizes are too small to effectively form a meniscus associated with a liquid surface tension as it is the case in MD processes. In this case, the Kelvin equation for the liquid-vapour equilibrium is not applicable, as the pure liquid saturation pressure above a convex liquid surface is essentially the same as the pressure above a flat surface. In other words, the pressure of the water molecules at the pore entrance is possibly the same as in the feed bulk liquid phase (i.e., hydrostatic pressure). Having said that, silica based membranes for PV desalination cannot truly be described as activated transport either, as is the case for these membranes in gas separation . Increasing feed bulk liquid pressure results in almost no water flux changes  as expected because changing the bulk feed pressure has a negligible effect on the vapour pressure of the feed; yet changing the vapour pressure of the feed, by increasing its temperature, delivers water flux improvements. Hence, in this case PV closely complies with Darcy’s law (N = K ΔP°) where the water flux (N) is proportional to the water vapour pressure (ΔP°) and coefficient K, which are in turn temperature dependent. Silica derived membranes are hydrophilic materials and the water transport can be described by a sorption-diffusion mechanism. In the case of silica-based membranes for PV, water molecules must preferentially access the pore entrances of the silica matrix to permeate through the membrane, a surface adsorption process. Hence, the water transport can be summarised in four successive steps, namely, (1) selective surface adsorption from the bulk liquid mixture, (2) selective access of water to the pore entrance at the membrane interface on the feed side, (3) diffusion of water from the feed side to the permeate side, (4) desorption of water into vapour phase at the membrane interface of the permeate side. Therefore, the physico-chemical properties of the silica membranes as well as their interaction with the water molecules are equally influential.
3. Features of Silica Based Membranes for Desalination
3.1. Features of Silica Based Membranes for Desalination
Amorphous silica materials that can be tailored to pore sizes in the range of 3–5 Å are highly suitable for selective membranes in water desalination applications. Several techniques have been widely developed to effectively control the pore size of silica derived membranes, including sol-gel methods [24–31] and chemical vapour deposition (CVD) [32–35]. Although remarkable progress in gas separation applications have been reported using both methods, to date only silica membranes derived via sol-gel processes have been investigated for desalination applications. One of the major reasons is that the sol-gel method is one of the most simple and cost effective routes, which still offers the flexibility to tailor the required porosity. Traditionally, the sol-gel method is a wet chemical process to fabricate metal oxide powders starting from a chemical solution which acts as a precursor for an integrated network (gel). This method is frequently adopted in membrane synthesis or membrane pore modification due to its controllability and homogeneity [24,30,36–38], and it includes various steps such as sol preparation, gel formation, drying and thermal treatment. Many types of silicon alkoxide precursors have been utilized, but the clear majority of research describes work using tetraethoxysilane (TEOS) [39–41]. The sol gel synthesis has been well described in a variety of reference materials , and so briefly it involves the hydrolysis (Equation (3)) and condensation reactions (Equations (4) and (5)) of a metal alkoxides to form a network. In the hydrolysis reaction, the alkoxide groups (OR, where R is an alkyl group, CxH2x+1) are replaced with hydroxyl groups (OH). The silanol groups (Si-OH) are subsequently involved in the condensation reaction producing siloxane bonds (Si-O-Si), alcohols (R-OH) and water. The desired microporous structure of the silica layer is thus partially determined by both the reactivity and the size of the precursors, but also by the appropriate selection of the precursor, water, alcohol and catalyst concentrations.
|≡Si-OR + H2O ↔ ≡Si-OH + ROH||(Hydrolysis)||(3)|
|≡Si-OR + HO-Si≡ ↔ ≡Si-O-Si≡ + ROH||(Alcohol condensation)||(4)|
|≡Si-OH + HO-Si≡ ↔ ≡Si-O-Si≡ + H2O||(Water condensation)||(5)|
Hydrolysis and condensation reactions are commonly catalysed by the use of a mineral base or acid. In the case of a silicon alkoxide, acidic conditions usually produce sols with fractal-like structures which have been shown to be more favorable for the formation of microporous silica with smaller pore sizes . Indeed, when the fractal dimension of those species is low enough, their interpenetration is not restricted during the gelation stage, which gives rise to the formation of weakly-branched structures with small pores . By contrast, basic conditions will otherwise favour the production of highly branched fractal structures and/or colloidal particles. This leads to the production of networks with larger pore sizes and is generally not used to prepare molecular sieveing silica membranes.
3.2. Membrane Preparation
Silica membranes are ultra-thin films (~250 nm) that are traditionally prepared on top of a support for mechanical strength to form an asymmetric structure (as depicted in Figure 3). The support quality plays a major role in the final morphology of the silica derived films as its homogeneity is fundamental in preparing thin films without defects. To achieve this aim, the substrate must have (i) small pore sizes, (ii) low surface roughness and (iii) low defect or void concentration . Substrates with large pores, voids and rough surfaces tend to induce mechanical stress in the films resulting in micro-cracks or pin-hole defects. In order to overcome support roughness, interlayers with smaller pores sizes are typically employed. According to the literature, only a few combinations of support and interlayers have been explored for silica-based membranes for PV desalination. Indeed, supports prepared from α-Al2O3 powders are currently the substrates of choice due to their high porosity and relatively low cost and high mechanical stability. Mesoporous γ-Al2O3, consisting of much smaller pore sizes of ~4 nm are as used in 2 μm thick intermediate layers, and are able to minimize the defect rate observed . However, γ-Al2O3 exhibits low hydrothermal stability , which is of concern if these materials are to be used in applications containing water vapour. Alternate intermediate layers include silica-zirconia composites developed by Tsuru and co-workers [46,47], which are typically more hydro-stable than γ-Al2O3 layers.
The coating of the substrate (or support) using the sol-gel process can be carried out by dip coating, spin coating and the pendulum method. Due to its flexibility to coat both flat and tubular geometries, in addition to small or large substrates, dip coating has been the preferred process to prepare silica based membranes. Scriven  extensively reviewed the dip coating process and proposed five stages: immersion, start-up, deposition, drainage and evaporation. Upon immersion of a substrate to a silica sol, the sol starts adhering to the surface of the substrate. During the withdrawal step, the sol deposits on the surface of the substrate leading to drainage of excess liquid and evaporation of the sol to forming a gel on the support surface. Brinker et al.  proposed that there is a sequential order of structural development that results from drainage accompanied by solvent evaporation, continued condensation reactions and capillary collapse. According to Brinker et al.  the concentration of the deposited film increases 18–36 fold due to evaporation. This causes the formed film to undergo very fast gelation and drying, thus suggesting structural reorganization of the film matrix.
Figure 3. SEM micrograph of the cross-section of a high quality asymmetric membrane structure—Reproduced by permission of The Royal Society of Chemistry (http://dx.doi.org/10.1039/B924327E) .
The withdrawal speed of the substrate from a sol, in addition to the viscosity of the sol, plays an important role in determining the silica thin film formation. Generally, withdrawal speeds reported by several research groups vary between 1 and 20 cm min−1, whilst prepared sols are diluted with ethanol up to 20 times the original sol volume. In this case (low withdrawal speed and low viscosity), the thickness of a film (h) is proportional to U2/3 (where U is the product of the viscosity and withdrawal speed), in accordance to the Landau and Levich equation . Hence, increasing the speed of withdrawal in the dip coating process will yield thicker films and vice versa. As the production of thicker films tends to lead to cracking upon evaporation and gelation, thinner sols of low viscosity with low withdrawal speeds are preferred.
Upon film coating, the membranes are calcined at high temperatures, generally up to 600 °C, in order to fix the silica structure. Higher temperatures tend to densify the silica film, resulting in extremely low fluxes. The calcination process can lead to thermal stresses between the substrate and the thin silica film, possibly causing film cracking and defects. Hence the heating ramp rate is of considerable importance and is typically low at around 1 °C min−1, although recent developments in rapid thermal processing for silica membranes in other applications are challenging this long held view [53,54]. As the thickness of the silica films are generally in the region of 30–50 nm, and possibly a single film may contain defects caused by either inhomogeneity in the support or interlayers, or calcination stresses, or environmental dust; the dip coating and calcination process is generally repeated at least 2–3 times to produce high quality membranes. As environmental dust affects thin film formation, de Vos and Verweij  demonstrated that the quality of silica membranes was greatly improved by simply coating in a clean room environment.
