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Ibiza seawater RO — special design features and operating data
DESALINATION Desalination 105 (1996) 125-134
ELSEVIER
Ibiza seawater RO - - special design features and operating data •
a*
Rudolf Edhnger , Femando Zaplana Gomilab aHydrotechnik Austria, Walter Simmer Strasse 4, A-5310 Mondsee, Austria Tel. +43 (6323) 518-7840; Fax +43 (6232) 501-087 bHydrotechnik Espana, SA, Planta Desaladora, Rotonda de Sta. Eulalia, E-07800 Ibiza, Spain Tel. +34 (71) 191193; Fax +34 (71) 191197
Received 31 May 1995; Accepted 30 June 1995
Abstract The paper presents the seawater reverse osmosis desalination plant for the town of Ibiza, Spain, that was constructed by Hydrotechnik Austria, now a member of the BWT Group, during the period of 1991-1992. This plant is the sister plant to the Fujairah desalination plant in the UAE. The plant has a nominal capacity of 9000 Tpd drinking water and is especially optimized for long-term and efficient operation for a period of more than 15 years on the principle of a BOT model. Besides a general explanation of the main components of the plant, special attention is given to special design criteria and features such as its own power generation, two-stage RO design, plant automatization and beach well seawater intake. Operating data confirm the optimum design with regards to chemical and specific power consumption as well as plant production flexibility. Keywords: RO desalination plant; Ibiza
1. History of p l a n t In 1989 the town council o f Ibiza, Baleares, Spain, decided to tender a seawater desalination plant with a nominal capacity o f 7500 Tpd to take serious counter-measures against the increasing salinity in water abstracted from their local well fields. The contract on a B O T basis was finally awarded to Hydrotechnik, Austria, in March 1990 and works c o m m e n c e d in spring 1991. The work was completed in December 1992, and the start-up o f the plant took place in *Corresponding author.
February/March 1994 after a period caused by delays in the construction o f the beach well intake system as well as due to political reasons. The plant was finally set in full commercial operation in June 1994. After negotiations with the provincial government o f the Baleares, the plant was sold to a newly formed Water Consortium, and Hydrotechnik is now operating the plant under separately with an operation and maintenance contract for the Consortium. Good quality drinking water is transferred to the consumers o f the town o f Ibiza.
0011-9164/96/$15.00 © 1996 ElsevierScienceB.V. All rights reserved PII S0011-9164(96)00066-5
126
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
The presently transferred drinking water quantities vary from 6,300 to 10,200 m3/d following a yearly plan to adapt to the seasonal variations of water consumption in the distribution system of Ibiza. 2. Special considerations for design and construction of the plant Based on the fact that the contract was originally a BOT (built, operate and transfer) contract for a period of 10 years operation of plant and return of investment, special attention was given to technical solutions and standards of materials and equipment efficiencies to improve operating costs for long-term operation rather than to accept weak technical solutions to save investment costs at the initial investment stage. Therefore, the following main criteria were especially taken into consideration for construction and design: Intake of seawater by use of a coastal well field to benefit from prefiltrated seawater with little biological activity and to reduce operating costs of chemicals in pretreatment of plant and to minimize the potential risk of biofouling in the whole system. • High-grade materials and standard of all equipment and especially in contact with seawater to avoid corrosion in order to reduce maintenance costs and to increase plant reliability and availability. Special care was also taken about the painting of all metallic surfaces which is a very important matter in saline atmospheres. • Standby equipment and redundant electrical systems to improve plant reliability and availability. Standardization of equipment throughout the plant as far as possible to reduce costs for spare parts and to improve plant availability. • Maximum efficiencies for pumps and turbines and electrical motors to reduce energy consumption and to improve specific power consumption. A Pelton-type energy recovery
turbine was used instead of reverse running pumps. • Automatization by visual display unit system with data logging and monitoring system as well as remote data transfer for remote control from Austria by use of a modem to reduce costs for personnel and to improve plant monitoring. • Own generation of electrical power for the plant by use of heavy fuel diesel generators to reduce the costs for electrical energy as the main operating cost factor for a RO plant. One fundamental reason, however, was the fact that the existing power generation system at the time of contract negotiation was not suitable to provide reliable power supply. • Two-stage RO system to optimize plant flexibility regarding permeate quality at different load factors, as well as to increase lifetime of first-stage elements.
