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Design features of a 20 migd SWRO desalination plant, Al Jubail, Saudi Arabia
DESALINATION
ELSEVIER
Desalination 118 (1998) 5-12
Design features of a 20 migd SWRO desalination plant, A1 Jubail, Saudi Arabia Mohammed Badrulla Baig*, Abdul Aziz A1 Kutbi Saline Water Conversion Corporation, PO Box 5968, Riyadh 11432, Saudi Arabia Tel. +966 (1) 463-1111; Fax +966 (I) 463-4546
Received 16 June 1998
Abstract The Saline Water Conversion Corporation (SWCC), Saudi Arabia, began operating reverse osmosis (RO) desalination plants as early as in 1978 with small capacities and is now operating the largest seawater RO plants in the world. The rich experience gained by SWCC in more than 20 years of operation of these plants has been used to address some of the problems faced earlier and in improving the design of the new RO plants. One of the recent RO plants, which is due for commissioning, is the 91,000 m3/d plant at A1 Jubail, on the coast of the Arabian Gulf. When placed in operation, it will be among the largest seawater RO desalination plants in the world. Considering the significance of a well designed pretrealment section for the successful operation of any RO plant, a pilot filtration plant was placed in operation during August 1993. The trials were carried out over a period of more than one year and the results were adapted in the plant design. This paper presents the salient design features of the plant and the improvements made. Keywords: Pretreatment; Seawater reverse osmosis (SWRO); Membranes; Coagulant; Filtration; Cartridge filters
1. Basic design criteria of AI Jubail SWRO plant The SWRO plant was envisaged as an extension to the existing 230 migd MSF desalination plant at A1 Jubail situated on the Arabian
Gulf, coast of Saudi Arabia. The plant's output will be pumped to the Qassim region through a 1500 mm dia pipeline. The basic design criteria of the RO plant is given below. Table 1 and Fig. 1 give the analysis of seawater and the process flow diagram, respectively.
*Corresponding author. Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier SeicmeeB.V. All rights reserved. PII SO011-9164(98)00068-X
6
M.B. Baig, A.A. AlKutbi/Desalination 118 (1998) 5-12 Sea water supply
Dual Media Filters
Backwash pumps
Filtered Wateg pumps
Filtered water ClearweH
Product water Storage tenlo
Lime dosing
Cartridge filters
HP Feed Permeators Pumps & ERTs
Bneldlow tank
Dechiorination
Product water Prooduct water Pumps CleorweH
Chlorination
Fig. 1. Process flow diagram of 20 migd RO desalination plant, AI Jubail. SWCC, Riyadh, Saudi Arabia. Table 1 Raw seawater analysis Component
Chloride, CiSulphate, SO~Bicarbonate, HCO~ Bromide, BrFluoride, FSodium, Na÷ Magnesium, Mg÷÷ Potassium, K+ Calcium, Ca÷+ Strontium, Sr÷+ Carbon dioxide, CO2 Dissolved oxygen TDS Temp., °C pH
Composition Min. value, mg/l
Max. value, mg/1
Average value, mg/i
23,120 3,150 151 73 1 11,910 1,430 428 450 16 1 6.4 42,000 I1 8.0
25,600 3,570 189 90 1 14,235 1,715 515 540 18 3 6.8 46,500 35 8.3
24,090 3,384 176 83 1 13,440 1,618 483 508 17 2 6.6 43,800 26.5 8.1
Net export capacity: 20 migd (90,910 m3/d) Product purity: TDS less than 450 mg/l (250 mg/l chlorides ) prior to passivation Seawater TDS: 46,500 mg/l max Seawater temperature: 20--35 °C Conversion rate: 3 5% No. of RO stages: Single
2. Pilot plant studies To validate the design of the pretreatment considered for the plant, a pilot filtration plant was set up. A self-contained container housing a scaled-down pretreatment section including a dualmedia filter column with backwash and air scour facilities, cartridge filters and suitable chemical dosing arrangements was placed in operation during August 1993. The pilot plant was also equipped with a high-pressure feed pump and three B-10 TWIN permeators representing the actual model installed in the main plant. A trial matrix was prepared with varying chemical dosing rates and different types of coagulant aids. The trials
M. B. Baig, A.A. AI Kutbi / DesaBnation 118 (1998) 5-12 were carried out over a period of more than one year, after which the following conclusions were drawn.
7
Several coagulant aids, which were locally available, were tried with Cyanamid C573 giving the best results.
2.1. Optimization o f media depths
2.3. Type of coagulant
The pilot plant trials were initiated with media depths as given in Table 2, assuming a 20 mg/1 total suspended solids (TSS) load with high level of organic and biological contamination. The seawater quality, however, turned out to be better than originally assumed with lower organic contamination (TOC, 3-4mg/l) and TSS lower than 1 mg/l. Based on this, the depth of coarser media anthracite is reduced and that of finer media sand is increased to have better retention of finer silt particles.
