Water is a important resource for survival and growth of life. With the growth of world population the need of fresh water has also increased substantially which has resulted in growth of desalination installation as well. The main reason for water shortage is uneven distribution of rain. Many other reasons are also there. Most of the water available is being polluted. So though there is water, we cannot use it as in the same form. For using the water we have to do some chemical and physical operation on this water. Logically the desalination activities are focus on those parts of the earth where availability of water is scarce.
Desalination of sea water has been used over 60 years and it is a one of the water supply in many countries. It is now feasible, technically and economically to produce large quantities of water of excellent quality from desalination processes. Reverse osmosis is one of the methods of desalination. In this project will mainly research about this method. 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 membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solution is allowed to pass to the other side.
Pretreatment is a key part of every membrane desalination. Pretreatment system is to remove particulate, organic, mineral…etc contained in the seawater and to prevent their accumulation on the downstream seawater reverse osmosis membranes. The content and nature of foulants contained in the source seawater depend on the type and location of the desalination plant intake. The need for water is rapidly increasing, and freshwater resources cannot be provide to all requirements. However, desalination method is the best way to modify this problem.
Background of desalination
Desalination is the wide range of processes design to remove salts from water of different qualities. Desalination technology is in use throughout the world for a wide range of intention, include providing potable fresh water for domestic and municipal purposes, treated water for industrial processes, and emergency water for refugees. The amount of the feed water discharged to waste as a brine discharge change from 20 to 70 percent of the feed flow, depends on the technology employed and the salt content of the water.
Desalination is economically as the technology improves. Desalination plants can be provided in a wide range of outputs to cater for small isolated communities or to contribute substantially to water supplies for large cities and even for irrigation.
Desalination methods
Desalting device is used for separates saline water into two streams: one with a low concentration of dissolved salts (the fresh water stream) and other contain the remaining dissolved salts (the concentrate stream). The device requires energy to operate and can be use a number of different technologies for the separation of the saline water. The desalination methods are listed below:
Thermal Process
Multi Stage Flash Distillation (MSF)
Multiple Effect Distillation (MED)
Vapour Compression Distillation (VC)
Membrane Process
Reverse Osmosis (RO)
Electrodialysis (ED)
Membrane processes have developed very quickly, and most new facilities use reverse osmosis technology. Membrane processes use semi-permeable membranes and pressure to separate salts from water. Membrane systems typically use less energy than thermal distillation, which has a reduction in overall desalination costs. Desalination remains energy intensive, however, costs will continue to depends on the price of both energy and desalination technology.
Desalination Plants
The lack of fresh water in some countries around the world is result shortage of natural resources. Hence, it needs to research new methods in this situation. For example, desalination technology can be providing large amount of fresh water that is appropriate for human, animal consumption and irrigation to use it. Desalination of the seawater has been usually used to solve the lack of water problems for industrial and human uses. Nowadays, about 12,500 desalination plants build in 157 countries. The total capacity of 100 largest desalination plants water about 21 million cubic meters per day (appendix A)
Good operation of RO desalination plants can obtain when maintain good plant performance. The quality and quantity of production will change in during operation due to expect or unexpected operating conditions
Ashkelon desalination plants
Ashkelon is the world largest reverse osmosis desalination plant which is successful and it is providing 330,000m3/d capacity of water in very low cost production. Ashkelon plant is part of desalination master plan created by Israel in 2009 to help solve the water resource problem.
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The desalination is located in the south Israel. It's contributing to supply to south Israeli city. This is about have 15% water consumption in the domestic use.
The function of plant includes raw water pretreatment, seawater pumping, membrane desalination.....etc. Although the plant is connect to electrical grid, the dedicated combined cycle gas turbines power station has setup. Around 56 MW will use for the desalination process. The advanced recovery system and good RO technology to reduce running costs has a competitive price, one of the lowest water prices is about ( $0.53/m3) offer for this plant. About 42% of this price covers energy costs, another 58% is come for operation and maintenance costs, chemicals and membranes costs.
