Seawater reverse osmosis plants operation

The approximate operating costs for brackish and seawater reverse osmosis plants are given in Table 5.3. These numbers are old, but improvements in membrane technology have kept pace with inflation so the costs remain reasonably current. 

Table 5.3 Operating costs for large brackish water and seawater reverse osmosis plants [49]. Capital costs are approximately US 1.25 per gal/day capacity for the brackish water plant and US 4-5 per gal/day capacity for the seawater plant...

Some of the largest plants for seawater desalination, wastewater treatment and gas separation are already based on membrane engineering. For example, the Ashkelon Desalination Plant for seawater reverse osmosis (SWRO), in Israel, has been fully operational since December 2005 and produces more than 100 million m3 of desalinated water per year. One of the largest submerged membrane bioreactor unit in the world was recently built in Porto Marghera (Italy) to treat tertiary water. The growth in membrane installations for water treatment in the past decade has resulted in a decreased cost of desalination facilities, with the consequence that the cost of the reclaimed water for membrane plants has also been reduced. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

To justify the assumptions, the estimated cost in power consumption, labor, and membrane replacement was compared with that in a seawater reverse osmosis desalination (SWRO) plant (Atikol et al., 2005). For SWRO, the reported cost for power was 0.04 US m, we estimated 0.022 US m for our system. The lower cost in energy consumption is mainly due to the low operating pressure and significantly higher water recovery in this system. The cost for pre-treatment was assumed to be lower than seawater plant due to the significantly much better water quality in the drinking water sources. The maintenance cost was adopted from the... 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

Reverse osmosis processes for desalination were first appHed to brackish water, which has a lower I DS concentration than seawater. Brackish water has less than 10,000 mg/L IDS seawater contains greater than 30,000 mg/L IDS. This difference in IDS 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 larger amounts of dissolved soHds led to the development of RO membranes capable of operating at pressures approaching 10.3 MFa (1500 psi). Desalination plants around the world process both brackish water and seawater (15). 

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). 

Figure 26-11 shows how seawater can be desalinated under pressure by reverse osmosis. The largest desalination plant in the world is located in Jubail, Saudi Arabia, where it produces 50% of the country s drinking water by the reverse osmosis of seawater from the Persian Gulf. Smaller desalination plants are in operation in Israel, California, and Rorida. 

Currently another process for the production of potable water from seawater is becoming established reverse osmosis (RO). The RO-process is particularly suitable for small plants. Therefore almost 70% of all plants operate according this principle, but they account for only 35% of the desalination capacity. In osmosis, water permeates... 

The DuPont Permasep Engineering Manual11 has published a "guide" for the capital, operating and maintenance costs for both a brackish water system and a seawater system. The brackish water system costs are shown in Table 4.10. They are based on a large brackish water system built in the southern United States in 1982. The estimated capital cost of the plant is 1.25 per gallon per day of product water installed. This cost includes the cost of wells, a reverse osmosis system with pretreatment, a building for the reverse osmosis systems and office. The above installed capacity cost does not include the cost of land nor an independent power source. 

The PA-300 membrane was the first composite reverse osmosis membrane to be used successfully in a major seawater desalination facility-the 3.2 MGD plant at Jeddah, Saudi Arabia.35 Much of the original membrane in this plant, as well as replacement membrane, is believed to be RC-100 (the TDI-based analog) because of its greater stability and retention of salt rejection. The Jeddah plant was truly a pioneer installation for spiral-wound composite membranes. Even though it was attended by a variety of start-up and operating problems, mostly unrelated to the membranes,36 it continues to operate successfully at this date. 

According to the American Water Works Association, more than 12,500 desalination plants in 120 countries were in operation in 2004 60% of these plants are in the Middle East. Two methods used to purify seawater are reverse osmosis and solar distillation. 

In addition, the seasonal population of McMurdo Station continued to grow which increased the demand for water. For that reason, the US Congress in 1960 authorized the construction of a nuclear-fission reactor in order to provide power for the desalination of seawater. The components arrived on December of 1961 and were installed in a building that was erected at a site on the slope of Observation Hill above the station (Fig. 2.9). This reactor, which was put into operation in March of 1962, provided the power required to operate a desahnation plant that converted seawater into fresh water (Neider 1974). However, in spite of the technological snperiority of this process, water continued to be in short supply and had to be rationed. Matters came to a head when the representatives of the Antarctic Treaty Nations determined that the nuclear reactor violated the Treaty and therefore had to be shut down, dismantled, and all parts of it had to be removed from Antarctica. The Office of Polar Programs (OPP) did what was required and all radioactive waste was shipped to CaUfomia The nuclear installation was replaced by a desahnation plant that is energized by fuel oil. The capacity of the present facility based on reverse osmosis is sufficient to provide an adequate... 

Typical seawater desalinahon plants using reverse osmosis produce 30 to 50 gfd of water [gal (U.S.) per square ft per day] (1 gfd = 4.72 x 10 up/m s). Suppose it is decided to double tire operating pressure from 50 to 100 atm. 

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