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Facts/Glossary
Distillation The intake water is exchanged from a liquid to vapor (steam). The vapor is then condensed to produce product water. Reverse Osmosis Pressure is applied to the intake water, forcing the water molecules through a semipermeable membrane. The salt molecules do not pass through the membrane, and the water that passes through becomes potable product water. Ranges In Plant Capacity (Other than off shore oil and gas platforms.) 16 - 112,000 AF/year (One acre-foot is equivalent to 326,000 gallons). This is equivalent to the amount of water that two to three households would consume in one year. In most cases, conversions can not be made directly from gallons per day (gpd) to acre-feet per year (AF/year) since most plants will not operate every day of the year. Costs Most sea water plants in California would produce water in the range of $1,300 to $2,200/AF. Energy Use 2,500 to 12,000 kWh/AF. Efficiency (Percent of product water recovery to input flow.) For every 100 gallons of sea water input, 15 to 73 gallons of fresh water would be produced. The remainder would be waste brine solution and solid wastes. Water Quality 2 to 500 ppm tds (the recommended California drinking water standard or maximum total dissolved solids level is 500 ppm.) Plant Size Area This varies according to how spread out the design of the plant is. Proposed or existing California plants range from 80 square feet for a 16 AF/year plant to 7.5 acres for a 5,000 AF/year plant. Height Typical distillation equipment 30 to 530 feet. (Typical reverse osmosis equipment 15 to 20 feet).
A unit for measuring the volume of water. One acre-foot equals 326,000 gallons. Biocide A chemical used to kill biological organisms, e.g. chlorine. Brine. Water that contains a high concentration of salt. Brine discharges from desalination plants may include constituents used in pretreatment processes, in addition to the high salt concentration seawater. Capacity Factor An electric utility's annual capacity factor is defined as the annual kilowatt-hour sales divided by the product of the total hours in a year and the rated capacity of the utility in kilowatts. Coagulation A pretreatment process in desalination plants. A substance, (e.g.,ferric chloride), is added to a solution to cause certain elements to become thickened into a coherent mass, so that they may be removed. Cogeneration A power plant that is designed to conserve energy by using the waste heat from generating electricity for another purpose.Deaeration Removal of oxygen. A pretreatment process in desalination plants to reduce corrosion. Distillation A process of desalination where the intake water is converted to steam. The steam is then condensed to produce a product water with low salt concentration. Electrodialysis A process of desalination whereby an electrical current is used to separate out salt and impurities in the intake water. Most of the impurities in water are present in an ionized (electrically charged) state and will conduct electric current. When high voltage is applied, the impurities migrate towards the positive and negative electrodes, and the intermediate area becomes purified product water. This technology is used for brackish waters, but is not currently available for desalting seawater on a commercial scale. Ion exchange A pretreatment process in desalination plants. An electrical charge is used to remove charged particles from solution. Kilowatt (kV&127; One thousand watts. The watt is a measure of power used by electricity generating plants. One watt is equivalent to 1 Joule/second or 3.412 BTU/hour. Multi-effect Distillation (MED) A form of distillation. Evaporators are in series, and vapor from one series is used to evaporate water in the next one. This technology is in several forms, one of the most common of which is the Vertical Tube Evaporator (VTE). Multi-stage Flash (MSF) A form of distillation. The intake water is pressurized and heated. It is discharged into a chamber maintained slightly below the saturation vapor pressure of the water, and a fraction of the water content flashes into steam. The steam condenses on the exterior surface of heat transfer tubing and becomes product water. The unflashed brine enters a second chamber, where brine flashes to steam at a lower temperature. Each evaporation and condensation series is called a stage. Reverse Osmosis (RO) A process of desalination where pressure is applied to the feedwater, forcing the water molecules through a semipermeable membrane. The water that has passed through the membrane leaves the unit as product water, and most of the dissolved impurities remain behind and are discharged in a waste stream. Scaling Salt deposits on the interior surfaces of a desalination plant. Total dissolved solids (tds) Total salt and calcium carbonate concentration in a sample of water. The state recommended Maximum Contaminant Level (MCL) drinking water standard for total dissolved solids is 500 ppm. The upper MCL is 1,000 ppm, and the short-term permitted level is 1,500 ppm. Vacuum Freezing (VF) ` A process of desalination where the temperature and pressure of the sea water is lowered so that the pure water forms ice crystals. The ice is then washed and melted to produce the product water. This technology is still being developed, and is not yet commercially competitive. Vapor Compression (VC) A form of distillation. The intake water is evaporated, and the vapor is sent to a compressor. Mechanical or thermal energy is used to compress the vapor, which increases its temperature. The vapor is then condensed to produce product water, and the heat that is given off is used to evaporate the feedwater.