Barcelona desalination plant
The Barcelona desalination plant is the largest in Europe. The plant is located next to Barcelona's commercial port than means will have hundreds of ships enter the port every day. This will cause water pollution by fuel. Therefore, the intake water area needs to build at some distance from these areas. Because its need to collect the clean possible water. The pilot is installed near to the plant that seawater quality to be assessed at intake and discharge area
The specific feature of the Barcelona desalination plants (Triple pretreatment)
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Pretreatment is an important process in reverse osmosis desalination, because it will affects the performance of the membrane systems. In the Barcelona desalination plant, a triple filtration system is design to remove any impurities. It can remove suspended solids, algae, pathogens….etc
The seawater will preliminary purify by flotation. The flocs formed, and then will separate from the water. At first the algae, suspended solids and organic matters will be coagulated. The residual clarification is collect at the surface of the float by overflow. Flotation will finish in the Seadaf. The advantage of this step is reducing the surface area than can be obtained via optimizing the hydraulics system. The water is filtered in Mediazur DM filters, which are double media filters. The water is pressurized before going to into a second double media filter stage with Seaclean filters. Each of the pressure filters can be treat around 900 m3/h flow rate. The filters can provide refined filtration before going into the cartridge filters.
Principle of Reverse osmosis
Reverse osmosis (RO) is a membrane process. The function of RO is remove dissolved minerals, most of the dissolved organic matter, biological and colloidal matter also can be remove from water having concentration from 50 ppm to 60,000 ppm. The application of pressure to overcome osmotic pressure reverses only the flow of solvent but not the direction of flow of solute.
In figure(1) show that, which represent a semi permeable membrane separating pure water and the concentrated solution is pure water pass in opposite direction. This process is natural osmosis. The driving force for the two flows is the difference in chemical potential between the two solutions. The water will go on until the pressure aerated by osmotic head equals-the osmotic pressure of salt solution in figure(2). These two liquids are in equilibrium by applying an external pressure; a saline in figure(3), the flow of solvent may be revised. The reversal of flow has given the process the name Reverse Osmosis.
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Reverse Osmosis : Process
In reverse osmosis, feed water is pumped at high pressure through permeable membranes, which separate salt and other minerals from the water. The pores in the membrane are large enough to allow water molecules to pass through, so too small to allow the passage of salt and other minerals. Reverse osmosis facilities generally involve four main processes: (1) pretreatment, (2) pressurization, (3) membrane separation, and (4) post-treatment stabilization. Pretreatment is use to remove suspended particles from the source water to keep the membrane surfaces clean and to treat the water to avoid growth of microbes on the membranes. The feed water is pressurized. The process that results in most of the energy demand for the reverse osmosis desalination method. The pressurized feed water is force through the reverse osmosis membrane. Product water quality is improved by passing the water through second set membranes. Once the feed water is separated into two streams, the product water is treated to meet drinking water requirements, and then to the water distribution or storage system.
Pretreatment
The incoming feed water is pretreated to be compatible with the membranes by removing suspended solids, adjusting the pH, and adding a threshold inhibitor to control scaling caused by constituents.
Pressurization
The pump raises the pressure of the pretreated feed water to an operating pressure appropriate for the membrane and the salinity of the feed water.
Membrane Separation
The permeable membranes inhibit the passage of dissolved salts while permitting the desalinated product water to pass through. Applying feed water to the membrane assembly results in a freshwater product stream and a concentrated brine reject stream.
Post-treatment
The product water from the membrane assembly usually requires pH adjustment and degasification before being transferred to the distribution system for use as drinking water.
Below figure show that desalination process flow:
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Table().
