Tuesday, 24 May 2011
Monday, 23 May 2011
How PX Pressure Exchanger Works?
A 3D animated look into the schematic diagram of an RO Process with and without isobaric energy recovery. And an inside look on how the PX Pressure Exchanger works.
ERI - Energy Recovery Turbine
The growth and importance of Seawater Reverse Osmosis (SWRO) desalination worldwide and how ERIs PX Technology is making desalination affordable.
Water Desalination Explained
You've heard the term before, but what exactly is water desalination? Our Planet 100 team explains.
Tuesday, 10 May 2011
Sea water: Our Only Hope for a Drink
Desalination of seawater has become a necessity, but it has to be done right.
As any globe will reveal, there's no shortage of water on Earth. Unfortunately, over 97 percent of it is too salty for us humans to drink, and only a tiny fraction of what remains is in the rivers, lakes, and groundwater that we're able to easily access.In much of the world, these freshwater supplies are growing scarce, and competition for these resources promises to be one of the hot-button geopolitical challenges of the next 50 years and beyond. As climate change worsens droughts, accelerates desertification, and whittles away glaciers (the water towers providing life to so much of the world), it's no wonder that some experts are looking towards that enormous pool of salty water for a drink.
It's not a novel idea. Nearly 50 years ago President John F. Kennedy noted, "If we could ever competitively, at a cheap rate, get fresh water from salt water, that it would be in the long-range interests of humanity which would really dwarf any other scientific accomplishment."
About 2,300 years before Kennedy said that, Aristotle was already experimenting with the idea. Since then, desalination-or the process of removing salts from ocean or brackish water-has been proven possible, and employed in some form for ages. Around 200 AD, sailors boiled seawater and captured the salt-free evaporation when they ran out of drinking water supplies. This "thermal desalination" process can be scaled, but the costs are, for most, prohibitively high; most of the larger-scaled projects that took root were in the oil-rich and water-poor Middle East.
In the past couple of decades, though, a more promising, scalable solution has surfaced-reverse osmosis. Bear with me as I revisit high school chemistry. Take a semi-permeable membrane that water molecules can travel through, but not larger sediments like salt. Put very salty water on one side and less salty water on the other, and water will travel through towards the salty side until the concentrations are even. That's osmosis. Alternately, apply pressure to the saltier side, and water flows through the membrane, but the salt gets stuck. That's reverse osmosis, and the result is fresh water. And that's how most modern day desalination plants work.
Today, there are over 13,000 desalination plants around the world, with a collective capacity to produce about 14
The upfront costs of building the plants are considerable-San Diego's Poseidon Plant is budgeted at $300 million; Melbourne is fixing to spend $2.9 billion on one that'd be amongst the world's largest-but after they're built, the chief expense is the energy it takes to push the seawater through the membranes.
Then there are the environmental costs, which are slowing down the approval processes in regulation-heavy places like California. As ocean water gets sucked into the system, aquatic organisms can get sucked up with it. Then, besides drinking water, there's the other byproduct of the process-very salty, and often hot, brine, which if released straight back into the ocean can create dead zones, worsening a problem already plaguing many coastal cities.
Both these problems can be addressed, albeit at some added expense. Sucking up seawater from beneath the sandy ocean floor avoids capturing unlucky creatures, and letting the brine mix with ocean water for awhile-as the Poseidon project is promising-before discharging it will prevent the dead zones.
But the chief environmental concern is certainly the energy it takes to run the system. There's a perverse logic in burning fossil fuels to make up for a shortage of freshwater-essentially worsening the problem you're trying to solve.
Of course, we can look to the wind and sun to power the desalination process. Offshore wind turbines make a lot of sense for plants that need to be located on the coast. Concentrated solar power could also do the trick. (Here's a study (pdf) that makes a very strong case for CSP powering desalination.)
The tough reality of the world's increasingly dire water crisis means that desalination isn't merely an option, but a necessity. The only sensible way to power these processes-without further contributing to one of the main causes of the freshwater shortages-is to do it without greenhouse gas emissions. Without exception, desalination needs to be coupled with clean energy.
What is Membrane?
How are membranes used?