•
3. Configuration of plant The technical configuration of the process plant can be described as follows.
3.1. Well field and transfer pipeline At the location of Punta Grossa, a coastal well field was constructed with eight wells of DN 500/400 perforated by a hydraulic hammer to a depth of 50 m into a limestone and dolomite aquifer with good fractures and dis-continuities with high to medium permeability. The most important factor for the location of the wells was to avoid increasing intrusion of seawater through the aquifers further inland, thus causing almost irreversible damage to existing potable water wells in the area. The individual wells are constructed with an UPVC well casing and screen and a concrete well head. The well pumps are made of marine bronze and installed at a level of approximately - 4 0 m. Local metering gear is provided in the well head.
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
127
cartridge filters dosing points for sulfuric acid and sodium bisulfite are provided. The CF bank with four (3+1) filters provides the second mechanical filtration before the membranes. The filters are also made of carbon steel internally hard rubber lined. Standard cotton cartridges 5 micron in size are used. After the CF bank the filtered water quality is metered (temperature, pH and redox with special sensor control system) and dumped through a dump valve should the parameters be out of the acceptable range for the RO system. Duplicate dosing systems for prechlorination with chlorine gas, sulfuric acid of 96% concentration, flocculating agent and SBS are installed in separate dosing rooms with all required safety gear.
3.3. Reverse osmosis section (see Figs. 2, 3)
Fig. 1. Ibiza desalination plant: location plan.
3.2. Pretreatment (see Figs. 2, 3) Prior to the pressure sand filters, the raw water quality is measured, and dosing points for chlorine and a flocculating agent with a static mixer are provided. For the first mechanical filtration stage, eight units of dual-media filters of 3.5 m diameter with nozzle floor are installed. The filters are made of carbon steel internally hard rubber lined to avoid corrosion. All valves are pneumatic and automatically controlled. Backwashing is performed with air scour and backwashing with filtered water. After the filter station a control valve adjusts the raw water system pressure to a constant value to avoid strong pressure fluctuations in the pipeline when well pumps are started or stopped. The filtered water is collected in a 400 m 3 filtered water tank built of specially coated bolted steel plates. The filtered water pumps (3+2) transfer the filtered water through the cartridge filters to the RO trains. Prior to the
The desalination of the seawater is performed in three identical RO trains with a nominal capacity of 3000 Tpd each. To overcome the osmotic pressure and to set proper operating conditions, a system pressure of max. 69 bar is required. The pressure is generated by a five-stage segment ring high pressure pump driven by an 800 kW HV motor. To recover energy from the brine, a Calder PT 3-1 Pelton-type energy recovery turbine is coupled on the same shaft. The energy recovery utilized with this type of turbine is approximately 34%. The hydraulic system parameters are adjusted by use of special high pressure valves. The materials of the pump and turbine as well as the valves are Duplex (DIN 1.4462) or higher grade. The PN 100 high pressure piping from the pump to the membrane rack and return to the ERT are made of VDM Chronifer 1925 HMO (DIN 1.4529) to avoid corrosion in this area. The first stage of the RO system consists of 44 pressure vessels suitable for eight elements. The element type installed at the first stage is TORAY SU 820. The nominal first stage recovery is initially 40%.
128
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
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R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
For increased plant flexibility and reduction of overall membrane replacement costs, a second stage of the system is installed. A booster pump takes permeate from the firststage permeate header and charges the second stage stack. Fifteen pressure vessels suitable for eight brackish water elements are provided in the second stage. The element type proposed is Toray SU 720 L. No energy recovery is used in the second stage (because it is not economic), but brine is recirculated to the feed of the first stage for optimization of the flow balance. The second stage recovery is up to 90%. At present 42 pressure vessels of the first stage are loaded with seven elements and one dummy which is well suited to produce the nominal permeate quantity while the second stage is not activated. When the second stage is activated, the first-stage recovery will be increased to approximately 43% to a maximum of 45% to keep the overall train recovery across both stages at 40% as initial. The number of pressure vessels in service is changed with the production requirements. Flushing pumps are installed to serve for permeate flushing, and a separate cleaning system with a 13 m 3 tank, cleaning pump and cartridge filter are provided.
3.4. Posttreatment The permeate from the RO trains is transferred to the 120 m 3 concrete surge suck-back tank which also acts as permeate storage for product transfer to the Water Consortium. The four product water transfer pumps (3+1) transfer the product through a 5 km long pipeline to a large elevated storage tank. A duplicate post-chlorination system and a lime dosing system are installed to turn the permeate into good quality drinking water. Special attention was also given to the problems of water hammer (critical at shut-down of all pumps); hence a sophisticated water hammer protection system with hydraulically dampened
non-return valves, air vent valves and overpressure discharge values is provided.
3.5. Brine discharge system The brine, sand filter backwash water and all other drainage water are collected in a 80 m 3 concrete underground tank which is divided into two compartments. The first compartment (with overflow to the second one) is used to feed the generator cooling pps (1+1) for the power station luboil and cooling water system. Brine transfer pumps (2+1) transfer the brine through a 3 m long pipeline returning to the sea at the cliffs of Punta Grossa.
