Introduction to Membrane Desalination
Table of Contents:
The idea of producing potable water from seawater was conceived in ancient times, when the crews of merchant ships, cruising across the oceans for long periods, met with formidable challenges of water shortage, while oddly enough, being surrounded by huge amounts of nondrinkable water. This contradiction constituted an incentive for conducting the first experiments that proved that distilled seawater does not carry any salt and is drinkable, and therefore, desalinated.
Desalination can therefore be defined as the process by which salts and minerals are removed from water, producing usable water (potable or for other uses such as agriculture).
The worldwide amount of water is limited and sustained through the water cycle process by which water that comes down as rain finally evaporates and recondenses.
With the amount of available water constant and the world’s population increasing, it is apparent that some parts of the world will become water-stressed.
World population growth, expected to exceed 10 billion by year 2040 (United Nations, DESA Population Division), introduces a major obstacle towards an effective management of water resources. Climate change is another stumbling block, which affects the Earth in ways yet to be discovered, and creates natural hazards - from severe draughts to hurricanes and monsoons - that seem to increase in severity and occurrence.
Moreover, 66% of the global population (approximately 4 billion people) currently live in conditions of severe water scarcity for at least one month per year.1
Additionally according to the UN, nearly 2.4 billion people, who constitute almost one third of the world’s current population, live within 100 km (60 miles) of coastlines, and therefore, seawater desalination seems to be a reasonable solution for supplying fresh water to water-stressed coastal regions.
Water use has been increasing worldwide by about 1% per year since the 1980s, driven by a combination of population growth, socioeconomic development, and changing consumption patterns. Global water demand is expected to continue increasing at a similar rate until 2050, accounting for an increase of 20% to 30% above the current level of water use, mainly due to rising demand in the industrial and domestic sectors. Water-stress levels will continue to increase as demand for water grows and the effects of climate change escalate.2
Studies analyzing 14 years of NASA’s satellite data (Gravity Recovery and Climate Experiment, GRACE) from freshwater resources showed that areas with limited access to water will likely become more stressed than before (Figure l.l).3
Water deficiency often leads to the use of degraded or low-quality deposits that result in public health risks. It is distressing that even nowadays, waterborne diseases such as cholera continue to menace large numbers of people. Estimates show that each year there are 1.3 million to 4.0 million cases of cholera and 21.000 to 143.000 deaths worldwide due to cholera.4Access to clean water will be able to greatly attenuate these numbers. According to the WHO “cholera transmission is closely linked to inadequate access to clean water and sanitation facilities. Typical at-risk areas include peri-urban slums, and camps for internally displaced persons or refugees, where minimum requirements of clean water and sanitation are not met."5
Under the critical conditions of global water scarcity, desalination processes have the potential to become a plentiful and feasible source of fresh water, effectively rising to the challenge of augmented water demands in the near future.
Nowadays, an estimated 15.906 desalination plants are fully operational (Figure 1.2), and are located in 177 countries and territories across all major world regions, producing around 95 million nvVday of desalinated water for human use, of which 48% is produced in the Middle East and North Africa.6
Desalination processes can be classified in two main categories: thermal systems and membrane systems.
In the next two sections, an outline of the most common methods in each category is given.
Thermal Desalination Processes
The main industrially used thermal desalination processes are the following:
FIGURE 1.1 Annotated map of TWS trends. Trends in TWS (in centimeters per year) obtained on the basis of GRACE observations from April 2002 to March 2016. The cause of the trend in each outlined study region is briefly explained and color-coded by category. The trend map was smoothed with a 150-km-radius Gaussian filter for the purpose of visualization; however, all calculations were performed at the native 3° w resolution of the data product.
ISource: Data from M. Rodell et al. Emerging trends in global freshwater availability, Nature, Vol. 557, p. 651, 2018].
FIGURE 1.2 Global distribution of large desalination plants by capacity, feedwater type, and desalination technology, (reproduced by permission from E. Jones ct al. Science of the Tolal Environment 657 (2019) 1343-1356).
Water that is heated enough eventually evaporates, leaving any salt content behind. Condensing this vapor via cooling results in distilled - freshwater production.
A simple still is sufficient to perform the aforementioned process. The heat energy required is the latent heat of evaporation, which is around 627 kWh /nr plus losses, making distillation a very energy demanding process.
