Desalination 153 (2002) 273–279
Design criteria of 10,000 m3/d SWRO desalination plant of Tajura, Libya Ibrahim Massaoud El-Azizi*, Abdu Alazizi Mohamed Omran Tajura Research Center, SWRO Desalination Plant, P.O.Box 30878, Tajur, Tripoli, Libya Tel. +218 (21) 607023; Fax +218 (21) 3614143; email:
[email protected]
Received 18 March 2002; accepted 30 March 2002
Abstract Tajura seawater reverse osmosis desalination plant with a capacity of 10,000 m3/d is the biggest RO plant in Libya. The plant is designed to produce high quality drinking and industrial water. The plant consists of a seawater intake, pretreatment, two stages of reverse osmosis membranes in two lines, post-treatment and product water storage tank. The plant has been working successfully with a capacity of 50% for over 18 years. Improvements of the plant will be made to work with the 100% capacity to minimize the operation cost and to eliminate the water problems of the area of Tajura. The objective of this paper is to present the design features of the RO plant and the improvements to be made. The design parameters, operational processes and operational data are described. Keywords: Seawater; Pretreatment; Reverse osmosis; Membranes; Intake
1. Basic design criteria of Tajura SWRO desalination plant The 10,000 m3/d SWRO desalination plant of Tajura, Libya was designed and built in 1983 by Deutscher Verfahrenstechink (DVT). The plant has now been working successfully for over 18 years. The basic design criteria of the SWRO *Corresponding author.
desalination plant are given below. Table 1 gives the analysis of the seawater and product water. The plant design parameters are as follows: Production capacity Product purity Seawater TDS Seawater temperature Number of RO stages Conversion
10,000 m3/d 232 mg/L 38,000 mg/L 15–35°C 2 in 2 lines 30%
Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Alexandria University Desalination Studies and Technology Center, Sharm El Sheikh, Egypt, May 4–6, 2002. 0011-9164/02/$– See front matter © 2002 Elsevier Science B.V. All rights reserved
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a seawater basin with a capacity of 1920 m3. The design parameters of the pipes are the following:
Table 1 Raw seawater and product water analysis
Component
Seawater composition, mg/L
Calcium Ca++ Magnesium Mg++ Sodium Na+ Potassium K+ Silica Si+ Chloride Cl– Biocarbonate HCO3– Sulphate SO4– Nitrate NO3– TDS pH
455 1427 11,600 419 2 20,987 163 2915 0 38,000 8.3
0 0 67 3 0 97 16.8
Seawater flow rate Number of seawater feed pumps Seawater pH Feed pressure Vacuum unit
0 232 8
1.1. Seawater intake Seawater from the Mediterranean Sea is fed by gravity through two submersed pipelines into
Dual media filters
1576 m3/h. 3 8.3 4.8 bar
2. Pretreatment The role of the pretreatment is to purify seawater to a quality acceptable by RO membranes. The pretreatment consists of the following main parts: • Dual media filters • Backwash pump
Cartridge filters
Feed pumps
PVC coated with iron 1300 m from the coast 60 cm
The seawater is then pumped to the pretreatment stage by three central controlled seawater pumps (two in duty, one in standby). The major design parameters of the seawater intake are the following:
A schematic diagram of the SWRO desalination plant is shown in Fig. 1.
Seawater basin
Pipe material Pipe length Pipe diameter
Product water composition, mg/L
HPP & ERTs
First stage RO membranes
Copper sulphate Sodium hydrogen sulphide Sulphuric acid Ferric chloride sulphate Polyelectrolyte
Sulphuric acid Antiscalants
Second stage RO membrane
Buffer tank HPP & ERTs
Product water storage tank Calcium hypochlorite lime
Fig. 1. Schematic diagram of the SWRO desalination plant.
