ABSTRACT
The new pressure exchanger (PX) device transfers the energy from the concentrate stream directly to the feed stream. This direct, positive displacement approach results in a net transfer efficiency of over 95%. Although application of the PX technology is simple in both theory and practice, in order to get the most benefit from this technology it is important to reconsider the SWRO design and operation approach. Pertinent design considerations include pre-filtration, conversion rate optimization; pump selection, and operating pressures. Some important operating procedures and characteristics are start up, high pressure regulation, conversion rate optimization, flow balancing, and shutdown. Furthermore, it will be shown that these considerations affect the design and operation of SWRO systems counter-intuitively and may possibly reverse given standards that have been developed over the past 20 years of SWRO design.
There has been a recent proliferation of commercially available energy recovery devices based on the positive displacement direct pressure exchange approach. This increased interest is driven by the fact that the technology can reduce the energy consumption of an SWRO system by as much as 60%. Since energy costs are rising and can consume as much as 75% of the total operating costs of an SWRO plant, it is important that the technology be encouraged and disseminated throughout the industry. Although the authors of this paper are directly associated with Energy Recovery, Inc., a leading company in pressure exchanger technology, the principles and theories presented in this paper will be applicable to all devices based on the positive displacement, direct pressure exchange approach.
4.0 START AND STOP PROCEDURES
Starting and stopping an SWRO system designed around the PX pressure exchanger device is actually simpler than with systems designed around other technologies such as regulating valves, turbos and Pelton wheels. This is because of the self-balancing nature of SWRO systems designed around pressure exchanger technologies.
4.2 System start up sequence
Start up of an SWRO plant designed around the PX device is very simple. The first step is to start the raw water supply pump. At this point the system will begin to fill with water and the PX may or may not start to spin. Next, start the high-pressure boost pump. The associated pressure drops through the RO membranes and PX combine with the high-pressure boost pump curve to dictate the reject flow rate into and seawater flow rate out of the PX unit. This means that once the high-pressure boost pump is running and has stabilized the reject flow is now running at or very near the normal flow rate for the plant. Now it is time to start the main high-pressure pump, which will pressurize the RO system. The system will reach the exact pressure required to produce the amount of product water being injected to the RO system by the main high-pressure pump. The membranes create the back-pressure in the system and now act like the pressure-regulating valve. It will take 5-10 seconds for a typical system to pressurize once the main high-pressure pump is started. It may be advisable to install a high-pressure bypass valve at the outlet of the RO membranes that can be closed slowly at start up. This will allow the operator to control of the rate at which the RO system reaches full operating pressure.
4.3 SYSTEM SHUT DOWN SEQUENCE
First stop the main high-pressure pump. After approximately 30 seconds the pressure in the RO system will drop to around 27 bar. At this point it is proper to stop the high-pressure booster pump and raw water supply pump. It should be noted that because the high-pressure side of the PX is sealed from the low pressure side of the PX the high pressure RO portion of the plant can maintain significant pressure for an extended period of time.
4.4 Fresh water flush
If the SWRO system is going to be shut down for an extended period of time it is required to fresh water flush the RO membranes and pressure exchanger in order to inhibit biological growth and fouling. Start by supply the RO system with un-chlorinated fresh water at the normal system feed pressure. Next run the high-pressure booster pump until all of the seawater has been purged from the RO membranes. It may also be desirable to also run the high-pressure pump for a few seconds during this process to ensure that it gets a complete flush as well.
5.0 FLOW CONTROL AND BALANCING THE SYSTEM
Flow rates and pressures in a SWRO plant will vary slightly over the life of a plant. Variations may be due to temperature, membrane fouling, seasonal feed salinity variations, etc. The following designs and procedures should be used to control these variables.
5.1 High Pressure Reject and Seawater Feed Flow Control
In order to control the high-pressure reject and feed flow rates, adjustment of the pressure and flow supplied by the high-pressure booster pump is typically required. Recommended practice is to use a high pressure booster pump with some additional capacity, and control its flow and pressure with a variable frequency drive or control valve. A high-pressure flow meter can be used to determine the amount of reject and feed water flowing through the high-pressure side of the pressure exchanger device. Remember that the high-pressure reject water and feed water are hydraulically connected and are separated only by a water barrier/piston. It is also possible to infer the high-pressure flow rates from the pressure drops across the pressure exchanger and/or high-pressure boost pump. Increasing and decreasing these flow rates is how we decrease and increase respectively the conversion rate of the RO system independently of the product water being produced.
