Seawater dissolved solids

Potable Water RO and NF both play a major role in providing potable water, defined either by the WHO criterion of <1000 ppm total dissolved solids (TDS) or the U.S. EPA limit of 500 ppm TDS. RO is most prominent in the Middle East and on islands where potable-water demand has outstripped natural supply. A plant awaiting startup at Al Jubail, Saudi Arabia produces over 1 mVs of fresh water (see Table 22-17). Small units are found on ships and boats. Seawater RO competes with multistage flash distillation (MSF) and multieffect distillation (MED) (see Sec. 13 Distillation ). It is too expensive to compete with conventional civil supply (canals, pipelines, w ls) in most locations. Low-pressure RO and NF compete with electrodialysis for the desalination of brackish water. The processes overlap economically, but they are sufficiently different so that the requirements of the application often favor one over the others. 

Applications RO is primarily used for water purification seawater desalination (35,000 to 50,000 mg/L salt, 5.6 to 10.5 MPa operation), brackish water treatment (5000 to 10,000 mg/L, 1.4 to 4.2 MPa operation), and low-pressure RO (LPRO) (500 mg/L, 0.3 to 1.4 MPa operation). A list of U.S. plants can be found at www2.hawaii.edu, and a 26 Ggal/yr desalination plant is under construction in Ashkelon, Israel. Purified water product is recovered as permeate while the concentrated retentate is discarded as waste. Drinking water specifications of total dissolved solids (TDS) < 500 mg/L are published by the U.S. EPA and of < 1500 mg/L by the WHO [Williams et ak, chap. 24 in Membrane Handbook, Ho and Sirkar (eds.). Van Nostrand, New York, 1992]. Application of RO to drinking water is summarized in Eisenberg and Middlebrooks (Reverse Osmosis Treatment of Drinking Water, Butterworth, Boston, 1986). 

The extension of inductively coupled plasma (ICP) atomic emission spectrometry to seawater analysis has been slow for two major reasons. The first is that the concentrations of almost all trace metals of interest are 1 xg/l or less, below detection limits attainable with conventional pneumatic nebulisation. The second is that the seawater matrix, with some 3.5% dissolved solids, is not compatible with most of the sample introduction systems used with ICP. Thus direct multielemental trace analysis of seawater by ICP-AES is impractical, at least with pneumatic nebulisation. In view of this, a number of alternative strategies can be considered ... 

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. 

Concentration ([ji.mol/L) Runoff (x10 moi/y) % Contribution by Mass to Totai Dissoived Soiids Concentration ((jtmol/L) Inventory (X10 mol) % Contribution by Mass to Total Dissolved Solids [Seawater] [Riverwater] ... 

Desalination. Desalination of seawater and brackish water has been and, as of the mid-1990s, is the primary use of RO. Driven by a need for potable water in areas of the world where there is a shortage, this industry has developed. Desalination involves the reduction of the total dissolved solids (TDS) concentration to less than 200 mg/L. RO offers several advantages over other possible desalination processes such as distillation (qv), evaporation (qv), and electro dialysis. The primary advantage of RO over the traditionally used method of distillation is the eneigy savings that is afforded by the lack of a phase change in RO. 

Reverse osmosis processes for desalination were first applied to brackish water, which has a lower TDS concentration than seawater. Brackish water has less than 10,000 mg/L TDS seawater contains greater than 30,000 mg/L TDS. This difference in TDS translates into a substantial difference in osmotic pressure and thus the RO operating pressure required to achieve separation. The need to process feed streams containing laiger amounts of dissolved solids led to the development of RO membranes capable of operating at pressures approaching 10.3 MPa (1500 psi). Desalination plants around the world process both brackish water and seawater (15). 

Alternatively, potable water can be extracted from seawater by freezing salts, which depress the freezing point of water, remain in the liquid phase. Generally, though, it is more practical to remove the relatively small amount of solutes (typically, 0.02% for river water) from the great excess of water, rather than vice versa. Seawater is an exceptional case, with about 3.5% dissolved solids. Water softening is concerned primarily with removal of Ca2+ and Mg2+, but for some purposes removal of all dissolved solids (deionization or demineralization) is necessary. 

Reverse osmosis is now extensively used to reduce salt concentrations in brackish waters and to treat industrial waste water, for example, from pulp mills. Reverse osmosis has also proved economical (the cost can be as low as about 1 per 1000 liters) for large-scale desalination of seawater, a proposition of major interest in the Middle East, where almost all potable water is now obtained by various means from seawater or from brackish wells. Thus, at Ras Abu Janjur, Bahrain, a reverse osmosis plant converts brackish feedwater containing 19,000 ppm dissolved solids to potable water with 260 ppm dissolved solids at a rate of over 55,000 m3 per day, with an electricity consumption of 4.8 kilowatt hours per cubic meter of product. On a 1000-fold smaller scale, the resort community on Heron Island, Great Barrier Reef, Australia, obtains most of its fresh water from seawater (36,000 ppm dissolved salts) directly by reverse osmosis, at a cost of about 10 per 1000 liters. 

Ar the southern end of San Francisco Bay are areas where the seawater has been partitioned off. These are evaporation ponds where the water is allowed to evaporate, leaving behind the solids that were dissolved in the seawater. These solids are further refined for commercial sale. The remarkable color of the ponds results from suspended particles of iron oxide and other minerals, which are easily removed during refining. 