Ferric chloride was used as the coagulant during the trials. Since the results were satisfactory and the required SDI values could be achieved, other types of coagulants were not tried.
2.2. Need of coagulant aid Trials were carried out to establish whether there is a need for coagulant aid and if so what type. The conclusions are as follows: • Lower filtrate SDI values were obtained when coagulant aid was dosed in association with the coagulant, ferric chloride. • Coagulant aid dosing helped in lowering ferric chloride dosing rates. Risk of carryover of colloidal ferric hydroxide is reduced, thereby ensuring longer life of the cartridge filter elements and minimizing the risk of colloidal fouling of the membrane surface.
2.4. Filtrate SDI values and optimization o f chemical dosing rates SDI values below 3, as required by the membrane manufacturer, were achieved on a continuous basis with the following chemical dosing rates: Coagulant (ferric chloride ): 0.8 mg/l as Fe Coagulant aid: 0.2-0.4 mg/l 2.5. Filter run time Filter run times of more than 24h were achieved without overloading the filter media. The maximum filter run time that is possible on a continuous basis could not, however, be established, as several trials had to be carried out in a specified time period.
3. Seawater supply system
Table 2 Media depths of the pilot filtration plant Media
Original design Revised Grain size, depth, mm depth, mm mm
Anthracite Sand Supporting gravel- 1 Supporting gravel-2
1200 800 150
600 1400 150
1.4-2.5 0.63-1.0 2.0-3.15
150
150
3.15-5.60
The required quantity of screened and chlorinated seawater for the RO plant is drawn from the existing seawater intake header of the MSF plant. Two 50% capacity pipelines transport seawater from the interface point to the dual-media filters of the RO plant. These pipes are buried for most of their length. Major design parameters are the following: SW flow rate: 12,000 mVh Number of SW feeders: 2 Pipe material: RTR fiberglass pipes
8
M.B. Baig, A.A. AI Kutbi/Desalination 118 (1998) 5-12
Line size: 1100 mm diameter Residual chlorine level: 0.25mg/l minimum SW SDI5 (200 ml): 16-19 (from pilot plant trial data) SW pH: 8.1-8.3 Static mixers are installed in each feeder line to ensure thorough mixing of sulfuric acid, coagulant and coagulant aid dosed in the system. Flow control valves are provided in each feeder to control seawater flow rate. Static mixers and valves are placed in pits to provide accessibility for inspection and maintenance. Special features: • Configuration of 2x50% capacity lines is chosen in preference over a single 100% capacity line to ensure continuity of plant production even in case of any damage to one of the pipelines. The plant will, however, be operating with reduced capacity till both lines resume operation. • The seawater supply system is designed to handle 10% excess flow over and above the normal demand. This allows more flexibility in the plant design for any future changes such as addition of second stage permeators, installation of improved capacity permeator models etc. 4. Dual media filters (DMF)
The DMF are provided to reduce the suspended solids in the raw seawater to SDI values of less than 3.0 as required by the membrane manufacturer. Coagulant and coagulant aid, dosed in the seawater intake lines upstream of the DMF, achieve "on-line flocculation and coagulation". Dosing of sulfuric acid for scale control upstream of the DMF also helps the filtration efficiency of the DMF. Seawater is chlorinated to avoid biological growth. The design of the DMF is validated by the pilot filtration plant trials carried out for more than one year during which SDI values of less than 3.0 were consistently achieved. The major design parameters of the DMF are
presented below: Filtration velocity:
7.79 m/h (with all beds in service) 8.39 m/h (during backwash) Back wash velocity: 53.5 m/h Air scour velocity: 50m/h Number of beds: 14 Area of each bed: l l 0 m 2(5x22m) Media depths and sizes: as per the "revised depth" column of Table 2 Special features: • Low filtration velocity • Covered filter beds to avoid algae growth under exposure to direct sunlight • Special elastomeric coating for all internal surfaces • First filtrate dump facility. The freshly backwashed bed gives higher filtrate SDI values for the first few minutes after placement in service. If this is adversely affecting the combined filtrate SDI, the filtrate from the particular bed can be dumped for the initial few minutes. • Filtrate trough located centrally in the filter bed. In case of filter bed with side trough configuration, the full width of the bed is not effectively utilized. This is avoided with central trough configuration dividing the bed in two halves, whereby the seawater flows on either side of the trough. • The filtrate trough has freeboard to allow 40% expansion, though the basic design of the beds is based on 23% expansion for the media size and backwash velocity selected. This extra margin allows increased backwash flow rates, if it becomes necessary at any stage.