5.2 Low Pressure Reject and Seawater Feed Flow Control
In order to control the flow rates of the low-pressure feed and reject water, adjustments to the low pressure seawater inlet pressure to the PX should be made. Recommended practice is to install a valve at the low-pressure seawater inlet of the PX unit(s). Remember that the low-pressure seawater and reject water are hydraulically connected and are separated only by a water barrier/piston. The low pressure seawater and reject flow rates can be determined by installing a flow meter at the low-pressure seawater inlet of the PX.
5.3 Balancing the pressure exchanger using flow meters
All flows in and out of the Pressure Exchanger must be approximately balanced. The following equation applies to this process:
High pressure seawater outlet flow = Low pressure seawater inlet flow
Determine the desired amount of high-pressure seawater outlet flow, which is approximately equal to the reject flow for your system. Adjust the variable frequency drive or control valve on the high pressure booster pump until that flow rate is achieved as seen at the high-pressure flow meter. In the absence of a high-pressure flow meter it is also possible to infer the flow rates from the pressure drops across the boost pump and/or pressure exchanger(s).
Adjust the low-pressure seawater inlet valve until the low-pressure seawater inlet flow equals the high pressure seawater outlet flow.
If the low pressure seawater inlet flow is less than the high pressure seawater outlet flow excessive intermixing of reject with the feed will occur which will result in lower quality permeate, increased feed pressure and higher energy consumption. If the low-pressure seawater inlet flow is greater than the high-pressure seawater outlet flow, treated feed water is being wasted and dumped to the low-pressure reject drain.
6.0 CONCLUSION
The PX is a new pressure exchanger device that promises to revolutionize SWRO design. The device affects the design and operation of SWRO systems in several counter-intuitive ways. As we have discussed, lower conversions rates in the order of 30-40% actually yield lower energy profiles than higher conversion rates. Furthermore, these systems operate at lower pressures and produce better water quality.
The fact that the main high pressure pump flow equals the permeate flow means that we can now achieve SWRO systems with nearly 100% conversion rates when considering the pumping power that is being applied. This fact also means that for any given high pressure pump, systems that are 2-3 times larger can be achieved with that same pump.
The PX device now makes it possible to adjust the conversion rate of an SWRO plant independently of product water production. This means that the conversion rate can now be simply and directly used to optimize the efficiency of the RO membranes rather than having to balance it against energy consumption, product quality, membrane pressure limits, and so on.
It is clear that this device is an extremely efficient approach to energy recovery, and that the SWRO systems of today and the future will consume far less energy than those of yesterday. However, the impact that this device will have on design concerns such as the conversion rate, water quality, and operating pressures will surely surprise us all.
The new pressure exchanger (PX) device transfers the energy from the concentrate stream directly to the feed stream. This direct, positive displacement approach results in a net transfer efficiency of over 95%. Although application of the PX technology is simple in both theory and practice, in order to get the most benefit from this technology it is important to reconsider the SWRO design and operation approach. Pertinent design considerations include pre-filtration, conversion rate optimization; pump selection, and operating pressures. Some important operating procedures and characteristics are start up, high pressure regulation, conversion rate optimization, flow balancing, and shutdown. Furthermore, it will be shown that these considerations affect the design and operation of SWRO systems counter-intuitively and may possibly reverse given standards that have been developed over the past 20 years of SWRO design.
There has been a recent proliferation of commercially available energy recovery devices based on the positive displacement direct pressure exchange approach. This increased interest is driven by the fact that the technology can reduce the energy consumption of an SWRO system by as much as 60%. Since energy costs are rising and can consume as much as 75% of the total operating costs of an SWRO plant, it is important that the technology be encouraged and disseminated throughout the industry. Although the authors of this paper are directly associated with Energy Recovery, Inc., a leading company in pressure exchanger technology, the principles and theories presented in this paper will be applicable to all devices based on the positive displacement, direct pressure exchange approach.
4.0 START AND STOP PROCEDURES
Starting and stopping an SWRO system designed around the PX pressure exchanger device is actually simpler than with systems designed around other technologies such as regulating valves, turbos and Pelton wheels. This is because of the self-balancing nature of SWRO systems designed around pressure exchanger technologies.