The small pores of the membranes can serve as a physical barrier, preventing passage of certain materials such as salt, bacteria and viruses while allowing the free passage of water and air. The desalination of water using reverse osmosis is a well known use of membranes as a filter.
Recently, recovery of water from sewage and recovery of whey protein from waste streams during cheese making have been carried out with ultrafiltration and microfiltation membranes which require much less pressure than reverse osmosis. While pressure is be used to drive filtration, electrical current, osmotic pressure, and temperature can also be used to preferentially allow one component in a mixture to pass freely through the membrane while retaining the rest. The membrane structure and chemistry can also serve to carry out other separations.
Membranes provide a high surface area material where chemical reactions or diffusion can take place. For example, bundles of hollow fiber membranes (membranes in a thin tubular form) are used in dialysis to purify the blood by removing certain toxins. Membranes can also be used to carry out solvent extraction and catalysis while also serving to separate the reactants.
Hydrophobic membranes can be used to prevent passage of liquid water but allow vapor to pass (like Goretex). This property has been exploited in membrane distillation where brackish water is heated using solar power and the pure water vapor passes through the membrane and condensed to produce very high quality water. This uses less energy than boiling and utilizes bountiful but low value energy in remote areas.
Types of Reverse Osmosis Membranes
As the pore size gets smaller, more pressure will be needed to start the filtration process. Also surface properties play an important role. For a hydrophobic (water rejecting) surface more pressure will be needed than with a hydrophilic (water attracting) surface.
MF and UF are typical particle filters, whereas NF and RO change the chemical or ionic composition of the water, as for instance the removal of dissolved minerals. See the filtration spectrum above.
Cellulosic Membrane
This is the kind of RO membrane that has been used during the early experiments on RO during the early years of the 1950s. These types of membranes are made up of thin surface layers that are dense.
They are also asymmetric, and they also have a porous structure that is thick. The main purpose of the dense layer is to increase the rejection rate of your membrane and thus the reverse osmosis treatment system while the porous substructure shall provide the strength that the membrane may need.
One of the greatest advantages of the cellulosic membranes compared to other varieties of reverse osmosis membranes is the fact that they are very cheap. They’re also very convenient to install. However, despite these benefits, there are also a number of limitations.
For one, this kind of membrane can easily be compacted, especially if there’s an increase in temperature or in pressure. It’s also very vulnerable to hydrolysis, which means you may only be able to use it at such limited range of pH level, usually between 3 and 8 pH level.
This will depend on the brand or make of your RO membranes. If the temperature goes as high as 35 degrees Celsius, the cellulosic membrane will degrade progressively.
Furthermore, the cellulosic membranes are very vulnerable to attacks of bacteria. This is because they tend to reject poorly the low molecular weight of the contaminants.
Thin Film Composites (TFC)
These kinds of membranes are made up of surface film that is dense and thin. It is commonly placed atop the porous substructure. You can customize the manufacturing process and the construction materials of these varieties so your reverse osmosis membranes will be able to function more effectively.
There are also several kinds of TFC today. These include the polyfurane cyanurate, aromatic polyamide, and alkyl-aryl poly urea.
TFCs are one of the most efficient reverse osmosis membranes available in the market. However, you should also be wary about them, more so when they are exposed to free chlorine. This is because they can actually degrade because of high oxidation levels.
You should also be very consistent when you’re going to maintain the thin composite films. One of the integral parts that need constant monitoring is your carbon prefilter.
It’s essential to your RO membranes as they do not only get rid of the bad smell, taste, and appearance of water, but it can also prevent sediments from moving on to your membranes. When they do, the RO membranes will get damaged easily.
Aromatic Polyamide Membrane
This kind of reverse osmosis membrane has been developed by Dupont. It basically looks like the cellulosic membrane since it’s also asymmetric in structure and is very thin.
However, it is much better than the latter because aromatic polyamide membrane has higher resistance to the occurrence of biological attacks and hydrolysis. It’s also able to sustain sudden rise in temperature, though constant exposure to such condition can damage it forever.
Membrane Technology in General
Membranes can be subdivided according to their filtration properties in 4 categories:
- Micro-Filtration (MF), range 0,1 – 1 micron (1 micron = 1/1000 mm);
- Ultra-Filtration (UF), range 0,001 – 0,1 micron;
- Nano-Filtration (NF), range 0,0001 – 0,001 micron (0,001 micron = 1 nanometer – nm);
- Reverse Osmosis(RO), bereik < 0,0001 micron.
