DESALINATION: CURRENT SITUATION AND FUTURE PROSPECTS
1. Introduction
This review presents the world situation concerning the production, processes, technologies and costs of desalination. Experience accumulated worldwide and in Israel enables the forecasting of developmental trends, namely, a continuation of cost reduction, when Israel will enter the era of large-scale desalination. To date, desalination in Israel has served local demand, mainly in Eilat and in the Negev Desert. However, during the next decade activation of this technology will be necessary for covering water deficits on a national scale (Dreizin, 1998).
2. Desalination in Israel and in the World
2.1. Installations in Israel
By early 2000, 13,600 desalination units were operating worldwide, with an output of 25.9 million cubic meters (MCM) per day (approximately 8 billion cubic meters annually). Eleven percent of these installations were erected within the past two years. Of these units, 38% are located in the Persian Gulf and 17% in the US (Wangnick, 2000). Plants installed in Israel over the last 30 years have a capacity of approximately 100,000 CM/day. However, production actually amounts to 60,000 CM/day, since some of the plants, mainly those based on evaporation technology, are not in operation.
2.2. Processes
The two most widely applied processes are Multi-Stage Flash (MSF) evaporation and Reverse Osmosis (RO), each of which produces approximately 43% of the total global output. Although RO has the same output, it is employed in 68% of the installations, with only 9% of the installations using the MSF process. The reason for this similar output in both methods is that the average MSF unit produces 8,800 CM/day, while the average RO unit produces 1,200 CM/day. The distribution of the processes, capacities and number of units in the world is presented in Figures 1 and 2 (Wangnick, 2000).
From these figures it can be seen that those installations operating with the RO and the MSF methods produce 86% of the desalinated water in the world. In addition, electro-dialysis (ED) plants produce 6%, vapor compression (VC) plants produce approximately 4% and multi effect distillation (MED) produces 4% of the desalinated water in the world.
Most MSF installations operate in the Persian Gulf, whereas RO plants are common in other parts of the world. Mediterranean countries, including Spain, Malta and Cyprus, as well as Israel, have reverted to the RO method during the past two decades. All desalination plants in Israel operate using the RO method (Fig. 3).
2.3 Developments
Improvements have been achieved in all technologies, by improved materials, improving pre-treatment of raw water and improving computerized management of operations. Progress in the RO method has been much greater than in the other technologies, since this technology is suitable for all types of water sources as well as due to the intensive competition between companies in this field.
LEGEND:
ED - Electro Dialysis
RO - Reverse Osmosis
MSF - Multi Stage Flash
MED - Multi Effect Distillation
VC - Vapor Compression
Fig. 1: Distribution of Desalination Installations in the World according to the Number of Units
2.4 Costs and Cost Reduction
Massive reductions in costs have occurred during the past decade in all three main cost areas: capital, energy, operation and maintenance. This is due mainly to:
· Technological developments
· Increasing size of plants
· Lower interest rate and energy costs
· Changes in managing enterprise performance
· Intense competition between equipment suppliers worldwide
Thus, for example, the cost of one cubic meter desalinated seawater dropped from one dollar in the early 90s to between 70-80 cents (Glueckstern, 1991; Leitner, 1991).
The investment in a typical plant with a capacity of 20,000 CM/day, such as the plant established at the beginning of the decade in the Canary Isles, was approximately $20 million. The cost of the desalinated water was approximately one dollar, distributed as follows:
· Capital cost according to 8% for 20 years 30 cents/CM
· Energy costs (5.5 kWh/CM) according to 8 cents 44 cents/CM
· Operation and maintenance 24 cents/CM
· Total 98 cents/CM
For comparison purposes, the capital cost of a plant of similar capacity in Eilat (Glueckstern, 1998), Israel, which began operations in 1997, and which will produce 20,000 cubic meters per day by 2003, is also $20 million. However, the cost of water is 25% lower, as indicated below:
· Capital cost according to 6.5% for 20 years 27 cents/CM
· Energy costs (4 kWh/CM) according to 6 cents 24 cents/CM
· Operation and maintenance 21 cents/CM
· Total 72 cents/CM
LEGEND:
ED - Electro Dialysis
RO - Reverse Osmosis
MSF - Multi Stage Flash
MED - Multi Effect Distillation
VC - Vapor Compression
Fig. 2: Capacity of Desalination Installations in the World According to Types of Processes
The lower water cost is mainly due to the energy component: the Eilat plant uses 30% less electricity, and the cost of electricity is also lower. Lower operating costs are mainly due to the lower cost of membranes and pre-treatment chemicals. Larger plants will achieve even larger cost reductions.