4.2 System start up sequence
Start up of an SWRO plant designed around the PX device is very simple. The first step is to start the raw water supply pump. At this point the system will begin to fill with water and the PX may or may not start to spin. Next, start the high-pressure boost pump. The associated pressure drops through the RO membranes and PX combine with the high-pressure boost pump curve to dictate the reject flow rate into and seawater flow rate out of the PX unit. This means that once the high-pressure boost pump is running and has stabilized the reject flow is now running at or very near the normal flow rate for the plant. Now it is time to start the main high-pressure pump, which will pressurize the RO system. The system will reach the exact pressure required to produce the amount of product water being injected to the RO system by the main high-pressure pump. The membranes create the back-pressure in the system and now act like the pressure-regulating valve. It will take 5-10 seconds for a typical system to pressurize once the main high-pressure pump is started. It may be advisable to install a high-pressure bypass valve at the outlet of the RO membranes that can be closed slowly at start up. This will allow the operator to control of the rate at which the RO system reaches full operating pressure.
4.3 SYSTEM SHUT DOWN SEQUENCE
First stop the main high-pressure pump. After approximately 30 seconds the pressure in the RO system will drop to around 27 bar. At this point it is proper to stop the high-pressure booster pump and raw water supply pump. It should be noted that because the high-pressure side of the PX is sealed from the low pressure side of the PX the high pressure RO portion of the plant can maintain significant pressure for an extended period of time.
4.4 Fresh water flush
If the SWRO system is going to be shut down for an extended period of time it is required to fresh water flush the RO membranes and pressure exchanger in order to inhibit biological growth and fouling. Start by supply the RO system with un-chlorinated fresh water at the normal system feed pressure. Next run the high-pressure booster pump until all of the seawater has been purged from the RO membranes. It may also be desirable to also run the high-pressure pump for a few seconds during this process to ensure that it gets a complete flush as well.
5.0 FLOW CONTROL AND BALANCING THE SYSTEM
Flow rates and pressures in a SWRO plant will vary slightly over the life of a plant. Variations may be due to temperature, membrane fouling, seasonal feed salinity variations, etc. The following designs and procedures should be used to control these variables.
5.1 High Pressure Reject and Seawater Feed Flow Control
In order to control the high-pressure reject and feed flow rates, adjustment of the pressure and flow supplied by the high-pressure booster pump is typically required. Recommended practice is to use a high pressure booster pump with some additional capacity, and control its flow and pressure with a variable frequency drive or control valve. A high-pressure flow meter can be used to determine the amount of reject and feed water flowing through the high-pressure side of the pressure exchanger device. Remember that the high-pressure reject water and feed water are hydraulically connected and are separated only by a water barrier/piston. It is also possible to infer the high-pressure flow rates from the pressure drops across the pressure exchanger and/or high-pressure boost pump. Increasing and decreasing these flow rates is how we decrease and increase respectively the conversion rate of the RO system independently of the product water being produced.
5.2 Low Pressure Reject and Seawater Feed Flow Control
In order to control the flow rates of the low-pressure feed and reject water, adjustments to the low pressure seawater inlet pressure to the PX should be made. Recommended practice is to install a valve at the low-pressure seawater inlet of the PX unit(s). Remember that the low-pressure seawater and reject water are hydraulically connected and are separated only by a water barrier/piston. The low pressure seawater and reject flow rates can be determined by installing a flow meter at the low-pressure seawater inlet of the PX.
5.3 Balancing the pressure exchanger using flow meters
All flows in and out of the Pressure Exchanger must be approximately balanced. The following equation applies to this process:
High pressure seawater outlet flow = Low pressure seawater inlet flow
Determine the desired amount of high-pressure seawater outlet flow, which is approximately equal to the reject flow for your system. Adjust the variable frequency drive or control valve on the high pressure booster pump until that flow rate is achieved as seen at the high-pressure flow meter. In the absence of a high-pressure flow meter it is also possible to infer the flow rates from the pressure drops across the boost pump and/or pressure exchanger(s).
Adjust the low-pressure seawater inlet valve until the low-pressure seawater inlet flow equals the high pressure seawater outlet flow.
If the low pressure seawater inlet flow is less than the high pressure seawater outlet flow excessive intermixing of reject with the feed will occur which will result in lower quality permeate, increased feed pressure and higher energy consumption. If the low-pressure seawater inlet flow is greater than the high-pressure seawater outlet flow, treated feed water is being wasted and dumped to the low-pressure reject drain.
6.0 CONCLUSION
The PX is a new pressure exchanger device that promises to revolutionize SWRO design. The device affects the design and operation of SWRO systems in several counter-intuitive ways. As we have discussed, lower conversions rates in the order of 30-40% actually yield lower energy profiles than higher conversion rates. Furthermore, these systems operate at lower pressures and produce better water quality.
