Economics of seawater desalination by reverse osmosis

Economics of seawater desalination by reverse osmosis

Desalination, 99 (1994) 39-55 39 Elsevier Science B.V. Amsterdam - Printed in The Netherlands Economics of seawater desa~ina~iun by reverse osmosis...

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Desalination, 99 (1994) 39-55

39

Elsevier Science B.V. Amsterdam - Printed in The Netherlands

Economics of seawater desa~ina~iun by reverse osmosis S. Ebrabim and M. AbdeHawad Kuwait Institute for ScientiJic Research, PO Box 24885, Safat 13309 (Kuwait) (Received November 3, 1993)

The ultimate goal in the scientific study of desalting and water purification is to design a process that produces potable water at the lowest possible cost. Presently two major techniques are commercially used in different parts of the worId: distillation and membrane processes. The dominant distill&ion process is multi-stage flash (MSF), and the main membrane process is reverse osmosis (RU), Ahhough cost factors vary by site, the total eost of producing potable water from seawater with the RO process is usually !ess than thermal desalting processes. In this paper results of a study carried out in Kuwait to compare the unit cost of water produced from one-stage and two-stage hoffow fiber (HF) and spiral wound (SW) seawater RO systems are presented. Results indicate that an average reduction in product water cost of about 22% could be achieved when onestage RU is used instead of two-stage RU to produce drinking water from seawater in Kuwait. Moreover, an average of 11.84 % of the capital investment cost can be saved by using a one-stage instead of a two-stage RO system. A greater reduction in product water cost is expected in the future as there is more room for improvement in the design, operation and rnai~t~~an~e of seawater desalination by RO technology.

~~1~9~64/94/$07.~a 0 1994 Elsevier Science B.V. Ali rights reserved, ~~~roorl-9164(94)00118-9

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The economic advantages of RO versus evaporation processes, such as multi-stage flash (MSF) distillation, are responsible for the rapid increase in use of seawater RO. The main contributing factor to this rapid increase. is that it is more efficient and much simpler than MSF. The MSF process is carried out at high temperatures (requiring a boiler) and involves changes of phase from liquid to vapor and then back to liquid. Consequently, it has relatively more scale and corrosion problems and, in addition to requiring highly skilled operating and maintenance personnel, needs relatively large spaces. The RO process, on the other hand, is carried out at ambient temperature and involves no change of phase; therefore, it has comparatively fewer scale and corrosion problems. Moreover, the phenomenal growth of RO systems is attributed, above all, to their low energy consumption~ RO with an energy recovery system requires only one-third to one-half the energy needed for distillation processes [l]. A study by Darwish et al. [2] indicates that the available energy consumption/m3 for the MSF unit is about three times that of the RO plant. Another study by Fosselard and Wagnick [3] shows that the energy cost/m3 for plant capacities between 2~-3~ m3/d is lower for RO ( = $1.75 to $1 .30/m3) than for multi-effect thermal vapor compression (ME-TVC) ( = $2.25 to $1.4/m3) and for mechanical vapor compression (MVC) (= $2.65 to $1.8/m3). The study also shows that the unit water cost/m3 for various desalting plants depends on local conditions and costs for electrical power and heavy fuel. At arbitrary costs for these two energy items, the multi-effect (ME) process is the cheapest. Even the ME-TVC process is superior to RO, whereas the MVC process is the most expensive process for the conditions selected. RO is the cheapest process, and the ME process is the most expensive when doubling the price for heavy fuel and halving the price for electric energy, The MVC process is then comparatively economical. According to Leitner [4], RO and ME are nearly equal in annual costs ($/m3) and more cost effective than MSF on the basis of base fuel cost ($1.26, $1.31 and $1.53 for RO, ME and MSF, respectively). On the basis of world fuel cost, the RO process would save over 10% compared with ME and 32% compared with MSF. This study was done for a plant capacity of 23,000 m3/d and based on feed’s TDS of 45,000 mg/l, which is similar to the Arabian Gulf feeds TDS. Previous studies by Tewari et al. [5] and Darwish and Al-Najam [6f