DESALINATION TECHNOLOGIES A number of technologies have been developed for desalination which include distillation, reverse osmosis (RO), electrodialysis, and vacuum freezing. Two of these technologies, distillation and reverse osmosis, are being considered by municipalities, water districts and private companies for development of sea water desalination. A description of these technologies are described below: Distillation In the distillation process, sea water is heated and then evaporated to separate out the dissolved salts. The most common methods of distillation include vapor compression (VC), multi-stage flash (MSF) and multi-effect distillation (MED). The vapor compression process involves evaporating the input water and then compressing the vapor. The vapor is then condensed, and the heat released is used to evaporate the input water. In the multi-stage flash process, saline feedwater is heated and the pressure is lowered; so that the water "flashes" into steam. This process constitutes one stage, and there are a number of stages in series, each of which is at a lower pressure. In multi-effect distillation, there are a number of evaporators in series, and vapor from one series is used to evaporate water in the next effect. Some distillation plants are a hybrid of more than one of these technologies. The waste product from these processes is a high salt concentration brine solution. Reverse Osmosis In reverse osmosis, the sea water is pre-treated to remove particles that would clog the membranes. The sea water is pumped at high pressure through membranes, separating the salt from the water. The quality of the water produced is dependent upon the pressure, the concentration of salts in the feedwater and the salt permeation constant of the membrane. The quality of the product water from reverse osmosis plants can be improved by adding a second stage of membranes. Input Water Desalination plants may use sea water from the ocean or brackish water from groundwater wells as input water. Brackish water has a lower salt concentration and may produce water at a lower cost than sea water plants. Brackish water plants typically use reverse osmosis technology. Therefore, most plants that are built or proposed in the coastal zone are distillation plants. Desalination intake pipes should be located away from sewage treatment plant discharges; but if some of these discharges or other types of pollutants are included in the intake, the pre-treatment and post-treatment processes should remove them. Product Water Distillation plants produce a high-quality product water that ranges from 2 to 50 parts per million (ppm) total dissolved solids (tds). (Note: The recommended California drinking water standard for maximum total dissolved solids level is 500 ppm). Reverse osmosis produces potable product water that ranges from 10 to 500 ppm total dissolved solids. In desalination plants that are producing water for domestic use, post treatment processes are often employed to ensure that the product water meets the health standards for drinking water. The desalination product water may be used in its pure form, (i.e., in power plant turbines, or it may be mixed with less pure water and used for irrigation, drinking water, or other uses. The water that is produced with desalination is usually more pure than the drinking water standards, so when the product water is intended for municipal use, it may be mixed with water that contains higher levels of total dissolved solids. The pure desalination water has a high acidity level, which is corrosive to pipes. Therefore, it needs to be adjusted for the correct ph level and hardness before it is piped offsite. If not, other sources of water that it is mixed with will have to be piped on site. Pretreatment Processes Pretreatment processes are needed to remove substances that would interfere with the reverse osmosis membranes. Algae and bacteria can grow in both reverse osmosis and distillation plants, so both require some type of biocide to clean to system (usually chlorine is added - about 1 ppm). Some reverse osmosis membranes cannot tolerate chlorine, so dechlorination techniques are required. Ozone or ultraviolet light may also be used to remove marine organisms. If ozone is used, it must be removed with chemicals prior to reaching the membranes. In reverse osmosis plants, particles in the inflow water must be removed to reduce fouling of the membranes. Metals in the inflow water are removed with acids or ion exchange. Suspended solids are removed with coagulation and tiltration. Filter Backwashing Membrane Cleaning and Storage and Pipe Cleaning The filters for pretreatment of plant water at a reverse osmosis plant must be cleaned every few days (backwashed) to clear the accumulation of sand and solids. The reverse osmosis membranes must be cleaned approximately four times a year and must be replaced every three to five years. Alkaline cleaners are used to remove organic fouling, and acid cleaners are used to remove scales and other inorganic precipitates. All or a portion of the plant must be shut down when the membranes are replaced. When reverse osmosis plants are not used continuously, the membranes must be stored in a chemical disinfection/preservation solution, which must he disposed of after use. For both reverse osmosis and distillation systems, the intake and outfall structures will become fouled with organisms and will have to be cleaned by applying chemicals or high temperatures. The plant components must be cleaned to reduce scaling. Scaling is a condition where salts deposit on pipe surfaces or other parts of the desalination plants. It is caused by the high salt concentration of the sea water and can result in reduced efficiency of the plants and corrosion of the pipes. Scaling can be reduced by introducing additives to inhibit crystal growth, reducing temperature and/or salt concentrations, removing the scale forming constituents, or seeding to form particles. Once scales have formed, they can be removed with chemical or mechanical means. Scaling is a concern for reverse osmosis plants and distillation plants unless lower temperatures are used, which reduces the potential for scaling. The input water may also be deaerated to reduce corrosion.
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