The fact that the main high pressure pump flow equals the permeate flow means that we can now achieve SWRO systems with nearly 100% conversion rates when considering the pumping power that is being applied. This fact also means that for any given high pressure pump, systems that are 2-3 times larger can be achieved with that same pump.
The PX device now makes it possible to adjust the conversion rate of an SWRO plant independently of product water production. This means that the conversion rate can now be simply and directly used to optimize the efficiency of the RO membranes rather than having to balance it against energy consumption, product quality, membrane pressure limits, and so on.
It is clear that this device is an extremely efficient approach to energy recovery, and that the SWRO systems of today and the future will consume far less energy than those of yesterday. However, the impact that this device will have on design concerns such as the conversion rate, water quality, and operating pressures will surely surprise us all.
ABSTRACT
The new pressure exchanger (PX) device transfers the energy from the concentrate stream directly to the feed stream. This direct, positive displacement approach results in a net transfer efficiency of over 95%. Although application of the PX technology is simple in both theory and practice, in order to get the most benefit from this technology it is important to reconsider the SWRO design and operation approach. Pertinent design considerations include pre-filtration, conversion rate optimization; pump selection, and operating pressures. Some important operating procedures and characteristics are start up, high pressure regulation, conversion rate optimization, flow balancing, and shutdown. Furthermore, it will be shown that these considerations affect the design and operation of SWRO systems counter-intuitively and may possibly reverse given standards that have been developed over the past 20 years of SWRO design.
There has been a recent proliferation of commercially available energy recovery devices based on the positive displacement direct pressure exchange approach. This increased interest is driven by the fact that the technology can reduce the energy consumption of an SWRO system by as much as 60%. Since energy costs are rising and can consume as much as 75% of the total operating costs of an SWRO plant, it is important that the technology be encouraged and disseminated throughout the industry. Although the authors of this paper are directly associated with Energy Recovery, Inc., a leading company in pressure exchanger technology, the principles and theories presented in this paper will be applicable to all devices based on the positive displacement, direct pressure exchange approach.
1.0 INTRODUCTION
The PX is a new pressure exchanger device that transfers the energy from the concentrate/reject stream directly to the feed stream in a cylindrical rotor with longitudinal ducts. The rotor spins inside a sleeve between two end covers that divide the rotor into high and low pressure halves. When designing a seawater RO system using the new pressure exchanger device it is only necessary to reconsider the high pressure portion of the system. This is because this is the only portion that differs from earlier RO designs. Pertinent design considerations include conversion rate optimization, pump selection, and operating pressures. Some important operating procedures and characteristics are start up, high pressure regulation, conversion rate optimization, flow balancing, and shutdown. Although application of the pressure exchanger is simple in both theory and practice it is vastly different from typical SWRO design and operation. These differences affect the design and operation of SWRO systems in surprising ways that can be used to improve other aspects of system performance beyond the scope of energy consumption alone.
2.0 PRINCIPLE OF OPERATION
The PX unit utilizes the principle of positive displacement to transfer the energy in the reject stream directly to the feed stream. It is interesting to note that the reject stream is continuously and directly connected to the feed stream. This direct connection allows a real net transfer efficiency of energy from the reject stream to the feed stream of over 95%. The PX device uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis to transfer the pressure energy from the concentrate/reject stream to the feed stream.
The rotor spins inside a sleeve between two end covers with port openings for low and high pressure. The low-pressure side of the rotor fills with seawater while the high-pressure side discharges seawater. The rotation simply facilitates the valving mechanism, which is to transport the ducts from one side to the other.
By rotation the ducts are exposed to the low pressure feed water, which fills the duct and displaces the reject water. The rotor continues to rotate and is exposed to the high-pressure concentrate, which fills the duct from the opposite direction, and displaces the feed water at high pressure. This rotational action is similar to a Gatling machine gun firing high-pressure bullets and being refilled with new seawater cartridges from the muzzle. A liquid piston moves back and forth inside each duct creating a barrier that inhibits mixing between the concentrated reject and new seawater streams. At 1500 rpm one revolution is completed every 1/25 second. Due to this short cycle time, membrane feed water concentrations typically increase only 1%-2%. See Figure 4-1 below.
Applying PX pressure exchanger technology to SWRO is different from conventional energy recovery device system design, but in practice is quite simple. The reject brine from the SWRO membranes is passed into the PX unit, where its pressure energy is transferred directly to a portion of the incoming raw seawater at up to 97% efficiency. This seawater stream, nearly equal in volume and pressure to the reject stream, then passes through a high-pressure booster pump, not the main high-pressure pump. This booster pump is making up the pressure losses across the RO membrane (approx. 2 bar), PX unit(s) (approx. 1 bar) and piping losses (approx. 0.5 bar). The total head provided by the boost pump is typically around 3.5 bar. See figure 4-2 and table 4-1 below.