3.6. Power generation system (Fig. 2) The electrical power for the whole plant (at present also including power for the well field) is generated by 2 ELIN synchronous generators with a voltage of 6.300 V and a power rating of 2.7 MVA. The generators are driven by heavy fuel diesel engines of the type MAK653M with 2.2 MW which is a six-cylinder turbo-charged diesel engine suitable for operation with heavy fuel (type Bunker C). Such engines are standard ship engines with fuel consumptions of about 190 g/kWh excluding auxiliary engine driven pumps. The design of the generators (electrical field and copper) as well as the speed controller and fly wheel was specifically optimized with the 800 kW HV motors driving the high pressure pumps to allow for defined start-up conditions. Technically it is highly difficult to start a motor of 800 kW with a 2.2 MW diesel engine under controlled conditions (defined max. voltage and frequency drops). Each engine was optimized to allow for start-up of 2 HV motors and for operation of the whole plant under full load to reach the optimum load factors and engine efficiencies at nominal plant operation. An automatic synchronization system for parallel operation and a special load sharing control system (manual or automatic) for
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
defined load sharing in parallel operation was provided to cope with all different operating conditions and to allow for controlled engine maintenance. The fuel (diesel for start-up and heavy fuel for normal operation) is bunkered in a 460 m 3 tank for the heavy fuel with integrated preheating system and a 16,000 It underground tank for the gas oil. The operation of diesel engines with heavy fuel is quite problematic because the fuel needs to be heated to reach a proper viscosity for processing. A special separation and filter system is necessary to clean the fuel from impurities (about 2-5%) prior to injection to the engine. The engines are started with 25 bar compressed air. A separate 415 kVA emergency diesel generator set is installed to provide energy in case both large engines are on service and for start-up of the power station. Originally, however, it was not planned to be totally isolated from the public electrical network, but problems with local infrastructure and legalization have forced us to install the emergency diesel generator. 3.7. Electrical switchgear and controls
The generators feed a high voltage switchgear with the motor starters for the high pressure pumps and two redundant transformers for the power supply of the low-voltage switchgear. The well field is also supplied with 6 kV by underground cable from the HV switchgear. Alternative supply from the local network is also foreseen at the well field substation. The LV switchgear is of the fully withdrawable MCC type to allow for quick maintenance in case of emergency. Duplicate supply lines are also provided for the different substations (dosing, station auxiliaries, fire fighting, etc.) to ensure maximum reliability of power supply. The plant control system gets its power through a special UPS system to allow troublefree operation in case of short power cuts or voltage drops due to changeover of engines or
131
engine faults. Batteries allow for continued 3 h alive system in case of a total power cut. The process parameters and status indications are displayed on a back-up process and power mimic. The automatic operation of the plant is accomplished using a process visualization system (VDU) with video screen, terminal and mouse. A separate printer prints all incoming faults and system alarms for the record. The controls of the individual plant sections are individually used by sub-PLC stations which are all linked together by a digital bus system. The overall control is made by the master station which communicates with the VDU system and the protocol system. All process data are also stored in the system for 36 h in the form of curves; a daily protocol with all minimum/maximum and average values goes through a modem to Austria. In case of emergency, it is even possible to set up a live data connection between the plant and a specialist in Austria to identify errors in the system software. The plant instrumentation is of the highest standard with local indications for all analytical parameters as well as flows and pressures. The REDOX monitoring system has a duplicate system with a self-check function to identify sensor faults and miscalibration. As previously explained, special focus was made on equipment reliability, efficiency and long-term performance in order to optimize plant reliability and operating costs.
4. Operational data and experience The first train was started on 22 February 1994. After completion of commissioning and testing, the plant was stopped and the membranes were conserved in bisulfite solution of about 500-800 ppm until the final start of commercial operation, which began in June 1994. Since then the plant has been in continuous service, and production was fixed to a
R. Edlinger, F. Zaplana Gomila /Desalination 105 (1996) 125-134
132
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Fig. 4. Ibiza SWRO plant. Incoming well water before sand filters: conductivity, temperature, SDI.
yearly schedule o f 6,400 Tpd to 10,200 Tpd monthly. The maximum production at present configuration is 10,800 Tpd with the first stage.
4.1. Raw water general data from well field (Fig. 4) Temperature: SDI: TDS: pH:
19°C average with a seasonal variation of ±1 °C 0.3-1 approx. 39,300-40,500 ppm 7.25
One of the biggest advantages of beach wells is the almost stable temperature throughout the year as well as the low SDI factor. The TDS in our case is higher than the usual seawater TDS probably due to a higher salinity in the ground. On the other hand, it is certain that almost no potable water is abstracted from the land side aquifers; this confirms that the location of the well field is correct.