Membrane Filtration
Microfiltration
Ultrafiltration
Nonofiltration
Reverse Osmosis
What is TDS?
What is TDS?
Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.
What Are Total Dissolved Solids?
“Dissolved solids” refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in water other than the pure water (H20) molecule and suspended solids. (Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as wood pulp.)
In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water.
Parts per Million (ppm) is the weight-to-weight ratio of any ion to water.
A TDS meter is based on the electrical conductivity (EC) of water. Pure H20 has virtually zero conductivity. Conductivity is usually about 100 times the total cations or anions expressed as equivalents. TDS is calculated by converting the EC by a factor of 0.5 to 1.0 times the EC, depending upon the levels. Typically, the higher the level of EC, the higher the conversion factor to determine the TDS. NOTE – While a TDS meter is based on conductivity, TDS and conductivity are not the same thing. For more information on this topic, please see our FAQ page.
Where Do Dissolved Solids Come From?
Some dissolved solids come from organic sources such as leaves, silt, plankton, and industrial waste and sewage. Other sources come from runoff from urban areas, road salts used on street during the winter, and fertilizers and pesticides used on lawns and farms.
Dissolved solids also come from inorganic materials such as rocks and air that may contain calcium bicarbonate, nitrogen, iron phosphorous, sulfur, and other minerals. Many of these materials form salts, which are compounds that contain both a metal and a nonmetal. Salts usually dissolve in water forming ions. Ions are particles that have a positive or negative charge.
Water may also pick up metals such as lead or copper as they travel through pipes used to distribute water to consumers.
Note that the efficacy of water purifications systems in removing total dissolved solids will be reduced over time, so it is highly recommended to monitor the quality of a filter or membrane and replace them when required.
Why Should You Measure the TDS Level in Your Water?
The EPA Secondary Regulations advise a maximum contamination level (MCL) of 500mg/liter (500 parts per million (ppm)) for TDS. Numerous water supplies exceed this level. When TDS levels exceed 1000mg/L it is generally considered unfit for human consumption. A high level of TDS is an indicator of potential concerns, and warrants further investigation. Most often, high levels of TDS are caused by the presence of potassium, chlorides and sodium. These ions have little or no short-term effects, but toxic ions (lead arsenic, cadmium, nitrate and others) may also be dissolved in the water.
Even the best water purification systems on the market require monitoring for TDS to ensure the filters and/or membranes are effectively removing unwanted particles and bacteria from your water.
The following are reasons why it is helpful to constantly test for TDS:
High TDS results in undesirable taste which could be salty, bitter, or metallic. It could also indicate the presence of toxic minerals. The EPA’s rescommended maximum level of TDS in water is 500mg/L (500ppm). More …
Test your water to make sure the reverse osmosis or other type of water filter or water purification system has a high rejection rate and know when to change your filter (or membrane) cartridges. More …
High TDS indicates Hard water, which causes scale buildup in pipes and valves, inhibiting performance. More …
A constant level of minerals is necessary for aquatic life. The water in an aquarium or tank should have the same levels of TDS and pH as the fish and reef’s original habitat. More …
TDS is the best measurement of the nutrient concentration in a hydroponic solution. More …
TDS levels must be monitored to prevent maintenance problems. More …
High TDS levels could impede the functions of certain applications, such as boilers and cooling towers, food and water production and more. More …
TDS levels must be controlled prior to making colloidal silver. More …
For a truly great cup of coffee, proper TDS levels must be maintained. More …
Have a washer with a spotless rinse? An inline dual TDS monitor will tell you when to change the filter cartridge or RO membrane. More …
*Chart values represent national U.S. averages. Actual TDS levels for geographic regions within the U.S. and other countries may vary.
Click here for the U.S. EPA’s list of National Secondary Drinking Water Regulations.