It is important to notice that the PX and associated boost pump are handling nearly 100% of the reject flow. The size of the main high-pressure pump has been reduced to a make up pump for the permeate flow that is exiting the RO system. Product water flow and reject flow are being provided by two independent pumping systems and therefore are independent of one another.
Since the PX unit is providing nearly 100% of the reject flow at over 95% efficiency there is very little energy penalty associated with increasing this flow and thereby lower the conversion rate of the RO system. At lower conversion rate the pressure required to produce the same amount of product water is lower. Since the main high-pressure pump flow equals the product water flow an energy savings is actually achieved at lower conversion rates.
3.0 CONVERSION OPTIMIZATION
There are many factors that effect RO conversion optimization but none has been more influential than energy consumption. This is because energy costs can be as much as 75% of the entire operating cost of an SWRO plant. In the past the sewater to fresh water conversion rate has had a major and direct impact on the energy consumption of an RO plant. This is because of the inherent shortcomings of the energy recovery and pumping devices that have been used such as the Pelton wheels, turbines, and pumps. These technologies have real/overall net transfer efficiencies of 40-70 percent and are designed to pump the entire feed flow of an RO plant. Therefore at lower conversion rates these inefficient devices are pumping more water. The only way to make these devices pump less water and thereby consume less energy is to increase the conversion rate of the RO system. This is all very logical, and with rising energy costs it is natural that SWRO systems are now being designed at the membrane challenging conversion rates of 50-60%.
System designs with the PX device are different. This is because the PX, a 95% efficient device, is pumping the reject water independently of the product water being produced. The overall energy consumption of an SWRO plant using the PX device has a low point at conversion rates typically between 30-40%. Outside these conversion points the plant will start to consume slightly higher amounts of power. See figure 3 below.
It is important to remember that with the PX device the main high pressure pump flow approximately equals the product water flow. At lower conversion rates it requires lower pressure to produce the same amount of product water. Therefore, the main high-pressure pump will consume less power pumping against less pressure at the lower conversion rates. This phenomenon yields the net energy decrease shown in Figure 3.
Another important point to consider is water quality. In the past, system design had to balance high conversion rate with good water quality. In high salinity applications this has been a difficult challenge. Figure 4 below shows how a lower conversion rate yields better water quality.
Logically combining the kWh/m3 vs Conversion Rate curve of Figure 3 with the diminishing water quality curve as the conversion rate increases in figure 4 shows us that there are good reasons now to consider SWRO designs with lower conversion rates. There are also additional benefits associated with lower conversion rate designs such as ease of operation, fewer cleaning cycles, longer membrane life and a better balance of flux (GFD) from the lead element to the end element in an RO pressure vessel. Figure 5 below shows how GFD is balanced as the system conversion rate lowers(5).
Of course decreasing the system conversion rate does have its disadvantages mainly in increasing the size of the pretreatment system. This effect is less significant in smaller systems under 1000 m3/day because of the less expensive piping materials, pressure media filtration systems and other components typically employed in these systems. However on larger plants using open seawater intakes with large-scale gravity feed media filtration systems, and when the chemical additions associated with coagulation are significant operating costs, conversion rates between 45-50% may be more practical.
The new pressure exchanger (PX) device transfers the energy from the concentrate stream directly to the feed stream. This direct, positive displacement approach results in a net transfer efficiency of over 95%. Although application of the PX technology is simple in both theory and practice, in order to get the most benefit from this technology it is important to reconsider the SWRO design and operation approach. Pertinent design considerations include pre-filtration, conversion rate optimization; pump selection, and operating pressures. Some important operating procedures and characteristics are start up, high pressure regulation, conversion rate optimization, flow balancing, and shutdown. Furthermore, it will be shown that these considerations affect the design and operation of SWRO systems counter-intuitively and may possibly reverse given standards that have been developed over the past 20 years of SWRO design.
There has been a recent proliferation of commercially available energy recovery devices based on the positive displacement direct pressure exchange approach. This increased interest is driven by the fact that the technology can reduce the energy consumption of an SWRO system by as much as 60%. Since energy costs are rising and can consume as much as 75% of the total operating costs of an SWRO plant, it is important that the technology be encouraged and disseminated throughout the industry. Although the authors of this paper are directly associated with Energy Recovery, Inc., a leading company in pressure exchanger technology, the principles and theories presented in this paper will be applicable to all devices based on the positive displacement, direct pressure exchange approach.