4. Novel Silica Based Membranes in Desalination
4.1. Hydro-stability and Current Strategies
Owing to the affinity of amorphous silica for water adsorption, silica derived membranes undergo structural degradation when exposed to water, leading to a loss of selectivity . Briefly, silica surface materials are prone to rehydration via a mechanism of physisorption of H2O molecules on silanol groups (Si-OH), followed by reaction with a nearby siloxane (chemisorption) [57,58]. As a result, H2O assists the breakage of siloxane groups, allowing for dissociative chemisorption via the hydrolysis reaction (the reverse of Equation (5)) . Therefore, hydrolysed surface siloxanes may become strained, which act as strong acid–base sites, having a rapid uptake of water and becoming mobile . As the silica seeks to reduce its surface energy under hydrothermal conditions , Duke and co-workers  proposed that the mobile and strained hydrolysed siloxane groups migrate to smaller pores where they undergo re-condensation to block the pore, whilst the larger pores become even larger. Hydro-stability is therefore a serious problem for the deployment of silica based membranes for water desalination. To address this problem, researchers have attempted to modify the surface properties of the silica, to minimize the interaction of water molecules with the membrane structure. A summary of the main strategies employed is displayed in Figure 4.
One strategy to solve this challenging problem is introducing non-covalently bonded, organic templates into the pure silica matrix [62–64]. Indeed, the presence of carbon moieties embedded into the silica framework can prevent the mobility of soluble silica groups under hydrolytic attack and consequently inhibits micropore collapse. This was demonstrated by Duke et al.  who successfully prepared carbonized-template molecular sieve silica membranes (CTMSS) by introducing the ionic surfactant (C6 hexyltriethyl ammonium bromide) during the silica sol synthesis. The carbon moieties trapped in the CTMSS matrix were formed by carbonization of the surfactant under vacuum or an inert atmosphere, leading to a hybrid silica/carbon membrane. Although CTMSS membranes still retained their hydrophilic properties, the resultant membranes showed great potential for attaining hydro-stability without compromising the selectivity for wet gas separation . Based on this approach, CTMSS membranes were subsequently tested for desalination performance, demonstrating high salt rejection from seawater .
In a similar study, Wijaya et al.  investigated the effect of the carbon chain length of ionic surfactants in CTMSS membranes for desalination by preparing sol-gels with hexyltriethyl ammonium bromide (C6), dodecyltrimethyl ammonium bromide (C12) and hexadecyltrimethyl ammonium bromide (C16). It was found that the CTMSS membrane prepared with the surfactant with the longest carbon chain (C16) delivered the highest salt rejection, whilst also given the largest pore volume and surface area, although interestingly, the average pore sizes were similar for the three surfactants used. These results suggest that the embedded carbon has a beneficial role in silica matrices and the amount embedded has a direct impact in terms of desalination performance, since the carbon content of the added surfactant is directly related to the amount of carbon remaining following carbonization. However, if the concentration of ionic surfactants is too high they form micelles  which drastically limits the possibility of using the sol-gel to dip coat substrates. In order to increase the carbon content in the silica framework, Ladewig et al.  proposed the use of a non-ionic surfactant such as a tri-block copolymer like polyethylene glycol–polypropylene glycol–polyethylene glycol (PEG-PPG-PEG), a high molecular weight polymer. Silica samples were mixed with 1–20 wt % PEG-PPG-PEG, and increasing the loading of the tri-block copolymer to 10 wt % effectively doubled the pore volume and surface area compared to pure silica, whilst still maintaining microporosity. Further increases in tri-block copolymer loading to 20 wt % altered the structure of the CTMSS materials to produce mesopores. Of greatest relevance to both the preceding studies and future research directions, the CTMSS membranes prepared with 10 wt % PEG-PPG-PEG (i.e., the highest carbon content sample, whilst still remaining microporous) also delivered high salt rejections and water fluxes.
Figure 4. Schematic representation of various strategies for silica modification.
Another approach to increase the hydrothermal stability of the pure silica membrane is by incorporating terminal methyl groups (≡Si-CH3) via various precursors used during the sol-gel synthesis (Figure 5). This was firstly reported by de Vos et al.  who synthesized methylated silica membranes derived by the copolymerization of TEOS and methyltriethoxysilane (MTES) in the presence of ethanol and water, with an acid catalysis. Again, the membranes were calcined under a non-oxidising environment to retain the carbon moieties in the silica matrix. These membranes showed remarkable stability for alcohol dehydration for 18 months, though severe degradation occurred at testing temperatures of ≥95 °C thereafter . Although the addition of methyl ligand groups to silica rendered hydrophobicity, the counter effect was the formation of larger micropores. Duke et al.  investigated the effect of both methyl ligand and non-ligand C6 surfactant as templates in silica membranes for desalination. They found that the CTMSS membrane outperformed the methylated-silica membrane, suggesting that carbonizing the C6 surfactants led to the formation of smaller pores than the covalently attached methyl groups.
Figure 5. Precursors used for the preparation of pure (TEOS), methylated (MTES) and hybrids (BTESE) silica membranes.
Following on from the methyl ligand work, significant hydrothermal improvement can be achieved when the siloxane bridges (Si-O-Si) are partially replaced by organic bridges (Si-CH2-CH2-Si) such as BTESE in Figure 5. In this method, alkyl groups (ethylene groups in Figure 5) between Si atoms, which cannot be hydrolyzed, can be used as a “spacer” to control the silica network size while minimizing the hydrophilicity of the silica pore surface. The sol synthesis for such membrane layers was first developed by Castricum et al.  and consisted of a two-step acid hydrolysis of BTESE/MTES mixtures. In this work they showed that the durability of the membrane network for the dehydration of n-butanol by PV was greatly improved by incorporating hydrolytically stable organic groups as integral bridging components into the nanoporous silica. These hybrid organosilica membranes were able to withstand long-term PV operation of up to 2 years at 150 °C. Recently, Tsuru et al. reported the potential of such BTESE membranes in RO and PV desalination processes .
Alternate efforts have focused on modifying the silica structure through the addition of metal oxides [73–77]. Recently Lin et al.  reported for the first time the potential of cobalt oxide silica (CoOxSi) membranes for desalination of waters from brackish to brine concentrations. CoOxSi xerogels were synthesised via the sol-gel method using TEOS, cobalt nitrate hexahydrate and hydrogen peroxide, at a range of pH from 3 to 6. The pH was altered by addition of ammonia during the sol-gel process. Initial hydrothermal exposure (<2 days) at 75 °C of xerogels resulted in the reduction of pore volume and surface area, although subsequent exposure proved that the pore structure of the xerogels was no longer significantly altered. The CoOxSi synthesized at pH 5 was the most resistant to the hydrothermal degradation, remaining stable and delivering high salt rejections for 570 hours of testing at temperatures up to 75 °C and NaCl salt concentrations up to 15 wt %.
4.2. Membrane Performance: Effect of Testing Conditions
A summary of the reported membranes performance in term of water flux and salt rejection is listed in Table 1. It must be stressed that comparing these results gives an indication of the general performance only. One should be aware of that these results are dependent upon several parameters related to testing condition including feed concentration, salt used, feed temperature, feed flow rate, cross-flow velocity, permeate vapour pressure and fouling/scaling tendencies. In addition, these listed membranes may have different geometries (flat or tubular and sizes) and architecture (thickness of top film, number interlayers number, porosity and substrate). As such, all these factors play a role in the final performance of the tested membranes.
a Feed pressurizing up to 7 bar and permeate vacuum pumping; b Permeate vacuum pumping, resulting in a pressure difference ΔP across the membrane less than 1bar; * Sea water.
The majority of membranes listed in Table 1 were tested for feed synthetic solutions containing NaCl dissolved in deionised water with concentrations ranging from 0.3 to 3.5 wt % in order to simulate the typical salt concentration of brackish water (0.3–1 wt %) and sea water (3.5 wt %). CTMSS membranes gave similar water fluxes varying from 1.4 to 6.3 kg m−2 h−1 with high salt rejections greater than 84%, depending on the operating conditions. Hybrid membranes (i.e., those prepared with terminal methyl groups or covalently bound carbon bridges) also gave similar water fluxes and salt rejections. The hybrid membranes prepared with BTESE delivered considerable high water fluxes at 34 kg m−2 h−1 at 90 °C and excellent salt rejection 99.9%. However, these membranes were tested at very low salt concentration (NaCl 0.2 wt %) and high feed temperature and when cooler feed temperatures (30 °C) were used, the water fluxes reduced considerably (one order of magnitude). In the only study of its kind so far, CoOxSi based silica membranes were also investigated for brine
processing conditions where the salt concentrations ranged from 7.5 to 15 wt %. In this case, an increase in salt concentration in the feed from 0.3 to 15 wt % resulted in a decline of the permeate flux from 1.8 to 0.55 kg m−2 h−1 at 75 °C. However, despite the high salt feed concentrations, the salt rejection remained high suggesting they were stable under these harsh testing conditions.
