Single-stage seawater reverse osmosis

As of the end of 1984, the desalination of brackish water accounted for 82% of capacity. This is due to the fact that early reverse osmosis membranes were incapable of single stage seawater desalination and, thus, they were limited to brackish water desalination. Within the last 10 years, significant advances have been made in both the flux and rejection capability of membranes and reverse osmosis is technically able to desalt seawater in a single stage. In the recent past, it has been an effective competitor to the distillation process in seawater desalination. In fact, reverse osmosis is now beginning to replace existing distillation capacity in the Middle East.4... 

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%. 

To overcome the problems of cellulose acetate membranes, many synthetic polymeric materials for reverse osmosis were proposed, but except for one material, none of them proved successful. The only one material, which could remain on the market, was the linear aromatic polyamide with pendant sulfonic acid groups, as shown in Figure 1.2. This material was proposed by DuPont, which fabricated very fine hollow fiber membranes the modules of this membrane were designated B-9 and B-10. They have a high rejection performance, which can be used for single-stage seawater desalination. They were widely used for mainly seawater or brackish water desalination and recovery of valuable materials such as electric deposition paints, until DuPont withdrew them from the market in 2001. 

Koyuncu et al. [56] presented pilot-scale studies on the treatment of pulp and paper mill effluents using two-stage membrane filtrations, ultrafiltration and reverse osmosis [56]. The combination of UF and RO resulted in very high removals of COD, color, and conductivity from the effluents. At the end of a single pass with seawater membrane, the initial COD, color and conductivity values were reduced to 10-20 mg/L, 0-100 PCCU (platinum cobalt color units) and 200-300 ps/cm, respectively. Nearly complete color removals were achieved in the RO experiments with seawater membranes. 

Desalination of seawater is one of the important applications of membrane processes. There are various ways to produce fresh water such as distillation, electrodialysis, membrane distillation, freezing, membrane bioreactor, and reverse osmosis. Among them, distillation is the most used technique, but RO is becoming more popular in the desalination industry. A flow diagram of a single-stage RO system is shown in Fig. 4. 

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. 

Flow- diagram for a 1(X)0 m /day single-stage reverse osmosis seawater desalination plant. 

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