1.0 INTRODUCTION
The PX is a new pressure exchanger device that transfers the energy from the concentrate/reject stream directly to the feed stream in a cylindrical rotor with longitudinal ducts. The rotor spins inside a sleeve between two end covers that divide the rotor into high and low pressure halves. When designing a seawater RO system using the new pressure exchanger device it is only necessary to reconsider the high pressure portion of the system. This is because this is the only portion that differs from earlier RO designs. Pertinent design considerations include conversion rate optimization, pump selection, and operating pressures. Some important operating procedures and characteristics are start up, high pressure regulation, conversion rate optimization, flow balancing, and shutdown. Although application of the pressure exchanger is simple in both theory and practice it is vastly different from typical SWRO design and operation. These differences affect the design and operation of SWRO systems in surprising ways that can be used to improve other aspects of system performance beyond the scope of energy consumption alone.
2.0 PRINCIPLE OF OPERATION
The PX unit utilizes the principle of positive displacement to transfer the energy in the reject stream directly to the feed stream. It is interesting to note that the reject stream is continuously and directly connected to the feed stream. This direct connection allows a real net transfer efficiency of energy from the reject stream to the feed stream of over 95%. The PX device uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis to transfer the pressure energy from the concentrate/reject stream to the feed stream.
The rotor spins inside a sleeve between two end covers with port openings for low and high pressure. The low-pressure side of the rotor fills with seawater while the high-pressure side discharges seawater. The rotation simply facilitates the valving mechanism, which is to transport the ducts from one side to the other.
By rotation the ducts are exposed to the low pressure feed water, which fills the duct and displaces the reject water. The rotor continues to rotate and is exposed to the high-pressure concentrate, which fills the duct from the opposite direction, and displaces the feed water at high pressure. This rotational action is similar to a Gatling machine gun firing high-pressure bullets and being refilled with new seawater cartridges from the muzzle. A liquid piston moves back and forth inside each duct creating a barrier that inhibits mixing between the concentrated reject and new seawater streams. At 1500 rpm one revolution is completed every 1/25 second. Due to this short cycle time, membrane feed water concentrations typically increase only 1%-2%. See Figure 4-1 below.
Applying PX pressure exchanger technology to SWRO is different from conventional energy recovery device system design, but in practice is quite simple. The reject brine from the SWRO membranes is passed into the PX unit, where its pressure energy is transferred directly to a portion of the incoming raw seawater at up to 97% efficiency. This seawater stream, nearly equal in volume and pressure to the reject stream, then passes through a high-pressure booster pump, not the main high-pressure pump. This booster pump is making up the pressure losses across the RO membrane (approx. 2 bar), PX unit(s) (approx. 1 bar) and piping losses (approx. 0.5 bar). The total head provided by the boost pump is typically around 3.5 bar. See figure 4-2 and table 4-1 below.
It is important to notice that the PX and associated boost pump are handling nearly 100% of the reject flow. The size of the main high-pressure pump has been reduced to a make up pump for the permeate flow that is exiting the RO system. Product water flow and reject flow are being provided by two independent pumping systems and therefore are independent of one another.
Since the PX unit is providing nearly 100% of the reject flow at over 95% efficiency there is very little energy penalty associated with increasing this flow and thereby lower the conversion rate of the RO system. At lower conversion rate the pressure required to produce the same amount of product water is lower. Since the main high-pressure pump flow equals the product water flow an energy savings is actually achieved at lower conversion rates.
3.0 CONVERSION OPTIMIZATION
There are many factors that effect RO conversion optimization but none has been more influential than energy consumption. This is because energy costs can be as much as 75% of the entire operating cost of an SWRO plant. In the past the sewater to fresh water conversion rate has had a major and direct impact on the energy consumption of an RO plant. This is because of the inherent shortcomings of the energy recovery and pumping devices that have been used such as the Pelton wheels, turbines, and pumps. These technologies have real/overall net transfer efficiencies of 40-70 percent and are designed to pump the entire feed flow of an RO plant. Therefore at lower conversion rates these inefficient devices are pumping more water. The only way to make these devices pump less water and thereby consume less energy is to increase the conversion rate of the RO system. This is all very logical, and with rising energy costs it is natural that SWRO systems are now being designed at the membrane challenging conversion rates of 50-60%.