5. Filtered water clearwell (FWC) and back wash tank
Filtered water from the DMF flows into an underground concrete clearwell. 3×50% capacity
M. B. Baig, A.A. AI Kutbi / Desalination 118 (1998) 5-12
vertical pumps, installed in the FWC, forward the filtered seawater to the RO trains through the cartridge filters. A backwash tank, constructed integrally with the FWC, is placed hydraulically between the DMF and FWC. Vertical pumps, 2× 100% capacity, installed in the backwash tank, supply required quantity of filtered seawater for the backwash of the filter beds. The pumps are supplied by Ingersoll Dresser (Spain). The rating and the materials of the pumps are presented below. FWC pumps
BW pumps
Rated capacity
8000 m3/h at 52 m TDH
8000 m3/hat 8.5 m TDH
Motor rating
1400 k
425 kW
Materials: Casing Impeller Shaft
AISI 317 L ASTM A743 CG8M Nitronic 50 (Cr-22, Ni-12.5, Mo-2.5, V-0.2)
The backwashing cycle is expected to be of 41 min duration, with the sequence: draining of the bed, 5 min ~ first backwash, 5 min ~ draining of the bed, 5 rain, - air scouring, (5 min) ~ dwell time (5 min), ~ second backwash (6 min), ~ prefiltrate dump, 10 min (only if required). Special features: • Combined backwash tank and filtered water clearwell concept is not only economical but also occupies less space. • Filtered water will be used for bed backwash. The attractive option of using brine for backwash was considered but not employed due to the risk of carryover of biofouling into the permeator feed stream. 6. Micron cartridge filters
The filtrate may collect some debris downstream of the dual media filters from the
9
clear well and the impurities from the dosed chemicals, which if not prevented would enter the permeators. Micron cartridge filters, provided after the dual-media filters, remove suspended matter of size 10 microns and above. Major design parameters : No. of cartridge filter vessels: 16 (15 in operation + 1 standby) Filter cartridges/vessel: 161 Cartridge size (ram): Length/O.D/I.D: 1000/63/29 Nominal filtration size: 5/~m (removal of 98% of 10/~m particles) Vessel size: Volume/dia/height): 3.4 m3/1250 mm/3100 mm Design pressure: 6.5 bar Maximum allowable pressure drop: 1.5 bar Materials -RTR Vessel housing: Stainless steel 904 L Vessel internals: Polypropylene Cartridges: Stainless steel 316 Lid clamps: Special feature: The vessel inlet is from the bottom. This avoids the breakage of tie rods of the cartridges, which was experienced with the side inlet vessels.
7. Reverse osmosis desalination units
The desalination section consists of 15 independent RO trains with respective high-pressure feed pumps and energy recovery turbines. Fifteen numbers of RO trains were chosen considering the market availability of commercially proven maximum pump capacities at the required heads. A single-stage configuration was adopted which is adequate to meet the guaranteed water quality of 250ppm chloride level maximum. The design recovery rate is 35%. The plant is controlled by the flow rates with fixed permeate and feed flows within the guaranteed temperature range of 2035 ° C. A pressure-temperature profile is set in the
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M. B. Baig, A.A. AI Kutbi / Desalination 118 (1998) 5-12
control room computers to ensure that the highpressure feed pump trips when the feed pressure exceeds the set criteria. Major design parameters: Number of RO trains: 15 Permeators/train: 205 Conversion rate: 35% Number of RO stages: Single DuPont B- 10 TWIN Permeator model: Configuration: Hollow fine fibre Aramid polymer Material: 84 bar (1200 psi) Maximum pressure: 40°C Maximum temp. limit: Maximum SDI limit: 3 Chlorine tolerance: Nil 53 ma/d Initial flux at test conditions: 99.35% Nominal salt rejection: Elements/shell: 2 0.623 MFRC (estimated): after 3 years of operation Special features: • Flexible rubber hoses reinforced with stainless steel are used for permeator connections on the high-pressure side in preference over rigid connections. This provided more flexibility in permeator installation and hook-up with the stainless steel manifolds and is expected to lower the possibility of leaks at the connections. The selected flexible hoses are manufactured to withstand a burst pressure of 500 bar and hydrotest of 130 bar. • Each train is divided into two halves which allows only one-half to be isolated for chemical cleaning. • The design ensures that the plant production rate does not drop even during the membrane cleaning cycle.
8. High-pressure feed pumps Major design parameters: Type: Horizontal, 4 stage horizontally split centrifugal
Manufacturer: Ingersoll Dresser, UK 900 m3/h at TDH of 810 m Rated capacity: Motor rating: 2450 kW Materials: Casing: IDP 885 (Cr-21, Ni-12, Mo-4,V-0.15,N-0.15) Impellers: IDP 885 (Cr-21, Ni-12, Mo-4,V-0.15,N-0.15) Shaft: Nitronic 50 (Cr-22, Ni-12.5, Mo-2.5, V- 0.2, N-0.3) IDP 885 is a nitrogen strengthened improved version of CG-3M (cast SS317 L) alloy which offers improved corrosion resistance and mechanical properties. Special feature: The pumps are designed for a capacity 15% excess of the duty flow corresponding to the duty TDH. This extra margin provides flexibility for installation of permeators with higher capacities/higher feed pressures at a later date.
