- Email: [email protected]
Desalination pre-treatment: Lowering the costs of desalination
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Column Feature
Desalination (a Filtration + Separation publication) Volume 4 Issue 1
Desalination pre-treatment:
Lowering the costs of desalination T
he new Thames Gateway desalination plant is going to be using a membrane for its pre-treatment which has been designed with the aim of lowering the whole life cost. We take a look at how this works and the particular difficulties relating to this plant.
Introduction Numerous tests around the world have proven that ultrafiltration (UF) provides optimum pre-treatment for seawater desalination based on reverse osmosis membranes (SWRO). Ultrafiltration removes all suspended solids and provides a substantial reduction in microbiological activities. The main obstacle against their use has always been the higher operating cost when compared with conventional pre-treatment.
Background The North Sea is on the European continental shelf of the Atlantic Ocean. It connects with the rest of the Atlantic through the Dover Strait and the English Channel in the south and through the Norwegian Sea in the north. The salinity of the water is dependent on place and time of year, but it is generally in the range of 15 to 25 parts per thousand (ppt) around river mouths and up to 32 to 35 ppt in the northern North Sea (Wikipedia, 2008). The North Sea is an area that is heavily populated with urban developments, many port facilities and several shore based industrial plants including ship-building yards, factories, storage tanks for chemical industries, paper mills and oil refineries. Domestic and industrial water consumption puts a stress on the available potable water supply. Furthermore domestic and industrial waste water discharges have a negative effect on the water quality of the North Sea.
The Thames Gateway desalination plant will be situated in Beckton, East London, on the north bank of the Thames.
Column Feature
Desalination (a Filtration + Separation publication) Volume 4 Issue 1
Alternative sources of water were historically very limited: seawater desalination by means of reverse osmosis (SWRO) was not an option because conventional pretreatment was not able to cope with the deteriorating water quality, especially the high organic content of the water. The high cleaning frequency that would be required for maintaining SWRO performance would increase the operational cost and decrease the SWRO life time to a point that makes it economically unfeasible.
Ultrafiltration In 2005 Norit X-Flow introduced an ultrafiltration membrane called Seaguard that is specifically designed for pre-treatment to SWRO. This membrane uses inside out hollow fibre technology. The membrane fibres are fed from the lumen side with the clean water permeating the wall of the fibres. The hollow fibre membrane technology is well suited for removing suspended solids and microbes. Ultrafiltration membranes will not remove any dissolved matter; therefore the mineral content will remain unchanged. The main characteristics for this membrane are: s Ultrafiltration for good removal of suspended solids, resulting in low turbidity and SDI. s Hydrophilic polyethersulfone/PVP, for high permeability and low fouling tendency. s Operated in dead-end mode in order to minimise the energy consumption. s Pressurised inside-out filtration, allowing for direct feed from the intake works into the RO high pressure pumps.
Table 1: Comparison of the throughput of the on line skids Design
Modified
Number of skids
21
42
Housings per skid
40
20
160
80
Membranes per skid Total membranes
3,360
3,360
Total membrane area
134,400
134,400
Net UF permeate flow
182 MLD
182 MLD
5%
2.5%
Flux variation
ultra- and microfiltration membranes were piloted in different configurations; with lamella clarifiers or with lamella clarifiers in combination with either media filters or granular activated carbon filters. The total duration of the pilot was twelve months. Following this pilot a contract for a 140 MLD desalination plant was awarded to Acciona (formerly Pridesa). The following processes were selected: During low tide water of reduced salinity is stored in basins – this water has approximately 1/3 of the salinity of normal seawater. In this way the operating pressure and the energy consumption of the RO unit can be reduced by a factor of two. During high tide the feed water intake from the river is stopped and feed water is taken from the storage basins. Pretreatment by means of coagulation followed by lamella clarification – the low salinity feed water is characterised by high turbidity and a high organic content.
s Use of short cleaning cycles, so called Chemically Enhanced Backwashes (CEB). This mode of operation minimises the flow fluctuations to the RO system. The modules have an outside diameter of 200 mm (8 inch) and they are installed in glass fibre reinforced epoxy (GRP) pressure vessels.
The Thames Gateway Thames Water is the UK’s largest water and wastewater services company. It supplies 2,600 million litres of tap water per day to 8.5 million customers across London and the Thames Valley. Thames Water also removes and treats 2,800 million litres of sewage for an area covering 13.5 million customers. It needed to augment its potable water supply with a new source, the most viable being surface water from the tidal River Thames. In 2003 Thames Water started an extensive piloting program targeting the optimum pretreatment solution for a 140 MLD SWRO plant located in East London. Four different
Original
A cross-section of Norit’s membrane.