Number in the figure
Function
1 - Seawater intake
Intake the seawater
2 - Pretreatment
Represses both calcium carbonate and calcium sulfate crystal formation process
Inhibits formation of other potential scalants such as oxides or hydroxides of iron, manganese and silica
Removes turbidity, suspended solids and other materials to prevent scaling or fouling of membrane
3 - Reverse Osmosis desalination
Pretreated seawater is forced under pressure through a semi-ermeable membrane for removal of chemical impurities and solids such as salt
4 - Post-treatment
Addition of carbon dioxide to increase alkalinity
Chlorine is added in the final stage for disinfection.
Stabilize corrosive characteristics of permeate water produced from the reverse osmosis process
5 - Injection
For disposal of the reject brine, off-spec. water and seawater feed bypass
Pretreatment (seawater and Brackish Water)
The objective of any reverse osmosis pretreatment system (seawater or brackish water) is to decrease the fouling propensity of the water in the reverse osmosis membrane system. Surface water resources (seawater and brackish water) typically have a greater propensity for membrane fouling and require more extensive pretreatment systems than groundwater resources. Normally, seawater reverse osmosis tends to use in surface water sources, while brackish water reverse osmosis often use in groundwater sources.
Why Pretreatment?
To decrease membrane fouling, RO systems need sufficient, dependable pretreatment to produce great quality RO feed water that will ensure stable. Ineffective or undependable pretreatment will conduct to problems with the RO system. For examples, high rates of membrane fouling, high frequency of membrane cleanings, lower recovery rates, high operating pressure, poor product quality and reduced membrane life. Above problems have a serious impact on plant productivity and operation costs. So that pretreatment optimization is the key part for a successful RO desalination system.
3.1.1.1. Conventional Pretreatment
Conventional pretreatment systems are consisting of an open seawater intake, acid addition, coagulant/flocculant addition, media filtration, and cartridge filtration. At first chemical additions, including acid, coagulant, and flocculant, prepares the feed water for media filtration. The acid treatment reduces the pH of the feed water (typical pH 5-7), which increases the solubility of calcium carbonate. It is the key potential precipitate in many feed waters. The role of coagulants is to effectively neutralize like charges and allow the suspended solids to organize together in flocs (large groups of loosely bound suspended particles). Therefore, coagulants are typically small, positively charged molecules. The primary objective of coagulation is to change the surface chemistry of the suspended particles so they connect well to media filter. When feed water has a high SDI, flocculation is often used with coagulation before media filtration. The process of flocculation and sedimentation is a well known method of particle removal in water treatment. Media filtration includes materials such as sand, anthracite, pumice, gravel, and so on. A combination of those materials is used in layers in the filtration bed to take advantage of materials different effective sizes. Cartridge filtration is used in last pretreatment step in conventional pretreatment. It acts as a final polishing step to remove larger particles that passed through media filtration.
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3.1.1.2 Membranes Pretreatment
Membrane filtration is often characterized by the pore size of the membrane in use. Typical operating pressure, and membrane configurations, i.e. tubular, spiral wound, hollow fiber. Membrane filtration is the process of removing particulate matter across a physical barrier. Particles larger than the pore opening of the membrane barrier are retained on the membrane surface while clean water and dissolved components pass through. Ultrafiltration membranes provide a positive barrier to particulates and pathogens and protect the RO by physically separating the solids. This ensures a consistent and excellent RO feed flow quality, regardless of raw water turbidity even during storm events and algae blooms and is also considered to be the best choice for pretreatment of seawater that contains colloidal silica.
In seawater RO plants equipped with immersed Ultrafiltration membrane pretreatment, chemical dosing is significantly reduced or eliminated. The seawater is typically supplied from an open intake line, which can be much shorter than for a conventional pretreatment, pre-filtered at least by a mechanical screen.