9. Energy recovery turbines (ERT) After a careful study, the reverse running pump type Francis turbine was preferred over the Pelton wheel for the following reasons: • Availability of standard pumps in seawaterresistant materials. • Operational experience with similarpumps and turbines in SWCC-RO plants. • Inadequate references on Pelton wheel turbines in the required capacity range for seawater applications. • Higher maintenance costs especially towards the replacement of runner blades in case of Pelton wheel turbines. Direct flange connection of ERT to the drive motor was compared with connecting via a clutch. Direct flange connection was preferred due to the problems faced with the clutches in other projects. ERTs required frequent stoppages for the replacement of rollers and for other maintenance needs.
M. B. Baig, A.A. AI Kutbi / Desalination 118 (1998) 5-12
Major design parameters: Type: Reverse running pump type, Francis turbine Manufacturer: Ingersoll Dresser, UK 485 mS/h Rated capacity: 71.5 bar Rated TDH: 760 kW (33%) Power recovery: Same as that of HP pumps Materials:
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to be less than 1.0mg/1 to achieve required SDI levels in the filtrate. Ferric chloride of 40% concentration will be procured in drums. 10.4. Coagulant aid dosing
Pilot plant trials established the need of polyelectrolyte dosing in support of ferric chloride to optimize the DM filter performance. Dosage rates of 0.2 to 0.4 mg/l are foreseen.
10. Chemical dosing systems 10.1. Seawater chlorination
10.5. Sodium bisulfite dosing
The seawater, tapped from the MSF seawater header, is already chlorinated with a minimum residual chlorine level of 0.25mg/l. However, during the pilot plant trials, the best DMF performance was achieved at residual chlorine levels of about 1.0 to 1.2mg/l in the filtrate. Additional chlorine dosing was, therefore, necessary which was achieved by the sodium hypochlorite drawn from the Jubail MSF plant chlorine generators. System capacity for additional chlorination is 2.0 mg/l.
Sodium bisulfite will be dosed downstream of the cartridge filters to dechlorinate the RO feed. Additional dosing point for the sodium bisulfite is provided at the HP feed pump suction to allow flexibility of operation. Dechlorination at the pump suction reduces the unchlorinated zone in the feed system considerably and reduces the risk of membrane biofouling. This option will be exercised only after ensuring that there is no risk to the permeators even in case of the failure of the dechlorination system.
10.2. Acid dosing
10. 6. Product chlorination
Sulfuric acid will be dosed to bring down the pH of seawater from 8.3 to 6.7 in the RO feed, thereby controlling the scaling of the pipelines and membranes mainly from CaCO3 salts. The acid dosing will be done in two steps. The dosing upstream of the DM filters will bring down pH to 7.0, and the secondary dosing downstream of the DM filters will bring the pH down further to 6.7. Enough flexibility exists in the system design to vary the pH levels of the filtrate and RO feed for optimum DM filter performance and efficient scale control.
Product water clearwell and the product water storage tanks are known to be the two potential areas of biological growth. To protect these two areas and the piping system from the biological growth, dosing of calcium hypochlorite is done at two locations--one at the inlet pipeline to the product water clearwell and the other at the discharge of the product water pumps. The dosing system provided can maintain residual chlorine level ofupto 1.5 mg/l.
10. 3. Coagulant dosing
Based on pilot plant results, ferric chloride is selected as the coagulant. Dosing rate is expected
10. 7. Product water passivation
Product water is passivated to maintain pH of 8.5, positive LSI of 0.1 to 0.3 and hardness of 2 German degrees. To achieve this, a lime handling and dosing plant of capacity 300 kg/h and
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M. B. Baig, A.A. AI Kutbi / Desalination 118 (1998) 5-12
a carbon dioxide dosing system of 200kg/h capacity is provided. Hydrated lime, received through tankers, will be pneumatically conveyed into the lime silos. As a first step, the lime powder is prepared into a 10% concentration lime milk and pumped into the saturators. The saturators, named "Lamella saturators", remove the impurities present in the lime powder to the extent of 1020%. The Lamella saturators are selected in preference over conventional lime saturators as they ensure lime water free of suspended solids and impurities. The discharge from the Lamella saturators is a high-quality lime, water free of impurities and of concentration corresponding to the solubility levels of lime at the corresponding temperatures. Carbon dioxide, in required quantities, is drawn from the available surplus capacity of the Jubail MSF C02 generation plant and dosed upstream of the product water pumps. 10.8. Membrane cleaning chemicals
During the operation of the plant, deposits of mineral scale, biological matter, silt and insoluble
organic matter build up on the membrane surface. This affects the membrane productivity, salt rejection and the bundle pressure drop. To restore the condition of the permeators, the RO trains are subjected to cleaning operations. A complete chemical cleaning system with necessary pumps, tanks, cooling arrangement etc., is provided to clean half train at a time.
11. Conclusions
The AI Jubail SWRO desalination plant has been built taking into account the experiences with other RO plants in operation. Improvements were made in the design to reduce/eliminate the known problems areas. Pretreatment was given due importance, and a pilot plant was run for more than one year to optimize the pretreatment. With this, it is expected that the Jubail SWRO desalination plant, when it goes on stream, will perform well.
