|Managing Water for Peace in the Middle East: Alternative Strategies (UNU, 1995, 309 pages)|
|3. Hydro-powered reverse-osmosis desalination in water-resources development in Kuwait|
3.7.1 Development alternatives
The predominant multi-stage flash (MSF) desalination facilities in Kuwait, which consume extensive materials and more energy than RO, will be replaced in stages after the completion their plant life of about 15-20 years. RO seawater desalting plants will replace the old MSF desalter units. The unit cost of RO brackish-groundwater desalination is also much lower than that of seawater desalination, implying lower energy consumption and less capital investment. Hydro-powered RO desalination is the method with the lowest cost, minimizing both energy consumption and capital cost. The development alternatives include the following:
>> MSF distilling of seawater (existing plant; highest cost);
>> RO desalination of seawater (experimental stage now being completed);
>> RO desalination of brackish groundwater (existing skid-mounted RO units);
>> Hydro-powered RO desalination of brackish groundwater (as proposed here; the lowest-cost method).
3. 7.2 Conjunctive-use plan
A number of old MSF plants in Kuwait are going to be phased out by the year 2000. Seawater RO will replace the old MSF in stages, but almost pure water from the existing MSF system will also be blended with RO product water with a salinity of about 500 mg of TDS per litre to obtain water of a quality suitable for drinking.
The salinity ranges of the product water from the various processes are as follows:
These may be compared with the WHO drinking-water standards of 250-500 mg of TDS per litre for Europe, the United States, and Japan and 500-1,000 mg/l, for the Middle East.
In a hybrid RO/MSF seawater desalination system, a seawater reverseosmosis plant is combined with either a new or an existing MSF co-generation plant, with the following advantages:
>> Both the capital and operating cost for the RO system are reduced.
>> A single-stage RO process can be used, and the life of the RO membrane can be extended by reducing the water-quality specification for the permeate.
>> The temperature of the MSF product water is reduced by blend ing the hot product water from MSF distillation with the RO permeate.
The combination of the new RO system with the existing MSF will be the key approach to developing the Kuwait water supply system in the 1990s.
Combining the proposed hydro-powered RO desalination system with the present MSF system would make more effective use of brackish-groundwater desalination at the lowest cost, taking into account the limited potential of brackish-groundwater resources and the unlimited potential source of seawater. The salinity of the permeate from brackish-groundwater RO desalination can be controlled in the range 100-500 mg of TDS per litre, while the brine reject water has a salinity as high as 10,000 mg/l or more. The salinity of the product water from MSF distillation is as low as 25-50 mg/l, Brackish-groundwater RO desalination will contribute (1) a direct supply of good quality drinking water to meet WHO standards and (2) an indirect supply by blending brine reject water from RO with almost pure water from the existing MSF system. The conjunctive-use plan suggests the following priority uses of the water and energy elements:
>> RO product water (permeate)-31.5 million m³ per year with 500 mg of TDS per litre-for direct use for drinking-water supply;
>> RO brine reject water-8 million m³ per year with 17,700 mg of TDS per litre-for indirect use to blend with MSF product water;
>> MSF product water-632,000 m³ per day maximum with 25-50 mg of TDS per litre-to be blended with RO brine reject water;
>> MSF brine reject water->45,000 mg of TDS per litre-to be safely disposed of offshore in Kuwait Bay;
>> energy recovered from the RO process-1.98 million kWh of electricity per year-to be used to supply electricity for treatment and/ or pumping.
A tentative conjunctive-use plan is illustrated as a flow diagram in fig. 3.10.
3.7.3 Remarks on future development planning
The desalination of saline water by a membrane separation with low energy requirements will play an increasingly important role in the water-resources planning of arid states in the twenty-first century. Reverse osmosis is the least costly process today, but it may not be the optimum solution, which may be neither reverse osmosis nor thermal desalination. Membrane desalination, however, will be a key application for water-resources planning in the twentyfirst century. A new desalination system using reverse osmosis either with or without the use of hydro-power will be incorporated in the existing MSF system in Kuwait by stages to make a reality of energy-saving desalination technology.