•
The slow activities of biofouling in the system (long SF backwashing intervals and long CF replacement rates) confirm that the pretreatment can be operated without chlorination for the future with close monitoring and continued bimonthly disinfection.
4.3. Operating data of RO trains without second stage The data in Table 1 are readings taken at the plant in 1995. The number o f pressure vessels on stream is different for the trains. With the second stage in operation, the permeate conductivities drop to 580/680 for ROT 2 and ROT 3, respectively.
4.4. Operating data of posttreatment • •
4.2. Operating data for pretreatment •
•
Chemical dosing: Chlorination stopped, disinfection of complete pretreatment every 6-8 weeks by shock chlorination; SBS dosing stopped; flocculation not required; acid dosing to set a filtered water pH of approx. 6.5-6.8 SF backwashing: once a week
CF replacement rates: approx, every 4-6 months
•
Total cond. product: approx. 760 I~S Post-chlorination: 0.5-1 ppm pH adjustment: approx. 15-10 ppm lime dosing to set the pH to about 8.5.
4.5. Specific consumption As per Figs. 5 and 6, the specific consumption of the plant varies with the scheduled monthly production from approximately 5 . 5 6.3 kWh/m 3. The reasons for these significant
R. Edlinger, F. Zaplana Gomila /Desalination 105 (1996) 125-134
133
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Fig. 5. Ibiza SWRO plant: 1995 plan for energy consumption and production.
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Fig. 6. Ibiza SWRO plant: 1996 plan for energy consumption and production (without wells). Table 1 Actual plants readings on 12.5.1995
4.6. Specific fuel consumption
Parameter
ROT 1
ROT 2
ROT 3
Flow feed, m 3 Flow permeate, m3/h Pressure feed, bar Diff. pressure RO, bar Cond. 1 st. stage, itS PVs on stream, nos. Spec. power cons. HPP kWh/m 3
337 134 66.5 1.9 320a 36 4.2
363 144 66.7 1.1 900 41 3.95
382 153 65.5 1.2 990 42 3.85
aWith new elements installed. variations are changing efficiencies o f the generators with the different load factors and the increased station auxiliaries when both engines are required to run.
In our case it is, however, m o r e interesting to calculate with the specific fuel consumption because fuel is required from external sources and needs to be procured by the plant operator. Such large diesel engines have their best efficiency at load factors o f 8 0 - 9 0 % ; in our case, due to an additional load required for the well field, we have to operate both engines with a lower efficiency for full plant operation with all three RO trains running. The actual specific fuel c o n s u m p t i o n for the plant, therefore, varies from a p p r o x i m a t e l y 1.21.5 i o f fuel/m 3 drinking water, depending on the actual load factor o f the diesel generators.
R. Edlinger, F. Zaplana Gomila /Desalination 105 (1996) 125-134
134
4. 7. Staffing of plant Due to the fact that power station facilities with diesel engines require more supervision and maintenance during operation, a more specialized staff with experience in ship engines is required than normally necessary for the RO plant. The grade of automatization for the engines and their auxiliary systems is also limited. The plant personnel are shown in Table 2. Table 2 Plant personnel (total staff members are currently 22) Duty
Remarks
Management
Plant manager (generator specialist) Chemist and process engineer Administration Assistant 3 per shill (2 at generators), 5 shifts in total For all routine analysis as per local regulations For all plant mechanical and electrical
Operators Laboratory Maintenance
No. of staff 1 1 1 1 15 1 2
5. Conclusions The Ibiza RO plant has fulfilled its prime target of supplying good-quality drinking water
to the network of the City o f Ibiza to overcome the huge problems with water quality the past years. The initial special considerations for design and construction o f the plant are proven to be reliable and efficient for industrial use during almost 1 year of continued operation. Hence there is no doubt that this plant can be considered as one of the most modern and technically advanced installations in Europe.
Acknowledgements The authors wish to thank all colleagues from their own companies, our geologist, as well as the equipment suppliers for their continued commitment and support for this project throughout all its stages.
References [1] The feasibility of supply seawater from a well field to the new Ibiza desalination plant, Report No. 606A/1291 for Hydrotechnik, Austria, by Engineering Geology Ltd., Godalming, Surrey GU7 1LG, UK. [2] Project documents, drawings, reports, etc. from Hydrotechnik, Austria. [3] Operating data from Hydrotechnik, Ibiza, Spain.