ELSEVIER
Desalination 118 (1998) 5-12
Design features of a 20 migd SWRO desalination plant, A1 Jubail, Saudi Arabia Mohammed Badrulla Baig*, Abdul Aziz A1 Kutbi Saline Water Conversion Corporation, PO Box 5968, Riyadh 11432, Saudi Arabia Tel. +966 (1) 463-1111; Fax +966 (I) 463-4546
Received 16 June 1998
Abstract The Saline Water Conversion Corporation (SWCC), Saudi Arabia, began operating reverse osmosis (RO) desalination plants as early as in 1978 with small capacities and is now operating the largest seawater RO plants in the world. The rich experience gained by SWCC in more than 20 years of operation of these plants has been used to address some of the problems faced earlier and in improving the design of the new RO plants. One of the recent RO plants, which is due for commissioning, is the 91,000 m3/d plant at A1 Jubail, on the coast of the Arabian Gulf. When placed in operation, it will be among the largest seawater RO desalination plants in the world. Considering the significance of a well designed pretrealment section for the successful operation of any RO plant, a pilot filtration plant was placed in operation during August 1993. The trials were carried out over a period of more than one year and the results were adapted in the plant design. This paper presents the salient design features of the plant and the improvements made. Keywords: Pretreatment; Seawater reverse osmosis (SWRO); Membranes; Coagulant; Filtration; Cartridge filters
1. Basic design criteria of AI Jubail SWRO plant The SWRO plant was envisaged as an extension to the existing 230 migd MSF desalination plant at A1 Jubail situated on the Arabian
Gulf, coast of Saudi Arabia. The plant's output will be pumped to the Qassim region through a 1500 mm dia pipeline. The basic design criteria of the RO plant is given below. Table 1 and Fig. 1 give the analysis of seawater and the process flow diagram, respectively.
*Corresponding author. Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier SeicmeeB.V. All rights reserved. PII SO011-9164(98)00068-X
6
M.B. Baig, A.A. AlKutbi/Desalination 118 (1998) 5-12 Sea water supply
Dual Media Filters
Backwash pumps
Filtered Wateg pumps
Filtered water ClearweH
Product water Storage tenlo
Lime dosing
Cartridge filters
HP Feed Permeators Pumps & ERTs
Bneldlow tank
Dechiorination
Product water Prooduct water Pumps CleorweH
Chlorination
Fig. 1. Process flow diagram of 20 migd RO desalination plant, AI Jubail. SWCC, Riyadh, Saudi Arabia. Table 1 Raw seawater analysis Component
Chloride, CiSulphate, SO~Bicarbonate, HCO~ Bromide, BrFluoride, FSodium, Na÷ Magnesium, Mg÷÷ Potassium, K+ Calcium, Ca÷+ Strontium, Sr÷+ Carbon dioxide, CO2 Dissolved oxygen TDS Temp., °C pH
Composition Min. value, mg/l
Max. value, mg/1
Average value, mg/i
23,120 3,150 151 73 1 11,910 1,430 428 450 16 1 6.4 42,000 I1 8.0
25,600 3,570 189 90 1 14,235 1,715 515 540 18 3 6.8 46,500 35 8.3
24,090 3,384 176 83 1 13,440 1,618 483 508 17 2 6.6 43,800 26.5 8.1
Net export capacity: 20 migd (90,910 m3/d) Product purity: TDS less than 450 mg/l (250 mg/l chlorides ) prior to passivation Seawater TDS: 46,500 mg/l max Seawater temperature: 20--35 °C Conversion rate: 3 5% No. of RO stages: Single
2. Pilot plant studies To validate the design of the pretreatment considered for the plant, a pilot filtration plant was set up. A self-contained container housing a scaled-down pretreatment section including a dualmedia filter column with backwash and air scour facilities, cartridge filters and suitable chemical dosing arrangements was placed in operation during August 1993. The pilot plant was also equipped with a high-pressure feed pump and three B-10 TWIN permeators representing the actual model installed in the main plant. A trial matrix was prepared with varying chemical dosing rates and different types of coagulant aids. The trials
M. B. Baig, A.A. AI Kutbi / DesaBnation 118 (1998) 5-12 were carried out over a period of more than one year, after which the following conclusions were drawn.
7
Several coagulant aids, which were locally available, were tried with Cyanamid C573 giving the best results.
2.1. Optimization o f media depths
2.3. Type of coagulant
The pilot plant trials were initiated with media depths as given in Table 2, assuming a 20 mg/1 total suspended solids (TSS) load with high level of organic and biological contamination. The seawater quality, however, turned out to be better than originally assumed with lower organic contamination (TOC, 3-4mg/l) and TSS lower than 1 mg/l. Based on this, the depth of coarser media anthracite is reduced and that of finer media sand is increased to have better retention of finer silt particles.