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confirmed the cost-effectiveness of the RO process over other desalting processes, Tewari et al. [5] concluded that water cost ($/ms) using the RO process is lower than the duaI~pur~se MSF process and, consequently, much lower than the single-purpose MSF process ($1.17, $1.41 and $1.60, respectively). Darwish and Al-Najam [6] showed that multi-effect boiling (MEB) and MSF systems in dual-purpose plants as well as RO have similar energy costs and have the lowest energy cost than any of the other compared systems. The energy cost of MVC foltows the feast followed by MSF and MEB fed by cogeneration steam turbines. Wade [7] recently compared five desalination schemes: (scheme No. 1) MSF+back pressure steam turbine (MSF+BPST); (scheme No. 2) MSF-t gas turbine and waste heat boiler (MSF-+GT/WHB); (scheme No. 3) MSF+~o-generation combined cycle (gas turbine+waste heat boiler +back pressure steam turbine) (MSF+CO-GEN); (scheme No. 4) RO one-stage with energy recovery (RO 1 stage) and (scheme No. 5) RO two-stage with energy recovery (RG 2 stage). The desaIination plant schemes and the reference power plant cycle were compared for a plant life of 20 years, discount rate of 10% PA, and annual amortization of 11.75% of capita]. Fuel cost was taken as $18 per barrel for the base case. Wade f7] concluded that the cost of water produced by the RO plant is less sensitive to increases in tie1 costs than the MSF plant in dual-purpose schemes. At fuel costs above $35 per barrel, there would be subs~ntial cost savings for the RO plant compared with the conventional dual-purpose plant. The variation in unit water cost with me1 cost at 45% cycle efficiency reference power plant is shown in Table I, Unit water cost of RO schemes (i.e., schemes no, 4 and 5) are lower than other schemes at a fuel cost of $18 per barrel and above. Leitner [S] compared the capital and total water costs for three large seawater RG plants in Malta, Las Palmas and Jeddah. He used a standard format for cost calculations that was recognized at the 1988 Bahrain Workshop on Production Cost of Water. He also normalized the total water casts for the three plants using equal fixed cost rates and electric power costs based on California’s ins~llations, Based on the standard format, the total water costs in $/m3 were 1.18, 1.62 and 0.59 for Malta, Las Palmas and Jeddah, respectively, whereas normalized water costs were 0.95, 1.19 and 1.01, respectively. The capital costs for the three plants (X 103) were $14,458, $80,850 and $65,031, respectively, The higher capital cost of the Las Palmas pfant was reflected In higher water costs. Glue&stem ]9]

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TABLE I Variation in unit water cost with fuel cost with 45% cycle efficiency power plant Fuel cost

reference

IJnit water cost ($/m3)

$/Barrel

$fG.I

Scheme 1 Scheme 2

0 18 35 70

0.0 2.8 5.4 10.8

1.20 1.89 2.52 3.83

1.21 1.96 2.96 4.12

Scheme 3 Scheme 4

Scheme 5

1.42 2.00 2.54 3.67

1.46 1.61 1.74 2.02

1.23 1.36 1.48 1.72

Source: Wade f73.

reported that the cost of desalted water from 20,000 m3/d seawater RO plants at 7.5% PA interest rate and an assumed plant life of 25 years is estimated to be $0.84-1.07fm3. The higher cost was obtained by assuming proven membrane technology and conservative assumptions regarding site development and indirect investment costs. The lower cost was obtained for a design based on less proven membrane technology and optimistic assumptions regarding site development (including feed water supply) and jindirect investment costs. He also reported that, based on a preliminary design, desalted water costs from very large (2~,~ m3/d) plants were estimated to range from $0.64 to $0.80/m3, From these studies, it is evident that the RO process should receive increasing attention in the coming years because it is the most energyefficient of the processes and will be of increasing importance as fuel prices rise. There are also opportunities for further reduction of costs through increasing membrane life and developing membranes that operate at higher pressures (up to 1500 psi) and higher conversion (50%) when operating on 45,~ mgll seawater.