ELSEVIER
Ibiza seawater RO - - special design features and operating data •
a*
Rudolf Edhnger , Femando Zaplana Gomilab aHydrotechnik Austria, Walter Simmer Strasse 4, A-5310 Mondsee, Austria Tel. +43 (6323) 518-7840; Fax +43 (6232) 501-087 bHydrotechnik Espana, SA, Planta Desaladora, Rotonda de Sta. Eulalia, E-07800 Ibiza, Spain Tel. +34 (71) 191193; Fax +34 (71) 191197
Received 31 May 1995; Accepted 30 June 1995
Abstract The paper presents the seawater reverse osmosis desalination plant for the town of Ibiza, Spain, that was constructed by Hydrotechnik Austria, now a member of the BWT Group, during the period of 1991-1992. This plant is the sister plant to the Fujairah desalination plant in the UAE. The plant has a nominal capacity of 9000 Tpd drinking water and is especially optimized for long-term and efficient operation for a period of more than 15 years on the principle of a BOT model. Besides a general explanation of the main components of the plant, special attention is given to special design criteria and features such as its own power generation, two-stage RO design, plant automatization and beach well seawater intake. Operating data confirm the optimum design with regards to chemical and specific power consumption as well as plant production flexibility. Keywords: RO desalination plant; Ibiza
1. History of p l a n t In 1989 the town council o f Ibiza, Baleares, Spain, decided to tender a seawater desalination plant with a nominal capacity o f 7500 Tpd to take serious counter-measures against the increasing salinity in water abstracted from their local well fields. The contract on a B O T basis was finally awarded to Hydrotechnik, Austria, in March 1990 and works c o m m e n c e d in spring 1991. The work was completed in December 1992, and the start-up o f the plant took place in *Corresponding author.
February/March 1994 after a period caused by delays in the construction o f the beach well intake system as well as due to political reasons. The plant was finally set in full commercial operation in June 1994. After negotiations with the provincial government o f the Baleares, the plant was sold to a newly formed Water Consortium, and Hydrotechnik is now operating the plant under separately with an operation and maintenance contract for the Consortium. Good quality drinking water is transferred to the consumers o f the town o f Ibiza.
0011-9164/96/$15.00 © 1996 ElsevierScienceB.V. All rights reserved PII S0011-9164(96)00066-5
126
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
The presently transferred drinking water quantities vary from 6,300 to 10,200 m3/d following a yearly plan to adapt to the seasonal variations of water consumption in the distribution system of Ibiza. 2. Special considerations for design and construction of the plant Based on the fact that the contract was originally a BOT (built, operate and transfer) contract for a period of 10 years operation of plant and return of investment, special attention was given to technical solutions and standards of materials and equipment efficiencies to improve operating costs for long-term operation rather than to accept weak technical solutions to save investment costs at the initial investment stage. Therefore, the following main criteria were especially taken into consideration for construction and design: Intake of seawater by use of a coastal well field to benefit from prefiltrated seawater with little biological activity and to reduce operating costs of chemicals in pretreatment of plant and to minimize the potential risk of biofouling in the whole system. • High-grade materials and standard of all equipment and especially in contact with seawater to avoid corrosion in order to reduce maintenance costs and to increase plant reliability and availability. Special care was also taken about the painting of all metallic surfaces which is a very important matter in saline atmospheres. • Standby equipment and redundant electrical systems to improve plant reliability and availability. Standardization of equipment throughout the plant as far as possible to reduce costs for spare parts and to improve plant availability. • Maximum efficiencies for pumps and turbines and electrical motors to reduce energy consumption and to improve specific power consumption. A Pelton-type energy recovery
turbine was used instead of reverse running pumps. • Automatization by visual display unit system with data logging and monitoring system as well as remote data transfer for remote control from Austria by use of a modem to reduce costs for personnel and to improve plant monitoring. • Own generation of electrical power for the plant by use of heavy fuel diesel generators to reduce the costs for electrical energy as the main operating cost factor for a RO plant. One fundamental reason, however, was the fact that the existing power generation system at the time of contract negotiation was not suitable to provide reliable power supply. • Two-stage RO system to optimize plant flexibility regarding permeate quality at different load factors, as well as to increase lifetime of first-stage elements.
•
3. Configuration of plant The technical configuration of the process plant can be described as follows.
3.1. Well field and transfer pipeline At the location of Punta Grossa, a coastal well field was constructed with eight wells of DN 500/400 perforated by a hydraulic hammer to a depth of 50 m into a limestone and dolomite aquifer with good fractures and dis-continuities with high to medium permeability. The most important factor for the location of the wells was to avoid increasing intrusion of seawater through the aquifers further inland, thus causing almost irreversible damage to existing potable water wells in the area. The individual wells are constructed with an UPVC well casing and screen and a concrete well head. The well pumps are made of marine bronze and installed at a level of approximately - 4 0 m. Local metering gear is provided in the well head.