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• Air scour blowers • Cartridge filters • Chemical dosing systems 2.1. Dual media filters The dual media filters (DMF) are provided to reduce the suspended solids, organic matter and inorganic particles in the raw seawater. The Tajura RO plant has eight parallel running dual media filters. The filters are automatically controlled and the data of each filter is indicated in the central control room. If the pressure loss of one filter is too high it can be backwashed with seawater and air while the other filters are still in operation. Media depth and grain size of DMF are presented in Table 2. Table 2 Media depth and grain size of DMF
Filtering material layer
Grain mix, mm
Layer depth, m
Supporting layer Quartz sand Hydro anthracite
0.7–1.2 1.4–2.5
0.3 0.85 0.85
The major design parameters of DMF are the following: Number of DMF Filtration velocity Backwash water velocity The backwash air volume Water flow rate through each filter Design pressure The volume of each filter Filter tank diameter/length Filter tank wall thickness Filtered medium
8 11.7 m/h 60 m3/m2 filter area/h 100 m3/m2 filter area/h 187 m3/h 4.5 bar 60 m3 4.5 m/2.912 m 16 mm Pretreated seawater
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of dosed chemicals of the size of 20 mm and above. Clogging of the cartridge filters is indicated by an increase of the differential pressure. Vessel inlets are from the bottom. The cartridge filter outlets are connected to the high-pressure pumps suction header. Replacement of the cartridges is required when the differential pressure reaches a specific level. The major design parameters of cartridge filters are the following: Number of cartridge filter vessels Nominal filtration size Capacity of each filter Design pressure Filtered medium Material: Vessel housing Nozzle Cartridges
5 20 mm 300 m3/h 4 bar Pretreated seawater Carbon steel Carbon steel Polypropylene
From the water analysis of the RO feed water it was noticed that the suspended solid in the water is about 3.5 mg/L. It means that the cartridge filters with nominal size of 20 µm cannot reject all suspended particles. It is recommended to change the cartridge to nominal filtration size of 5 µm instead of 20 µm to avoid any particle deposition (fouling) in RO membranes. 2.3. Chemical dosing systems Depending on the seawater composition, there are several chemical pretreatment processes required for the reduction of suspended and dissolved organic and inorganic particles. The dosing quantities of the required chemicals are automatically controlled in accrodance with the feed water quality. 2.3.1. Seawater disinfection Copper sulfate solution (CuSO4) is added at the beginning of the pretreatment for disinfection of the raw seawater.
2.2. Micron cartridge filters Micron cartridge filters are provided after DMF to remove suspended matters and impurities
2.3.2. Dechlorination Sodium bisulfate (NaHSO3) is added to the feed
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water if it contains free chlorine. Dechlorination process reduces the risk of membrane biofouling. 2.3.3. Acid dosing Acidification process is necessary to prevent precipitation of CaCO3 scale in the RO membranes. Sulphuric acid (H2SO4) is added to the raw feed water in two steps. The first dosing is upstream of the DMF to bring the pH value from 8.3 to 7 for the neutralization process. The second dosing is downstream of the DMF to bring the pH to 6.5. Now H2SO4 is dosed downstream only to reduce the consumption quantity of sulphuric acid. 2.3.4. Coagulant dosing Ferric chloride sulphate solution (FeClSO4) is dosed for destabilization and agglomeration of the colloidal particles. The destabilization and the agglomeration of the particles are necessary to get filterable particle sizes. Polyelectrolyte solution is dosed to support this process. Coagulation and flocculation processes were
not used because the SDI of the feed water is about 4.2%. 2.3.5. Antiscalants AF200 is added to inhibit CaSO4 precipitation on the surface of the RO membranes. Antiscalants are added before the cartridge filters. 3. Reverse osmosis membrane systems The Tajura SWRO desalination plant consists of two RO stages in two lines to produce 10,000 m3/d of desalted water with a quality of 170 mg/L TDS. The racks are arranged in two lines to run the plant either with 50% or with 100% capacity. The first RO stage consists of four parallel RO racks with 99 pressure vessels each. Each pressure vessel contains six spiral wound RO membranes. The design recovery rate of the plant is 30%. The product water of the first stage is collected in the buffer tank with a capacity of 50 m3. The major design parameters of RO membrane systems are presented in Table 3.
Table 3 Design parameters of first and second stage RO membranes
Item
First stage
Second stage
Number of RO racks Pressure vessels configuration Number of pressure vessels Number of membranes Number of membranes per pressure vessel Nominal diameter, inch Membrane model Design pressure, bar Working pressure, bar pH Maximum temperature, °C Feed flow, m3/h Permeate flow, m3/h Concentrate flow, m3/h Design salt rejection, % Recovery, % Permeate salinity, mg/L Feed salinity, mg/L
4 1 stage 396 2376 6 6 TFC 1501 PA 69 55 5–6 45 1576 552 1024 98.6 35 1940 36,204
2 3 stages (24-12-6) 84 504 6 8 TFC 8600 PA 41 25 5–6 45 552 468 84 98 85 170 1940
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4. High-pressure feed pumps — first stage
7. High-pressure feed pump — second stage
There are six high-pressure feed pumps in the RO plant with four pumps feeding the first stage and two pumps feeding the second stage. These pumps generate the pressure required for desalination. The major design parameters of the first stage high-pressure feed pumps are the following:
The major design parameters of the first stage high-pressure feed pumps are the following:
Manufacturer Type Pumping medium Flow rate Pump efficiency Working pressure Construction Power required
KSB, Germany HDAO 150 Seawater 394 m3/h 76% 71 bar Horizontal type, multistage, with vertical split casing 1040 kW
5. Power recovery turbines (ERT) The concentrate discharged through the concentrate line is still highly pressurized. In order to recover this pressure energy, the concentrate is fed to the turbine, which is in fact a reverse motion pump. This pump is coupled through a free-wheel clutch to the high-pressure pump and, once on line, supplies about 30% of the high-pressure pump’s energy requirement. The major design parameters of the energy recovery turbine are the following: Manufacturer Type Rated capacity Pumping medium Efficiency Construction
KSB, Germany HDANO 100 256 m3/h Seawater 78% Horizontal type, multistage, with vertically split casing
6. Buffer tank Tank volume Tank diameter Tank length Tank wall thickness Tank material Medium Construction
Manufacturer Type Pumping medium Flow rate Pump efficiency Working pressure Construction Power required
KSB, Germany HDAO 150 Permeate of second stage 275 m3/h 72% 45 bar Horizontal type, multistage, with vertical split casing 500 kW
8. Post-treatment Desalted water after passing the second RO stage is fed to an intermediate storage tank with a capacity of 21 m3. From this tank the desalted water is pumped to the decarbonater where dissolved CO2 is eliminated by air from a blower with a capacity of 14,000 m3 air/h. After the desalted water passes the decarbonator, the following chemicals are dosed: 8.1. Product chlorination Calcium hypo chlorite [Ca(OCl)2] solution is added to the product water to prevent any biological growth in the pipelines and in product water storage tank. The dosing system provided can maintain residual chlorine level of up to 0.3 mg/L. 8.2. Product neutralization Soda (NaOH) is added to maintain pH value of 8 and for perfect neutralization. 9. Control system
50 m 3.2 m 6.1 m 10 mm Fiberglass-reinforced plastic Permeate of first stage Cylindrical, vertical 3
Simatic Step 5 (S5-150 K) is the central controller system designed to provide the operator intervention for safe operation. The control system of the SWRO plant consists of a PLC basis system with a software package for ease of operation. This operating and controlling system was manufactured and installed by Siemens Company in 1983.
