|Managing Water for Peace in the Middle East: Alternative Strategies (UNU, 1995, 309 pages)|
|4. Hydro-powered reverse-osmosis desalination in water-resources planning in Jordan|
Aqaba, with a population of 42,400, is the largest city in the Ma'an governorate and the fourth largest city in Jordan (DSJ 1988).
The port of Aqaba is Jordan's only access to the sea and therefore of strategic importance to commerce and industry. The highest growth of water demand is projected in the Ma'an governorate, from 11 million m³ per year in 1990 to 29 million m³ in 2005, with the greatest increase being in water demand for industrial use (World Bank 1988). Aqaba regional development will be even more constrained by water shortage for municipal and industrial use because of complete dependence on non-renewable or fossil groundwater in the deep sandstone Disi aquifer, about 50 km north-east from the city of Aqaba.
This section examines the application of mini-hydro-power from groundwater for brackish-groundwater reverse-osmosis desalination, proposing that mini-hydro-power plants and a RO desalting plant should be added to the existing Disi-Aqaba water pipeline system. This new proposal for co-generation would include the following objectives to sustain regional economic development:
>> recovery of the potential energy in the existing groundwater pipeline (trunk main) system, which is being wasted;
>> conservation of the non-renewable fresh groundwater in the Disi aquifer, replacing it by developing the brackish groundwater in the Kurnub sandstones;
>> desalting the brackish groundwater by hydro-powered reverse osmosis, using some of the recovered hydro-potential energy in the existing pipeline;
>> testing the technical feasibility and cost-effectiveness of the proposed cogenerating application with mini-hydro-power and RO desalination;
>> conservation of energy and water resources by introducing hybrid hydropowered RO desalination with an energy-recovery system.
4.5.1 Background of the Aqaba water supply
Aqaba is situated at the head of the Gulf of Aqaba on the Red Sea, at the southern end of Wadi Araba (fig. 4.2). Only 40 years ago Aqaba was a sleepy little fishing village whose small population lived in mudbrick houses nestling among palm groves which are still a delightful feature of the town.
However, as well as being Jordan's only outlet to the sea, Aqaba occupies a strategic position, providing an important link between the Middle East and East Africa. The port of Aqaba was a strategic point in the war between Iraq and Iran in the 1980s, and again in the Gulf war of 1990-91. It now handles all the sea imports and exports of Jordan as well as much of those for Iraq, Syria, and Lebanon. The volume of traffic through the port has increased spectacularly since just before the Gulf war, and Aqaba is still an important commercial centre. This expansion has been accompanied by rapid growth in industrial development along Jordan's limited coastline.
As a small fishing town, its water needs were readily met from shallow wells dug near the sea which produced sufficient quantities of good fresh water permeating to the sea through the alluvial fan of Wadi Araba. But shortly after World War II, as the demand for water increased, boreholes were drilled further inland. Well No. 1 was constructed in 1958, 2 km north of the sea. In 1964 well No. 2 was drilled further inland and water pumped to a 2,250 m³ reservoir, augmenting the supply.
Over-pumping of these wells resulted in the intrusion of seawater. To satisfy the increasing demand, additional holes were drilled in the deep alluvial deposits of Wadi Yutm. Until the middle 1970s these wells provided the entire water supply for Aqaba, but, with the limited yield of the alluvial aquifer, there have been increasing shortages, especially during the hot summer months, and rationing has been necessary for a number of years.
4.5.2 The Disi aquifer
Since the heart of the project is the water source, and the success of the scheme depends entirely on a correct assessment of the yield of the aquifer, intensive hydro-geological studies have been carried out since 1976 (NRA] 1977, 1978).
Groundwater flow through the Disi area originates in the Um Sahm mountains, discharging in a north-easterly direction around each end of the geological feature called the Kharawi dyke, which forms a natural underground barrier. The new wellfield at Qa Disi will intercept a large proportion of the flow at present passing round the northwestern limit of the dyke and will slowly develop in the groundwater a large depression, centred at Disi. The extent and rate of development of this depression has been simulated by digital computer models (NRAJ 1982). From the model simulation studies that have been carried out it was concluded that the aquifer will support a maximum abstraction from the Qa Disi area of between 17 and 19 million m³ per year for at least fifty years. The maximum capacity of the scheme has therefore been fixed at 17.5 million m³ per year.