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Post treatment
Product water from desalination plants is characteristically to low in mineral content, alkalinity, and pH. Therefore, desalinated water must be post-treated before to final distribution and use. Normally, post-treatment of product water includes following processes: 1) Stabilization; 2) Corrosion inhibition; 3) Re-mineralization by blending with high mineral content water; 3) Disinfection; 4) Water quality polishing for enhanced removal of specific compounds. Post-treatment of permeate produce by the desalination system are require for two key reasons: to protect public health and to safeguard the integrity of the water distribution system. In some cases the same post-treatment process and chemicals allow achievement of both objectives. Usually, the ultimate application dosage of any multipurpose chemical is determined by the minimum dosage required to achieve all purposes for which the conditioning chemical is added.
If the use of the same chemical is not found to be cost effective to achieve both the public health and the corrosion protection objectives, than a combination of chemicals that give the lowest overall cost of water production may be use to meet all post treatment objective.
Membrane Fouling
Membrane Fouling, the feed water is depends on water source. It may contain different of suspended solids and dissolved matter. Suspended solids may include biological debris, inorganic particles and colloids. For example: algae and microorganisms. Dissolved matter may include high soluble salts. For example: silica, sparingly soluble salts, sulfates, carbonates and chlorides. In the RO process, as the volume of feed water decreases, the dissolved ions and suspended particles will increases. Sparingly soluble salts may speed up from the concentrate stream, It will occur scale on the membrane surface, so that will result in lower water permeability pass through the RO membranes. Suspended particles maybe stuck on the membrane surface. Therefore, it will block the feed channels and increase friction loss pass thought the system. Above problem will result bad performance of the RO system. The objective of the feed water pretreatment process is improving the quality of the feed water to the level. Therefore, it would result in dependable operation of the RO membranes.
Figure.
Damage is caused by oxidation and hydrolysis of the membrane material because some matter present in the feed water. Most of the membranes cannot afford residual chlorine, this is use in desalination processes to prevent the biological (bacteria) growth on membranes. Typically, it is necessary to use mineral acid for adjusting the pH to recommended values. Beside, the method to prevent membrane damage is using the reducing chemical to remove oxidizing agents and dissolved oxygen.
Biological fouling is caused by bacteria or algae on the membrane. Because chlorine can use on cellulose only. Washing the membrane with biocides is necessary. Since the biomass will produce.
Scaling is the haste and sediment of sparingly soluble salts on the membranes. In operating process, under particular conditions, some of the components in concentrate stream may exceed the solubility limits. The components include: calcium sulphate, magnesium carbonate, silica….etc
Membrane Cleaning
Membrane cleaning is an important part which can extend the membrane's life and keep the efficiency of a membrane process. That means membrane should be clean when the permeate flow drop off, water flux, the desalted water have change and the differential pressure across membrane increase. These can be maintaining quality levels and productivity. If one or more factor had reached unacceptable values, it is need to clean the membrane completely or in part, to fix the flux.
Membrane cleaning can be divided by following groups:
Chemical cleaning
Mechanical cleaning
Air or water cleaning
Hydrodynamic cleaning
Chemical cleaning is mainly use for remove membrane foulants by hydrolyzing, dissolving, inactivating, oxidizing and denaturalizing the membrane fouling. The cleaning agents are using alkaline cleaning agents to dissolve inorganic to prevent scaling. Acid cleaning agents is use for dissolve organic deposits. Detergents is use for reduce the surface tension of the water.
Mechanical cleaning is completed by launching the high shear forces at the membrane surface. For example: using the sponges to cleaning the tubular membranes.
Air or water cleaning is using the mixtures of water or air when the membrane cleaning. That can increase the turbulence, at the membrane surface will occur high shear force.
Hydrodynamic cleaning is using the water flow to occur high shear forces at the membrane surface. In special conditions, backwashing the membranes is quite effective method to remove layers of fouling.
Costs
Economics of desalination
Economics is hard to find out the full range of the costs and the benefits of source of water. Generally, provide water supply is easy find out the economics. For example: the cost of the running and maintaining the water system, the capital cost of build the treatment plants and the pipelines. Also the environmental cost is lost stream flow or reduces the watershed life which cause export out of the area. Following described some primary influences desalination cost.