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
127
cartridge filters dosing points for sulfuric acid and sodium bisulfite are provided. The CF bank with four (3+1) filters provides the second mechanical filtration before the membranes. The filters are also made of carbon steel internally hard rubber lined. Standard cotton cartridges 5 micron in size are used. After the CF bank the filtered water quality is metered (temperature, pH and redox with special sensor control system) and dumped through a dump valve should the parameters be out of the acceptable range for the RO system. Duplicate dosing systems for prechlorination with chlorine gas, sulfuric acid of 96% concentration, flocculating agent and SBS are installed in separate dosing rooms with all required safety gear.
3.3. Reverse osmosis section (see Figs. 2, 3)
Fig. 1. Ibiza desalination plant: location plan.
3.2. Pretreatment (see Figs. 2, 3) Prior to the pressure sand filters, the raw water quality is measured, and dosing points for chlorine and a flocculating agent with a static mixer are provided. For the first mechanical filtration stage, eight units of dual-media filters of 3.5 m diameter with nozzle floor are installed. The filters are made of carbon steel internally hard rubber lined to avoid corrosion. All valves are pneumatic and automatically controlled. Backwashing is performed with air scour and backwashing with filtered water. After the filter station a control valve adjusts the raw water system pressure to a constant value to avoid strong pressure fluctuations in the pipeline when well pumps are started or stopped. The filtered water is collected in a 400 m 3 filtered water tank built of specially coated bolted steel plates. The filtered water pumps (3+2) transfer the filtered water through the cartridge filters to the RO trains. Prior to the
The desalination of the seawater is performed in three identical RO trains with a nominal capacity of 3000 Tpd each. To overcome the osmotic pressure and to set proper operating conditions, a system pressure of max. 69 bar is required. The pressure is generated by a five-stage segment ring high pressure pump driven by an 800 kW HV motor. To recover energy from the brine, a Calder PT 3-1 Pelton-type energy recovery turbine is coupled on the same shaft. The energy recovery utilized with this type of turbine is approximately 34%. The hydraulic system parameters are adjusted by use of special high pressure valves. The materials of the pump and turbine as well as the valves are Duplex (DIN 1.4462) or higher grade. The PN 100 high pressure piping from the pump to the membrane rack and return to the ERT are made of VDM Chronifer 1925 HMO (DIN 1.4529) to avoid corrosion in this area. The first stage of the RO system consists of 44 pressure vessels suitable for eight elements. The element type installed at the first stage is TORAY SU 820. The nominal first stage recovery is initially 40%.
128
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
/"
T
T
LI
I
/
.3 :! t
oDoDd
,,..:.
0
P_, p
I
ej
i
/
o o t~
/
.-d e,,i
R. Edlinger, F. Zaplana Gomila /Desalination 105 (1996) 125-134
129
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R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
For increased plant flexibility and reduction of overall membrane replacement costs, a second stage of the system is installed. A booster pump takes permeate from the firststage permeate header and charges the second stage stack. Fifteen pressure vessels suitable for eight brackish water elements are provided in the second stage. The element type proposed is Toray SU 720 L. No energy recovery is used in the second stage (because it is not economic), but brine is recirculated to the feed of the first stage for optimization of the flow balance. The second stage recovery is up to 90%. At present 42 pressure vessels of the first stage are loaded with seven elements and one dummy which is well suited to produce the nominal permeate quantity while the second stage is not activated. When the second stage is activated, the first-stage recovery will be increased to approximately 43% to a maximum of 45% to keep the overall train recovery across both stages at 40% as initial. The number of pressure vessels in service is changed with the production requirements. Flushing pumps are installed to serve for permeate flushing, and a separate cleaning system with a 13 m 3 tank, cleaning pump and cartridge filter are provided.
3.4. Posttreatment The permeate from the RO trains is transferred to the 120 m 3 concrete surge suck-back tank which also acts as permeate storage for product transfer to the Water Consortium. The four product water transfer pumps (3+1) transfer the product through a 5 km long pipeline to a large elevated storage tank. A duplicate post-chlorination system and a lime dosing system are installed to turn the permeate into good quality drinking water. Special attention was also given to the problems of water hammer (critical at shut-down of all pumps); hence a sophisticated water hammer protection system with hydraulically dampened
non-return valves, air vent valves and overpressure discharge values is provided.
3.5. Brine discharge system The brine, sand filter backwash water and all other drainage water are collected in a 80 m 3 concrete underground tank which is divided into two compartments. The first compartment (with overflow to the second one) is used to feed the generator cooling pps (1+1) for the power station luboil and cooling water system. Brine transfer pumps (2+1) transfer the brine through a 3 m long pipeline returning to the sea at the cliffs of Punta Grossa.
