Seawater reverse osmosis types

Kumar, M., S. S. Adham, and W. R. Pearce. 2006. Investigation of seawater reverse osmosis fouling and its relationship to pretreatment type. Environ, Sci. Technol. 40 2037-2044. 

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

All commercial types of processes, with the exception of freezing, namely, distillation, reverse osmosis and electrodialysis, are being applied in the above units with various kinds of distillation processes being used for seawater desalting. Two of them, horizontal tube multieffect distillation and vapor compression units were developed and manufactured locally by the Israel Desalination Engineering Ltd. Recently, two small RO units with a combined capacity of approx. 100 cu. m/day were also used to desalt seawater. The main aim of these units is to test and demonstrate the feasibility of this new technology. 

A variety of liquids have been treated with reverse osmosis and ultrafiltration membranes ranging from seawater, to wastewater, to milk and yeast suspensions. Each liquid varies in composition and in the type and fraction of the solute(s) to be retained by the membrane. Complicating factors include the presence of substances such as oil in seawater and wastewater [12-15]. The presence of the oil normally necessitates an additional pretreatment step further complicating the fouling process. The presence of humic acids in surface water and wastewater also needs special attention [16,17]. The fouling phenomena, the preventive means (i.e., pretreatment), and the frequency and type of membrane cleaning cycle are all dependent on the type of liquid being treated. 

Membrane materials for reverse osmosis and ultrafiltration applications range from polysulfone and polyethersulfone, to cellulose acetate and cellulose diacetate [12,18-23]. Commercially available polyamide composite membranes for desalination of seawater, for example, are available from a variety of companies in the United States, Europe, and Japan [24]. The specific choice of membrane material to use depends on the process (e.g., type of liquid to be treated and operating conditions) and economic factors (e.g., cost of replacement membranes and cost of cleaning chemicals). The exact chemical composition and physical morphology of the membranes may vary from manufacturer to manufaemrer. Since the liquids to be treated and... 

Membrane processes of the reverse osmosis (hyperfiltration) or electrodialysis types are used, but usually for smaller scale facilities (Fig. 5.4). Reverse osmosis units use high pressures of brackish water or seawater charging on one side of a semipermeable membrane, sufficient to exceed the osmotic... 

Reverse osmosis is a process that transforms an unusable water supply into a usable resource. It is capable of renovating a broad spectrum of feed-waters from municipal water supplies that need polishing for industrial purposes to seawater that is refined into a potable water supply. Table 4.23 shows the different types of feedwater being processed by reverse osmosis units. Seawater is considered to have a nominal total dissolved solids (TDS) content of 35,000 mg/C. Wastewater is from industrial or municipal sources and the TDS is variable. Brackish water is defined for the purposes of Table 4.2 only as a water that may have a TDS from that of municipal water supplies up to 10,000 mg/G. 

Figure 5.16, adapted from Kurihara,79 80 shows a comparison of several types of commercial reverse osmosis membranes in terms of salt rejection and permeate flow rate under seawater test conditions (35,000 ppm, 800 psi, 25°C). This chart emphasizes the capability of PEC-1000 to provide complete single-stage seawater desalting. In a test at Toray s Ehime desalination test facility on 42,000 ppm seawater (equivalent to Red Sea salinity), PEC-1000 spiral elements operated at 35% recovery produced a permeate having an average salinity of only 220 ppm, well below WHO standards. Average salt rejection was 99.5%. 

A typical sample of seawater contains 31.6 g dm of sodium chloride. Other substances are present in much smaller amounts and can be neglected. If seawater is contained in a cylinder with a semipermeable membrane at one end and a piston at the other, application of a pressure greater than the osinotic pressure will cause pure water to pass through the membrane.- Estimate the minimum work that is required to produce 1 mol (18 cm ) of water by the desalination of seawater at 25° C. (This process is known as reverse osmosis, and when suitable membranes are developed this type of desalination may be economic. Note that the work required is very much less than... 

The plant can also be configured to produce potable water using a two-stage reverse osmosis system for seawater desalination. The amount of potable water produced could also be selected in response to demand at each site, but the maximum capabilities of two types of the 4S to produce potable water are 34 000 m /day and 170 000 mVday respectively, when all generated energy is utilized for desalination. 

Water treatment systems have now come to include plastics, such as Du Pont s Permasep permeators for reverse osmosis (RO) water desalination, introduced in 1969 (see Figs. 12-5 and 12-6). Since that time such systems have been used in thousands of installations around the world for desalination of brackish water and seawater and to treat waste effluents. These permeators come in four product types, according to the type of water to be treated (see Table 12-1). 

Example 3.4.2 Earlier literature on reverse osmosis desalination of seawater at a high AP = 102 atm provided the following information for a cellulose acetate membrane A = 8.03 X 10 gmol H20/cm -s-atm it(C,y) = bCif = 45.7 atm DimKim/Sm) for Salt = 1.774 X10 cm/s Xif= 17.7 X 10 Xip = 0.4 X 10 . The membrane has a high rejection for the solute i, which is NaCl. Calculate the values of water flux and the salt flux through the membrane. Calculate the salt concentration on two sides of the membrane, assumed to be of the solution-diffusion type. Calculate the salt rejection R. [Noter. Current membranes are more productive. Therefore AP is around 54.4-68 atm (800-1000 psi). One should calculate A based on the final concentration of seawater desired.)... 

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