Energy
Energy is the largest cost in the produce desalinated water. In the desalination production, energy is occupied half of the cost. Desalination also needs energy to transport the water to end users. Water is a commodity and energy is use to pump water to uphill or transport to long distance, so it can be the largest expense.
Desalination methods
This is depends on what methods used, for example: reverse osmosis and distillation, distillation will spend more than the reverse osmosis. Because distillation requires to heat source water. The cost different is between which methods can be reduce somewhat.
Cleaning and Maintenance
Desalination facilities are requires to regular maintenance, cleaning and antifouling treatment. This is also depends on which desalination method used and type of materials used. Also the treated water is store in the distribution system, so that the system is needed to keep clean. Because it needs to provide to the end users, so there are continuous costs relate with maintenance and cleaning the water supply system.
Transportation
The desalination system needs to be connecting to another distribution system or the distribution system to transport water to end users.
Time of the operation
The system maybe needs operating full time for the backup supply, so the maintenance cost need to pay when the system is producing the water.
Water source
The source of water is use seawater that the cost is free. But some of the desalination system may use the recycle or reclaimed water, so it may purchase from suppliers. However, recycle or reclaimed water re easy and not expensive to treat to drinking water. Because these sources have low minerals, salts…etc, so the cost of the treatment is not expensive.
Energy use in desalination
Energy is the largest cost in producing desalinated water. Although the amount of energy needed have reduced in last ten years, the energy still occupied about half of the cost in desalination production. The energy use in desalination depends on follow factors.
Source of energy
This is base on the desalination using which source of energy. For example: electricity, natural gas , cogeneration processes or other sources.
Quality of the production
Desalination process not only produces the drinking water, so energy requirements is depends on the production of the water. For example: some production is for irrigation, manufacturing.
Type of desalination methods
Each desalination methods will have different energy requirements, like the energy use of reverse osmosis is less than distillation.
Type of the source water
To desalinate the seawater is use more energy than desalination of brackish water or groundwater. Because the salinity of seawater is higher, so it need use more energy on it. Also the cooler water requires more energy than the warmer water.
Efficiency and scale of the desalination equipments
The scale of equipments is mainly function of how many product units are combined together. Also, many desalination equipments are composed of discrete units which can produce sufficient amount of water.
Above factors has described most of the energy used in desalination process, also energy will use for transmission that is transporting the water to end users. These energy used in pumping and storage.
Environmental
Environmental impacts of desalination process
Desalination is a treatment process, it can remove dissolved minerals from water. Desalination plants have the possible problem to affect the environment in different ways. The position of the desalination plant should choose and build carefully to reduce the effect on habitats near the desalination plants.
Effect of the increase water temperature
After the desalination process, the plants will discharge the waste water. It is possible to rise up the temperature of waters. This is a problem to affect the water quality and the marine life.
Noise pollution
When the energy recover turbine and the high pressure pump are running, it will make a lot of noise, so that the plants necessary to locate not near the population area.
Effect the marine habitats
In the desalination process, a lot of chemical used in the desalination plants. For example: effluent is a heavily concentrated brine solution. The brine solution after discharged, it may kill the marine organisms. Another the problems is from membrane cleaning and the pretreatment, because above process also need chemicals, so it can harm the habitats. Also the plants may use the biocides to clean the pipes and pre- treat the water. These chemicals need to be treated before discharge to the sea.
Effect the aquifer
In the plants, there will have long pipes to conduct the brine and seawater. It may have potential to penetrate and leakage of salt water to aquifer.
To avoid above environmental impacts, can be preliminary planning and choose suitable location to decrease the impact of desalination process. Since the hydrogeology and biological nature of the sea location, the sensitivity of these disturbances is different from place to place.
Conclusion