3.6. Power generation system (Fig. 2) The electrical power for the whole plant (at present also including power for the well field) is generated by 2 ELIN synchronous generators with a voltage of 6.300 V and a power rating of 2.7 MVA. The generators are driven by heavy fuel diesel engines of the type MAK653M with 2.2 MW which is a six-cylinder turbo-charged diesel engine suitable for operation with heavy fuel (type Bunker C). Such engines are standard ship engines with fuel consumptions of about 190 g/kWh excluding auxiliary engine driven pumps. The design of the generators (electrical field and copper) as well as the speed controller and fly wheel was specifically optimized with the 800 kW HV motors driving the high pressure pumps to allow for defined start-up conditions. Technically it is highly difficult to start a motor of 800 kW with a 2.2 MW diesel engine under controlled conditions (defined max. voltage and frequency drops). Each engine was optimized to allow for start-up of 2 HV motors and for operation of the whole plant under full load to reach the optimum load factors and engine efficiencies at nominal plant operation. An automatic synchronization system for parallel operation and a special load sharing control system (manual or automatic) for
R. Edlinger, F. Zaplana Gomila / Desalination 105 (1996) 125-134
defined load sharing in parallel operation was provided to cope with all different operating conditions and to allow for controlled engine maintenance. The fuel (diesel for start-up and heavy fuel for normal operation) is bunkered in a 460 m 3 tank for the heavy fuel with integrated preheating system and a 16,000 It underground tank for the gas oil. The operation of diesel engines with heavy fuel is quite problematic because the fuel needs to be heated to reach a proper viscosity for processing. A special separation and filter system is necessary to clean the fuel from impurities (about 2-5%) prior to injection to the engine. The engines are started with 25 bar compressed air. A separate 415 kVA emergency diesel generator set is installed to provide energy in case both large engines are on service and for start-up of the power station. Originally, however, it was not planned to be totally isolated from the public electrical network, but problems with local infrastructure and legalization have forced us to install the emergency diesel generator. 3.7. Electrical switchgear and controls
The generators feed a high voltage switchgear with the motor starters for the high pressure pumps and two redundant transformers for the power supply of the low-voltage switchgear. The well field is also supplied with 6 kV by underground cable from the HV switchgear. Alternative supply from the local network is also foreseen at the well field substation. The LV switchgear is of the fully withdrawable MCC type to allow for quick maintenance in case of emergency. Duplicate supply lines are also provided for the different substations (dosing, station auxiliaries, fire fighting, etc.) to ensure maximum reliability of power supply. The plant control system gets its power through a special UPS system to allow troublefree operation in case of short power cuts or voltage drops due to changeover of engines or
131
engine faults. Batteries allow for continued 3 h alive system in case of a total power cut. The process parameters and status indications are displayed on a back-up process and power mimic. The automatic operation of the plant is accomplished using a process visualization system (VDU) with video screen, terminal and mouse. A separate printer prints all incoming faults and system alarms for the record. The controls of the individual plant sections are individually used by sub-PLC stations which are all linked together by a digital bus system. The overall control is made by the master station which communicates with the VDU system and the protocol system. All process data are also stored in the system for 36 h in the form of curves; a daily protocol with all minimum/maximum and average values goes through a modem to Austria. In case of emergency, it is even possible to set up a live data connection between the plant and a specialist in Austria to identify errors in the system software. The plant instrumentation is of the highest standard with local indications for all analytical parameters as well as flows and pressures. The REDOX monitoring system has a duplicate system with a self-check function to identify sensor faults and miscalibration. As previously explained, special focus was made on equipment reliability, efficiency and long-term performance in order to optimize plant reliability and operating costs.
4. Operational data and experience The first train was started on 22 February 1994. After completion of commissioning and testing, the plant was stopped and the membranes were conserved in bisulfite solution of about 500-800 ppm until the final start of commercial operation, which began in June 1994. Since then the plant has been in continuous service, and production was fixed to a
R. Edlinger, F. Zaplana Gomila /Desalination 105 (1996) 125-134
132
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24
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Fig. 4. Ibiza SWRO plant. Incoming well water before sand filters: conductivity, temperature, SDI.
yearly schedule o f 6,400 Tpd to 10,200 Tpd monthly. The maximum production at present configuration is 10,800 Tpd with the first stage.