Analyzing the results reported in Table 1, the trends are very clear with increasing temperature yielding increased water flux whilst increasing salt concentration results in decreasing water flux. For instance, at 0.3 wt % salt feed concentration, the water flux increased by 77% (from 0.4 to 1.8 kg m−2 h−1) as the feed temperature was raised from 20 °C to 75 °C. This can be explained through the thermodynamics of the system in that as the temperature increases, so does the water vapour pressure in the feed stream, leading to an increase in the driving force for water permeation across the membrane. Likewise the water vapour pressure decreases as a function of the salt concentration, partially explaining the decreased flux observed under seawater and brine feed concentrations. However, the water vapour pressure change as a function of the salt concentration at constant temperature is not large enough to justify the large reduction of flux as reported by several groups in Table 1. For instance, in the case of carbonized template CTMSS (ionic C6), experiment was conducted at a fixed temperature of 20 °C . Indeed, water flux was reduced by more than half (56%) by increasing the feed concentration from 0.3 wt % to 3.5 wt %. In that case, the change in vapour pressure driving force of an ideal salt solution will change from 2.3 kPa to 2.28 kPa, representing a decrease of 0.08%  far smaller than the decline in flux, thus demonstrating that salt and temperature polarization are also likely occurring. In this case a boundary layer of more concentrated salt forms at the membrane surface due to the permeation of water through the membrane being faster than the diffusion for fresh water from the bulk to the membrane surface. Likewise thermal boundary layers can form through the conduction and convection of sensible heat and the transfer of latent heat through the vapourisation of water through the membrane. This phenomenon, along with temperature polarization, is commonly observed for MD processes. Interestingly, temperature polarization, whereby the heat flow across the membrane from conduction and convection is sufficient to reduce the temperature at the membrane surface in comparison to the bulk feed, is the more commonly reported problem . The fact that salt concentration polarization is strongest suggest that (a) the silica-based membrane is more insulating than typical polymeric MD counterparts, and that (b) the cross flow velocities investigated were not sufficient to disturb or reduce the mass transfer boundary layer.
The purity of the water in the permeate stream is a fundamental parameter in terms of potable water. As the salt rejection is generally a ratio of salinities (Equation (2)), a high salt rejection for a high feed salt stream does not necessarily translating into potable water. According to the World Health Organization portable water should have a factor called total dissolved solids (TDS) < 600 ppm with an upper limit of TDS < 1000 ppm . To assess the performance of the membranes in Table 1 in terms of water quality, the permeate water concentration was calculated as shown in Figure 6. All the membranes listed in Table 1 produce good quality drinking water (TDS < 600 ppm) for slightly saline water conditions (0.3 wt %). However, only the CoOxSi silica base membranes were able to meet the requirement of 600 ppm for seawater and brine feed conditions. For those membranes with TDS in excess of 600 or 1000, a second pass becomes necessary to achieve potable water requirements. As discussed previously the theory of PV operation necessitates that the permeate stream should be free of salt, regardless of the feed conditions.
The observation that the vast majority of silica-based membranes tested under PV desalination conditions do not give pure water in the permeate stream is strong evidence that research focusing on improving the hydro-stability of the silica as well as the integrity of the membrane layer itself should continue to receive high priority. However, only a handful of authors have reported preliminary stability measurements as listed in Table 1. In the longest performance evaluation reported so far, Lin et al. sequentially tested cobalt oxide silica (CoOxSi) membranes with solutions containing salt at 1 wt % (288 h), 3.5 wt % (144 h), 7.5wt % (72 h) and 15 wt % (72 h), leading to a total of 575 hours . Despite a significant variation in water flux observed during the first 120 h, the water flux tended to stabilize after 5 days of measurement. This was attributed to initial textural and/or structural changes in the CoOxSi matrix and was also observed in nitrogen sorption and FTIR analyses. However, this long term testing successfully demonstrated the improved hydro-stability of CoOxSi membranes at several temperature points and feed concentrations. In the only other studies reported thus far, Duke et al. reported stable performance over 5 h of the CTMSS (Ionic 6) membrane ; and Ladewig et al. showed stable performance over 12 h, suggesting the benefit of the carbonized templating method to improve the hydro-stability of amorphous silica membranes .
Figure 6. Comparison of water quality in the permeate stream.
4.3. Future Challenges
Silica based membranes for desalination applications are still at the embryonic stages of research and development. Therefore, this type of membrane requires significant improvements to be able to compete against both alternate membranes and alternate technologies. Indeed, the RO process using polymeric membranes is now a mature technology, having undergone major research, development and deployment in the last 30 years. This developmental advantage implies that RO will continue to dominate the large desalination plants around the world. However, RO cannot process all feed concentrations, in particular the pressure requirements for brine processing are prohibitive and can even destroy the polymeric membranes. Thus silica based membranes (especially metal oxide silica membranes, such as CoOxSi) operating under PV conditions, could have a niche market in the processing of brines or even the processing or drying of mineral salts such as potash or lithium brines.
In order to be able to compete against polymeric RO membranes, the water fluxes of silica based membranes for processing seawater (NaCl 3.5 wt %) must be significantly increased, by an order of magnitude on average. At the moment high water fluxes in excess of 20 kg m−2 h−1 have been demonstrated for BTESE silica membranes only, although only for slightly saline feed concentrations (NaCl 0.2 wt %) and high temperatures at 90 °C. This raises the second major impediment to silica based membrane PV, the issue of temperature, and ultimately energy consumption. PV is a thermal process and raising the temperature of feed translates into higher vapour pressures which should likewise increase water flux and water production. The problem here is that heat must be generated to increase the temperature of the water (and ultimately vapourise it), which together with the energy required to condense the water vapour explains why the PV process uses more energy per liter of water produced than RO processes which use only pump energy to pressurize the saline water feed. If this heat is supplied through conventional means, the cost will be prohibitive. However, there are several options available to reduce the cost of energy by utilising waste heat from industrial sites and thermal power plants, salt gradient solar ponds or solar heat [80–84]. These options may be attractive to deploy PV using silica based membranes.
A vital aspect of any membrane technology is long term operation and stability. At the moment, CoOxSi silica membranes have demonstrated stability up to 575 hours of operation. Similar tests must also be undertaken for CTMSS and hybrid silica based membranes to show proof of concept. To some extent, the CoOxSi silica membranes showed superior performance than MFI zeolites, which may be viewed as a competing membrane technology. In a recent study, Dobrek and co-workers  reported the dissolution of both S-1 and ZSM-5 top layers in MFI zeolite membranes after 560 hours testing in PV desalination. This was attributed to the combined effects of ion exchange and water dissolution mechanisms. The loss of membrane performance due to the quality of the saline waters can therefore cause deterioration of the materials such as in zeolite membranes, or fouling and scaling as is the case of polymeric membranes . Currently, there is no fouling work reported for silica based membranes mainly due to the embryonic nature of the testing which has occurred under laboratory conditions using synthetic salt solutions. Given the scale of the problem for RO membranes, this is a problem that will require substantial research to ensure that silica based membranes can be deployed in an industrial context to process saline waters to potable quality.
Microporous silica based membranes have been shown to provide excellent molecular sieving properties for gas separation applications but their reported use in water treatment processes, such as desalination, have been limited, primarily due to the lack of stability when exposed to water. However, innovative concepts have been developed in the last two decades to realize the potential of silica based membranes for desalination via PV. In particular, research into silica based membrane desalination has focussed on three distinct methods of stabilising the structure including carbon templated silica, hybrid organic-inorganic silica and metal oxide silica. Whilst these methods have all been successfully trialed for desalination via PV, only metal oxide silica membranes have demonstrated significant potential with high salt rejections under all feed concentrations, reasonable fluxes and unaltered performance for over 575 hours of operation. Indeed they were the only membranes capable of producing potable water from highly concentrated brine feed streams. The target areas of research for membrane scientists is therefore on the materials development to further improve water fluxes (in order to compete with RO processes), to stabilize the silica structure to ensure no reductions in long term performance and to produce defect-free membranes to ensure high salt rejections, at low cost. The final challenge for the membrane research community is to establish the conditions under which PV desalination using silica based membranes is most technically and economically viable. The energy requirements of PV systems are considerable in comparison to RO processes and analysis of the thermodynamics indicates that parity will never be reached when utilizing primary energy sources. However, if PV processes are successfully integrated with waste heat or solar heat sources then the technology may be attractive for niche applications such as brine processing or salt recovery. Regardless, the separation and purification of potable water from desalination is a paramount task which the membrane research community must endeavour to address before water supply becomes a global crisis.
