|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|
As has been indicated, Kuwait has been producing fresh water by distilling seawater since the 1950s. The multi-stage flash desalting process, which has been used exclusively in the Arabian peninsula, has proved to be very simple and reliable, but it requires extensive materials and energy. The MSF system reached its maturity with very few improvements. It seems, however, that the race for the second generation of seawater desalters will be won by reverse osmosis and low-temperature multi-effect horizontal-tube evaporators (Darwish and Jawad 1989a). Both systems are characterized by their low energy requirements, as compared with MSF. Energy consumption is the largest single cost item in desalination. Intensive efforts were made in the 1980s to evaluate the feasibility of RO desalination of seawater, including the installation of a pilot RO plant in Doha, where a cost analysis was made to compare the costs of the experimental RO and the existing MSF systems.
3.4.2 The Doha experimental RO plant
This pilot plant, with an installed capacity of 3,000 m³ per day, was installed in 1984 to evaluate membrane and operating systems. It has three lines, equipped with different types of modules, namely spiral wound, hollow fine FIBRE, and plate-frame (Darwish and Jawad 1989b):
The feed seawater to the RO plant usually contains high concentrations of inorganic salts and foreign materials that can foul membranes and decrease their productivity. The main foulants associated with feed seawater are due to biological slime formation, suspended solids, colloids, metal oxide, and scale formation. Pre-treatment is essential to control the life of the membranes. Different methods of conventional pre-treatment were examined in each line.
Since the beginning of the plant's operation, pre-treatment has been running satisfactorily, with an availability of more than 96%. Most of the time, it has been successfully controlled to give a silt density index of less than 4, but in some cases it has failed to produce an acceptable quality, owing to clogging of the dual-media filters, absence of or overdosing with FeClSO4, breakdown of the destabilizer mixer, and climatic conditions such as temperature, dust storms, and wind.
3.4.3 Cost evaluation
The cost effectiveness of the membrane (RO) process can be assessed by comparison with the cost of the predominant thermal (MSF) process. These costs have two major components: (1) direct capital cost, and (2) operation and maintenance costs. The cost of equipment forms a major part of the capital cost, while the cost of the energy and chemicals consumed forms a major part of the operating and maintenance cost.
An evaluation was undertaken to compare the cost per unit of water produced by large-scale MSF and RO plants of typical design, each with an installed capacity of 27,300 m³ (6 mig) per day. The feed water is assumed to be of the quality of seawater in the Arabian Gulf, with concentrations per litre of 45,000 mg of TDS, 800 mg of Ca++, 1,700 mg of Mg++, 12,500 mg of Na+, 500 mg of K+, 3,600 mg of SO4--, 24 mg of CO3--, 24,000 mg of Cl-, 180 mg of HCO3 ,12 mg of Sr++, and 0.04 mg of Ba++. The evaluation further assumes the unit electric energy cost to be US$0.07/kWh, the rate of replacement of the membranes 20% per year, twenty years' plant life, a 90% load factor, and an interest rate of 10% a year. The results of the comparison were as follows (Darwish and Jawad 1989b):
FACILITIES. The seawater intake size and flow rate of the MSF unit are twice those of the RO unit.
The volume of the MSF unit is about three times that required for the RO permeators. The land area required for the MSF unit is at least four times that required for the RO permeators.
Extensive and heavy materials are used in the MSF unit, which are more than ten times those required for the RO unit. The heavy weight of the MSF unit requires heavy foundations and extensive civil engineering work.
ENERGY CONSUMPTION. Thermal energy is consumed only by the MSF unit, and amounts to 89 MW. This thermal energy can be very expensive if it is obtained directly from boilers (not extracted from steam turbines).
The energy consumption for pumping seawater to the pre-treatment system and the high-pressure feed pump in the RO plant was estimated to be 0.25 and 7.98 kWh/m³ respectively, or a total of 8.23 kWh/m³, which is about 25% more than that required for the MSF unit. However, the pumping energy for RO can be decreased about 30%, from 8.23 to 5.9 kWh/m³, by installing an energy-recovery unit such as a reversed centrifugal pump or Pelton wheel.
The average energy consumption per cubic metre of the product water for the MSF unit was 15.27 kWh/m³, which was about three times as high as the rate of 5.9 kWh/m³ for the RO plant.
UNIT COST OF PRODUCT WATER. From the above analysis, the on-site unit water costs of seawater desalination were estimated to be US$2.7/ m³ by MSF and US$1.7/m³ by RO. These are about twice as high as the costs of US$1.00/m³ for municipal water supply and of US$0.95/m³ for waste-water treatment in Japan (MFJ 1991), which are often used as a world standard for comparison.
CONJUNCTIVE USE PLAN FOR MSF AND RO. Introducing RO seawater desalting plants in Kuwait does not mean phasing out older desalting units. A combination of new RO and existing MSF units could be cost effective in a water-supply plan, as illustrated in fig. 3.10 in section 3.6 below.