COST ESTIMATE PARAMETERS Capital and operating costs are the two main parameters in cost estimates for any desalting process. For the seawater RO process, the com~nen~ of these two parameters are presented in Table f-f.

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TABLE PI

Capital and operating cost components of Capital cost component l l

* l

* * l

. l

Site development Building and offices Seawater intake structure Pipeline and transfer pumps Pretreatment High-pressure pumps RO racks and permeators Brine discharge and facilities Product water treatment/storage

a seawaterRO system Operating cost component * * * * *

Energy Chemicals Filter cartridges Other consumables Labor l ~ernb~ne replacement * Amortization

Seawater RO capital cost varies depending upon location, system capacity, degree of pretreatment, feedwater salinity and temperature, and overall scope of the particular project. Table III presents some prices compiled from available public sources for typical capital costs for various desalination plants [4]. Energy is the largest com~nent of desalination process operating costs. A large seawater RO plant with an energy recovery system requires only one-third to one-half the energy required for the most efficient MSF plant; 2530% of the total system energy input could be recovered with energy recovery equipment. As plant sizes increase, this percentage should increase with increased efficiency of pumps and motors. Operating costs for chemicals, filter cartridges and other consumables varies de~nding on type of pretreatment and size of the plant. Types and quantities of chemicals required are dete~ined by the feed water sources. Sea-well feed water will require little chemical treatment, wbereas surface water sources taken from an open sea intake wilt require more pretreatment and more chemicals and filters. Membrane replacement rate depends on type of membrane, quality of feedwater and mode of operation. Most membranes manufacturers give around a S-year guarantee on their membranes; however, average membrane life for most of the available membranes exceeds the guaranteed period. A 10% annual membrane replacement rate is a common practice in seawater RO plants. This replacement can be scheduled far in advance and can be made without shutting down the entire system.

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TABLE III Typical capital costs for desalination plants Year

Location

Capacity

PliCe

(mai)

@/ill

US$f GPD

25 20 25 50 6 27 60

107 328& 146 468’ 3od 128 275b 14Y

4.28 16.40 5.85 9.36 4.80 4.74 4.58 11.25

97 132

3.86 5.28 4.00 5.71 4.92 7.95

MSF plants 1979 1979 1981 1982 1985 1986 1987 1989

US OWRT Cst, Update Jeddah III, Saudi Arabia US OWRT Cst, Update Jeddah IV, Saudi Arabia Las Palmas, Spain Assir, Saudi Arabia Al Khobar III, Saudi Arabia Al Ghubrah. Oman

12

MED plants 1979 1981 1981 1985 1985 1989

US OWRT Cst, Update US OWRT Cst, Update St. Thomas, St. Croix Las Palmas, Spain Curacao, New Antilles Curacao, New Antilles

25 25 3.75 6.0 2.64 2.64

$4 13 21

SWRO plants 1978 1979 1982 1981 1984 1985 1985 1987 1988 1989

Jeddah, Saudi Arabia US OWRT Cst, Update US OWRT Cst, Update Key West, Florida, US Al Dur, Bahrain Las Palmas, Spain Jeddah I Rehab., Saudi Arabia Jubail III, Saudi Arabia Jubail IV, Saudi Arabia Fuiaira, United Arab Emirates

‘Adjusted price to delete power generation. bBid price. ‘Bid price, adjusted to delete power generation. dBid price; this process not selected. Source: Leitner [4].

