Water seawater desalination plants

Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. Raw water from a reservoir, river, lake, borehole or a seawater desalination plant is fed to the steam system. However, it needs to be treated before it can be used for steam generation. The treatment required depends both on the quality of the raw water and the requirements of the utility system. The principal problems with raw water are1,2 ... 

The concentrations of seawater and brackish water can vary significantly, and as such there is a difference between the concentrate produced from seawater desalination plants and brackish water desahnation plants. Seawater typically has a level of total dissolved solids (TDS) between 33,000-37,000 mg/L. The average major ion concentration of seawater is shown in Table 2.1 along with water from the Mediterranean Sea, and water from Wonthaggi off the southern coast of Australia. Seawater sahnity increases in areas where water evaporates or freezes, and it decreases due to rain, river runoff, and melting ice. The areas of greatest salinity occur and latitudes of 30° N and S where there are high evaporation rates. 

Discharge to surface water is the most economical form of concentrate management for seawater desalination plants, regardless of the discharge volume. Due to the availability of ocean discharge for seawater desalination plants, the cost of disposal tends to be less costly than for inland desalination. Costs include pumps and pipes. 

Concentrate can be harmful to the environment due to either its higher than normal salinity, or due to pollutants that otherwise would not be present in the receiving body of water. These include chlorine and other biocides, heavy metals, antisealants, coagulants and cleaning chemicals. Of particular concern is the effect of pollutants on delicate ecosystems and endangered or threatened species. However, with appropriate measures in place, the discharge of concentrate to surface water can remain a viable method for seawater desalination plants. 

Examples of desalination plants that currently blend their concentrate with treatment plant outfall include the Thames Water Desalination Plant in London (150,000 m /day capacity) and the Barcelona Seawater Desalination Plant (200,000 m /day capacity). 

Nonetheless a few commercially successful noncellulosic membrane materials were developed. Polyamide membranes in particular were developed by several groups. Aliphatic polyamides have low rejections and modest fluxes, but aromatic polyamide membranes were successfully developed by Toray [25], Chemstrad (Monsanto) [26] and Permasep (Du Pont) [27], all in hollow fiber form. These membranes have good seawater salt rejections of up to 99.5 %, but the fluxes are low, in the 1 to 3 gal/ft2 day range. The Permasep membrane, in hollow fine fiber form to overcome the low water permeability problems, was produced under the names B-10 and B-15 for seawater desalination plants until the year 2000. The structure of the Permasep B-15 polymer is shown in Figure 5.7. Polyamide membranes, like interfacial composite membranes, are susceptible to degradation by chlorine because of their amide bonds. 

The concept of a nuclear seawater desalination plant is shown in Fig. 16. The sea water desalination plant is planned based on a two stage reverse osmosis system with a capacity of240000mVday x 7 lines by using a single 4S plant. The plant can be constmcted on a site of about 210m x 140m. 

Application of NF to soften RO feed water in hybrid RO-multistage flash (MSF) seawater desalination plants such as shown in Figure 3.6 is relatively recent [6]. In the pre-treatment of high salinity seawater, for example, NF reduced total hardness from 7500 to... 

NF-RO or RO-MSF seawater desalination plants has a potential for significant reduction in the cost of drinking water ... 

This process is widely used in a seawater desalination plant, where purified water is obtained against a high salt concentration of seawater. In the metal-making industry this purification method is used in the oil-water mixed jet-cutting tool emulsions that contain high concentration of metals. A reverse osmosis unit separates the oil from water to be reused again. 

Catalina Island s plant may be just the beginning. The city of Santa Barbara opened a 40 million desalination plant in 1992 that can produce 8 million gallons of drinking water per day. The southern California city of Carlsbad opened a reverse osmosis desalination plant in 2012 that can produce 50 million gallons of drinking water daily from seawater. Desalination plants are also in the woiks for Huntington Beach, California, and Camp Pendleton, a military base just north of Carlsbad. 

Because of their lack of susceptibility to chloride-induced stress corrosion cracking, nickel-copper alloys are used in seawater desalination plants as pipes for evaporators or heat exchangers. An evaporator made of alloy 400 (NiCu 30 Fe, 2.4360) exhibited a corrosion rate of > 0.01 mm/a (> 0.4 mpy) after an exposure time of 225 days to a CaCl2 concentration of up to 35 % and a temperature of 433 K (160 C). The corrosion rate was 0.07 mm/a (2.76 mpy) for a NaCl solution saturated with water vapor and air at 366 K (93 °C) [73]. 