Tenders for Build-Own-Operate (BOO) or Build-Own-Operate Transfer (BOOT) plants have recently been published as well as a tender for a large plant with a capacity of 76,000 CM/day (25-27 million cubic meters per year), to be erected in Tampa, Florida (Leitner, 1998). This approach leads to water cost reductions. Of the five international concerns bidding for the Tampa plant, the three bids proposing the establishment of RO plants were the lowest cost bids. The other two bids proposed the establishment of a Vapor Compression plant and a Multiple Effect plant together with a power plant. These were more expensive proposals.
Investments per cubic meter per year according to these proposals vary between $2.5 (the lowest) and $7.5 (the highest). The cost of water produced by the BOO method for 30 years, varies between 61 cents/CM and $1.55/CM. The higher price refers to the proposal of a combined water desalination plant and power station. The average cost of water from the proposed RO units will be 70 cents/CM, i.e. less than half the most expensive proposal (Fig. 4).
2.5 Development During the Past Decade
The decrease in desalinated water costs discussed above have been achieved as a result of a combination of technological development and changes in macro-economic parameters such as the prices of fuel and capital, and the transition to larger units that enjoy the "advantages of size". The most extensive progress took place in the RO process, mainly in the area of performance and salt rejection, durability and lower membrane costs per output unit.
Fig. 3: Distribution of Mekorot's Desalination Sites - Active and Reserve Installations (1999)
Thus, for example, salt rejection has risen from 98.5% ten years ago to 99.7% today, whereas the output per single unit within a series of membranes installed in a single pressure vessel has risen from 60 cubic meters per day in 1991 to 84 CM/day in 1998. The increasing demand and the competition have led to a reduction of approximately 50% in cost per unit output. Furthermore, in 1998 the membrane manufacturers extended the guaranteed membrane life span from two-three years to six and even ten.
The effect of membrane price and life span on costs is presented in Fig. 5. It can be seen that the cost of replacing membranes has declined from a range of 10-16 cents/CM to between five and eight cents and has decreased further in 1998 to between 2.5-3.5 cents/CM.
Another component that affected desalination cost reductions is the significant decrease in unit energy costs, resulting from improvements in pump and turbine technology for recovery of energy from the reject brine. This factor, combined with the trend of building larger installations, enabled a reduction in energy requirements in 1998 from 5.5-6.0 kWh/CM to 4.0-4.5 kWh/CM (Fig. 6). This development, together with the lower cost of electricity resulting from the decrease in fuel prices, has led to a 30%-40% reduction in desalination energy costs.
3. Future Prospects
3.1. The next ten years
A forecast for the next ten years is presented herewith. This forecast is based on an analysis of the last decade's developments in plant size, improvements and cost reductions of membranes, energy costs and overall costs per cubic meter. It will take into account recent international tenders and the preliminary design by Mekorot (Israel's National Water Company) concerning large desalination plants along Israel's coast, as well as current R&D programs in Israel and abroad, which should bear fruit within the next few years (Glueckstern, 2000).
Fig. 4: Comparison between the Specific Investment and Water Costs in the Various Proposals of the Tampa Tender
3.1.1. Unit Size
Today larger desalination units can be built mainly due to the availability of high-pressure pumps and large turbines for energy recovery. Reasonable estimates indicate that the country will need plants with a capacity of 150,000-300,000 CM desalinated water per day (50-100 million CM/year). Planning for such a scope must take into consideration 10-20 units, each unit yielding 15,000-30,000 CM desalinated water per day.
3.1.2. Membranes
The increase in output per unit membrane will be achieved mainly by improvements in the treatment of raw water, by the implementation of a new pre-treatment method for feed-water using membrane processes such as Micro-Filtration (MF) or Ultra-Filtration (UF).