Ferric chloride was used as the coagulant during the trials. Since the results were satisfactory and the required SDI values could be achieved, other types of coagulants were not tried.
2.2. Need of coagulant aid Trials were carried out to establish whether there is a need for coagulant aid and if so what type. The conclusions are as follows: • Lower filtrate SDI values were obtained when coagulant aid was dosed in association with the coagulant, ferric chloride. • Coagulant aid dosing helped in lowering ferric chloride dosing rates. Risk of carryover of colloidal ferric hydroxide is reduced, thereby ensuring longer life of the cartridge filter elements and minimizing the risk of colloidal fouling of the membrane surface.
2.4. Filtrate SDI values and optimization o f chemical dosing rates SDI values below 3, as required by the membrane manufacturer, were achieved on a continuous basis with the following chemical dosing rates: Coagulant (ferric chloride ): 0.8 mg/l as Fe Coagulant aid: 0.2-0.4 mg/l 2.5. Filter run time Filter run times of more than 24h were achieved without overloading the filter media. The maximum filter run time that is possible on a continuous basis could not, however, be established, as several trials had to be carried out in a specified time period.
3. Seawater supply system
Table 2 Media depths of the pilot filtration plant Media
Original design Revised Grain size, depth, mm depth, mm mm
Anthracite Sand Supporting gravel- 1 Supporting gravel-2
1200 800 150
600 1400 150
1.4-2.5 0.63-1.0 2.0-3.15
150
150
3.15-5.60
The required quantity of screened and chlorinated seawater for the RO plant is drawn from the existing seawater intake header of the MSF plant. Two 50% capacity pipelines transport seawater from the interface point to the dual-media filters of the RO plant. These pipes are buried for most of their length. Major design parameters are the following: SW flow rate: 12,000 mVh Number of SW feeders: 2 Pipe material: RTR fiberglass pipes
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M.B. Baig, A.A. AI Kutbi/Desalination 118 (1998) 5-12
Line size: 1100 mm diameter Residual chlorine level: 0.25mg/l minimum SW SDI5 (200 ml): 16-19 (from pilot plant trial data) SW pH: 8.1-8.3 Static mixers are installed in each feeder line to ensure thorough mixing of sulfuric acid, coagulant and coagulant aid dosed in the system. Flow control valves are provided in each feeder to control seawater flow rate. Static mixers and valves are placed in pits to provide accessibility for inspection and maintenance. Special features: • Configuration of 2x50% capacity lines is chosen in preference over a single 100% capacity line to ensure continuity of plant production even in case of any damage to one of the pipelines. The plant will, however, be operating with reduced capacity till both lines resume operation. • The seawater supply system is designed to handle 10% excess flow over and above the normal demand. This allows more flexibility in the plant design for any future changes such as addition of second stage permeators, installation of improved capacity permeator models etc. 4. Dual media filters (DMF)
The DMF are provided to reduce the suspended solids in the raw seawater to SDI values of less than 3.0 as required by the membrane manufacturer. Coagulant and coagulant aid, dosed in the seawater intake lines upstream of the DMF, achieve "on-line flocculation and coagulation". Dosing of sulfuric acid for scale control upstream of the DMF also helps the filtration efficiency of the DMF. Seawater is chlorinated to avoid biological growth. The design of the DMF is validated by the pilot filtration plant trials carried out for more than one year during which SDI values of less than 3.0 were consistently achieved. The major design parameters of the DMF are
presented below: Filtration velocity:
7.79 m/h (with all beds in service) 8.39 m/h (during backwash) Back wash velocity: 53.5 m/h Air scour velocity: 50m/h Number of beds: 14 Area of each bed: l l 0 m 2(5x22m) Media depths and sizes: as per the "revised depth" column of Table 2 Special features: • Low filtration velocity • Covered filter beds to avoid algae growth under exposure to direct sunlight • Special elastomeric coating for all internal surfaces • First filtrate dump facility. The freshly backwashed bed gives higher filtrate SDI values for the first few minutes after placement in service. If this is adversely affecting the combined filtrate SDI, the filtrate from the particular bed can be dumped for the initial few minutes. • Filtrate trough located centrally in the filter bed. In case of filter bed with side trough configuration, the full width of the bed is not effectively utilized. This is avoided with central trough configuration dividing the bed in two halves, whereby the seawater flows on either side of the trough. • The filtrate trough has freeboard to allow 40% expansion, though the basic design of the beds is based on 23% expansion for the media size and backwash velocity selected. This extra margin allows increased backwash flow rates, if it becomes necessary at any stage.