System designs with the PX device are different. This is because the PX, a 95% efficient device, is pumping the reject water independently of the product water being produced. The overall energy consumption of an SWRO plant using the PX device has a low point at conversion rates typically between 30-40%. Outside these conversion points the plant will start to consume slightly higher amounts of power. See figure 3 below.
It is important to remember that with the PX device the main high pressure pump flow approximately equals the product water flow. At lower conversion rates it requires lower pressure to produce the same amount of product water. Therefore, the main high-pressure pump will consume less power pumping against less pressure at the lower conversion rates. This phenomenon yields the net energy decrease shown in Figure 3.
Another important point to consider is water quality. In the past, system design had to balance high conversion rate with good water quality. In high salinity applications this has been a difficult challenge. Figure 4 below shows how a lower conversion rate yields better water quality.
Logically combining the kWh/m3 vs Conversion Rate curve of Figure 3 with the diminishing water quality curve as the conversion rate increases in figure 4 shows us that there are good reasons now to consider SWRO designs with lower conversion rates. There are also additional benefits associated with lower conversion rate designs such as ease of operation, fewer cleaning cycles, longer membrane life and a better balance of flux (GFD) from the lead element to the end element in an RO pressure vessel. Figure 5 below shows how GFD is balanced as the system conversion rate lowers(5).
Of course decreasing the system conversion rate does have its disadvantages mainly in increasing the size of the pretreatment system. This effect is less significant in smaller systems under 1000 m3/day because of the less expensive piping materials, pressure media filtration systems and other components typically employed in these systems. However on larger plants using open seawater intakes with large-scale gravity feed media filtration systems, and when the chemical additions associated with coagulation are significant operating costs, conversion rates between 45-50% may be more practical.
Western Australia's Water Corporation is on track to produce drinking water form a 45GL/a desalination plant located in Kwinana, 25km south of Perth. The plant is being built by proAlliance - a 50/50 joint venture between West Australian construction company Multiplex and French water treatment company Degremont. Degremont will operate the 140,000m3/d plant for 25 years under a public-private partnership with the West Australian government-owned Water Corporation. The construction cost is $387 million.
To reduce the environmental impact of the project electricity for the desalination plant will be produced from a wind farm located 30km east of Cervantes in WA's midwest. The 80MW wind farm has been engineered by the Queensland government-owned power generation company Stanwell Corporation and WA private company Griffin Energy. The wind farm will be operated by the WA government-owned power utility Western Power.
When complete Perth's desalination plant will ease pressure on WA's Integrated Water Supply Scheme. 45GL represents the single biggest water source feeding into the IWSS. To maintain water supplies for Perth's growing population the Water Corporation is pursuing a strategy of "security through diversity" and so is progressing engineering on a further desalination plant as well as developing the underground Yarragadee Aquifer to the south of Perth.
The Corporation has had to accelerate installation of water desalination due to reduced rainfall in Perth's catchment areas.
The technology to be used for the desalination plant is reverse osmosis. Osmosis is a natural phenomenon that occurs when water diffuses through a semipermeable membrane to equalise the concentration of salt in a solution. The transfer of water is from the dilute to the concentrated solution. By applying energy in the form of water pressure water can be made to move in reverse from a concentrate solution to dilute solution - hence the term reverse osmosis.
A semipermeable membrane acts like a molecular sieve allowing water particles to pass through while stopping dissolved salts, viruses and bacteria. To reduce the area required to house the membrane, the membrane is wound onto a spiral. Water is pumped under pressure down the spiral and migrates to the centre.
In recent years improvements in membrane manufacture and energy recovery devices have reduced the capital and operating costs. The Perth plant will produce drinking water at less than $0.95/kl at the fence. Electrical power consumption is expected to be less than 4.5Wh/kl.
An important part of reducing power consumption is the energy recovery system which uses a ceramic pressure recovery device. A device called the Pressure Exchanger (PX), a trademark of US firm Energy Recovery, uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside the sleeve between two end covers with port openings for both streams. Pressure energy is transferred directly from the high-pressure concentrate/reject stream to the low-pressure feed/seawater stream. A liquid piston moves back and forth inside each duct creating a barrier that inhibits mixing between the streams. The low-pressure side of the rotor fills with seawater while the high-pressure side discharges seawater. This rotational action is similar to that of an old-fashioned machine gun firing high-pressure bullets that is refilled with new seawater cartridges while spinning around a central axis.