Multistage Flash (MSF)
MSF desalination is a thermally driven process that consists of an array of elements, called stages. Each stage is divided into two parts: the heat exchanger and distiller. In each stage, the seawater that flows in the pipes of the heat exchanger is heated by the steam that condenses on the pipes’ surface. At the end, the preheated feed is driven in a brine heater to raise its temperature to nearly the saturation temperature at the maximum system pressure. Afterwards, the feedwater enters the first stage distiller, through an orifice, where there is low pressure. Since the water was at the saturation temperature for a higher pressure, it becomes superheated and flashes into steam. Only a small part of this feed is transformed into water vapor, based on the level of pressure at this stage. The produced vapor passes through a wire mesh (demister) to remove any entrained brine droplets and then through the heat exchanger, where it condenses and drips into a distillate tray. Due to partial evaporation, the temperature of the remaining feed is decreased, so that in the evaporation process to be continued, the pressure of the subsequent stages would be successively lower.
Current commercial installations are designed with 10-30 stages with a typical 2°C -temperature drop per stage. The principle of operation is shown in Figure 1.3.
FIGURE 1.3 Principle of operation of the multistage flash (MSF) system.
(reproduced with permission from Sotcris Л. Kalogirou, in Solar Energy Engineering Elsevier 2014).
Multieffect Distillation (MED)
Similar to the MSF process, desalination takes place in a series of vessels (known as effects), each one operating at a successively lower pressure. The main difference between MED and MSF is that in MED. as seen in Figure 1.4, the preheated seawater is sprayed onto the hot surface of the heat exchanger’s bank of tubes (feed with steam). In each effect, a portion of the feedwater evaporates where the rest of the concentrated seawater is collected at the bottom of the effect. The vapor produced enters the next effect, which operates at a lower pressure than the previous pressure, generating secondary steam. The steam condenses in the tubes and is withdrawn as a product. This operation is repeated along the plant from stage to stage. The primary steam condensate is usually returned to the boiler of the power station since it is of extremely high quality. Up to 8 or 16 effects can be used in this way. MED plants are typically built in units of 2,000 to 10,000 nvVday capacity, with a top temperature (first effect) of about 70°C.
Vapor Compression (VC)
Vapor compression systems are divided into two main categories: mechanical vapor compression (MVC) and thermal vapor compression (TVC) systems. In these plants, the necessary heat for the evaporation of water is supplied by compression of vapor, rather than by direct heat transfer from the steam produced in a boiler. A simplified outline of a VC is depicted in Figure 1.5. The water vapors produced in the distillation chamber (distiller) are drawn by the vacuum line to the suction of the compressor in order to be compressed.
FIGURE 1.4 Principle of operation of the MED system.
(Reproduced from: A. H. M. Saadat. “Desalination Technologies for Developing Countries: Л Review”, J. Sci. Res. 10 (1), 77-97, 2018).
FIGURE 1.5 Basic layout of MVC.
Thereafter, these vapors (as steam) enter the internal of a tube buddle where preheated saline water is sprinkled at the outside surface of these tubes, resulting in a partial evaporation. The partial condensed steam exiting the distiller buddle passes through a heat exchanger preheating the cold feed seawater. As a result, the steam fully condenses and is collected as fresh water. The warm saline feed is further heated by passing through another heat exchanger where it receives energy from the relatively hot discharged brine. Finally, the hot saline feed is mixed with a portion of the brine collected in the bottom of the distillation chamber, while the remaining brine is directed to the saline preheater and finally discharged (brine level in the evaporation chamber must be monitored carefully).
A number of M VC-based desalination plants have been installed worldwide that produce fresh water for industrial and municipal purposes. These plants, however, have the disadvantage of restricted capacity due to scale limitations for large size vapor compressors. Another imperative issue is compressor maintenance resulting from the high possibility of corrosion; consequently, many MVC plants must operate at relatively low temperatures in order to minimize the formation of scaling and corrosion of materials.
It is generally recognized that the work entitled “Sea Water Demineralization by Means of an Osmotic Membrane” by Sidney Loeb and Srinivasa Sourirajan, presented in 19637 was a major starting point for the commercialization of membranes for desalination purposes in the 1970s, which eventually resulted in the dominance of membranes over thermal processes.
Depending on the applied driving force (chemical potential), membrane desalination processes can be grouped as:
Reverse Osmosis (RO)
RO is a membrane filtration process and, in contrast to the previously described thermal desalination processes, does not involve vaporization of water, which in turn, is much more energy-efficient than thermal methods.