Muthia Elma specially thanks for the scholarship provided by the University of Queensland. The authors acknowledge financial support from the Australian Research Council (DP110101185). Simon Smart also acknowledges funding support from the Australian Research Council (DP110103440).
- World Health Organization and United Nations International Children’s Fund. Progress on Sanitation and Drinking-Water in Joint Monitoring Programme for Water Supply and Sanitation; World Health Organization: Geneva, Switzerland, 2010.
- Lee, K.P.; Arnot, T.C.; Mattia, D. A review of reverse osmosis membrane materials for desalination-development to date and future potential. J. Membr. Sci. 2011, 370, 1–22.
- Duke, M.C.; O’Brien-Abraham, J.; Milne, N.; Zhu, B.; Lin, J.Y.S.; Diniz da Costa, J.C. Seawater desalination performance of mfi type membranes made by secondary growth. Sep. Purif. Technol. 2009, 68, 343–350.
- Greenlee, L.F.; Lawler, D.F.; Freeman, B.D.; Marrot, B.; Moulin, P. Reverse osmosis desalination: Water sources, technology, and today’s challenges. Water Res. 2009, 43, 2317–2348.
- Li, D.; Wang, H. Recent developments in reverse osmosis desalination membranes. J. Mater.
Chem. 2010, 20, 4551–4566.
- Igi, R.; Yoshioka, T.; Ikuhara, Y.H.; Iwamoto, Y.; Tsuru, T. Characterization of co-doped silica for improved hydrothermal stability and application to hydrogen separation membranes at high temperatures. J. Am. Ceram. Soc. 2008, 91, 2975–2981.
- Tsuru, T.; Igi, R.; Kanezashi, M.; Yoshioka, T.; Fujisaki, S.; Iwamoto, Y. Permeation properties of hydrogen and water vapor through porous silica membranes at high temperatures. AIChE J. 2011, 57, 618–629.
- Uhlmann, D.; Smart, S.; Diniz da Costa, J.C. H2S stability and separation performance of cobalt oxide silica membranes. J. Membr. Sci. 2011, 380, 48–54.
- Yacou, C.; Smart, S.; Diniz da Costa, J.C. Long term performance cobalt oxide silica membrane module for high temperature H2 separation. Energy Environ. Sci. 2012, 5, 5820–5832.
- Li, L.X.; Dong, J.H.; Nenoff, T.M.; Lee, R. Reverse osmosis of ionic aqueous solutions on a mfi zeolite membrane. Desalination 2004, 170, 309–316.
- Lin, J.; Murad, S. A computer simulation study of the separation of aqueous solutions using thin zeolite membranes. Mol. Phys. 2001, 99, 1175–1181.
- Leboda, R.; Mendyk, E. Hydrothermal modification of porous structure of silica adsorbents.
Mater. Chem. Phys. 1991, 27, 189–212.
- Wang, L.K.; Chen, J.P.; Hung, Y.-T.; Shammas, N.K. Handbook of Environmental Engineering: Membrane and Desalination Technology; Humana Press: New York, NY, USA, 2011; Volume 13.
- Elimelech, M.; Phillip, W.A. The future of seawater desalination: Energy, technology, and the environment. Science 2011, 333, 712–717.
- Pina, M.P.; Mallada, R.; Arruebo, M.; Urbiztondo, M.; Navascués, N.; de la Iglesia, O.; Santamaria, J. Zeolite films and membranes. Emerging applications. Microporous Mesoporous Mater. 2011, 144, 19–27.
- Yang, W.; Li, Y. Zeolite membranes. In Inorganic Membranes for Energy and Environmental Applications; Bose, A.C., Ed.; Springer: New York, NY, USA, 2009; pp. 275–286.
- Lawson, K.W.; Lloyd, D.R. Membrane distillation. J. Membr. Sci. 1997, 124, 1–25.
- Bolto, B.; Hoang, M.; Xie, Z. A review of membrane selection for the dehydration of aqueous ethanol by pervaporation. Chem. Eng. Process. Process Intensif. 2011, 50, 227–235.
- Duke, M.C.; Mee, S.; Diniz da Costa, J.C. Performance of porous inorganic membranes in non-osmotic desalination. Water Res. 2007, 41, 3998–4004.
- Wee, S.L.; Tye, C.T.; Bhatia, S. Membrane separation process-pervaporation through zeolite membrane. Sep. Purif. Technol. 2008, 63, 500–516.
- Lin, C.X.C.; Ding, L.P.; Smart, S.; Diniz da Costa, J.C. Cobalt oxide silica membranes for desalination. J. Colloid Interface Sci. 2012, 368, 70–76.
- Drobek, M.; Motuzas, J.; Julbe, A.; Ding, L.; Yacou, C.; Diniz da Costa, J.C. Long term pervaporation desalination of tubular mfi zeolite membranes. J. Membr. Sci. 2012, 415–416, 816–823.
- De Lange, R.S.A.; Keizer, K.; Burggraaf, A.J. Analysis and theory of gas transport in microporous sol-gel derived ceramic membranes. J. Membr. Sci. 1995, 104, 81–100.
- Basile, A.; Gallucci, F. Membranes for Membrane Reactors; John Wiley and Sons: Singapore, 2011; Volume 1.
- Kesmez, Ö.; Burunkaya, E.; Kiraz, N.; Çamurlu, H.E.; Asiltürk, M.; Arpaç, E. Effect of acid, water and alcohol ratios on sol-gel preparation of antireflective amorphous SiO2 coatings. J. Non-Cryst. Solids 2011, 357, 3130–3135.
- Klein, L.C. Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes; William Andrew Publishing/Noyes: Park Ridge, NJ, USA, 1988.
- Kreiter, R. Hydrothermal stability of silica-based membranes. In Proceedings of Sol-Gel Conference, Porto de Galinhas, Brasil, 22–27 August 2009; Energy Reasearch Center of the Netherland: Porto de Galinhas, Brasil,. 2009.
- Kreiter, R.; Rietkerk, M.; Bonekamp, B.; van Veen, H.; Kessler, V.; Vente, J. Sol-gel routes for microporous zirconia and titania membranes. J. Sol Gel Sci. Technol. 2008, 48, 203–211.
- Lange, R.S.A.; Kumar, K.N.P.; Hekkink, J.H.A.; Velde, G.M.H.; Keizer, K.; Burggraaf, A.J.; Dokter, W.H.; Garderen, H.F.; Beelen, T.P.M. Microporous SiO2 and SiO2/MOx (M = Ti, Zr, Al) for ceramic membrane applications: A microstructural study of the sol-stage and the consolidated state. J. Sol Gel Sci. Technol. 1994, 2, 489–495.
- Livage, J.; Sanchez, C. Sol-gel chemistry. J. Non Cryst. Solids 1992, 145, 11–19.
- Nair, B.N.; Yamaguchi, T.; Okubo, T.; Suematsu, H.; Keizer, K.; Nakao, S.-I. Sol-gel synthesis of molecular sieving silica membranes. J. Membr. Sci. 1997, 135, 237–243.
- Favennec, L.; Jousseaume, V.; Rouessac, V.; Fusalba, F.; Durand, J.; Passemard, G. Porous extreme low κ (Elκ) dielectrics using a PECVD porogen approach. Mater. Sci. Semicond. Process. 2004, 7, 277–282.
- Khatib, S.J.; Oyama, S.T.; de Souza, K.R.; Noronha, F.B. Chapter 2—Review of silica membranes for hydrogen separation prepared by chemical vapor deposition. In Membrane Science and Technology; Oyama, S.T.; Susan, M.S.-W., Eds.; Elsevier: New York, NY, USA, 2011; Volume 14, pp. 25–60.
- Nomura, M.; Aida, H.; Gopalakrishnan, S.; Sugawara, T.; Nakao, S.-I.; Yamazaki, S.; Inada, T.; Iwamoto, Y. Steam stability of a silica membrane prepared by counterdiffusion chemical vapor deposition. Desalination 2006, 193, 1–7.
- Nomura, M.; Ono, K.; Gopalakrishnan, S.; Sugawara, T.; Nakao, S.-I. Preparation of a stable silica membrane by a counter diffusion chemical vapor deposition method. J. Membr. Sci. 2005, 251, 151–158.