The “Glossary of Salt Water” published by the Water Quality Association classifies water as follows:
Fresh: 350 -1,000 ppm TDS
Brackish: 1,000-5,000 ppm TDS
Highly Brackish: 5,000-15,000 ppm TDS
Saline: 15,000-30,000 ppm TDS
Sea Water: 30,000-40,000 ppm TDS
Brine: 40,000-300,000+ ppmTDS
Brackish: 1,000-5,000 ppm TDS
Highly Brackish: 5,000-15,000 ppm TDS
Saline: 15,000-30,000 ppm TDS
Sea Water: 30,000-40,000 ppm TDS
Brine: 40,000-300,000+ ppmTDS
PPM: Parts Per Million
TDS: Total Dissolved Solids
TDS: Total Dissolved Solids
How Do You Reduce or Remove the TDS in Your Water?
Common Water Treatment Systems and Water Purification Systems:
Reverse osmosis works by forcing water under great pressure against a semi-permeable membrane that allows water molecules to pass through while excluding most contaminants. RO is the most thorough method of large-scale water purification available.
Distillation
Distillation involves boiling the water to produce water vapor. The water vapor then rises to a cooled surface where it can condense back into a liquid and be collected. Because the dissolved solids are not normally vaporized, they remain in the boiling solution.
Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity de-ionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.
Saturday, 7 May 2011
What is (R O ) Reverse Osmosis ?
Download Macromedia Flash Player to View Above Animation.
Reverse osmosis (RO) is a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.
Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other substances from the water molecules. This is the reverse of the normal osmosis process, in which the solvent naturally moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The movement of a pure solvent to equalize solute concentrations on each side of a membrane generates a pressure and this is the "osmotic pressure." Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis.
The process is similar to membrane filtration. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure, and water flux rate
History
The process of osmosis through semipermeable membranes was first observed in 1748 by Jean Antoine Nollet. For the following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1949, the University of California at Los Angeles (UCLA) first investigated desalination of seawater using semipermeable membranes. Researchers from both UCLA and the University of Florida successfully produced fresh water from seawater in the mid-1950s, but the flux was too low to be commercially viable[2]. By the end of 2001, about 15,200 desalination plants were in operation or in the planning stages worldwide.[1]
Process
A semipermeable membrane coil used in desalinization.
Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases, the membrane is designed to allow only water to pass through this dense layer, while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure that must be overcome. This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but since the early 1970s it has also been used to purify fresh water for medical, industrial, and domestic applications.
Osmosis describes how solvent moves between two solutions separated by a semipermeable membrane to reduce concentration differences between the solutions. When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential. Osmosis is an example of diffusion.
In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.
Applications
Drinking water purification
Around the world, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.
Such systems typically include a number of steps:
- a sediment filter to trap particles, including rust and calcium carbonate
- optionally, a second sediment filter with smaller pores
- an activated carbon filter to trap organic chemicals and chlorine, which will attack and degrade TFC reverse osmosis membranes
- a reverse osmosis (RO) filter, which is a thin film composite membrane (TFM or TFC)
- optionally, a second carbon filter to capture those chemicals not removed by the RO membrane
- optionally an ultra-violet lamp for disinfecting any microbes that may escape filtering by the reverse osmosis membrane
In some systems, the carbon prefilter is omitted, and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon postfilter is needed to remove chlorine from the final product water.
Portable reverse osmosis (RO) water processors are sold for personal water purification in various locations. To work effectively, the water feeding to these units should best be under some pressure (40 psi or greater is the norm). Portable RO water processors can be used by people who live in rural areas without clean water, far away from the city's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (saline water may need special membranes). Some travelers on long boating, fishing, or island camping trips, or in countries where the local water supply is polluted or substandard, use RO water processors coupled with one or more UV sterilizers. RO systems are also now extensively used by marine aquarium enthusiasts. In the production of bottled mineral water, the water passes through an RO water processor to remove pollutants and microorganisms. In European countries, though, such processing of Natural Mineral Water (as defined by a European Directive citation required) is not allowed under European law. In practice, a fraction of the living bacteria can and do pass through RO membranes through minor imperfections, or bypass the membrane entirely through tiny leaks in surrounding seals. Thus, complete RO systems may include additional water treatment stages that use ultraviolet light or ozone to prevent microbiological contamination.
