Water production costs seawater

Seawater is increasingly being used as a RW source for industrial water, drinking water, and boiler FW because of both a lack of suitable alternatives in some areas of the world and constantly improving RO water production-cost ratios. Seawater TDS levels vary around the world, from approximately 36,000 to more than 45,000 ppm. As TDS levels increase, the RO applied pressure requirement typically may increase from 800 to 1000 psig or more to maintain recovery rates (usually 25-35%). 

El-Zanati and El-Khatib (2007) suggested an IMS consisting of a NF unit as pre-treatment section of an RO unit, while MD was used as a system to concentrate the two brine streams from both NF and RO. The integration of these systems improved the performance of the seawater desalination unit, leading to a water recovery factor of 76.2%. The water production cost was estimated at around 0.92 m. This cost was considered competitive in comparison with those of potable water produced in a RO system for seawater treatment. 

SMART desalination plant incorporates the falling film multi-effect evaporation with horizontal tubes and a steam jet ejector (thermal vapour compressor). The desalination unit is designed with a plant life of 30 years, performance ratio of 19.6, acid cleaning to be performed once in 12 months, maximum brine temperature of 65°C, and the supplied seawater temperature of 33°C. Thermal vapour compressor is introduced to improve thermal efficiency of the process steam. The advantages of this design are high heat transfer coefficients and a relatively simple operation system. The performance ratio of desalination plant, one of the most important coupling parameters, was optimized based on the sensitivity analysis of water production cost and on the requirements to the SMART desalination plant. 

A state-of-the-art RO seawater system processes 50 million gallons per day with 50% feedwater recovery as potable water product using a 940-psi ( 65 bar) feed pressure [12]. These high pressures and flows are now routinely accommodated economically with compact vessels and high productivity membranes. An optimized thermal distillation plant with the same feedwater requires 1014 Btu/gal [78.5 (kwh)/m3] of water produced [8], while the state-of-the-art seawater RO system has an energy cost of only 2.2 (kwh)/m3 [8,12]. Using the current paradigm... 

The cost of potable water production from seawater is mainly dependent upon the cost of the energy consumed. It is, however, considerably higher than that for potable water produced from freshwater, a factor of 4 in Europe. 

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. 

In addition, an economic analysis of the desalination plant was performed to investigate economic viability of the SMART desalination plant. The results show that SMART is competitive with other power options, particularly with a gas fired combined plant, within a limited range of electricity generation. The calculated unit cost of fresh water production under desalination capacity of 40,000 mVday using the MED process were in the range of 0.56-0.88 /m for 80% plant availability, which is close to the results of studies performed in other countries. These results indicate that SMART can be considered as a competitive choice for seawater desalination. 

Table 3.8 compares the estimated costs of potable water production through seawater desalination cogeneration with conventional and VHTR power plants (Sato et al., 2014). The conventional plant is based on a modem gas turbine combined cycle (GTCC) power plant at 55% power generation efficiency. The VHTR cogeneration system is that described in Section 3.4.2.2. The costs were evaluated by an original equipment manufacturer (OEM) vendor active in the Middle East desalination plant constmction. The vendor carried out the plant equipment design and evaluated the required operation and maintenance. The cost estimation was then developed based on the vendor construction and operation know-how of comparable-scale MSF plants. 

Source water quality has a key influence on the suitability of using seawater desalination for industrial water supply. The water quality parameters that have a significant impact on the desalination system design, operations, and cost of water production are the concentration of TDS, chlorides, turbidity, silt density index (SDl), organic content, nutrients, algae, bacteria, temperature, boron, sUica, barium, calcium, and magnesium. 

Reverse osmosis is now extensively used to reduce salt concentrations in brackish waters and to treat industrial waste water, for example, from pulp mills. Reverse osmosis has also proved economical (the cost can be as low as about 1 per 1000 liters) for large-scale desalination of seawater, a proposition of major interest in the Middle East, where almost all potable water is now obtained by various means from seawater or from brackish wells. Thus, at Ras Abu Janjur, Bahrain, a reverse osmosis plant converts brackish feedwater containing 19,000 ppm dissolved solids to potable water with 260 ppm dissolved solids at a rate of over 55,000 m3 per day, with an electricity consumption of 4.8 kilowatt hours per cubic meter of product. On a 1000-fold smaller scale, the resort community on Heron Island, Great Barrier Reef, Australia, obtains most of its fresh water from seawater (36,000 ppm dissolved salts) directly by reverse osmosis, at a cost of about 10 per 1000 liters. 

---