Desalination, 97 (1994) 301-311 Elsevier Science B.V., Amsterdam - Printed in The Netherlands

301

Hydro-powered reverse osmosis (RO) desalination for co-generation: A Middle East case study

Masahiro Murakami

Nippon Koei Co. Ltd. Consulting Engineers, 54 Koojimachi Chiyoda-ku, Tokyo IO2 (Japan)

SUMMARY

A new co-generation application of the hydro-power development with application of reverse osmosis (RO) desalination is demonstrated in this paper, in which the technical feasibility of the hydro-powered reverse osmosis desalination system is examined in the Aqaba-Disi water pipeline project in Jordan and Galala-Red Sea seawater pumped-storage scheme in Egypt. Substantial reduction in the operating cost and energy could improve the cost constraints of the desalination technology. The unit cost of the hydro-powered reverse osmosis desalination is preliminarily estimated to be US$o.4/m3 for brackish groundwater and US$O.68/m3 for seawater.

INTRODU(JIION

Most countries in the Middle East already have a water deficit. They consume every drop of the rechargeable (annually renewed) water available to them in the rivers and subsurface aquifers and are rapidly mining underground fossil water that can be used once and then is gone for good.

Jordan and many other Arab states will also soon be depleting their own renewable sources if current patterns of water consumption am not quickly and radically altered.

OOll-9164/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved. SsDIOOll-9164(94)00094-5

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Non-conventional water resources and energy development, including desalination of seawater and brackish waters by a co-generation method, will be a key issue of the water resources planning in arid to semi-arid countries in the 21st century. The application of groundwater-hydro and seawater pumped-storage with RO desalination, which is a new type of co-generation system proposed herewith, is likely to be a key technological development in this region for the strategic objectives of saving fossil energy and the global environment. Two case studies of hydro-powered brackish groundwater RO desalination in Aqaba-Disi water pipeline project in Jordan and hydro-powered seawater RO desalination in Galala-Red Sea pumped-storage scheme in Egypt are examined in this study.

AQABA WATER SUPPLY PROJECT IN JORDAN

Aqaba is situated at the head of the Gulf of Aqaba on the Red Sea and at the southern end of Wadi Araba (Fig. 1) Aqaba is an important commercial center of Jordan with expansion being accompanied by a rapid growth of industrial development along Jordans limited coastline. Owing to a hyper-arid climate of the southern Jordan, water supply has been a major constraint of the Aqaba regional development. The Disi wellfield, which is located at 50 km northeast of Aqaba with an elevation at 840 m, was selected for the source of water supply scheme.

The Disi is a non-renewable (fossil) aquifer, however, with less salinity in the range between 300 and 400 ppm of total dissolved solids. From a model simulation study, the aquifer has been estimated to sup a maximum abstraction in the range between 17 x 106m3 to 19 x 10 B

ort m3

per annum for at least 50 years [ 11. A ductile iron trunk main 800-450 mm in diameter and 92 km long carries the water from Disi to Aqaba and southwards to the Fertilizer Factory near the Saudi border. Pressure is broken at three locations along the pipeline to limit pressure to a maximum of 25 (kg/cm*) as shown in profile of the trunk main on Fig. 1.

DISI-AQABA HYDRO-POWERED REVERSE OSMOSIS DESALINATION

SCHEME

A proposed hydro-powered RO desalination is a non-conventional application of co-generating system by annexing groundwater-hydro system with RO desalination unit.

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Fig. 1. Aqaba-Disi water supply scheme for co-generation.

The pioneer research on the brackish groundwater RO desalination in Jordan includes the following objectives; 1) Development of potential energy in a water pipeline (trunk main) to

conserve and/or retrieve clean energy. The existing pipeline system

304

comprises 5.2 MW of theoretical hydro-potential for a total head of 840 m, which is being wasted.