3.17 5.0 5.0 3.0 12.0 6.0 15.0 30.0 24.0 2.4

30.0 22.0 22.0 8.0 49.1 26.7 43.0 1 13.0b 154.0b 15.0

9.46 4.40 4.40 2.66 4.09 4.23 2.87 3.76 6.42 6.25

resuft of plant size and plant location. As the size of the plant becomes smaller and not isolated, it can be highly automated, and the number of laborers can be reduced signi~cantly, The percentage contribution of major components of operating cost to the actual cost of operation varies depending on plant size, location, method of operation, type of pretreatment and energy cost. The contribution of components of operating cost ($/m3) to the water unit cost is shown in Table IV. Energy is the largest component in operating costs (42.7%) followed by membrane replacement (20%). Labor and spare parts are equal at about ll%, followed by chemicals, maintenance and filters at 5,5%, L&or

cost will vary as a

TABLE IV Operating cost c~m~nents Comwnents

s/m3

% cont~bution

Energy Chemicals Labor and overhead Filters Membrane replacement Spare parts Maintenance

0.39 0.05 0.10 0.04 0.182 0.10 0.0s

42.7 5.5 Il.0 4.3 20.0 11.0 5.5

Total

0.912

100,o

TABLE V eating

cost components

Com~ne~t

% cont~b~tion

Energy Chemicals Filter cartridges Other consumables Labor Membrane replacement

$5 6 5 4 10 20

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5.5 % and 4.3 %, respectively. Another estimate for percent contribution of operating components to the total operating cost is given by PEM [l] and shown in Table V; energy contributes about 55 %, membrane replacement 20%) labor lO%, chemicals 6%, filters 5 % and other consumables 4% This estimate is close to the estimate presented by Darwish et al. [2] and is also shown in Table IV.

CASESTUDY

- T~D~HAR~PLA~{DRoP)

Several studies were conducted by the Kuwait Institute for Scientific Research (KISR) to assess the economics of seawater desalination by RO technology under Kuwaiti conditions. These studies were carried out at the Doha RO plant (DROP), which consists of three pilot RO lines, each of 1000 m3/d production capacity. One of these studies [lo] presents the actual cost of water produced from DROP using the two-stage RO system. The study was based on the same criteria and procedures used to calculate water cost by the MSF system used in Kuwait. The common criteria used for both MSF and RO were depreciation (method and asset service life), labor wage rate, inflation rate, energy price, construction rate and other s~i~~onomic p~ameters. Water costs (by both RO and MSF) were calculated by dividing the annual fixed and variable costs by total production. The study was done for six RO plant sizes ranging from 4546-27,276 m3/d using three scenarios for plant availability: 90% (base line scenario), 70% and 50%. Another study was carried out to compare the economics of two-stage with one-stage spiral-wound (SW) and hollow-fiber (HF) RO systems using two plant sizes of 4546 m3/d and 27,278 m3/d. Table VI presents technical details for these two systems. The results of this study are summarized in Tables VII-X. These results are based on data ob~ined from the HF and SW systems at DROP. The SW system was operated on single-stage continuously from July I989 to December 1989, whereas the HF system was operated continuously from January 1989 to April 1989. It was stopped from May 1989 to October 1989, and then it was put on-line during November and December in the same year. Energy consumption for line 2 (HF) is adjusted based on the ratio of operating pressure and pump maximum pressure (i.e., reported kWh consumption per cubic meter was multiplied by a factor of 0.7 as first-stage pump design pressure is 120 bar

date

High pressure pumps High pressure pump’s motors Type of membrane permcators No.fsize of membrane permeators Feed water fiow (m3/d) Permeator recovery Operating pressure (bar) Salt rejection TDS of permeate (mgll)

Second stage:

1984

Two Two UOP-PA-86~ 4218” diameter 1176 85% 28 98% 67

Two One Two UOP-PA-1501 20016” diameter 3922 30% 65 98.9% 1080

1000 Less than SO0 27% 94.8%

December

Two One Two Filmt~-~CSW30-8040 120 4032 28% 69 99.1% 400

1000 400 28% 94.5%

April 1989

Two Two B-9 (DuPont) No. 0840 1518‘*diameter 1270 85% 28 90% 90

Two Two Not used B-10 ~Dupont) #6840 1501’8” diameter 4534 28% 64 98% 900

1000 Less than 500 25% 93.8%

August 1984

Two-stage

Two-stage One-stage

RO line 2 (hollow fiber)

