Seawater reverse osmosis operations

Figure 16. Decrease of separation (or increase of solute permeabilities) of seawater reverse osmosis desalination at several concentration levels of NaOCl. Initial membrane constants pure water permeability constant = 97.0 nmol m Pa s and the solute permeability constant for NaCl = 0.9 X 10 cm s . Operational conditions k = 7.(97 X 10 cm s Ap = 6.0 MPa, and T = 25°C.

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. 

Brackish waters contain between 0.05 and 1 wt % TDS. Their lower osmotic pressures allow reverse osmosis operation between 15 and 30 bar. Less expensive pressure equipment and energy consumption translate to more favorable water production economics than those for seawater desalination. 

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). 

The greatest use of membranes is for reverse osmosis desalination of seawater and purification of brackish waters. Spiral wound and hollow fiber equipment primarily are applied to this service. Table 19.6 has some operating data, but the literature is very extensive and reference should be made there for details of performance and economics. 

Reverse osmosis is nsed as a method of desalting seawater, recovering wastewater from paper mill operations, pollution control, industrial water treatment, chemical separations, and food processing. This method involves application of pressure to the surface of a saline solution, thus forcing pure water to pass from the solution through a membrane that is too dense to permit passage of sodium and chlorine ions. Hollow fibers of cellulose acetate or nylon are used as membranes, since their large surface area offers more efficient separation. 

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

Lorain, O., B. Hersant, F. Persin, A. Grasmick, N. Brunard, and J. M. Espenan. 2007. Ultrafiltration membrane pre-treatment benefits for reverse osmosis process in seawater desalting. Quantification in terms of capital investment cost and operating cost reduction. Desalination 203 277-285. 

Reverse osmosis membranes can be divided into subclasses according to their solute/water selectivity and operating pressure regimes. Figure 30 shows a number of commercial membranes developed for seawater and brackish desalination, and for nanofiltration. These include cellulose ester and polyamide asymmetric membranes available since the 1960s, and high-performance composite membranes developed in the 1970s. Collectively, they make it possible to produce potable water from virtually all saline water sources. 

Due to the added resistance of the membrane, the applied pressures required to achieve reverse osmosis are significantly higher than the osmotic pressure. For example, for 1,500 ppm TDS brackish water, RO operating pressures can range from about 150 psi to 400 psi. For seawater at 35,000 ppm TDS, RO operating pressures as high as 1,500 psi may be required. 

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... 

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