- Wojcik, A.B.; Klein, L.C. Transparent inorganic/organic copolymers by the sol-gel process: Copolymers of tetraethyl orthosilicate (TEOS), vinyl triethoxysilane (VTES) and (meth)acrylate monomers. J. Sol Gel Sci. Technol. 1995, 4, 57–66.
- Hench, L.L.; West, J.K. The sol-gel process. Chem. Rev. 1990, 90, 33–72.
- Brinker, C.J.; Scherer, G.W. Sol-gel-glass: I. Gelation and gel structure. Non Cryst. Solids 1985,
- Boonstra, A.H.; Baken, J.M.E. Relation between the acidity and reactivity of a teos, ethanol and water mixture. J. Non Cryst. Solids 1990, 122, 171–182.
- Chang, S.Y.; Ring, T.A. Map of gel times for three phase region tetraethoxysilane, ethanol and water. J. Non Cryst. Solids 1992, 147–148, 56–61.
- Estella, J.; Echeverría, J.C.; Laguna, M.; Garrido, J.J. Silica xerogels of tailored porosity as support matrix for optical chemical sensors. Simultaneous effect of ph, ethanol:Teos and water:Teos molar ratios, and synthesis temperature on gelation time, and textural and structural properties. J. Non Cryst. Solids 2007, 353, 286–294.
- Brinker, C.J.; Scherer, G.W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, MA, USA, 1990.
- Bonekamp, B.C. Preparation of asymmetric ceramic membrane supports by dip-coating.
In Fundamentals of Inorganic Membrane Science and Technology; Burggraaf, A.J., Cot, L., Eds.; Elsevier: Amsterdam; The Netherlands, 1996; pp. 141–226.
- Bonekamp, B.C.; van Horssen, A.; Correia, L.A.; Vente, J.F.; Haije, W.G. Macroporous support coatings for molecular separation membranes having a minimum defect density. J. Membr. Sci. 2006, 278, 349–356.
- Van Gestel, T.; Vandecasteele, C.; Buekenhoudt, A.; Dotremont, C.; Luyten, J.; van der Bruggen, B.; Maes, G. Corrosion properties of alumina and titania NF membranes. J. Membr. Sci. 2003, 214, 21–29.
- Tsuru, T.; Izumi, S.; Yoshioka, T.; Asaeda, M. Temperature effect on transport performance by inorganic nanofiltration membranes. AIChE J. 2000, 46, 565–574.
- Tsuru, T.; Wada, S.I.; Izumi, S.; Asaeda, M. Silica-zirconia membranes for nanofiltration.
J. Membr. Sci. 1998, 149, 127–135.
- Scriven, L.E. Physics and applications of dip coating and spin coating. In Proceedings of Better Ceramics through Chemistry III, Reno, NV, USA, 5–8 April 1988; Brinker, C.J., Clark, D.E., Ulrich, D.R., Society., M.R., Eds.; Materials Research Society: Pittsburgh, PA, USA, 1988; pp. 717–729.
- Brinker, C.J.; Hurd, A.J.; Schunk, P.R.; Frye, G.C.; Ashley, C.S. Review of sol-gel thin film formation. J. Non Cryst. Solids 1992, 147, 424–436.
- Brinker, C.J.; Clark, D.E.; Ulrich, D.R.; Society, M.R. Symposium on Better Ceramics through Chemistry III; Proceedings of Better Ceramics through Chemistry III, Reno, NV, USA, 5–8 April 1988;. Materials Research Society: Pittsburgh, PA, USA, 1988; pp. 1–847.
- Smart, S.; Lin, C.X.C.; Ding, L.; Thambimuthu, K.; Diniz da Costa, J.C., Ceramic membranes for gas processing in coal gasification. Energy and Environmental Science 2010, 3, 268-278.
- Brinker, C.J.; Scherer, G.W. Sol-gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, MA, USA, 1990; pp. 787–838.
- Kappert, E.J.; Nijmeijer, A.; Benes, N.E. Expeditious calcination of inorganic membranes by an instant temperature increment. Microporous Mesoporous Mater. 2012, 151, 211–215.
- Schillo, M.C.; Park, I.S.; Chiu, W.V.; Verweij, H. Rapid thermal processing of inorganic membranes. J. Membr. Sci. 2010, 362, 127–133.
- De Vos, R.M.; Verweij, H. Improved performance of silica membranes for gas separation.
J. Membr. Sci. 1998, 143, 37–51.
- Giessler, S.; Jordan, L.; Diniz da Costa, J.C.; Lu, G.Q. Performance of hydrophobic and hydrophilic silica membrane reactors for the water gas shift reaction. Sep. Purif. Technol. 2003, 32, 255–264.
- Iler, R.K. Chemistry of Silica; Wiley: New York, NY, USA, 1979.
- Young, G.J. Interaction of water vapor with silica surfaces. J. Colloid Sci. 1958, 13, 67–85.
- Burneau, A.E.; Gallas, J.-P. Vibrational Spectroscopies—Hydroxyl groups on silica surfaces In The Surface Properties of Silicas; Legrand, A.P., Ed.; John Wiley: Chichester, NY, USA, 1998; pp. 147–234.
- Duke, M.C.; Diniz da Costa, J.C.; Do, D.D.; Gray, P.G.; Lu, G.Q. Hydrothermally robust molecular sieve silica for wet gas separation. Adv. Funct. Mater. 2006, 16, 1215–1220.
- Fournier, R.O.; Rowe, J.J. Solubility of amorphous silica in water at high-temperatures and high-pressures. Am. Mineral. 1977, 62, 1052–1056.
- Raman, N.K.; Brinker, C.J. Organic “template” approach to molecular sieving silica membranes.
J. Membr. Sci. 1995, 105, 273–279.
- Xomeritakis, G.; Naik, S.; Braunbarth, C.M.; Cornelius, C.J.; Pardey, R.; Brinker, C.J. Organic-templated silica membranes: I. Gas and vapor transport properties. J. Membr. Sci. 2003, 215, 225–233.
- Giessler, S.; Diniz da Costa, J.C.; Lu, G.Q. Hydrophobicity of templated silica xerogels for molecular sieving applications. J. Nanosci. Nanotechnol. 2001, 1, 331–336.
- Duke, M.C.; Diniz da Costa, J.C.; Lu, G.Q.; Petch, M.; Gray, P. Carbonised template molecular sieve silica membranes in fuel processing systems: Permeation, hydrostability and regeneration. J. Membr. Sci. 2004, 241, 325–333.
- Wijaya, S.; Duke, M.C.; Diniz da Costa, J.C. Carbonised template silica membranes for desalination. Desalination 2009, 236, 291–298.
- Raman, N.K.; Anderson, M.T.; Brinker, C.J. Template-based approaches to the preparation of amorphous, nanoporous silicas. Chem. Mater. 1996, 8, 1682–1701.
- Ladewig, B.P.; Tan, Y.H.; Lin, C.X.C.; Ladewig, K.; Diniz da Costa, J.C.; Smart, S. Preparation, characterization and performance of templated silica membranes in non-osmotic desalination. Materials 2011, 4, 845–856.
- De Vos, R.M.; Maier, W.F.; Verweij, H. Hydrophobic silica membranes for gas separation.
J. Membr. Sci. 1999, 158, 277–288.
- Campaniello, J.; Engelen, C.W.R.; Haije, W.G.; Pex, P.P.A.C.; Vente, J.F. Long-term pervaporation performance of microporous methylated silica membranes. Chem. Commun. 2004, 10, 834–835.
- Castricum, H.L.; Sah, A.; Kreiter, R.; Blank, D.H.A.; Vente, J.F.; ten Elshof, J.E. Hydrothermally stable molecular separation membranes from organically linked silica. J. Mater. Chem. 2008, 18, 2150–2158.
- Xu, R.; Wang, J.; Kanezashi, M.; Yoshioka, T.; Tsuru, T. Development of robust organosilica membranes for reverse osmosis. Langmuir 2011, 27, 13996–13999.
- Battersby, S.; Smart, S.; Ladewig, B.; Liu, S.; Duke, M.C.; Rudolph, V.; Diniz da Costa, J.C. Hydrothermal stability of cobalt silica membranes in a water gas shift membrane reactor. Sep. Purif. Technol. 2009, 66, 299–305.
- Kanezashi, M.; Asaeda, M. Stability of H2-permeselective Ni-doped silica membranes in steam at high temperature. J. Chem. Eng. Jpn. 2005, 38, 908–912.