All formation waters contain dissolved solids, primarily sodium chloride. The water sometimes is called brine or salt water. However, oilfield brines bear no relationship to seawater, either in the concentration of solids or in the distribution of the ions present. Generally, oilfield waters contain much higher concentrations of solids than seawater does. Formation waters have been reported with total solid concentrations ranging from as little as 200 ppm to saturation, which is approximately 300,000 ppm. Seawater contains about 35,000 ppm total solids. 

DESALINATION. As generally used, the term describes the production of water appropriate for human consumption from seawater and hraekish water. Seawater averages about 35,(MX) ppm of total dissolved solids (TDS). Brackish waters TDS range from 2000 ppm upwards. The maximum TDS of water considered tolerable and acceptable for continued human consumption is about 500 ppm. although water containing up lo 1000 ppm TDS may be consumed for short periods. 

Concentration of Seawater by ED. In terms of membrane area, concentration of seawater is the second largest use. Warm seawater is concentrated by ED to 18 to 20% dissolved solids using membranes with monovalent-ion-selective skins. The EDR process is not used. The osmotic pressure difference between about 19% NaQ solution and partially depleted seawater is about 20,000 kPa (200 atm) at 25°C, which is well beyond the range of reverse osmosis. Salt is produced from the brine by evaporation and crystallization at seven plants in Japan and one each in South Korea, Taiwan, and Kuwait. A second plant is soon to be built in South Korea. None of the plants are justified on economic grounds compared to imported solar or mined salt. 

Brine Water that is saturated or nearly saturated with sodium chloride and perhaps other salts. Brines contain considerably more dissolved solids than seawater. 

Land (1987) has reviewed and discussed theories for the formation of saline brines in sedimentary basins. We will summarize his major relevant conclusions here. He points out that theories for deriving most brines from connate seawater, by processes such as shale membrane filtration, or connate evaporitic brines are usually inadequate to explain their composition, volume and distribution, and that most brines must be related, at least in part, to the interaction of subsurface waters with evaporite beds (primarily halite). The commonly observed increase in dissolved solids with depth is probably largely the result of simple "thermo-haline" circulation and density stratification. Also many basins have basal sequences of evaporites in them. Cation concentrations are largely controlled by mineral solubilities, with carbonate and feldspar minerals dominating so that Ca2+ must exceed Mg2+, and Na+ must exceed K+ (Figures 8.8 and 8.9). Land (1987) hypothesizes that in deep basins devolatilization reactions associated with basement metamorphism may also provide an important source of dissolved components. 

Osmotic pressure (typically represented by jt (pi)) is a function of the concentration of dissolved solids. It ranges from 0.6 to 1.1 psi for every 100 ppm total dissolved solids (TDS). For example, brackish water at 1,500 ppm TDS would have an osmotic pressure of about 15 psi. Seawater, at 35,000 ppm TDS, would have an osmotic pressure of about 350 psi. 

Seawater membranes are used to treat high-salinity (35,000 to 50,000 ppm total dissolved solids (TDS)) feed waters. These membranes can operate at pressures up to 1,500 psi. Typical membrane test conditions are as follows ... 

Salinity— The conventional salinity (S) of - seawater is defined relative to the chlorinity (Cl), which is the chlorine equivalent of the total mass of halides that can be precipitated from 1 kg of seawater by the addition of silver nitrate. The relationship is S = 1.80655 x CL The total mass of dissolved solids (St) is related to the conventional salinity by the equation St = 1.00544 x S. 

Salinity is defined here as the grams of dissolved solids (or inorganic dissolved compounds) per kg of seawater, or parts per thousand (or as a per thou-sand%o). Alternatively, it can be defined as the mg/L or mmol/L of the major ions (i.e., those present in concentrations above 1 ppm). The total concentration of dissolved solids ranges from 7,000 ppm for the Baltic Sea, to an average of 35,000 ppm in large oceans, and up to 40,000 ppm in regions where evaporation is high and inputs are low, such as the Red Sea. 

On the other hand, pressure is applied in reverse osmosis to drive the solvent (water) out of the high-concentration side into the low-concentration side this facilitates de-watering insoluble species for their removal. This process produces high-quality water and concentrated refuse. It separates and removes dissolved solids, organics, pyrogens, colloidal matter, viruses, and bacteria from water in the particle range 10 4—10—2 pm. Reverse osmosis can remove up to 95%-99% of the total dissolved solids (TDS) and 99% of all bacteria. It is used for the ob-tention of drinking water from seawater and for the production of ultra pure water in various industries. 

GB is completely miscible with water (DA, 1974). Its rate of hydrolysis is dependent on temperatnre, pH, and other water qnality parameters (Epstein, 1974 Morrill et al., 1985 Clark, 1989). At 20°C, the half-life ranges from 461 hr at pH 6.5 to 46 hr at pH 7.5. At 25°C, the half-life is 237 hr at pH 6.5 and 24 hr at pH 7.5. GA is much more persistent at low temperature at 0°C, its half-life is 8,300 hours at pH of 6.5. The rate of hydrolysis under natnral conditions is accelerated by the presence of ions (dissolved solids) in solntion. Metal cations such as copper and manganese in seawater increase the rate of hydrolysis (Epstein, 1974). 

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