Seawater desalination plants

The copper-chelating abihty of sahcylaldoxime has been used to remove copper from brine in a seawater desalination plant effluent. A carbon—sorbate bed produced by sorption of the oxime on carbon proved to be extremely effective in the continuous process (99). In another apphcation, the chelating abihty of sahcylaldoxime with iron and copper was used to stabilize bleaching powders containing inorganic peroxide salts (100). 

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. 

Figure 10.4 Cost estimate of a common RO seawater desalination plant.

A 100-million-m3 RO-based seawater desalination plant requires an electrical energy supply of less than 50 MW. A dedicated power station can work at a much higher efficiency than a regular power station for this purpose since it is operated constantly without the known sine wave, representing day-night, summer-winter... 

Glueckstern, P. and Priel, M. (1999) Experience, capability and plans for erecting large seawater desalination plants. Proceedings of the 2nd Annual IDS Conference, Haifa. 

As pressure on existing freshwater supplies tightens, seawater desalination plants, using multieffect vacuum distillation or reverse osmosis, are required in increasing numbers for provision of freshwater. The residual evaporated brines from these plants contain much higher salt concentrations than ordinary seawater and this is also obtained near potential salt markets. 

The largest seawater desalination plant in the World operates in Jeddah, Saudi Arabia. It has a capacity of 56,800,000 liters per day, and the TDS content of the seawater is approximately 44,000ppm. Pretreatment modules for this plant include chlorine treatment, a dual media filter, and a cartridge filter for the coagulation and filtration of dead cells of microorganisms (Figure 8). 

Smith, R., Purnama, A., Al-Barwani, H. H. (2007). Sensitivity of hypersaline Arabian Gulf to seawater desalination plants. Applied Mathematical Modelling, 31, 2347-2354. 

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. 

Characterization of uncertainties in the operation and economies of the proposed seawater desalination plant in the Gaza Strip was made by using a Bayesian belief network (BBN) approach [80]. In particular, the model was used to (1) characterize the different uncertainties involved in the RO process, (2) optimize the RO process reliability and cost, and (3) study how uncertainty in unit capital cost, unit operation and maintenance (O M) cost, and permeate quality was related to different input variables. The minimum specific capital cost was found to be 0.224 0.064 US /m, and the minimum O M cost was found to be 0.59 0.11 US /m. This unit cost was for a production capacity of 140,000 mVday. 

Kimura S, Ohya H, Murayama Y, Kikuchi K, Hirai M, Toyoda M, Sonoda T, and Setogawa S, Five years operating experience of a 800 cubic meters per day R.O. seawater desalination plant. Desalination 1985, 54, 45-54. 

Ghabayen S, McKee M, and Kemblowski M, Characterization of uncertainties in the operation and economics of the proposed seawater desalination plant in the Gaza Strip, Desalination 2004, 161, 191-201. 

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... 

Salts recovery from seawater desalination plants... 

Brine concentrate stream from seawater desalination plants is typically discharged to the sea. Flowever, the brine can be processed to recover salts for the chemical industry, e.g.,... 

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

The efficiency of seawater desalination plants is low, 10—25% as compared with the efficiency of other major industrial plants [7]. For example, the efficiency of cogeneration power generating plants is 50%. The efficiency of small desalination plants such as used on ships is in the low range. The energy consumption of the wind-powered 140,000 m /day SWRO plant in Perth is 3.56 kWh/m [38]. Using the same feed and outlet conditions, the minimum work consumed is 0.951 kWh/m. Hence, the second law efficiency of the plant is about 26.7% [7]. The SWRO plant efficiency would increase if the plant energy consumption of2.2 kWh/m is achieved [5]. Alternate SWRO plant and system designs... 

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