- Kanezashi, M.; Asaeda, M. Hydrogen permeation characteristics and stability of Ni-doped silica membranes in steam at high temperature. J. Membr. Sci. 2006, 271, 86–93.
- Uhlmann, D.; Liu, S.; Ladewig, B.P.; Diniz da Costa, J.C. Cobalt-doped silica membranes for gas separation. J. Membr. Sci. 2009, 326, 316–321.
- Ikuhara, Y.H.; Mori, H.; Saito, T.; Iwamoto, Y. High-temperature hydrogen adsorption properties of precursor-derived nickel nanoparticle-dispersed amorphous silica. J. Am. Ceram. Soc. 2007, 90, 546–552.
- Sparrow, B.S. Empirical equations for the thermodynamic properties of aqueous sodium chloride.
Desalination 2003, 159, 161–170.
- Zhang, J.; Li, J.-D.; Duke, M.; Xie, Z.; Gray, S. Performance of asymmetric hollow fibre membranes in membrane distillation under various configurations and vacuum enhancement. J. Membr. Sci. 2010, 362, 517–528.
- Banat, F.; Jwaied, N. Economic evaluation of desalination by small-scale autonomous solar-powered membrane distillation units. Desalination 2008, 220, 566–573.
- Blanco Gálvez, J.; García-Rodríguez, L.; Martín-Mateos, I. Seawater desalination by an innovative solar-powered membrane distillation system: The medesol project. Desalination 2009, 246, 567–576.
- Blanco, J.; Malato, S.; Fernández-Ibañez, P.; Alarcón, D.; Gernjak, W.; Maldonado, M.I. Review of feasible solar energy applications to water processes. Renew. Sustain. Energy Rev. 2009, 13, 1437–1445.
- Chen, T.-C.; Ho, C.-D. Immediate assisted solar direct contact membrane distillation in saline water desalination. J. Membr. Sci. 2010, 358, 122–130.
- Mericq, J.-P.; Laborie, S.; Cabassud, C. Evaluation of systems coupling vacuum membrane distillation and solar energy for seawater desalination. Chem. Eng. J. 2011, 166, 596–606.
- Tang, C.Y.; Chong, T.H.; Fane, A.G. Colloidal interactions and fouling of nf and ro membranes: A review. Adv. Colloid Interface Sci. 2011, 164, 126–143.
Muthia Elma, Christelle Yacou, David K. Wang, Simon Smart and João C. Diniz da Costa *
Films and Inorganic Membrane Laboratory, School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia; E-Mails: email@example.com (M.E.); firstname.lastname@example.org (C.Y.); email@example.com (D.K.W.); firstname.lastname@example.org (S.S.)
* Author to whom correspondence should be addressed; E-Mail: email@example.com; Tel.: +62-7-33656960; Fax: +62-7-33654199.
© 2012 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).
TweetDesalination refers to processes that remove some amount of salt and other minerals from saline water. Salt water is desalinated to produce fresh water suitable for human consumption or irrigation. One potential byproduct of desalination is salt. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused […]
Desalination refers to processes that remove some amount of salt and other minerals from saline water.
Salt water is desalinated to produce fresh water suitable for human consumption or irrigation. One potential byproduct of desalination is salt. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use. Along with recycled wastewater, this is one of the few rainfall-independent water sources.
Large-scale desalination typically uses large amounts of energy and specialized, expensive infrastructure, making it more expensive than fresh water from conventional sources, such as rivers or groundwater.
Desalination is particularly relevant to countries such as Australia, which traditionally have relied on collecting rainfall behind dams to provide their drinking water supplies.
According to the International Desalination Association, in 2009, 14,451 desalination plants operated worldwide, producing 59.9e6 cubic meters (2.12×109 cu ft) per day, a year-on-year increase of 12.3%. It was 68 million m3 in 2010, and expected to hit 120 million m3 by 2020; some 40 million m3 is planned for the Middle East. The world’s largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates.
Schematic of a multistage flash desalinator
A – steam in
B – seawater in
C – potable water out
D – waste out
E – steam out
F – heat exchange
G – condensation collection
H – brine heater
Plan of a typical reverse osmosis desalination plant
The traditional process used in these operations is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. This is because the boiling of a liquid occurs when the vapor pressure equals the ambient pressure and vapor pressure increases with temperature. Thus, because of the reduced temperature, energy is saved. Multistage flash distillation, a leading method, accounted for 85% of production worldwide in 2004.
The principal competing processes use membranes to desalinate, principally applying reverse osmosis technology. Membrane processes use semipermeable membranes and pressure to separate salts from water. Reverse osmosis plant membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. Desalination remains energy intensive, however, and future costs will continue to depend on the price of both energy and desalination technology.
Cogeneration is the process of using excess heat from power production to accomplish another task. For desalination, cogeneration is the production of potable water from seawater or brackish groundwater in an integrated, or “dual-purpose”, facility in which a power plant becomes the source of energy for desalination. Alternatively, the facility’s energy production may be dedicated to the production of potable water (a stand-alone facility), or excess energy may be produced and incorporated into the energy grid (a true cogeneration facility). Cogeneration takes various forms, and theoretically any form of energy production could be used. However, the majority of current and planned cogeneration desalination plants use either fossil fuels or nuclear power as their source of energy. Most plants are located in the Middle East or North Africa, which use their petroleum resources to offset limited water resources. The advantage of dual-purpose facilities is they can be more efficient in energy consumption, thus making desalination a more viable option for drinking water.
The Shevchenko BN350, a nuclear-heated desalination unit
In a December 26, 2007, opinion column in The Atlanta Journal-Constitution, Nolan Hertel, a professor of nuclear and radiological engineering at Georgia Tech, wrote, “… nuclear reactors can be used … to produce large amounts of potable water. The process is already in use in a number of places around the world, from India to Japan and Russia. Eight nuclear reactors coupled to desalination plants are operating in Japan alone … nuclear desalination plants could be a source of large amounts of potable water transported by pipelines hundreds of miles inland…”
Additionally, the current trend in dual-purpose facilities is hybrid configurations, in which the permeate from a reverse osmosis desalination component is mixed with distillate from thermal desalination. Basically, two or more desalination processes are combined along with power production. Such facilities have already been implemented in Saudi Arabia at Jeddah and Yanbu.
A typical aircraft carrier in the US military uses nuclear power to desalinate 400,000 US gallons (1,500,000 l; 330,000 imp gal) of water per day.
Factors that determine the costs for desalination include capacity and type of facility, location, feed water, labor, energy, financing, and concentrate disposal. Desalination stills now control pressure, temperature and brine concentrations to optimize efficiency. Nuclear-powered desalination might be economical on a large scale.
While noting costs are falling, and generally positive about the technology for affluent areas in proximity to oceans, a 2004 study argued, “Desalinated water may be a solution for some water-stress regions, but not for places that are poor, deep in the interior of a continent, or at high elevation. Unfortunately, that includes some of the places with biggest water problems.”, and, “Indeed, one needs to lift the water by 2,000 meters (6,600 ft), or transport it over more than 1,600 kilometers (990 mi) to get transport costs equal to the desalination costs. Thus, it may be more economical to transport fresh water from somewhere else than to desalinate it. In places far from the sea, like New Delhi, or in high places, like Mexico City, high transport costs would add to the high desalination costs. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh and Harare. In many places, the dominant cost is desalination, not transport; the process would therefore be relatively less expensive in places like Beijing, Bangkok, Zaragoza, Phoenix, and, of course, coastal cities like Tripoli.” After being desalinated at Jubail, Saudi Arabia, water is pumped 200 miles (320 km) inland through a pipeline to the capital city of Riyadh. For coastal cities, desalination is increasingly viewed as an untapped and unlimited water source.
In Israel as of 2005, desalinating water costs US$ 0.53 per cubic meter. As of 2006, Singapore was desalinating water for US$ 0.49 per cubic meter. The city of Perth began operating a reverse osmosis seawater desalination plant in 2006, and the Western Australian government announced a second plant will be built to serve the city’s needs. A desalination plant is now operating in Australia’s largest city, Sydney, and the Wonthaggi desalination plant was under construction in Wonthaggi, Victoria.
The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm. A wind farm at Bungendore in New South Wales was purpose-built to generate enough renewable energy to offset the Sydney plant’s energy use, mitigating concerns about harmful greenhouse gas emissions, a common argument used against seawater desalination.