2) Conservation of non-renewable fresh groundwater in Disi aquifer to replace it by developing brackish groundwater in the Khreim and/or Kurnub sandstones.

3) Hydro-electric development to supply a part of peak power electricity, using 400 m of water head difference in the existing trunk main.

4) Brackish groundwater RO desalination to supply fresh with direct use of hydro-potential energy at 200 m of water head difference in the existing trunk main.

The brackish groundwater with salinity about 4,000 ppm/TDS would be exploited from the potential wellfields in the Khreim and/or Kumub formations near the Disi, of which the potential is preliminary estimated to be 1 m3/s to replace it with the fossil groundwater in the Disi aquifer. The average discharge of the trunk main is assumed to be 17.5 x 106/y (0.555 ms/s), which is equivalent to a design capacity of 0.663 m3/s with a unit operating time of 21 hours per day. The brackish water flows down from collecting reservoir (E.L = 840 m) to desalination plant as terminal reservoir (EL = 220 m), through existing pipeline system passing by two hydro-power stations by steps; the 1st hydra-power station (E.L = 630 m) and the 2nd hydro-power station (E.L = 410 m). The installed capacity and annual power output of the two stations are estimated to be 2,078 kW and 15,900 MWh per annum in total, respectively (Table I). The following equations are used by assuming a 5% of friction head loss, 0.80 of synthesized efficiency and 0.873 of generating efficiency;

Pth = 9.8*Q*He (1)

P = Pth*Ef (2)

WP = 365*24*GPkP (3)

where Pth : Hydro-potential (kw) Q : Flow discharge (m3/s) He : Effective difference head of water (m) P : Installed capacity (kW) Ef : Synthesized efficiency (-) wp : Potential power generation per annum (kWh) Gf : Generating efficiency

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TABLE I

Installed capacity and annual power output of Disi-Aqaba scheme

Power Elevation Effective head Installed capacity Potential power generation station (m) (m) 0 VW)

No. 1 630 200 1,039 7,946* No. 2 410 200 1,039 7,946* No. 3 220 180 81&

(Total) (2,078) (woc9* @lo)**

Remarks: * Hydro-power potential of groundwater-hydro ** Energy recovery fkom hydro-powered RO

desalination. 1) Friction head loss is assumed to be 5 5 of the total head. 2) Synthesized efficiency is assumed to be 0.80. 3) Load factor of mini hydra-power generation at 83.7 % (0.555Kl.663). 4) Elevation of the collecting reservoir is at 840 m above sea level. 5) Elevation of the terminal reservoir is at 220 m above sea level.

The hydro-powered RO system is composed of three parts; 1st part of the pre-treatment unit, 2nd part of the pressure pipeline unit, and the 3rd part of the RO unit, of which the system profile is shown in Fig. 1. The pre-treatment unit is to be sited just beside the outlet of the 2nd mini- hydro-power station (E.L = 410 m), including dual-media filters (hydro- anthracite & fine sands), and cartridge filters (5 micron size). After passing through the cartridge filter, the flow water is connected with a pressure pipeline (trunk main between 410 m and 220 m) to obtain the hydraulic pressure at 18 kg/cm2, which is directly used to transfer the osmosis pressure to be needed to permeate the RO membrane. The main heart of the RO unit is a membrane, which is a low-pressure type, spiral wounded compost type with 8 inch diameter, including the specifications; i) salt rejection rate of 87.5%, ii) design operating pressure of 18 kg/cm2, iii) design amount of permeate of 30 ms per day, and iv) maximum operating water temperature at 4OOC, and v) pH of feed water between 6.0 and 6.5.

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A unit line of the RO vessel consists of a series circuit with six modules. Recovery is estimated to be 70 % of the feedwater, including 41,100 m3/d of permeate with salinity at 500 ppm of TDS and 10,200 ms/d of brine reject with TDS at 17,700 ppm. The effective pressure of the brine reject is estimated to be 15 kg/cm2 by assuming the friction loss of 3 kg/cm2 in the RO circuit. The potential energy recovery from the RO brine reject is preliminary estimated to be 136 kW (= 9.8* (10,200/86,400) * 15*9.8*0.8), which is equivalent to generate the electricity of 810,000 kWh per annum by assuming the generating efficiency of 68% (Table I).