RO was introduced in the early 1950s as an alternative process for seawater desalination based on cellulose acetate membranes.8 In 2000, the volumes of desalinated water produced by thermal technologies (dominated by MSF) and RO were approximately equal to 11.6 million nrVday and 11.4 nr/day respectively, together accounting for 93% of the globally produced volume of desalinated water (Figure 1.6a). Since then, both the number and capacity of RO plants has risen exponentially, while thermal technologies have only experienced marginal increases (Figure 1.6b). The current production of desalinated water from reverse osmosis stands at
65.5 million m Vday, accounting for 69% of the volume of desalinated water.
FIGURE 1.6 Trends in global desalination by (a) number and capacity of total and operational desalination facilities and (b) operational capacity by desalination technology through the period 1960-2020.
(reproduced bv permission from E. Jones ct al. Science of the Total Environment 657 (2019) 1343-1356).
Osmosis is a surprisingly powerful phenomenon; the osmotic pressure of typical seawater is around 26 bar, and this is the pressure that the pump must overcome in order to reverse the direction of freshwater flux. In practice, a significantly higher pressure is used, typically 50-70 bar, in order to achieve a generous flow of freshwater, which is the product (permeate), while the rejected seawater is known as concentrate or brine.
The ratio of product flow to that of the feed is defined as the recovery ratio. In seawater RO. a recovery ratio of 30% is typical, meaning that the remaining 70% is the concentrate, which is commonly returned to the sea.
The desalination industry makes a distinction between seawater and brackish water. Seawater typically has a salt concentration in the order of 36,000 mg/L total dissolved solids (TDS), while brackish water, usually from underground, might be between 3,000 and 10,000 mg/L TDS.
Forward Osmosis (FO)
The basic principle of FO is the osmotic pressure gradient between seawater and a draw solution. This solution has greater osmotic pressure than seawater and therefore, water molecules permeate through a membrane to its side. By this process, a seawater concentrate and a diluted draw solution are created using low (if any) hydraulic pressure.
In order to recover fresh water from the draw solution, a posttreatment step such as membrane distillation is necessary. As a whole, FO requires less energy than any RO process.
ED is a process in which solute ions move across membranes by application of an electric field. This electrical potential drives the electrolytes into a concentrated solution, leaving behind a diluted solution. Unlike other desalination technologies, in electrodialysis the salts are removed from the feed- water. The feed water should be free of suspended solids, organic matter, and nonionic contaminants that accumulate in the product. A typical electrodialysis module consists of anion and cation selective membranes that are stacked alternately with spacers interposed between them, as depicted in Figure 1.7. The saltwater is fed into the spacer layers on one side of the stack, and a DC voltage is applied to the stack as a whole. Electrodialysis was introduced as a desalination process during the 1960s and is widely used today for desalination of brackish water. The energy consumption depends very much on the concentration of the feedwater, and for this reason, electrodialysis is rarely used for seawater desalination.
Membrane Distillation (MD)
MD is a fairly new concept that is steadily gaining popularity in the field of desalination research; as the term membrane distillation indicates, MD combines established desalination methods of membrane filtration and heating.
FIGURE 1.7 Representative schematic illustration of a conventional electrodialysis stack
(Reproduced with permission from J. Ran ct al. Journal of Membrane Science 522, pp 267-291, 2017).
A porous hydrophobic membrane is used as a nonselective interface placed between an aqueous heated solution on the one hand (feed or retentate) and a condensing phase (permeate or distillate) on the other. The hydrophobic nature of the membrane, generally polymeric, prevents penetration of the pores by aqueous solutions due to surface tension, and allows the establishment of a vapor - liquid interface at the entrance of each pore. The temperature gradient between the two streams leads to a vapor pressure difference that causes volatile compounds (most commonly water) to evaporate on the hot feed solution - membrane interface, transfer of the vapor phase through the membrane pores, and condensation on the cold side membrane - permeate solution interface.
MEMBRANE PREPARATION TECHNIQUES
Asymmetric membranes are mostly fabricated by a process called phase inversion, which can be achieved through four principal methods: immersion precipitation (wet-casting), vapor-induced phase separation, thermally-induced phase separation, and dry-casting. In all of these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects, and the subsequent phase separation concludes to a polymer-lean and a polymer- rich phase. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, specifies the area where the pores are generated (reproduced with permission from:
Sacide Alsoy Altinkaya & Bulent Ozbas, Journal of Membrane Science Volume 230, Issues 1-2, 15 February 2004, Pages 71-89).9
Thin Film Composite (TFC)
Thin film nanocomposite (TFN) membranes are a new type of composite membranes prepared via interfacial polymerization (IP) processes. Nanoparticles are incorporated within the thin polyamide (PA) dense layer of the TFC membrane with the aim of improving the characteristics of the interfacially polymerized layer as in Figure l .8.