4.5.3 Disi-Aqaba water supply scheme
The Aqaba water supply scheme comprises four main elements: (1) the wellfield and headworks complex, (2) the trunk main from Disi to Aqaba, (3) the trunk distribution main from Aqaba to a fertilizer factory near the Saudi border, and (4) a distribution network within the town (fig. 4.4). The scheme was completed and has been in operation since the end of 1981.
HEADWORKS. For the first-stage development to exploit 10 million m³ per year, seven boreholes 400 m deep were drilled to penetrate the Disi sandstone aquifers. The finished diameter of the upper half of the boreholes is 219 mm and of the lower half 171 mm. Each borehole is equipped with twin submersible pumps delivering water through collecting mains into a reservoir from where the water gravitates to Aqaba. Power for the pumps is provided by a power station equipped with four diesel generating sets of 550 kW each.
TRUNK MAINS. A ductile iron trunk main 800-450 mm in diameter and 92 km long carries the water to Aqaba and southwards to the fertilizer factory near the Saudi border. Pressure is broken at three locations along the pipeline, as shown in the profile of the trunk main (fig. 4.5), to limit pressure to a maximum of 25 bar, which is the ceiling bearing capacity of the ductile iron steel pipe used in this project.
A large reservoir of 9,000 m³ capacity, sited immediately north of Aqaba, provides a buffer to absorb fluctuation in demand downstream and reservoir storage in the event of a pipeline failure. A 4,500 m³ reservoir is constructed at the fertilizer factory to provide service storage for the factory and for other industrial developments expected in the same area.
PROJECT COST. The total cost of the Disi-Aqaba water supply project was estimated at ID 11 million (US$44 million at 1978 prices of the Jordan Water Supply Corporation), including the following main cost elements:
The total cost of the project is estimated to be US$74.8 million, assuming price escalation at 170% from 1978 to 1990 (IMP international financial statistics, 1990/1978).
4.5.4 Introduction of mini-hydro development
The theoretical hydro-potential of the Qa Disi wellfield, which is situated at an elevation of 840 m above sea level, is preliminarily estimated to be 5.2 MW by assuming a flow discharge of 0.663 m³/sec with an effective differential head of water of 800 m (95 % of the total head). This hydro-potential energy is being wasted by breaking the water pressure at three locations along the pipeline as described above.
This study aims to evaluate the effectiveness of using the hydropotential in the trunk main between Disi and Aqaba by installing a series of mini-hydro stations in the existing trunk main at each point with a difference head of water of about 200 m. The head difference between the collecting reservoir (840 m) and the terminal reservoir (220 m) is 620 m. The hydro-potential of the existing trunk main between the collecting reservoir and the terminal reservoir is estimated to be 3.2 MW, using the equations given in section 3.6.3 to evaluate the hydro-potential and power.
The flow discharge is assumed to be 17.5 million m³ per year (0.555 m³/sec), which is equivalent to a design capacity of 0.663 m³/sec with a unit operating time of 21 hours per day. The effective differential head of water is estimated to be 589 m, assuming a 5% friction head loss.
From the optimal layout of the pressure pipeline system (fig. 4.5), two hydro-power stations would be installed, at ground elevations of 630 m and 410 m respectively.
By assuming a synthesized efficiency of 0.80 and a generating efficiency of 0.873, the installed capacity and annual power output are estimated to be 2 MW and 15,900 MWh per year respectively. The details are shown in table 4.1.