The investment cost of the proposed hydra-powered RO desalination scheme is preliminary estimated to be US$56,088,000. The annual cost is estimated to be US$ 2,677,OOO per annum for the capital and US$ 2,631,OOO per annum for the operation and maintenance (Table II). This cost estimate is based on 1990 price and 8% of interest, assuming 1) three years of construction of the plant, 2) plant life of 20 years, 3) membrane life (replacement) of 3 years, 4) excluding cost benefit from energy recovery, 5) excluding costs for groundwater development and pipeline/distribution The unit cost of the permeate, which will supply 14.6 x 106 ms per annum of fresh water to the Aqaba municipality, is preliminary estimated to be US$O.4l/ms.

GAL,AL,A-RED SEA PUMPED-STORAGE SCHEME

Two unique ideas of the seawater pumped-storage development have been discussed in Egypt, including Qatara solar-hydro scheme and Galala-Red Sea pumped-storage scheme (see Fig. 2), to supply peak power electricity in the county.

The scheme involves flooding a natural depression in the Western desert (the Qatara) through a canal or tunnel from the Mediterranean Sea, 56 km away (see Fig. 2) At its lowest point, the depression is 134 m below sea level. The plan envisages generating power utilizing the fall in water to the lake which will eventually be formed, and of which surface will be 60 m below sea level, with an area of 19,500 km2. The theoretical hydro-potential has been estimated to be 315 MW, which includes the pumped-storage alternatives with installed capacity at 900- 2,000MW [2].

Construction of new thermal or nuclear power stations in Egypt has encouraged the Electric Authority to build a pumped-storage plant. In 1989, feasibility study on the 600 MW of seawater pumped-storage scheme was carried out in the North Galala plateau, 55 km south of Suez

TABLE II

Major cost elements of Aqaba-Disi scheme

Major capital cost element US$ (1990 price)

Pre-treatment Desalting plant RO membrane/equipment Control and operating system Appurtenant works Powerline and substation Energy recovery/turbine

Sub-total Design and construction management: Financial expenditure:

6,468,OOO 10,306,ooo 12,417,000

871,000 3,954,ooo 1,143,ooo

255,000 35,414,ooo 9,111,ooo

11,563,000

Major O&M cost element US$ per annum

Labor Material supply Chemicals Membrane replacement

sub-total

544,000 272,OQO

1,089,OOO 726,000

2,63 1,000

(see Fig. 2). The scheme will utilize seawater which would be pumped directly to a natural basin located 587 m above sea level with a storage capacity of 8.2 x 106 m3 [2]. To compare with the Qatara solar-hydra scheme, Galala-Read Sea seawater pumped-storage scheme has two advantages as shown below; i) Delete a substantial capital cost of intake tunnel or canal with a

length 60 to 80km. ii) Minimize the environmental problems of the artificial lake.

The world first seawater pumped-storage scheme, which has been conceived in the early 1980s in Egypt, includes some technical problems such as corrosion of pipe and turbine system. This unique application of non-conventional hydro-power, however, would be marginally feasible in the arid region where deficit of the peak power demand is substantial.

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Fig. 2. Location map of Qatara Depression and Galala Highland.

APPLICATION OF HYDRO-POWERED REVERSE OSMOSIS DESALINATION

IN A SEAWATER PUMPED-STORAGE SCHEME?

The co-generation system is an application of RO desalination annexing to the seawater pumped-storage scheme with 600 MW of installed capacity at differential head of 587 m, of which the schematic profile is shown in Fig. 3. The design flow discharge of the seawater pumped-storage scheme is preliminary estimated to be 119 m3/s (= 600 x 103/ (9.8 x 1.03 x 587 x 0.85)), by assuming specific weight of seawater of 1.03 and synthesized efficiency of 0.85.