In the 1970s, composite membranes comprising ultrathin polyamide films - formed via in situ polycondensation on porous polysulfone supports - were developed to replace integrally skinned, asymmetric RO membranes - formed by phase inversion of cellulose acetate. A great advantage of TFC technology is that it allows development and successful handling of extremely thin layers of barrier materials formed from almost any conceivable chemical combination . In addition, the ultrathin barrier layer and the porous support can be independently optimized with respect to structure, stability, and performance.
Over the last 30 years, water flux and solute rejection by polyamide TFC membranes have continually improved, but reverse osmosis processes remain relatively energy-intensive and fouling-prone. A lack of significant innovations in RO membrane materials persists despite the pressing needs for desalination membranes with (1) increased water permeability for energy savings, (2) improved control of selectivity in membrane design, and (3) more fouling-resistant surfaces. These constraints remain in the face of rising worldwide demand for clean water and the sustainability imperatives to control energy consumption.10
DESALINATION MEMBRANE SHAPES AND MODULES
In industrial membrane plants, an active membrane area of 100.000 m2 is required to perform effective separations and achieve sufficient water recovery. There are several ways to economically and efficiently package membranes to provide a large surface area for effective separation. From the perspective of
FIGURE 1.8 Crosscut images of commercial TFC desalination membranes, (reproduced with permission from Journal of Membrane Science 318 (2008) 458-466).
overall cost mitigation, both the cost of membranes per unit area and the cost of the containment vessel into which they are mounted, are equally important. Basically, the challenge is to achieve the packing of the highest possible area of membranes into the smaller possible volume to minimize the cost of the containment vessel, along with providing acceptable flow hydrodynamics in the vessel. These packages are called membrane modules. Plate-and-frame, tubular, spiral-wound and hollow fiber are the most popular categories of modules.
These modules are now generally limited to ultrafiltration applications, for which the benefit of resistance to membrane fouling outweighs the high cost. Tubular membranes contain as many as five to seven smaller tubes, each 0.5-1.0 cm in diameter, nested inside a single larger tube. In a typical tubular membrane system, a large number of tubes are manifolded in series. The permeate is removed from each tube and sent to a permeate collection header.
Industrial-scale modules contain several membrane envelopes, each with an area of 1-2 nr, wrapped around the central collection pipe (Figure 1.9). Multienvelope designs minimize the pressure drop encountered by the permeate traveling toward the central pipe. The standard industrial spiral-wound module is 8 in. (20 cm) in diameter and 40 in. (~lm) long.
The module is placed inside a tubular pressure vessel. The feed solution passes across the membrane surface and a portion of the feed permeates into the membrane envelope, where it spirals toward the center and exits through the collection tube. Normally, four to six spiral-wound membrane modules are connected in series inside a single pressure vessel.
FIGURE 1.9 Spiral wound membrane module.
(reproduced with permission from Balstcr J. (2013) Spiral Wound Membrane Module. In: Drioli E., Giorno L. (cds) Encyclopedia of Membranes. Springer, Berlin, Heidelberg).
Hollow-fiber modules (Figure 1.10) are characteristically 4-8 in. (10-20 cm) in diameter and 3-5 ft (1.0-1.6 m) long. Hollow-fiber units are almost always run with the feed stream on the outside of the fiber.
Water passes through the membrane into the inside or lumen of the fiber. A number of hollow fibers are collected together and potted in an epoxy resin at both ends and installed into an outer shell. Hollow-fiber membrane modules are formed in two basic geometries: (a) shell-side feed design and (b) bore-side feed design.
The source of sea water for the desalination process is either from open intake or beach well intake. The source water composition varies due to various factors such as industrial discharge, water depth and temperature, ocean currents, and algae content. The various foulants in the sea water are chemical foulants, which are responsible for membrane scaling; particulate matter which is responsible for particle deposition on membrane surfaces; biological matter, which is responsible for biofouling of membranes; and dissolved organic matter, which interacts with membranes.
Figure 1.11 shows the conventional pretreatment process. The key contaminants to be filtered in the pretreatment stage are particulate/suspended matter, microbial contaminants, and dissolved organic matter.
FIGURE 1.10 Hollow fiber module.
(reproduced with permission from Norfamilabinti Che Mat ct.al. Current Opinion in Chemical Engineering 2014, 4:18-24).
FIGURE 1.11 Conventional pretreatment system for RO plant.
(reproduced with permission from J. Kavitha et al. Journal of Water Process Engineering 32 (2019) 100926).
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