0011-9164/02/$– See front matter © 2002 Elsevier Science B.V. All rights reserved
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.
*Corresponding author.
Design criteria of 10,000 m
3/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, LibyaTel. +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
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 10,000 m3/d Product purity 232 mg/L Seawater TDS 38,000 mg/L Seawater temperature 15–35°C Number of RO stages 2 in 2 lines Conversion 30%
Buffer tank HPP & ERTs Second stage RO membrane Calcium hypochlorite lime Product water storage tank Sulphuric acid Antiscalants Copper sulphate
Sodium hydrogen sulphide Sulphuric acid Ferric chloride sulphate
Polyelectrolyte HPP & ERTs First stage RO membranes Cartridge filters Dual media filters Feed pumps Seawater basin Table 1
Raw seawater and product water analysis
Component Seawater composition, mg/L Product water composition, mg/L Calcium Ca++ 455 0 Magnesium Mg++ 1427 0 Sodium Na+ 11,600 67 Potassium K+ 419 3 Silica Si+ 2 0 Chloride Cl– 20,987 97 Biocarbonate HCO3– 163 16.8 Sulphate SO4– 2915 Nitrate NO3– 0 0 TDS 38,000 232 pH 8.3 8
A schematic diagram of the SWRO desalination plant is shown in Fig. 1.
1.1. Seawater intake
Seawater from the Mediterranean Sea is fed by gravity through two submersed pipelines into
a seawater basin with a capacity of 1920 m3. The
design parameters of the pipes are the following: Pipe material PVC coated with iron Pipe length 1300 m from the coast Pipe diameter 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:
Seawater flow rate 1576 m3/h. Number of seawater feed pumps 3
Seawater pH 8.3 Feed pressure 4.8 bar Vacuum unit
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 pumpFig. 1. Schematic diagram of the SWRO desalination plant.
•
Air scour blowers•
Cartridge filters•
Chemical dosing systems2.1. 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 0.3 Quartz sand 0.7–1.2 0.85 Hydro anthracite 1.4–2.5 0.85
The major design parameters of DMF are the following:
Number of DMF 8 Filtration velocity 11.7 m/h
Backwash water velocity 60 m3/m2 filter area/h The backwash air volume 100 m3/m2 filter area/h Water flow rate through 187 m3/h
each filter
Design pressure 4.5 bar The volume of each filter 60 m3 Filter tank diameter/length 4.5 m/2.912 m Filter tank wall thickness 16 mm
Filtered medium Pretreated seawater
2.2. Micron cartridge filters
Micron cartridge filters are provided after DMF to remove suspended matters and impurities
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 5 Nominal filtration size 20 mm Capacity of each filter 300 m3/h Design pressure 4 bar
Filtered medium Pretreated seawater Material:
Vessel housing Carbon steel Nozzle Carbon steel Cartridges 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 sus-pended 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 orga-nic and inorgaorga-nic particles. The dosing quantities of the required chemicals are automatically con-trolled 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.
2.3.2. Dechlorination
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 Table 3
Design parameters of first and second stage RO membranes
Item First stage Second stage
Number of RO racks 4 2
Pressure vessels configuration 1 stage 3 stages (24-12-6)
Number of pressure vessels 396 84
Number of membranes 2376 504
Number of membranes per pressure vessel 6 6
Nominal diameter, inch 6 8
Membrane model TFC 1501 PA TFC 8600 PA
Design pressure, bar 69 41
Working pressure, bar 55 25
pH 5–6 5–6
Maximum temperature, °C 45 45
Feed flow, m3/h 1576 552
Permeate flow, m3/h 552 468
Concentrate flow, m3/h 1024 84
Design salt rejection, % 98.6 98
Recovery, % 35 85
Permeate salinity, mg/L 1940 170
Feed salinity, mg/L 36,204 1940
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.
4. High-pressure feed pumps — first 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 desali-nation. The major design parameters of the first stage high-pressure feed pumps are the following:
Manufacturer KSB, Germany Type HDAO 150 Pumping medium Seawater Flow rate 394 m3/h Pump efficiency 76% Working pressure 71 bar
Construction Horizontal type, multistage, with vertical split casing Power required 1040 kW
5. Power recovery turbines (ERT)
The concentrate discharged through the con-centrate 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 KSB, Germany Type HDANO 100 Rated capacity 256 m3/h Pumping medium Seawater Efficiency 78%
Construction Horizontal type, multistage, with vertically split casing
6. Buffer tank
Tank volume 50 m3 Tank diameter 3.2 m Tank length 6.1 m Tank wall thickness 10 mm
Tank material Fiberglass-reinforced plastic
Medium Permeate of first stage Construction Cylindrical, vertical
7. High-pressure feed pump — second stage
The major design parameters of the first stage high-pressure feed pumps are the following:
Manufacturer KSB, Germany Type HDAO 150
Pumping medium Permeate of second stage Flow rate 275 m3/h
Pump efficiency 72% Working pressure 45 bar
Construction Horizontal type, multistage, with vertical split casing Power required 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 m3air/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
Simatic Step 5 (S5-150 K) is the central con-troller 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 manu-factured and installed by Siemens Company in 1983.
Siemens Company will install a new control system (Simatic Step 7) this year. The new Simatic S7 consists of a power supply unit, a computer-processing unit (CPU), and input and output modules.
10. Product water storage tank
The product water is pumped to an under-ground storage tank. The capacity of this tank is 50,000 m3. The storage tank consists of
pre-fabricated 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 mem-branes were replaced in 1998 by a Canadian com-pany 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 90 Number of membranes 540 Number of membranes per 6
pressure vessel
Nominal diameter 8 inch
Membrane material PA
Membrane model TFC 2822SS-360 Element construction High area Designed salt rejection 99.6% Design permeate productivity 22.7 m3/d Maximum operating pressure 82.8 bar Allowable operating pH range 4–11
Pressure vessel type Codeline No. 45 fiber-glass-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 pro-duction 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.
Fig. 2. Quantity of industrial water produced per year, 1984– 2001. 0 100000 200000 300000 400000 500000 600000 700000 800000 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 Year Q uantity (m 3)
14. Improvements of the plant
In order to operate the plant with 100% pro-duction 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.15. Conclusions
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.