4.5.5 Conservation of fossil groundwater in Disi
Disi is currently exploited for M&I water supply for Aqaba (8.5 million m³ per year) and for irrigated agriculture. The recorded extraction for 1986 is 14.5 million m³, but 3,000 ha have now been developed for agriculture, implying an extraction of over 30 million m³ per year, and licences have been granted to drill wells for the irrigation of over 20,000 ha, implying an annual extraction of over 200 million m³. It should be noted that the aquifer is extremely expensive to develop for irrigation for growing wheat: the water table lies 250-300 m below the surface and wells have to be drilled to a depth of 500-1,000 m. Furthermore, Disi represents, with Al-Wuheda dam, Jordan's last substantial unexploited water resource and deserves to be regarded as a strategic water reserve. A World Bank study (1988) recommended that the aquifer be monitored at present abstraction levels to confirm the most reasonable long-term yield for M&I supply in south Jordan.
Table 4.1 Installed capacity ana annual power output of Disi-Aqaba groundwder-hydro scheme
|Station||Elevation (m)||Effective head (m)||Installed capacity (kW)||Potential power generation (MWh/year)|
Note: Friction head loss assumed to be 5% of the total head. Synthesized efficiency assumed to be 0.80. Load factor of mini hydro-power generation, 83.7% (0.555/0.663). Elevation of collecting reservoir, 840 m above sea level. Elevation of terminal reservoir, 220 m above sea level.
a. Groundwater hydropower potential.
b. Energy recovery from hydro-powered RO desalination.
4.5.6 Brackish groundwater resources
The Kurnub group of Lower Cretaceous age underlies almost all of Jordan. It is composed of sandstones with poorly to very well cemented facies interbedding silts, clays, shales, and occasionally dolomitic layers. It is thought to be a deep aquifer unit with a large storage potential and a maximum thickness of about 1,000 m or more. The water table, however, is about 200300 m or more below the ground level, and permeability and salinity vary in place and depth. The quality of the groundwater varies from 300 to 2,800 mg of TDS per litre, but it is considered to be mostly brackish except for minor recharging areas in the north-western highlands.
In southern Jordan, the Disi aquifer is unconformably overlain by the Khreim formation, which is about 100-300 m thick and stores brackish groundwaterin its upper to middle sections. Brackish groundwater is also found in the Kurnub formation along the southern edges of the Jafr basin about 25 km north from the Disi (fig. 4.5). The depth of the pumping water level will range from 100 to 250 m in the Khreim formation between Disi and Muddawwara, while it is as deep as 230325 m in the Kurnub formation along the southern fringe of the Jafr basin. These brackish waters with salinity between 1,000 and 5,000 mg of TDS per litre in southern Jordan would mostly be fossil with limited amounts of natural recharge from rain. The storage potential, however, has been estimated to be as large as 16,600 million m³ (NRAJ 1986). Brackish-groundwater development with desalination may thus be able to replace existing fossil-groundwater abstraction from the Disi aquifer.
4.5.7 Hydro-powered brackish-groundwater desalination by RO
Co-generation, that is annexing of a brackish-groundwater RO desalination unit to a groundwater-hydro system would develop the hydro-potential energy in the differential head of water between the Disi wellfield at 840 m and the Aqaba terminal reservoir at 220 m. Two thirds of the differential head of water would be used to generate hydro-powered electricity and one-third to produce the hydraulic pressure for permeating the RO (fig. 4.5). The proposed co-generating system for the Disi-Aqaba water supply scheme would include the following objectives and measures:
>> developing clean hydro-potential energy in the existing water trunk main between Disi and Aqaba, amounting to 620 m of differential head of water, thus indirectly conserving fossil (oil) energy in generating electricity;
>> co-production of hydro-powered electricity and fresh water for Aqaba M&I water supply;
>> development of brackish groundwater resources in the Khreim and/ or Kurnub formations, conserving fossil groundwater in the Disi aquifer, which has been being abstracted for M&I water supply for Aqaba since 1970;
>> direct use of hydro-potential energy for generating pressure for reverse osmosis, utilizing part of the hydro-potential energy at 150250 m of differential head of water in the trunk main, whose pressure at 15-25 kg/cm² is the optimum requirement for operating reverse osmosis;
>> pioneer research on brackish-groundwater RO desalination in Jor dan, evaluating the cost-effectiveness of minimizing operation and maintenance costs, which are a major cost factor in desalination engineering.