RO Line 1 (spiral wound)

for RO lines 1 and 2 at DROP

High-pressure pumps Energy recovery turbine High pressure pump’s motors Type of membrane permeators No.lsize of permeators Feed water flow rate (m3/d) Permeator recovery Operating pressure (bar) Salt rejection TDS of permeate (mgll)

First stage:

Product capacity (m3/d) Final TDS (mg/l) Overall recovery Overall availability

Commissioning

Parameters

Technical sp~i~cations

TABLE VI

1989

Two Two Not used BtOT (DuPont) #684OT 7.518” diameter 3572 28% 70 99% 500

1000 so0 28% 93.4%

January

One-stage

P ti

297.837

(KD/m3/d)

cost

unit

30.53 1.27

413,364 17,270

100.00

53.16

719,796

1,353,364

15.04

% of total

203,537

Total cost

Buildings and construction Machinery and equipment RO membranes Other assets

KD/m3

220.252

6,007,599

2,133,852 72,411

2,861,628

939,709

KD/m3

100.00

35.52 1.21

47.63

15.64

% of total

258.212

1,173,834

272,820 17,270

680,207

203,537

KD/m3

4546 m3/d

4546 m3/d 27,276 m31d

One-stage

Two-stage

100.00

23.24 1.47

57.95

17.34

% of total

Capital investment cost of seawater desalination by one-stage and two-stage hollow fiber systems

TABLE VII

188.353

5,137,503

1,421,145 72,411

2,704,238

939,709

KD/m3

100.00

27.66 1.41

52.64

18.29

% of total

27,276 m3/d

4,234,188 1,609,566 72,411 6,855,873

71.48

15.83 0.99

loo.00

1,243,284

275,310 17,270

1,739,44x

382.622

Total cost

unit

cost

(KD/m3/d)

251.352

939,709

11.70

203,537

Buildings and construction Machinery and equipment RO membranes Other assets 100.00

23.48 1.06

61.76

13.71

347.352

1,579,066

183,356 17,270

1,174,903

203,537

KD/m3

KD/m3

KD/m3 % of total

4546 m3/d

27,276 m3/d

4546 m3/d

% of total

One-stage

Tw-o-stage

4,001,307 1,071,971 72,411 6,085,398

74.40 11.61 1.10 100.00

223.105

939,709

KD/m3

12.29

% of total

100.00

17.62 1.19

65.75

15.44

% of total

27,276 m3/d

Capital investment cost of seawater desalination by one-stage and two-stage spiral wound RO system

TABLE VIII

0.263 0.046 0.033 0.173 0.011

0.351

Variable costs Labor Chemicals Energy Maintenance and spares

Total cost

“RR, energy recovery,

0.088 0.033 0.055

Fixed costs: Depreciation RO membranes

100

74.89 13.13 9.44 49.21 3.12

25.11 9.42 15.69

0.301

0.231 0.017 0.033 0.173 0.008

0.070 0.023 0.047

100

76.69 5.58 11.01 57.43 2.67

23.31 7.55 15.76

% of total

0.405

0.317 0.046 0.033 0.227 0.011

0.088 0.033 0.055

KD/ m3

100

78.27 11.36 8.15 56.05 2.71

21.73 8.15 13.58

% of total

KD/ m3

KD/ m3

% of total

4546 m3/d

27.276 m3/d

4546 m3/d

0.355

0.285 0.017 0.033 0.227 0.008

0.070 0.023 0.047

KD/ m3

100

80.28 4.79 9.30 63.94 2.25

19.72 6.48 13.24

% of total

27,276 m3fd

Two-stage (without ER)

Two-stage (with 24% ERja

0.302

0.234 0.046 0.033 0.144 0.011

0,068 0,032 0.036

KD/ m3

100

77.48 15.23 10.93 47.68 3.64

22.52 10.60 11.92

%of total

4546 m3/d

0.255

0.202 0.017 0.033 0.144 0.008

0.053 0.022 0.031

KD/ m3

100

79.22 6.67 12.94 56.47 3.f4

20.79 8.63 12.16

% of total

27,276 m31d

One-stage (without ER)

Product water cost of seawater desalination by one-stage and two-stage hollow fiber systems

TABLE IX

X

loo

*ER, energy recovery.