A combination of nuclear power reactor and seawater desalination plant could be realized in a more economical way, since the higher is the temperature and pressure of steam used in a turbine, the lower is the cost of electricity produced. On the other hand, steam at low temperature and pressure is needed for fractional desalination, and the greater part of inputted heat is the latent heat of steam. Therefore, the power production and desalination systems may be advantageously combined. A 100 MWe FBNR when realized within a cogeneration plant for the production of both power and potable water could produce 70 MWe of electricity and... 

The demineralized (DM) water make up requirement of a 300 MW(e) AHWR is about 350 m /d. An additional requirement of about 150 m /d.of fresh water for drinking and other purposes is envisaged. It is therefore proposed to set up a 500 m /d low temperature multi effect distillation (LT-MED) seawater desalination plant utilizing low pressure steam from the turbine to meet the DM water requirements. Figure XI-8 provides a schematic flow sheet of the desalination plant of AHWR. 

Alawadhi, A., Kannari, T., Katsube, M., Umemori, R, and Fujiwara, N. (2005). Elimination of biological fouling in seawater desalination plant in Bahrain. In Proceedings of IDA World Congress on Desalination and Water Reuse. IDA, Swissotel The Stamford, Singapore. 

Seawater desalination is the production of fresh, low-salinity potable or industrial-quality water from a saline water source (sea, bay, or ocean water) via membrane separation or evaporation. Over the past 30 years, desalination technology has made great strides in many arid regions of the world such as the Middle East and the Mediterranean. Today, desalination plants operate in more than 120 countries worldwide, and some desert states, such as Saudi Arabia and the United Arab Emirates, rely on desalinated water for over 70% of their water supply. According to the 2004 desalination plant inventory report prepared by the International Desalination Association (Wagnick Consulting, 2004), by the end of 2003 worldwide there were over 17,000 desalination units with total installed treatment capacity of 37.8 million m /day. Seawater desalination plants contribute approximately 35% (13.2 million m /day) of this capacity. 

The largest SWRO plant in North America, which obtains source water fiom beach wells, is the 15,000-m /day water supply facility for the Pemex Salina Cruz refinery in Mexico. This plant also has the largest existing seawater intake wells—three Raimey-type horizontal collectors with capacities of 15,000 m /day each. Key considerations for the selection of the type of intake most suitable for the site-specific conditions of a given SWRO plant and guidelines for the development of subsurface intakes for seawater desalination plants are discussed elsewhere (AWWA, 2007 Wright and Missimer, 1997 Voutchkov, 2004a). 

This issue is very significant for pretreatment systems for seawater desalination plants with open-ocean intakes. Often the source seawater contains small sharp objects (such as shell particles), which can easily puncture the pretreatment membranes and result in a very quick loss of their integrity, unless the damaging particles are removed upstream of the membrane pretreatment system. As discussed previously, to remove sharp seawater particles that can damage the membranes from the source water, the SWRO plant intake system has to incorporate a microscreening system that can remove particles larger than 120 p,m. 

SWRO system. For example, a partial second-pass configuration is nsed at the 95,000-m / day Tampa Bay seawater desalination plant. The second pass at this facility is designed to treat up to 30% of the permeate produced by the first-pass SWRO system as needed in order to maintain the concentration of chlorides in the plant product water always below lOOmg/L. The partial second pass at the Tampa Bay seawater desalination plant was installed to provide operational flexibility and to accommodate the wide fluctuations of source water salinity (16,000-32,000mg/L) and temperature (18-40°C). Typically, the product water quality target chloride concentration of 100 mg/L at this plant is achieved by only operating the first pass of the system. However, when source water TDS concentration exceeds 28,000 mg/L and/or the source water temperature exceeds 35°C, the second pass is activated to maintain adequate product water quality. The percent of first-pass permeate directed for additional treatment through the second pass is a function of the actual combination of source water TDS and temperature and is adjusted based on the plant product water chloride level. 

An example of a full-scale two-pass, two-stage RO system application is the 170,500-m /day Fujairah seawater desalination plant (Rovel, 2003). A general treatment process schematic of this plant is depicted on Figure 3.10. The plant uses Gulf of Oman, Indian Ocean, seawater (see Table 3.2 for water quality characteristics). 

The entire volume of permeate from the first pass of the Point Lisas SWRO system is further treated in a second-pass RO system to meet the final product water quality specifications. The second-pass system also consists of two stages—each equipped with BWRO membranes. The Point Lisas seawater desalination plant has the same number of first-pass and second-pass RO membrane trains. 

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