Therefore extensive pre-treatment will be necessary. s Media filtration – seventeen horizontal pressure sand filters. s Precoat filters – the precoat filters use diatomaceous earth as a filtration aid to achieve a filtering efficiency down to approximately 3 micron. s First stage SWRO system – nine two stage RO units, each with inter-stage booster pumps s Second stage RO system – three two stage RO units, each with inter-stage booster pumps. The RO systems are concentrate staged in order to maximise the overall recovery. The use of the low salinity feed water allows for an overall RO recovery of 80%, which is unusually high for seawater. The contractor initially opted for precoat filters because of the lower investment costs. The desalination plant will be operated for approximately 40% of the time, therefore the
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22
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Desalination (a Filtration + Separation publication) Volume 4 Issue 1
operating costs and environmental impact of the desalination plant. s More flexible operation – the UF system can be switched on and off instantaneously. The precoat filters need a ripening period during which production cannot be started.
Figure 1: The arrangement of the skids at the Thames Gateway desalination plant.
replacement costs of the diatomaceous earth are offset by the low amortisation costs.
Project delays Due to environmental concerns, the permitting process for the Thames Gateway project took more time than anticipated. The Seaguard membranes did allow Acciona to re-evaluate the pre-treatment options. The membranes provided a solution with similar investment costs compared to the precoat filters while achieving several benefits: s Reduction in chemical consumption – no disposal of diatomaceous earth. This lowers
The modification of the design of the Thames Gateway Desalination Plant at this late stage had to be kept to an absolute minimum. The building had received planning permission and civil construction was started already. The floor plan of the desalination building had been constructed already. Any alterations to the building would have required a new application for planning permission and major civil modifications. The original process design featured a complete pressurised feed supply line from the lamella clarifiers up to the SWRO high pressure pumps, without any intermediate buffer tanks. The X-Flow Seaguard membranes are installed in GRP membrane housings and can be operated under pressure. Dimensionally the Seaguard UF system could be retrofitted into the space that was allocated for the precoat filters. The
existing media filters could be maintained; this improved the water quality being fed to the UF system. This allowed for integrating the UF system into the pressurised feed line between the media filters and the SWRO high pressure pumps. The major challenge was to design the UF system for a constant flow. During backwash of individual UF skids, the throughput of the on line skids has to be increased in order to maintain feed flow and feed pressure to the SWRO high pressure pumps. This was accomplished by doubling the number of the skids and decreasing the size of the individual skids, compared to the original design: This decreased the flow rate changes to within acceptable limits. The skids are arranged two by two (see Figure 1). The project is currently in the construction phase and on schedule for completion in 2009. Hand over to the customer is planned for early 2010. Contact: Wouter Brouze Norit X-Flow Email: [email protected] www.xflow.com
Norit X-Flow seawater pre-treatment with SEAGUARD membranes, as seen in the Palm Jumeirah desalination plant in Dubai.
Column Feature
Desalination (a Filtration + Separation publication) Volume 4 Issue 1
Desalination pre-treatment:
Lowering the costs of desalination T
he new Thames Gateway desalination plant is going to be using a membrane for its pre-treatment which has been designed with the aim of lowering the whole life cost. We take a look at how this works and the particular difficulties relating to this plant.
Introduction Numerous tests around the world have proven that ultrafiltration (UF) provides optimum pre-treatment for seawater desalination based on reverse osmosis membranes (SWRO). Ultrafiltration removes all suspended solids and provides a substantial reduction in microbiological activities. The main obstacle against their use has always been the higher operating cost when compared with conventional pre-treatment.
Background The North Sea is on the European continental shelf of the Atlantic Ocean. It connects with the rest of the Atlantic through the Dover Strait and the English Channel in the south and through the Norwegian Sea in the north. The salinity of the water is dependent on place and time of year, but it is generally in the range of 15 to 25 parts per thousand (ppt) around river mouths and up to 32 to 35 ppt in the northern North Sea (Wikipedia, 2008). The North Sea is an area that is heavily populated with urban developments, many port facilities and several shore based industrial plants including ship-building yards, factories, storage tanks for chemical industries, paper mills and oil refineries. Domestic and industrial water consumption puts a stress on the available potable water supply. Furthermore domestic and industrial waste water discharges have a negative effect on the water quality of the North Sea.
The Thames Gateway desalination plant will be situated in Beckton, East London, on the north bank of the Thames.
Column Feature
Desalination (a Filtration + Separation publication) Volume 4 Issue 1
Alternative sources of water were historically very limited: seawater desalination by means of reverse osmosis (SWRO) was not an option because conventional pretreatment was not able to cope with the deteriorating water quality, especially the high organic content of the water. The high cleaning frequency that would be required for maintaining SWRO performance would increase the operational cost and decrease the SWRO life time to a point that makes it economically unfeasible.
Ultrafiltration In 2005 Norit X-Flow introduced an ultrafiltration membrane called Seaguard that is specifically designed for pre-treatment to SWRO. This membrane uses inside out hollow fibre technology. The membrane fibres are fed from the lumen side with the clean water permeating the wall of the fibres. The hollow fibre membrane technology is well suited for removing suspended solids and microbes. Ultrafiltration membranes will not remove any dissolved matter; therefore the mineral content will remain unchanged. The main characteristics for this membrane are: s Ultrafiltration for good removal of suspended solids, resulting in low turbidity and SDI. s Hydrophilic polyethersulfone/PVP, for high permeability and low fouling tendency. s Operated in dead-end mode in order to minimise the energy consumption. s Pressurised inside-out filtration, allowing for direct feed from the intake works into the RO high pressure pumps.