4.1. Raw water general data from well field (Fig. 4) Temperature: SDI: TDS: pH:
19°C average with a seasonal variation of ±1 °C 0.3-1 approx. 39,300-40,500 ppm 7.25
One of the biggest advantages of beach wells is the almost stable temperature throughout the year as well as the low SDI factor. The TDS in our case is higher than the usual seawater TDS probably due to a higher salinity in the ground. On the other hand, it is certain that almost no potable water is abstracted from the land side aquifers; this confirms that the location of the well field is correct.
•
The slow activities of biofouling in the system (long SF backwashing intervals and long CF replacement rates) confirm that the pretreatment can be operated without chlorination for the future with close monitoring and continued bimonthly disinfection.
4.3. Operating data of RO trains without second stage The data in Table 1 are readings taken at the plant in 1995. The number o f pressure vessels on stream is different for the trains. With the second stage in operation, the permeate conductivities drop to 580/680 for ROT 2 and ROT 3, respectively.
4.4. Operating data of posttreatment • •
4.2. Operating data for pretreatment •
•
Chemical dosing: Chlorination stopped, disinfection of complete pretreatment every 6-8 weeks by shock chlorination; SBS dosing stopped; flocculation not required; acid dosing to set a filtered water pH of approx. 6.5-6.8 SF backwashing: once a week
CF replacement rates: approx, every 4-6 months
•
Total cond. product: approx. 760 I~S Post-chlorination: 0.5-1 ppm pH adjustment: approx. 15-10 ppm lime dosing to set the pH to about 8.5.
4.5. Specific consumption As per Figs. 5 and 6, the specific consumption of the plant varies with the scheduled monthly production from approximately 5 . 5 6.3 kWh/m 3. The reasons for these significant
R. Edlinger, F. Zaplana Gomila /Desalination 105 (1996) 125-134
133
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Fig. 5. Ibiza SWRO plant: 1995 plan for energy consumption and production.
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Fig. 6. Ibiza SWRO plant: 1996 plan for energy consumption and production (without wells). Table 1 Actual plants readings on 12.5.1995
4.6. Specific fuel consumption
Parameter
ROT 1
ROT 2
ROT 3
Flow feed, m 3 Flow permeate, m3/h Pressure feed, bar Diff. pressure RO, bar Cond. 1 st. stage, itS PVs on stream, nos. Spec. power cons. HPP kWh/m 3
337 134 66.5 1.9 320a 36 4.2
363 144 66.7 1.1 900 41 3.95
382 153 65.5 1.2 990 42 3.85
aWith new elements installed. variations are changing efficiencies o f the generators with the different load factors and the increased station auxiliaries when both engines are required to run.
In our case it is, however, m o r e interesting to calculate with the specific fuel consumption because fuel is required from external sources and needs to be procured by the plant operator. Such large diesel engines have their best efficiency at load factors o f 8 0 - 9 0 % ; in our case, due to an additional load required for the well field, we have to operate both engines with a lower efficiency for full plant operation with all three RO trains running. The actual specific fuel c o n s u m p t i o n for the plant, therefore, varies from a p p r o x i m a t e l y 1.21.5 i o f fuel/m 3 drinking water, depending on the actual load factor o f the diesel generators.
R. Edlinger, F. Zaplana Gomila /Desalination 105 (1996) 125-134
134
4. 7. Staffing of plant Due to the fact that power station facilities with diesel engines require more supervision and maintenance during operation, a more specialized staff with experience in ship engines is required than normally necessary for the RO plant. The grade of automatization for the engines and their auxiliary systems is also limited. The plant personnel are shown in Table 2. Table 2 Plant personnel (total staff members are currently 22) Duty
Remarks
Management
Plant manager (generator specialist) Chemist and process engineer Administration Assistant 3 per shill (2 at generators), 5 shifts in total For all routine analysis as per local regulations For all plant mechanical and electrical
Operators Laboratory Maintenance
No. of staff 1 1 1 1 15 1 2
5. Conclusions The Ibiza RO plant has fulfilled its prime target of supplying good-quality drinking water
to the network of the City o f Ibiza to overcome the huge problems with water quality the past years. The initial special considerations for design and construction o f the plant are proven to be reliable and efficient for industrial use during almost 1 year of continued operation. Hence there is no doubt that this plant can be considered as one of the most modern and technically advanced installations in Europe.
Acknowledgements The authors wish to thank all colleagues from their own companies, our geologist, as well as the equipment suppliers for their continued commitment and support for this project throughout all its stages.
References [1] The feasibility of supply seawater from a well field to the new Ibiza desalination plant, Report No. 606A/1291 for Hydrotechnik, Austria, by Engineering Geology Ltd., Godalming, Surrey GU7 1LG, UK. [2] Project documents, drawings, reports, etc. from Hydrotechnik, Austria. [3] Operating data from Hydrotechnik, Ibiza, Spain.