In December 2007, the South Australian government announced it would build a seawater desalination plant for the city of Adelaide, Australia, located at Port Stanvac. The desalination plant was to be funded by raising water rates to achieve full cost recovery. An online, unscientific poll showed nearly 60% of votes cast were in favor of raising water rates to pay for desalination.
A January 17, 2008, article in the Wall Street Journal stated, “In November, Connecticut-based Poseidon Resources Corp. won a key regulatory approval to build the $300 million water-desalination plant in Carlsbad, north of San Diego. The facility would produce 50,000,000 US gallons (190,000,000 l; 42,000,000 imp gal) of drinking water per day, enough to supply about 100,000 homes … Improved technology has cut the cost of desalination in half in the past decade, making it more competitive … Poseidon plans to sell the water for about $950 per acre-foot [1,200 cubic metres (42,000 cu ft)]. That compares with an average [of] $700 an acre-foot [1200 m³] that local agencies now pay for water.” Each $1,000 per acre-foot works out to $3.06 for 1,000 gallons, or $.81 per cubic meter.
While this regulatory hurdle was met, Poseidon Resources is not able to break ground until the final approval of a mitigation project for the damage done to marine life through the intake pipe is received, as required by California law. Poseidon Resources has made progress in Carlsbad, despite an unsuccessful attempt to complete construction of Tampa Bay Desal, a desalination plant in Tampa Bay, FL, in 2001. The Board of Directors of Tampa Bay Water was forced to buy Tampa Bay Desal from Poseidon Resources in 2001 to prevent a third failure of the project. Tampa Bay Water faced five years of engineering problems and operation at 20% capacity to protect marine life, so stuck to reverse osmosis filters prior to fully using this facility in 2007.
In 2008, a San Leandro, California company (Energy Recovery Inc.) was desalinating water for $0.46 per cubic meter.
A Jordanian-born chemical engineering doctoral student at University of Ottawa, Mohammed Rasool Qtaisha, invented a new desalination technology that is alleged to produce between 600% and 700% more water output per square meter of membrane than current technology. General Electric is looking into similar technology, and the U.S. National Science Foundation funded the University of Michigan to study it, as well. Patent issues and details of the technology were unresolved as of 2008.
While desalinating 1,000 US gallons (3,800 l; 830 imp gal) of water can cost as much as $3, the same amount of bottled water costs $7,945.
In the United States, due to a recent court ruling under the Clean Water Act, ocean water intakes are no longer viable without reducing mortality of the life in the ocean, the plankton, fish eggs and fish larvae, by 90%. The alternatives include beach wells to eliminate this concern, but require more energy and higher costs, while limiting output.
All desalination processes produce large quantities of a concentrate, which may be increased in temperature, and contain residues of pretreatment and cleaning chemicals, their reaction byproducts, and heavy metals due to corrosion. Chemical pretreatment and cleaning are a necessity in most desalination plants, which typically includes the treatment against biofouling, scaling, foaming and corrosion in thermal plants, and against biofouling, suspended solids and scale deposits in membrane plants.
To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean, such as the outfall of a wastewater treatment or power plant. While seawater power plant cooling water outfalls are not as fresh as wastewater treatment plant outfalls, salinity is reduced. If the power plant is medium-to-large sized and the desalination plant is not enormous, the power plant’s cooling water flow is likely to be at least several times larger than that of the desalination plant. Another method to reduce the increase in salinity is to mix the brine via a diffuser in a mixing zone. For example, once the pipeline containing the brine reaches the sea floor, it can split into many branches, each releasing brine gradually through small holes along its length. Mixing can be combined with power plant or wastewater plant dilution.
Brine is denser than seawater due to higher solute concentration. The ocean bottom is most at risk because the brine sinks and remains there long enough to damage the ecosystem. Careful reintroduction can minimize this problem. For example, for the desalination plant and ocean outlet structures to be built in Sydney from late 2007, the water authority stated the ocean outlets would be placed in locations at the seabed that will maximize the dispersal of the concentrated seawater, such that it will be indistinguishable beyond between 50 and 75 meters (160 and 246 ft) from the outlets. Typical oceanographic conditions off the coast allow for rapid dilution of the concentrated byproduct, thereby minimizing harm to the environment.
The Kwinana Desalination Plant opened in Perth in 2007. Water there and at Queensland’s Gold Coast Desalination Plant and Sydney’s Kurnell Desalination Plant is withdrawn at only 0.1 meters per second (0.33 ft/s), which is slow enough to let fish escape. The plant provides nearly 140,000 cubic meters (4,900,000 cu ft) of clean water per day.
Alternatives without pollution
Some methods of desalination, particularly in combination with evaporation ponds and solar stills (solar desalination), do not discharge brine. They do not use chemicals in their processes nor the burning of fossil fuels. They do not work with membranes or other critical parts, such as components that include heavy metals, thus do not cause toxic waste (and high maintenance). A new approach that works like a solar still, but on the scale of industrial evaporation ponds is the Integrated Biotectural System. It can be considered “full desalination” because it converts the entire amount of saltwater intake into distilled water. One of the unique advantages of this type of solar-powered desalination is the feasibility for inland operation. Standard advantages also include no air pollution from desalination power plants and no temperature increase of endangered natural water bodies from power plant cooling-water discharge. Another important advantage is the production of sea salt for industrial and other uses. Currently, 50% of the world’s sea salt production still relies on fossil energy sources.
Alternatives to desalination
Increased water conservation and efficiency remain the most cost-effective priorities in areas of the world where there is a large potential to improve the efficiency of water use practices. Wastewater reclamation for irrigation and industrial use provides multiple benefits over desalination. Urban runoff and storm water capture also provide benefits in treating, restoring and recharging groundwater.
A proposed alternative to desalination in the American Southwest is the commercial importation of bulk water from water-rich areas either by very large crude carriers converted to water carriers, or via pipelines. The idea is politically unpopular in Canada, where governments imposed trade barriers to bulk water exports as a result of a claim filed in 1999 under Chapter 11 of the North American Free Trade Agreement (NAFTA) by Sun Belt Water Inc., a company established in 1990 in Santa Barbara, California, to address pressing local needs due to a severe drought in that area.
Experimental techniques and other developments
Many desalination techniques have been researched, with varying degrees of success.
One such process was commercialized by Modern Water PLC using forward osmosis, with a number of plants reported to be in operation.
The US government is working to develop practical solar desalination.
The Passarell process uses reduced atmospheric pressure rather than heat to drive evaporative desalination. The pure water vapor generated by distillation is then compressed and condensed using an advanced compressor. The compression process improves distillation efficiency by creating the reduced pressure in the evaporation chamber. The compressor centrifuges the pure water vapor after it is drawn through a demister (removing residual impurities) causing it to compress against tubes in the collection chamber. The compression of the vapor causes its temperature to increase. The heat generated is transferred to the input water falling in the tubes, causing the water in the tubes to vaporize. Water vapor condenses on the outside of the tubes as product water. By combining several physical processes, Passarell enables most of the system’s energy to be recycled through its subprocesses, namely evaporation, demisting, vapor compression, condensation, and water movement within the system.
Geothermal energy can drive desalination. In most locations, geothermal desalination beats using scarce groundwater or surface water, environmentally and economically.
Nanotube membranes may prove to be effective for water filtration and desalination processes that would require substantially less energy than reverse osmosis.
Biomimetic membranes are another approach.
On June 23, 2008, Siemens Water Technologies announced technology based on applying electric fields that purports to desalinate one cubic meter of water while using only 1.5 kWh of energy. If accurate, this process would consume only one-half the energy of other processes. Currently, Oasis Water, which developed the technology, still uses three times that much energy.
Freeze-thaw desalination uses freezing to remove fresh water from frozen seawater.
In 2009, Lux Research estimated the worldwide desalinated water supply will triple between 2008 and 2020.
Desalination through evaporation and condensation for crops
The Seawater Greenhouse uses natural evaporation and condensation processes inside a greenhouse powered by solar energy to grow crops in arid coastal land.
Low-temperature thermal desalination
Originally stemming from ocean thermal energy conversion research, low-temperature thermal desalination (LTTD) takes advantage of water boiling at low pressures, potentially even at ambient temperature. The system uses vacuum pumps to create a low-pressure, low-temperature environment in which water boils at a temperature gradient of 8–10 °C (46–50 °F) between two volumes of water. Cooling ocean water is supplied from depths of up to 600 meters (2,000 ft). This cold water is pumped through coils to condense the water vapor. The resulting condensate is purified water. LTTD may also take advantage of the temperature gradient available at power plants, where large quantities of warm wastewater are discharged from the plant, reducing the energy input needed to create a temperature gradient.