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Siemens Company will install a new control system (Simatic Step 7) this year. The new Simatic S7 consists of a power supply unit, a computerprocessing unit (CPU), and input and output modules. 10. Product water storage tank The product water is pumped to an underground storage tank. The capacity of this tank is 50,000 m3. The storage tank consists of prefabricated concrete elements coated with a thin film foil. The flat roof of the tank is covered with stones for good insulation. 11. Membrane cleaning system Mineral scale, biological matter and insoluble organic matter build up on the membrane surface during the operation process. This affects the membrane productivity, salt rejection and the bundle pressure drop. The complete cleaning system is provided. 12. Replacement of one RO stage Eight-inch RO membranes were widely used, especially in large plants, because they provide a designed salt rejection level of 99.6%, whereas the 6-inch membranes offer a design salt rejection level of 98% only. Two RO skids with an 8-inch diameter pressure vessels, and seawater RO membranes were replaced in 1998 by a Canadian company Jadmedic to produce potable water with maximum TDS of 200 mg/L at a rate of 3000 m3/d for each RO skid using one stage only. The design parameters of recently replaced two RO skids are: Number of pressure vessels Number of membranes Number of membranes per pressure vessel Nominal diameter
90 540 6 8 inch
Membrane material Membrane model Element construction Designed salt rejection Design permeate productivity Maximum operating pressure Allowable operating pH range Pressure vessel type
PA TFC 2822SS-360 High area 99.6% 22.7 m3/d 82.8 bar 4–11 Codeline No. 45 fiberglass-reinforced plastic vessels
13. Production quantity of industrial water The quantity of high quality water required by Tajura Research Center (TRC) is limited. The normal consumption of high quality water by TRC is about 500 m3/d. The quantity of industrial water produced by the SWRO plant during 1984–2001 is shown in Fig. 2. From this figure, one can notice that the production quantity of industrial water in the first 4 years is high due to the high quantity requirement of industrial water by Tajura Research Center. The production quantity has decreased since 1989 and was almost stable until 2000 (less consumption). Since 2001 the production quantity of industrial water started to increase due to the demand of high quality water by Tajura Research Center, Libo Car Factory and other companies located near the plant.
800000 700000
Quantity (m3)
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600000 500000 400000 300000 200000 100000 0 1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
Year
Fig. 2. Quantity of industrial water produced per year, 1984– 2001.
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14. Improvements of the plant
15. Conclusions
In order to operate the plant with 100% production capacity and to produce high quality industrial water (max. 200 mg/l TDS) necessary for Tajura Research Center and other companies, and to provide potable water to the area of Tajura, the following improvements will be made this year: • Replacement with new complete two SWRO skids with 6000 m3/d capacity. • Replacement with new complete two brackish water RO skids with 10,000 m3/d capacity. • Replacement of new control system (Simatic S7). • Refurnishment material of the pretreatment, post-treatment, and all instrumentation in the field. • Replacement of demineralization plant.
The Tajura RO plant has been working successfully for 18 years. The plant is designed to operate continuously to produce drinking and industrial water. Now the plant is working with a capacity of 50% and is producing only industrial water with 231 mg/l TDS necessary for the Tajura Research Center, Libo Car Factory and other companies. Improvements of the plant by replacement with a new RO skids and new control system will be made this year to meet the designed production capacity. Increasing production capacity of the plant to 100% is necessary to produce drinking and industrial water, to minimize the operating costs and to eliminate the water problems in the area of Tajura. Drinking water produced will meet the WHO standards.