0.348

Total cost

74.95

25.05 14.51 10.54

13.24 9.52 47.98 4.21

0.087 0.051 0.037

Variable costs: 0.261 Labor 0.046 0.033 Chemicals Energy 0.167 Maintenance 0.015 and spares

Fixed costs: Depreciation RO membranes

0.292

0.226 0.017 0.033 0.167 0.009

0.066 0.030 0,036

100

77.40 5.74 11.33 57.11 3.22

22.60 10.38 12.23

% of total

OAQO

0,313 0.046 0.033 0.219 0.015

0.087 0.051 0.037

K.D/ m” KD/ m3

100

0.344

78.25 0.278 11.50 0.017 8.25 0.033 54.75 0.219 3.75 0.009

21.75 0.066 12.75 0.030 9.00 0.036

%of total

KD/ m3

KD/ m3

% of total

4546 m3/d

27,276 m3/d

4546 m3/d

loo

80.81 4.94 9.59 63.66 2.62

19.19 8.72 10.47

% of total

27,276 m3/d

Two-stage (without ER)

Two-stage (with 24% ER)”

0.327

0.254 0.046 0.033 0.170 0.015

0.073 0.049 0.024

KD/ m3

100

75.15 13.61 9.76 50.29 1.49

24.85 14.49 10.36

% of total

4546 m3/d

0.290

0.237 0.017 0.033 0.170 0.009

0.053 0.029 0.024

KD/ m3

100

79.00 5.67 11.00 56.67 5.66

21.00 9.67 11.33

% of total

27,276 m3/d

One-stage (withoutER)

Product water cost of seawaterdesalinationby one-stage and two-stage spiral wound RO systems (24% energy recovery)

TABLE

52

whereas operating pressure of line 2 during 1989 was about 70 bar). The basis of the study was on the two-stage RO system [lo]. From this study and Tables XII-X, the following findings were made: l The capital investment costs for the two-stage and the one-stage SW and HF RO systems are as follows:

One-stage

Two-stage

Spiral wound RO Hollow fiber RO

4546

27,276

4546

27,276

(m3/d)

(m3/d)

(m3/d)

(m3/d)

382.622a 297.837

251.352 220.252

347.352 258.212

223.105 188.353

‘Expressed as KD/m3/d (i.e., capital cost per unit capacity.)

The higher costs per unit capacity for the SW system with the HF system is attributed mainly to the higher capital investment costs of the pretreatment system for the SW system. For plant sizes similar to that of the MSF operating in Kuwait (i.e., commercial size), costs of membranes account for 35.5% and 27.6% of the total capital investment cost of two-stage and one-stage HF systems, respectively, whereas the costs of the membranes for two-stage and onestage SW systems are 23.5% and 17.6%, respectively. Costs of machines and equipment account for 47.6% and 52.6% for two-stage and one-stage HF systems, resistively; and for 61.7 % and 65.7% for two-stage and onestage SW systems, respectively. The high share of machinery and equipment to the total capital investment costs in the case of the SW system is due to the high capital inves~ent cost of the pretreatment system (mainly carbon filters) , l

l ~achine~ and equipment costs were reduced for both systems by an average of 5.5% because the second stage is not needed whereas costs of RO membranes were reduced by 33%. Accordingly, the percentage reduction in capital investment cost if one-stage RO is used instead of twostage is estimated at 9.2% and 11.24% for the SW system for a 4546 m3/d and 27,276 m3/d plant, respectively, whereas the percentage saving in capital cost for the HF system is 13.3% and 14.48% for the small and large

53

RO plants, respectively. On average, about 11.84% of the capital investment cost can be saved when one-stage RO is used to desalt seawater. l Product water costs (in KD/m3) for the two-stage, with and without energy recovery (ER), and for the one-stage (without ER) for both the SW and HF systems are as follows:

Two-stage

(with ER)