Experiments were conducted in the US and Japan to test the approach. In Japan, a spray-ﬂash evaporation system was tested by Saga University. In Hawaii, the National Energy Laboratory tested an open-cycle OTEC plant with fresh water and power production using a temperature difference of 20 C° between surface water and water at a depth of around 500 meters (1,600 ft). LTTD was studied by India’s National Institute of Ocean Technology (NIOT) from 2004. Their first LTTD plant opened in 2005 at Kavaratti in the Lakshadweep islands. The plant’s capacity is 100,000 liters (22,000 imp gal; 26,000 US gal)/day, at a capital cost of INR 50 million (€922,000). The plant uses deep water at a temperature of 7 to 15 °C (45 to 59 °F). In 2007, NIOT opened an experimental, floating LTTD plant off the coast of Chennai, with a capacity of 1,000,000 litres (220,000 imp gal; 260,000 US gal)/day. A smaller plant was established in 2009 at the North Chennai Thermal Power Station to prove the LTTD application where power plant cooling water is available.
In October 2009, Saltworks Technologies, a Canadian firm, announced a process that uses solar or other thermal heat to drive an ionic current that removes all sodium and chlorine ions from the water using ion-exchange membranes.
Shared under the Creative Commons Attribution-ShareAlike License, original text and illustrations found here: http://en.wikipedia.org/wiki/Desalination
TweetHow many of these wastewater treatment technologies are you familiar with? What is the most effective combination of processes? How do you measure results? Who’s doing the best wastewater treatment research? Is this the best way? Or can the processes below be recombined, rethought, and retooled into something better? Activated sludge systems Advanced oxidation process […]
How many of these wastewater treatment technologies are you familiar with? What is the most effective combination of processes?
How do you measure results? Who’s doing the best wastewater treatment research?
Is this the best way? Or can the processes below be recombined, rethought, and retooled into something better?
Activated sludge systems
Advanced oxidation process
Aerobic granular reactor
Aerobic treatment system
API oil-water separator
Bioconversion of biomass to mixed alcohol fuels
Coarse bubble diffusers
Dissolved air flotation
Expanded granular sludge bed digestion
Fine bubble diffusers
Flocculation & sedimentation
Lamella clarifier (Inclined Plate Clarifier) 
Microbial fuel cell
NERV (Natural Endogenous Respiration Vessel)
Parallel plate oil-water separator
Rotating biological contactor
Sedimentation (water treatment)
Sequencing batch reactor
Submerged aerated filter
Upflow anaerobic sludge blanket digestion
MyronLMeters.com serves the wastewater treament industry with the finest handheld and inline water quality meters.
Please continue this discussion in our Linkedin Users group here:
or on Facebook here:
Tweet A Book On Desalination Plant Concentrate Management by Nikolay Voutchkov, PE, BCEE of Water Globe Consulting and one of Myron L Meters valued customers This book provides an overview of the alternatives for management of concentrate generated by brackish water and seawater desalination plants, as well as site specific […]
A Book On Desalination Plant Concentrate Management by Nikolay Voutchkov, PE, BCEE of Water Globe Consulting and one of Myron L Meters valued customers
This book provides an overview of the alternatives for management of concentrate generated by brackish water and seawater desalination plants, as well as site specific factors involved in the selection of the most viable alternative for a given project, and the environmental permitting requirements and studies associated with their implementation. The book focuses on widely used alternatives for disposal of concentrate, including discharge to surface water bodies; disposal to the wastewater collection system; deep well injection; land application; evaporation; and zero liquid discharge. Direct discharge through new outfall; discharge through existing wastewater treatment plant outfall; and co-disposal with the cooling water of existing coastal power plant are thoroughly described and design guidance for the use of these concentrate disposal alternatives is presented with engineers and practitioners in the field of desalination in mind. Key advantages, disadvantages, environmental impact issues and possible solutions are presented for each discharge alternative. Easy-to-use graphs depicting construction costs as a function of concentrate flowrate are provided for all key concentrate management alternatives.
Mr. Voutchkov is a registered professional engineer and a board certified environmental engineer (BCEE) by the American Academy of Environmental Engineers. He has over 25 years of experience in planning, environmental review, permitting and implementation of large seawater desalination, water treatment and water reclamation projects in the US and abroad. Mr. Voutchkov has extensive expertise with all phases of seawater desalination project delivery: from conceptual scoping, pilot testing and feasibility analysis; to front-end and detailed project design; environmental review and permitting; contractor procurement; project construction and operations oversight/asset management. Mr. Voutchkov is President of Water Globe Consulting a private company specialized in providing expert advisory services in the field of seawater desalination and reuse. For over 11 years prior to establishing his project advisory firm, Mr. Voutchkov was a Chief Technology Officer and Corporate Technical Director for Poseidon Resources, a private company involved in the development of the largest seawater desalination projects in the USA. In recognition of his outstanding efforts and contribution to the field of seawater desalination, Mr. Voutchkov has received a number of prestigious awards from the International Desalination Association, the International Water Association and the American Academy of Environmental Engineers. He is one of the principal authors of the American Water Works Association s Manual of Water Supply Practices on Reverse Osmosis and Nanofiltration and of the World Health Organization s Guidance for the Health and Environmental Aspects Applicable to Desalination. Mr. Voutchkov has published over 40 technical articles in the field of water and wastewater treatment and reuse, and is co-author of several books and manuals of practice on membrane treatment and desalination. He wrote a book on “Seawater Pretreatment”, which was published by Water Treatment Academy in 2010.
Desalination Plant Concentrate Management
By Nikolay Voutchkov, PE, BCEE
ISBN: 978-974-496-357-4, 181 pages, Hardcover, Published by Technobiz Communications
Ch. 1. Introduction to Concentrate Management
Ch. 2. Desalination Plant Discharge Characterization
2.1. Desalination Plant Waste Streams
2.3. Spent Pretreatment Backwash Water
2.4. Chemical Cleaning Residuals
Ch. 3. Surface Water Discharge of Concentrate
3.1. New Surface Water Discharge
3.2. Potential Environmental Impacts
3.3. Concentrate Treatment Prior to Surface Water Discharge
3.4. Design Guidelines for Surface Water Discharges
3.5. Costs for New Surface Water Discharge
3.6. Case Studies of New Surface Water Discharges
3.7. Co-Disposal with Wastewater Effluent
3.8. Co-Disposal with Power Plant Cooling Water
Ch. 4. Discharge to Sanitary Sewer
4.2. Potential Environmental Impacts
4.3. Effect on Sanitary Sewer Operations
4.4. Effect on Wastewater Treatment Operations
4.5. Effect on Water Reuse
4.6. Design and Configuration Guidelines
4.7. Costs for Sanitary Sewer Discharge
Ch. 5. Deep Well Injection
5.2. Potential Environmental Impacts
5.3. Criteria and Methods for Feasibility Assessment
5.4. Design and Configuration Guidelines
5.5. Injection Well Costs
Ch. 6. Land Application
6.2. Potential Environmental Impacts
6.3. Criteria and Methods for Feasibility Assessment
6.4. Design and Configuration Guidelines
6.5. Land Application Costs
Ch. 7. Evaporation Ponds
7.2. Potential Environmental Impacts
7.3. Criteria and Methods for Feasibility Assessment
7.4. Design and Configuration Guidelines
7.5. Evaporation Pond Costs
Ch. 8. Zero Liquid Discharge Concentrate Disposal Systems
8.2. Potential Environmental Impacts
8.3. Criteria and Methods for Feasibility Assessment
8.4. Design and Configuration Guidelines
8.5. Zero Liquid Discharge Costs
Ch. 9. Beneficial Use of Concentrate
9.1. Technology Overview
9.2. Feasibility of Beneficial Concentrate Use
Ch. 10. Regional Concentrate Management
10.1. Types of Regional Concentrate Management Systems
10.2. Use of Brackish Water Concentrate in SWRO Plants
Ch. 11. Non-Concentrate Residuals Management
11.1. Spent Pretreatment Backwash Water
11.2. Chemical Cleaning Residuals
Ch. 12. Comparison of Concentrate Management Alternatives
12.1. Selection of Concentrate Management Approach
12.3. Environmental Impacts
12.4. Regulatory Acceptance
12.5. Ease of Implementation
12.6. Site Footprint
12.7. Reliability and Operational Constraints
12.8. Energy Use
You can order your book here: http://talloaks.com/Zencart/index.php?main_page=product_info&cPath=1_9&products_id=85