Table 1: Comparison of the throughput of the on line skids Design
Modified
Number of skids
21
42
Housings per skid
40
20
160
80
Membranes per skid Total membranes
3,360
3,360
Total membrane area
134,400
134,400
Net UF permeate flow
182 MLD
182 MLD
5%
2.5%
Flux variation
ultra- and microfiltration membranes were piloted in different configurations; with lamella clarifiers or with lamella clarifiers in combination with either media filters or granular activated carbon filters. The total duration of the pilot was twelve months. Following this pilot a contract for a 140 MLD desalination plant was awarded to Acciona (formerly Pridesa). The following processes were selected: During low tide water of reduced salinity is stored in basins – this water has approximately 1/3 of the salinity of normal seawater. In this way the operating pressure and the energy consumption of the RO unit can be reduced by a factor of two. During high tide the feed water intake from the river is stopped and feed water is taken from the storage basins. Pretreatment by means of coagulation followed by lamella clarification – the low salinity feed water is characterised by high turbidity and a high organic content.
s Use of short cleaning cycles, so called Chemically Enhanced Backwashes (CEB). This mode of operation minimises the flow fluctuations to the RO system. The modules have an outside diameter of 200 mm (8 inch) and they are installed in glass fibre reinforced epoxy (GRP) pressure vessels.
The Thames Gateway Thames Water is the UK’s largest water and wastewater services company. It supplies 2,600 million litres of tap water per day to 8.5 million customers across London and the Thames Valley. Thames Water also removes and treats 2,800 million litres of sewage for an area covering 13.5 million customers. It needed to augment its potable water supply with a new source, the most viable being surface water from the tidal River Thames. In 2003 Thames Water started an extensive piloting program targeting the optimum pretreatment solution for a 140 MLD SWRO plant located in East London. Four different
Original
A cross-section of Norit’s membrane.
Therefore extensive pre-treatment will be necessary. s Media filtration – seventeen horizontal pressure sand filters. s Precoat filters – the precoat filters use diatomaceous earth as a filtration aid to achieve a filtering efficiency down to approximately 3 micron. s First stage SWRO system – nine two stage RO units, each with inter-stage booster pumps s Second stage RO system – three two stage RO units, each with inter-stage booster pumps. The RO systems are concentrate staged in order to maximise the overall recovery. The use of the low salinity feed water allows for an overall RO recovery of 80%, which is unusually high for seawater. The contractor initially opted for precoat filters because of the lower investment costs. The desalination plant will be operated for approximately 40% of the time, therefore the
21
22
Column Feature
Desalination (a Filtration + Separation publication) Volume 4 Issue 1
operating costs and environmental impact of the desalination plant. s More flexible operation – the UF system can be switched on and off instantaneously. The precoat filters need a ripening period during which production cannot be started.
Figure 1: The arrangement of the skids at the Thames Gateway desalination plant.
replacement costs of the diatomaceous earth are offset by the low amortisation costs.
Project delays Due to environmental concerns, the permitting process for the Thames Gateway project took more time than anticipated. The Seaguard membranes did allow Acciona to re-evaluate the pre-treatment options. The membranes provided a solution with similar investment costs compared to the precoat filters while achieving several benefits: s Reduction in chemical consumption – no disposal of diatomaceous earth. This lowers
The modification of the design of the Thames Gateway Desalination Plant at this late stage had to be kept to an absolute minimum. The building had received planning permission and civil construction was started already. The floor plan of the desalination building had been constructed already. Any alterations to the building would have required a new application for planning permission and major civil modifications. The original process design featured a complete pressurised feed supply line from the lamella clarifiers up to the SWRO high pressure pumps, without any intermediate buffer tanks. The X-Flow Seaguard membranes are installed in GRP membrane housings and can be operated under pressure. Dimensionally the Seaguard UF system could be retrofitted into the space that was allocated for the precoat filters. The
existing media filters could be maintained; this improved the water quality being fed to the UF system. This allowed for integrating the UF system into the pressurised feed line between the media filters and the SWRO high pressure pumps. The major challenge was to design the UF system for a constant flow. During backwash of individual UF skids, the throughput of the on line skids has to be increased in order to maintain feed flow and feed pressure to the SWRO high pressure pumps. This was accomplished by doubling the number of the skids and decreasing the size of the individual skids, compared to the original design: This decreased the flow rate changes to within acceptable limits. The skids are arranged two by two (see Figure 1). The project is currently in the construction phase and on schedule for completion in 2009. Hand over to the customer is planned for early 2010. Contact: Wouter Brouze Norit X-Flow Email: [email protected] www.xflow.com
Norit X-Flow seawater pre-treatment with SEAGUARD membranes, as seen in the Palm Jumeirah desalination plant in Dubai.