309

Fig. 3. Schematic profile of pumped-storage scheme for co-generation.

The pumped-storage scheme is designed to generate peak power of 4-8 hours a day. The marginal operation of the RO system is designed to retrieve the hydro-potential energy in a penstock pipeline with 587 m of head difference during 16-20 hours a day of the off-peak tune.

The feed water requirements to produce 100 x lo6 m3 per annum of permeate with 1,000 mgIl of TDS are estimated to be 333 x lo6 m3 per annum by assuming 30% of recovery ratio (70 % for brine reject). The installed capacity of the RO unit is estimated to be 322,300 m3/d with load factor at 85%. The main heart of the RO unit is a membrane module which is designed to have 1) salt rejection rate of 99%, 2) operating pressure at 5Okg/cm2, and 3) design quantity of permeate of 13 ms/d. The requirement of the RO modules will amount to 30,000 in total.

The energy recovery potential from the brine reject is estimated to be 28,280 kW (= 9.8 x 1.03 x (233 x 106/ 365/86400) x (587 x 0.95 x 0.8) x 0.85) by assuming friction loss of 20% in the RO circuit. Annual product of the electricity from the RO brine is estimated to be annual generation of 168 x lo6 kWh of electricity with load factor at 68%. The recovered energy (electricity) will be used to supply electricity for the

310

post-treatment process or others to save the electricity from the national grid.

The total investment cost for the proposed hydro-powered seawater RO desalination unit is preliminary estimated to be US$ 389,355,OOO. The annual cost is estimated to be US$ 18,568,OOO of the capital and US$44,387,000 of the operation and maintenance (O&M) as shown in Table III.

The unit water cost of the hydro-powered seawater reverse osmosis desalination to produce fresh water of 100 x 106 m3 per annum is preliminary estimated to be US$O.68/m3.

TABLE III

Major cost elements of Galala-Red Sea scheme

Major capital cost element US$ (1990 price)

Pm-treatment Desalting plant RO membrane/equipment Control and operating system Appurtenant works Powerline and substation Energy recovery/turbine

sub-total Design and construction management Financial expenditure

44,195,ooo 70,4 14,ooo 84,835,ooo 5,952,OOO

27,013,OOO 11,427,OOO 2,999,ooo

246,835,OOO 62,250,OOO 80~70,ooo

Major O&M cost element US$ per annum

Labor 3,718,ooo Material supply 1,860,OOO Chemicals 7,440,ooo Power (pumped-storage for RO feedwater) 3,100,OOO Membrane replacement 28,269,OOO

sub-total 44,387,OOO

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CONCLUDDJG REMARKS

Application studies on the hydro-powered RO desalination including two case studies in Jordan (brackish groundwater) and Egypt (seawater) suggests a substantial reduction in operating cost and energy which has long been a major constraints in the desalination practice. The desalination of saline water by the membrane process with low energy requirement, will play an increasingly important role in the water resources planning of the next decade. This study attempts to evaluate some new non-conventional approaches to water resources which need to taken into account in building the new peace of the Middle East. These new approach offer the opportunity to introduce new applications of well-tried technology to solve long-standing water problems which are at the center of many of the potential source of conflict.

ACKNOWLEDGMENTS

The author wishes to express his deep appreciation to Professor Dr. Katsumi Musiake of University of Tokyo for his guidance, valuable advice and concern in all matters related to this study.

REFERENCES

1 RNA - Howard Humphereys Ltd., Groundwater Resources Study in the Shidiya Area, Main

Report, pp. 49-112 (1986).

2 WPDC, World News, Egypt Plans Seawater Pumped-Storage Plant, Water Power & Dam

Construction, January (1989), p. 4.