Membrane pore sizes can vary from 0.1 to 5,000 nanometers (nm) depending on filter type. "Particle filtration" removes particles of 1,000 nm or larger. Microfiltration removes particles of 50 nm or larger. "Ultrafiltration" removes particles of roughly 3 nm or larger. "Nanofiltration" removes particles of 1 nm or larger. Reverse osmosis is in the final category of membrane filtration, "hyperfiltration", and removes particles larger than 0.1 nm.
In the United States military, reverse osmosis water purification units (ROWPUs), are used on the battlefield and in training. Capacities range from 1500 gallons per day (GPD) to 150,000 GPD, depending on the need. The most common of these are the 600 gallons per hour (GPH) and the 3,000 GPH units. Both are able to purify salt water and water contaminated with nuclear/biological/chemical (NBC) contamination from the water. During a normal 24-hour period, one unit can produce 12,000 to 60,000 gallons of water, with a required 4-hour maintenance window to check systems, pumps, RO elements and the engine generator. A single ROWPU can sustain a force the size of a battalion, or roughly 1,000 to 6,000 soldiers.
Water and Waste Water Purification
Rain water collected from storm drains is purified with reverse osmosis water processors and used for landscape irrigation and industrial cooling in Los Angeles and other cities, as a solution to the problem of water shortages.
In industry, reverse osmosis removes minerals from boiler water at power plants. The water is boiled and condensed repeatedly. It must be as pure as possible so that it does not leave deposits on the machinery or cause corrosion. The deposits inside or outside the boiler tubes may result in under-performance of the boiler, bringing down its efficiency and resulting in poor steam production, hence poor power production at turbine.
It is also used to clean effluent and brackish groundwater. The effluent in larger volumes (more than 500 cu. meter per day) should be treated in an effluent treatment plant first, and then the clear effluent is subjected to reverse osmosis system. Treatment cost is reduced significantly and membrane life of the RO system is increased.
The process of reverse osmosis can be used for the production of deionized water.
In 2002, Singapore announced that a process named NEWater would be a significant part of its future water plans. It involves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into the reservoirs.
Food Industry
In addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids (such as fruit juices) than conventional heat-treatment processes. Research has been done on concentration of orange juice and tomato juice. Its advantages include a lower operating cost and the ability to avoid heat-treatment processes, which makes it suitable for heat-sensitive substances like the protein and enzymes found in most food products.
Reverse osmosis is extensively used in the dairy industry for the production of whey protein powders and for the concentration of milk to reduce shipping costs. In whey applications, the whey (liquid remaining after cheese manufacture) is concentrated with RO from 6% total solids to 10-20% total solids before UF (ultrafiltration) processing. The UF retentate can then be used to make various whey powders, including whey protein isolate used in bodybuilding formulations. Additionally, the UF permeate, which contains lactose, is concentrated by RO from 5% total solids to 18–22% total solids to reduce crystallization and drying costs of the lactose powder.
Although use of the process was once avoided in the wine industry, it is now widely understood and used. An estimated 60 reverse osmosis machines were in use in Bordeaux, France in 2002. Known users include many of the elite classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux.
Car Washing
Because of its lower mineral content, reverse osmosis water is often used in car washes during the final vehicle rinse to prevent water spotting on the vehicle. Reverse osmosis is often used to conserve and recycle water within the wash/pre-rinse cycles, especially in drought stricken areas where water conservation is important. Reverse osmosis water also enables the car wash operators to reduce the demands on the vehicle drying equipment, such as air blowers.
Maple Syrup Production
In 1946, some maple syrup producers started using reverse osmosis to remove water from sap before being further boiled down to syrup. The use of reverse osmosis allows approximately 54-42% of the water to be removed from the sap, reducing energy consumption and exposure of the syrup to high temperatures. Microbial contamination and degradation of the membranes has to be monitored.
Hydrogen Production
For small scale production of hydrogen, reverse osmosis is sometimes used to prevent formation of minerals on the surface of electrodes.
Reef Aquariums
Typical RO/DI unit used for an Aquarium
Many reef aquarium keepers use reverse osmosis systems for their artificial mixture of seawater. Ordinary tap water can often contain excessive chlorine, chloramines, copper, nitrogen, phosphates, silicates, or many other chemicals detrimental to the sensitive organisms in a reef environment. Contaminants such as nitrogen compounds and phosphates can lead to excessive, and unwanted, algae growth. An effective combination of both reverse osmosis and deionization (RO/DI) is the most popular among reef aquarium keepers, and is preferred above other water purification processes due to the low cost of ownership and minimal operating costs. Where chlorine and chloramines are found in the water, carbon filtration is needed before the membrane, as the common residential membrane used by reef keepers does not cope with these compounds.