Spiral wound Hollow fiber

Two-stage (without ER)

One-stage (without ER)

4546 (m3/d)

27,276 (m3/d)

4546 (m3/d)

27,276 (m3/d)

4546 (m3/d)

27,276 (m3/d}

0.348 0.351

0.292 0.301

0.400 0.405

0.344 0.355

0.327 0,302

0.290 0.255

One-stage RO cost calculations are based on two-stage RO costs assuming no energy recovery. One-stage energy costs are based on actual power consumption, assuming an electricity price of 18 fils/kWh. Costs of membrane replacement were reduced by 33 %, which represents the share of second-stage membrane cost in total cost of membranes for a two-stage RO system. Depreciation cost of two-stage RO was reduced by 5.5%, which represents the share of second-stage RO (without membranes) in total capital cost, Hence, the percentage reduction in product water costs when one-stage RO is used is estimated at 18.25% and 15.7% for the SW system for a 4546 m3/d and 27276 m3/d plant, respectively. The percentage saving in product water cost for one-stage HF is 25.4% and 28.2% for a 4546 m3/d and 27,276 m3/d plants, respectively. On average, about 22% reduction in product water cost could be achieved when one-stage RO is used instead of two stages to produce drinking water from seawater. 0 Production costs of water by both systems are not generally sensitive to plant service life and capital investment costs (i.e., moderate inflations levels). Also, production costs are not sensitive to reasonable decreases in plant availability (down to 70%) and membrane service life (down to three years). l As energy cost is the most s~gni~cant item, production costs are sensitive to increases in electricity price and/or energy-specific require-

54

ments. Energy-specific consumption and cost can be reduced by instaIling an energy recovery system within the RO plant. * Economics of scale exists in both HF and SW systems. The scalingup factors associated with a six-fold increase in capacity and based on total capital investment costs are 0.72 and 0,714 for HF and SW systems, respectively. An average scaling-up factor of 0.70 can be used to estimate the plant capital investment costs when the capacity is within the commercial-scale range.

CONCLUSIONS l The analyses in this study show that drinking water can be produced in Kuwait by RO technology (HF and SW systems) at reasonable costs, although the SW system requires higher initial capital investment. The cost of drinking water could be further reduced by using one-stage RO that produces drinking water of the same quality as that of two-stage. l When one-stage RO is used instead of two-stage RO, savings would be made in both capital investment and product water costs. l Based on the preliminary analyses in this study, capital investment cost of one-stage RO is about 11.84% less than capital investment cost of two-stage RO. l Product water cost by one-stage RO is 22% less than that by twostage RO. One-stage RO requires less energy, capital investment and membrane replacement. l It is likely that both RO systems wiI1 offer greater cost reduction opportunities and hence better economies of scale in the future as research efforts improve the design, operation and maintenance of the system. l A standard format for water cost Gal~ulations for RO and other desalination technologies should be adopted so that a more realistic cost assessment can be made.

1 2 3 4

Permasep Engineering Manual (PEM). E-1. DuPont de Nemours & Co., 1989. M.A. Darwish, M. Abdel-Jawad and G.S. Aly, Desalination, 76 (1989) 281. G. Fossefard and K. Wagnick, Desalination 76 (1989) 215. G.F. Leitner, Desalination 76 (1989) 201,

55

5 P.K. Tewari, M.S. Hanra and M.P.S. Ramani, Desalination 64 (1987) 203. 6 M.A. Darwish and N.M. Al-Najem, Desalination 64 (1987) 83. 7 N.M. Wade, Proc., Twelfth International Symposium on Desalination and Water Re-use, 1 (1991) 3. 8 G.F. Leitner, TechnicalProc., IDA World Conferenceon Desalination and Water Re-use, 2 (1991) 1. 9 P. Glueckstern, Proc., Twelfth International Symposium on Desalination and Water Reuse, 1 (1991) 49. 10 S. Akashah, M. Abdel-Jawad, M.M. Abdul-Halim and J. Dahdah, Desalination, 64 (1987) 65.