5. Filtered water clearwell (FWC) and back wash tank
Filtered water from the DMF flows into an underground concrete clearwell. 3×50% capacity
M. B. Baig, A.A. AI Kutbi / Desalination 118 (1998) 5-12
vertical pumps, installed in the FWC, forward the filtered seawater to the RO trains through the cartridge filters. A backwash tank, constructed integrally with the FWC, is placed hydraulically between the DMF and FWC. Vertical pumps, 2× 100% capacity, installed in the backwash tank, supply required quantity of filtered seawater for the backwash of the filter beds. The pumps are supplied by Ingersoll Dresser (Spain). The rating and the materials of the pumps are presented below. FWC pumps
BW pumps
Rated capacity
8000 m3/h at 52 m TDH
8000 m3/hat 8.5 m TDH
Motor rating
1400 k
425 kW
Materials: Casing Impeller Shaft
AISI 317 L ASTM A743 CG8M Nitronic 50 (Cr-22, Ni-12.5, Mo-2.5, V-0.2)
The backwashing cycle is expected to be of 41 min duration, with the sequence: draining of the bed, 5 min ~ first backwash, 5 min ~ draining of the bed, 5 rain, - air scouring, (5 min) ~ dwell time (5 min), ~ second backwash (6 min), ~ prefiltrate dump, 10 min (only if required). Special features: • Combined backwash tank and filtered water clearwell concept is not only economical but also occupies less space. • Filtered water will be used for bed backwash. The attractive option of using brine for backwash was considered but not employed due to the risk of carryover of biofouling into the permeator feed stream. 6. Micron cartridge filters
The filtrate may collect some debris downstream of the dual media filters from the
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clear well and the impurities from the dosed chemicals, which if not prevented would enter the permeators. Micron cartridge filters, provided after the dual-media filters, remove suspended matter of size 10 microns and above. Major design parameters : No. of cartridge filter vessels: 16 (15 in operation + 1 standby) Filter cartridges/vessel: 161 Cartridge size (ram): Length/O.D/I.D: 1000/63/29 Nominal filtration size: 5/~m (removal of 98% of 10/~m particles) Vessel size: Volume/dia/height): 3.4 m3/1250 mm/3100 mm Design pressure: 6.5 bar Maximum allowable pressure drop: 1.5 bar Materials -RTR Vessel housing: Stainless steel 904 L Vessel internals: Polypropylene Cartridges: Stainless steel 316 Lid clamps: Special feature: The vessel inlet is from the bottom. This avoids the breakage of tie rods of the cartridges, which was experienced with the side inlet vessels.
7. Reverse osmosis desalination units
The desalination section consists of 15 independent RO trains with respective high-pressure feed pumps and energy recovery turbines. Fifteen numbers of RO trains were chosen considering the market availability of commercially proven maximum pump capacities at the required heads. A single-stage configuration was adopted which is adequate to meet the guaranteed water quality of 250ppm chloride level maximum. The design recovery rate is 35%. The plant is controlled by the flow rates with fixed permeate and feed flows within the guaranteed temperature range of 2035 ° C. A pressure-temperature profile is set in the
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M. B. Baig, A.A. AI Kutbi / Desalination 118 (1998) 5-12
control room computers to ensure that the highpressure feed pump trips when the feed pressure exceeds the set criteria. Major design parameters: Number of RO trains: 15 Permeators/train: 205 Conversion rate: 35% Number of RO stages: Single DuPont B- 10 TWIN Permeator model: Configuration: Hollow fine fibre Aramid polymer Material: 84 bar (1200 psi) Maximum pressure: 40°C Maximum temp. limit: Maximum SDI limit: 3 Chlorine tolerance: Nil 53 ma/d Initial flux at test conditions: 99.35% Nominal salt rejection: Elements/shell: 2 0.623 MFRC (estimated): after 3 years of operation Special features: • Flexible rubber hoses reinforced with stainless steel are used for permeator connections on the high-pressure side in preference over rigid connections. This provided more flexibility in permeator installation and hook-up with the stainless steel manifolds and is expected to lower the possibility of leaks at the connections. The selected flexible hoses are manufactured to withstand a burst pressure of 500 bar and hydrotest of 130 bar. • Each train is divided into two halves which allows only one-half to be isolated for chemical cleaning. • The design ensures that the plant production rate does not drop even during the membrane cleaning cycle.
8. High-pressure feed pumps Major design parameters: Type: Horizontal, 4 stage horizontally split centrifugal
Manufacturer: Ingersoll Dresser, UK 900 m3/h at TDH of 810 m Rated capacity: Motor rating: 2450 kW Materials: Casing: IDP 885 (Cr-21, Ni-12, Mo-4,V-0.15,N-0.15) Impellers: IDP 885 (Cr-21, Ni-12, Mo-4,V-0.15,N-0.15) Shaft: Nitronic 50 (Cr-22, Ni-12.5, Mo-2.5, V- 0.2, N-0.3) IDP 885 is a nitrogen strengthened improved version of CG-3M (cast SS317 L) alloy which offers improved corrosion resistance and mechanical properties. Special feature: The pumps are designed for a capacity 15% excess of the duty flow corresponding to the duty TDH. This extra margin provides flexibility for installation of permeators with higher capacities/higher feed pressures at a later date.