3.1.3. Energy
A reduction in energy consumption from 4-4.5 kWh/CM to 3.5-3.8 kWh/CM (Fig. 7) will be achieved using larger and more efficient pumps as well as improved pre-treatment in order to minimize fouling, which obligates use of higher pressure. Energy costs will decrease if the units are linked to an extra-high voltage network instead of the high voltage network and with on-site power production, since this electricity is cheaper than network electricity, except during off-peak hours.
3.1.4. Investments
Large installations which will use the cooling water and existing infrastructure of power stations (Glueckstern & Priel, 1998b), will reduce the investment per unit (by 2005) from $3-$3.5 per CM/year to $2.3-$2.8 per CM/year (Fig. 7). Another factor in capital cost is the life span of the desalination plants. It can be assumed that the life span of the plants will increase due to the use of improved building materials, lower interest rates, use of more mature technologies and prolonged plant life span. The risk factor in project financing will thus decrease and the capital cost component will decrease considerably.
Fig. 5: Membrane Replacement Cost Vs. Unit Capacity Cost and Membrane Life Span
3.1.5. Operation and Maintenance
Reduction of this cost factor will result mainly from a reduction in manpower in the large plants. A smaller saving will ensue from the decrease in the cost of membrane replacement and of the chemicals needed for pre-treatment.
3.1.6. Total Costs Per Cubic Meter
All these savings in cost should lower the cost of desalinated water by the year 2005 by approximately 30%, from a range of 67-84 cents per cubic meter to 48-56 cents. The effect of these changes in energy, costs, energy consumption and membrane replacement costs are presented in Fig. 7.
3.2. Long Range Possibilities
The general concern over global ecology has led to the examination of renewable energy sources in various parts of the world, especially in the fields of solar and nuclear energy. In Israel and Jordan, the most likely source is the exploitation of the altitude difference of approximately 400 meters between the Mediterranean or Red Sea and the Dead Sea, known as the "Dead Sea Hydro Project" (Glueckstern, 1995). The evaporation capacity at the Dead Sea enables the establishment of a desalination plant at the end of a canal beginning either at the Mediterranean or at the Red Sea. Exploitation of this potential will enable the desalination of 800 to 1,000 million cubic meters of water a year, which will be supplied to Jordan, the Palestinian Authority, as well as to Israel (to the eastern Negev). Due to the huge investments on the one hand and the excessive (compared to demand in the next decade) quantities of water on the other hand, this possibility will be considered in the more distant future, especially in view of concern for the disappearance of the Dead Sea.
Fig. 6: Relation between Specific Energy Consumption in Reverse Osmosis Plants for Seawater Vs. Pump-Turbine Efficiency, Process Pressure and Recovery Factor
3.3. Trends in Israel
Concomitantly to embarking on a massive seawater desalination program, Israel should exploit lower cost brackish water sources. Tens of millions of cubic meters of brackish water are found in various areas such as the Dead Sea region, Mishor Rotem, the Carmel coast, the Sea of Galilee region and the Western Negev. This water can be desalinated at a cost of 20-50 cents per cubic meter. The lower price refers to large desalination plants and low pumping costs, whereas the higher cost will result from deeper wells with higher drilling and pumping costs. Other marginal brackish water sources, such as polluted river water, agricultural drainage water, fishpond effluents and eventually wastewater will also be considered (Glueckstern & Priel, 2000, 2000b). Desalination of these sources may complement a substantial part of the required capacity at a lower cost and will be friendlier to the environment.
4. Conclusion
The accelerated progress in desalination technologies has substantially lowered water costs. This factor, together with increased demand and the trend to establish bigger plants, will lead to a reduction in water costs in the coming years.
Israel has recently jointed an international research network, with wide-ranging collaboration between research institutions and industry, intended to advance research in various aspects of desalination. These projects are likely to accelerate progress in the treatment of polluted brackish waters other than seawater, such as drainage from agricultural and urban waste sources.
A goal of desalinated seawater at 50 cents per cubic meter is not a "Mission Impossible", if the planning, preparation and management are carried out properly.
Fig. 7: Changes in Cost Components and Total Cost of Seawater Desalination in the Present Decade and Predictions for the Next Decade
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