Brackish groundwater would be pumped at a rate of 0.663 m³/sec from the Khreim and/or Kurnub formations underlying the area of Disi-MuddawwaraShidiya, and be conveyed to a collecting reservoir at an elevation of 840 m. For the purpose of this case study, the design value of the salinity of the feed water is assume to be 4,000 mg of TDS per litre.
The brackish water would flow down from the collecting reservoir at 840 m elevation to the desalination plant and terminal reservoir at 220 m through the existing pipeline system, passing two mini-hydropower stations by steps at 630 m and 410 m respectively. The installed capacity of the two stations is estimated to be 2,078 kW and the annual power output 15,900 MWh.
The hydro-powered RO system would have three parts: a pretreatment unit, a pressure pipeline unit, and the RO unit itself. The pretreatment unit would be sited just beside the outlet of the second minihydro-power station at 410 m elevation and would include dual-media filters (hydro-anthracite and fine sands) and cartridge filters (5-microm size). After passing through the cartridge filter, the flow would pass through a pressure pipeline (the trunk main) between 410 m and 220 m to obtain a hydraulic pressure of 18 kg/cm², which would be used directly to overcome the osmotic pressure and permeate the RO membrane. The main heart of the RO unit is a low-pressure-type membrane, spiral-wound in a composite-type 8-inch-diameter module, with the following specifications:
A unit line of the RO vessel would consist of a series circuit of six modules. Recovery is estimated to be 70% of the feed water, including 40,100 m³ per day of permeate with a salinity of 500 mg TDS per litre and 10,200 m³ of brine reject with 17,700 mg/l, The effective pressure of the brine reject is estimated to be 15 kg/cm², assuming a friction loss of 3 kg/cm² in the RO circuit. The potential energy recovery from the RO brine reject is preliminarily estimated to be 136 kW, assuming the total efficiency of the turbine generator to be 80%, which would generate 810,000 kWh of electricity per year with a load factor of 68%. Another alternative would be to develop 0.72 m³/sec of brackish groundwater to produce 43,400 m³ of permeate per day, which is equivalent to the current water supply volume of 15.8 million m³ per year.
UNIT WATER COST. The total investment cost for the proposed hydropowered RO desalination, based on 1990 prices with 8% interest during three years' construction, is preliminarily estimated to be US$56,088,000, with an annual capital cost of US$2,677,000, comprising the following major elements:
The annual cost of the operation and maintenance is estimated to be US$2,631,000, including the following main cost elements:
The above cost estimates are based on 1990 prices and the following assumptions:
The unit water cost for 14.6 million m³ of design annual product water is estimated to be US$0.41/m³.
COST FOR GROUNDWATER HYDRO-POWER. The capital cost of the proposed two mini-hydro-power stations, each equipped with a 1 MW Pelton turbine, is preliminarily estimated to be US$2 million, accounting for only 5.7% of the capital cost of the RO unit. The generated power of 16 million kWh per year will be used in part to supply elec tricity for pumping the groundwater wells and in part to recover the investment cost of the plants. The costs of existing hydraulic structures such as pipelines and reservoirs are referred in section 4.5.3 above.
OTHER DEVELOPMENT ALTERNATIVES. After yielding its hydropotential energy in the recovery unit, the pressure-free brine water at 17,700 mg of TDS per litre would be discharged directly into the Gulf of Aqaba, where it would combine harmlessly with seawater at 45,000 mall; or it could be used for blending with distilled water if a thermal or solar seawater desalination system were constructed. The desalination of seawater at Aqaba will be an important means of supplying fresh water from non-conventional sources, which may include the following four options:
Non-conventional water-resources development alternatives including hydro-powered brackish-groundwater desalination and seawater desalination at Aqaba can therefore be integrated into the framework of a regional water master plan that would make the region self-supporting.
A pumped-storage application with seawater RO desalination for cogeneration in the context of an inter-state Aqaba regional economic development plan is discussed in section 5.6.