9. Energy recovery turbines (ERT) After a careful study, the reverse running pump type Francis turbine was preferred over the Pelton wheel for the following reasons: • Availability of standard pumps in seawaterresistant materials. • Operational experience with similarpumps and turbines in SWCC-RO plants. • Inadequate references on Pelton wheel turbines in the required capacity range for seawater applications. • Higher maintenance costs especially towards the replacement of runner blades in case of Pelton wheel turbines. Direct flange connection of ERT to the drive motor was compared with connecting via a clutch. Direct flange connection was preferred due to the problems faced with the clutches in other projects. ERTs required frequent stoppages for the replacement of rollers and for other maintenance needs.
M. B. Baig, A.A. AI Kutbi / Desalination 118 (1998) 5-12
Major design parameters: Type: Reverse running pump type, Francis turbine Manufacturer: Ingersoll Dresser, UK 485 mS/h Rated capacity: 71.5 bar Rated TDH: 760 kW (33%) Power recovery: Same as that of HP pumps Materials:
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to be less than 1.0mg/1 to achieve required SDI levels in the filtrate. Ferric chloride of 40% concentration will be procured in drums. 10.4. Coagulant aid dosing
Pilot plant trials established the need of polyelectrolyte dosing in support of ferric chloride to optimize the DM filter performance. Dosage rates of 0.2 to 0.4 mg/l are foreseen.
10. Chemical dosing systems 10.1. Seawater chlorination
10.5. Sodium bisulfite dosing
The seawater, tapped from the MSF seawater header, is already chlorinated with a minimum residual chlorine level of 0.25mg/l. However, during the pilot plant trials, the best DMF performance was achieved at residual chlorine levels of about 1.0 to 1.2mg/l in the filtrate. Additional chlorine dosing was, therefore, necessary which was achieved by the sodium hypochlorite drawn from the Jubail MSF plant chlorine generators. System capacity for additional chlorination is 2.0 mg/l.
Sodium bisulfite will be dosed downstream of the cartridge filters to dechlorinate the RO feed. Additional dosing point for the sodium bisulfite is provided at the HP feed pump suction to allow flexibility of operation. Dechlorination at the pump suction reduces the unchlorinated zone in the feed system considerably and reduces the risk of membrane biofouling. This option will be exercised only after ensuring that there is no risk to the permeators even in case of the failure of the dechlorination system.
10.2. Acid dosing
10. 6. Product chlorination
Sulfuric acid will be dosed to bring down the pH of seawater from 8.3 to 6.7 in the RO feed, thereby controlling the scaling of the pipelines and membranes mainly from CaCO3 salts. The acid dosing will be done in two steps. The dosing upstream of the DM filters will bring down pH to 7.0, and the secondary dosing downstream of the DM filters will bring the pH down further to 6.7. Enough flexibility exists in the system design to vary the pH levels of the filtrate and RO feed for optimum DM filter performance and efficient scale control.
Product water clearwell and the product water storage tanks are known to be the two potential areas of biological growth. To protect these two areas and the piping system from the biological growth, dosing of calcium hypochlorite is done at two locations--one at the inlet pipeline to the product water clearwell and the other at the discharge of the product water pumps. The dosing system provided can maintain residual chlorine level ofupto 1.5 mg/l.
10. 3. Coagulant dosing
Based on pilot plant results, ferric chloride is selected as the coagulant. Dosing rate is expected
10. 7. Product water passivation
Product water is passivated to maintain pH of 8.5, positive LSI of 0.1 to 0.3 and hardness of 2 German degrees. To achieve this, a lime handling and dosing plant of capacity 300 kg/h and
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a carbon dioxide dosing system of 200kg/h capacity is provided. Hydrated lime, received through tankers, will be pneumatically conveyed into the lime silos. As a first step, the lime powder is prepared into a 10% concentration lime milk and pumped into the saturators. The saturators, named "Lamella saturators", remove the impurities present in the lime powder to the extent of 1020%. The Lamella saturators are selected in preference over conventional lime saturators as they ensure lime water free of suspended solids and impurities. The discharge from the Lamella saturators is a high-quality lime, water free of impurities and of concentration corresponding to the solubility levels of lime at the corresponding temperatures. Carbon dioxide, in required quantities, is drawn from the available surplus capacity of the Jubail MSF C02 generation plant and dosed upstream of the product water pumps. 10.8. Membrane cleaning chemicals
During the operation of the plant, deposits of mineral scale, biological matter, silt and insoluble
organic matter build up on the membrane surface. This affects the membrane productivity, salt rejection and the bundle pressure drop. To restore the condition of the permeators, the RO trains are subjected to cleaning operations. A complete chemical cleaning system with necessary pumps, tanks, cooling arrangement etc., is provided to clean half train at a time.
11. Conclusions
The AI Jubail SWRO desalination plant has been built taking into account the experiences with other RO plants in operation. Improvements were made in the design to reduce/eliminate the known problems areas. Pretreatment was given due importance, and a pilot plant was run for more than one year to optimize the pretreatment. With this, it is expected that the Jubail SWRO desalination plant, when it goes on stream, will perform well.