Desalination
Areas that have either no or limited surface water or groundwater may choose to desalinate seawater or brackish water to obtain drinking water. Reverse osmosis is the most common method of desalination, although 85 percent of desalinated water is produced in multistage flash plants.[3]
Large reverse osmosis and multistage flash desalination plants are used in the Middle East, especially Saudi Arabia. The energy requirements of the plants are large, but electricity can be produced relatively cheaply with the abundant oil reserves in the region. The desalination plants are often located adjacent to the power plants, which reduces energy losses in transmission and allows waste heat to be used in the desalination process of multistage flash plants, reducing the amount of energy needed to desalinate the water and providing cooling for the power plant.
Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that has been commercially used since the early 1970s. Its first practical use was demonstrated by Sidney Loeb and Srinivasa Sourirajan from UCLA in Coalinga, California. Because no heating or phase changes are needed, energy requirements are low in comparison to other processes of desalination, but are still much higher than those required for other forms of water supply (including reverse osmosis treatment of wastewater).[citation needed]
The Ashkelon seawater reverse osmosis (SWRO) desalination plant in Israel is the largest in the world.[4][5] The project was developed as a BOT (Build-Operate-Transfer) by a consortium of three international companies: Veolia water, IDE Technologies and Elran.[6]
The typical single-pass SWRO system consists of the following components:
- Intake
- Pretreatment
- High pressure pump
- Membrane assembly
- Remineralisation and pH adjustment
- Disinfection
- Alarm/control panel
Pretreatment
Pretreatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. The material is engineered in such a fashion as to allow only one-way flow through the system. As such, the spiral wound design does not allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems, they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. Pretreatment in SWRO systems has four major components:
- Screening of solids: Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pump components.
- Cartridge filtration: Generally, string-wound polypropylene filters are used to remove particles between 1 - 5 micrometres.
- Dosing: Oxidizing biocides, such as chlorine, are added to kill bacteria, followed by bisulfite dosing to deactivate the chlorine, which can destroy a thin-film composite membrane. There are also biofouling inhibitors, which kill bacteria, but simply prevent them from growing slime on the membrane surface and plant walls.
- Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater result in a scaling tendency when they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acid form.
- CO3-2 + H3O+ = HCO3- + H2O
- HCO3- + H3O+ = H2CO3 + H2O
- Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium carbonate scaling tendency is estimated using the Langelier saturation index. Adding too much sulfuric acid to control carbonate scales may result in calcium sulfate, barium sulfate or strontium sulfate scale formation on the RO membrane.
- Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of all scales compared to acid, which can only prevent formation of calcium carbonate and calcium phosphate scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales, disperse colloids and metal oxides, and specialty products can be to inhibit silica formation.
High pressure pump
The pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 375 psi (15.5 to 26 bar, or 1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 psi (55 to 81.5 bar or 6 to 8 MPa). This requires a large amount of energy.
Membrane Assembly
The layers of a Membrane.
The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. RO membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and hollow-fiber.
Remineralisation and pH adjustment
The desalinated water is very corrosive and is "stabilized" to protect downstream pipelines and storages, usually by adding lime or caustic to prevent corrosion of concrete lined surfaces. Liming material is used to adjust pH between 6.8 and 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control.
Disinfection
Posttreatment consists of stabilizing the water and preparing it for distribution. Desalination processes are very effective barriers to pathogenic organisms; however, disinfection is used to ensure a "safe" water supply. Disinfection (sometimes called germicidal or bactericidal) is employed to sterilise any bacteria, protozoa and viruses that have bypassed the desalination process into the product water. Disinfection may be by means of ultraviolet radiation, using UV lamps directly on the product, or by chlorination or chloramination (chlorine and ammonia). In many countries, either chlorination or chloramination is used to provide a "residual" disinfection agent in the water supply system to protect against infection of the water supply by contamination entering the system.
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