When complete the plant will employ 16 people and provide valuable technical expertise in operating a large-scale reverse osmosis plant. A comprehensive environmental monitoring program is an operating license condition to ensure that the marine environment, near the seawater inlet and brine outlet, will not be harmed.
To reduce the environmental impact of the project electricity for the desalination plant will be produced from a wind farm located 30km east of Cervantes in WA's midwest. The 80MW wind farm has been engineered by the Queensland government-owned power generation company Stanwell Corporation and WA private company Griffin Energy. The wind farm will be operated by the WA government-owned power utility Western Power.
When complete Perth's desalination plant will ease pressure on WA's Integrated Water Supply Scheme. 45GL represents the single biggest water source feeding into the IWSS. To maintain water supplies for Perth's growing population the Water Corporation is pursuing a strategy of "security through diversity" and so is progressing engineering on a further desalination plant as well as developing the underground Yarragadee Aquifer to the south of Perth.
The Corporation has had to accelerate installation of water desalination due to reduced rainfall in Perth's catchment areas.
The technology to be used for the desalination plant is reverse osmosis. Osmosis is a natural phenomenon that occurs when water diffuses through a semipermeable membrane to equalise the concentration of salt in a solution. The transfer of water is from the dilute to the concentrated solution. By applying energy in the form of water pressure water can be made to move in reverse from a concentrate solution to dilute solution - hence the term reverse osmosis.
A semipermeable membrane acts like a molecular sieve allowing water particles to pass through while stopping dissolved salts, viruses and bacteria. To reduce the area required to house the membrane, the membrane is wound onto a spiral. Water is pumped under pressure down the spiral and migrates to the centre.
In recent years improvements in membrane manufacture and energy recovery devices have reduced the capital and operating costs. The Perth plant will produce drinking water at less than $0.95/kl at the fence. Electrical power consumption is expected to be less than 4.5Wh/kl.
An important part of reducing power consumption is the energy recovery system which uses a ceramic pressure recovery device. A device called the Pressure Exchanger (PX), a trademark of US firm Energy Recovery, uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside the sleeve between two end covers with port openings for both streams. Pressure energy is transferred directly from the high-pressure concentrate/reject stream to the low-pressure feed/seawater stream. A liquid piston moves back and forth inside each duct creating a barrier that inhibits mixing between the streams. The low-pressure side of the rotor fills with seawater while the high-pressure side discharges seawater. This rotational action is similar to that of an old-fashioned machine gun firing high-pressure bullets that is refilled with new seawater cartridges while spinning around a central axis.
When complete the plant will employ 16 people and provide valuable technical expertise in operating a large-scale reverse osmosis plant. A comprehensive environmental monitoring program is an operating license condition to ensure that the marine environment, near the seawater inlet and brine outlet, will not be harmed.
Seven Seas Water received a production Pressure Exchanger for evaluation in conjunction with the ADA workshop held in St. Croix, USVI in October of 1998.
A paper dealing with the performance of the device was delivered by Mr. Leif J. Hauge, President of Energy Recovery Inc. at the workshop. Tours were conducted at the test facility where the Pressure Exchanger was in operation.
TESTING PARAMETERS
It was decide, in discussions with Mr. Hauge, to deviate from the standard system design and determine if the Pressure Exchanger could be used to increase the capacity of an existing plant by utilizing the available waste energy. The pressure boost pump would also be eliminated to further simplify installation and evaluation. The Exchanger would act as a stand-alone pump using only waste energy to operate a separate bank of membranes. The Exchanger would be supplied with a separate source of filtered seawater.
INSTALLATION
The first impression of the Pressure Exchanger is the compact size and the simplicity of external design. The simplest way of mounting the system was to fabricate a mounting system that would allow a horizontal, wall mount of the device. This was completed and piping fabricated to meet installation requirements. Instrumentation was added to allow evaluation of the performance.
START-UP OF THE SYSTEM
The pre-existing membrane system was being operated at 940 PSI with a feed flow of 39 GPM. Recovery rate was at 39%. Pre-treated seawater feed pressure was 25-32 PSI depending upon state of filters.
Without displaying the math, we had available 23.8 gallons per minute at 930 PSI (10 PSI differential) to operate a bank of three, eight-inch membranes, a very marginal feed supply. Also the Exchanger was designed for an optimum flow rate of 40 GPM. Since the Pressure Exchanger is primarily centrifugal in design, it was expected that some portion of the feed water pressure would translate to the discharge of the pump.
In starting the system, it took the Pressure Exchanger several minutes to ‘wind up’ as it does any turbine device. Within 15 minutes, the entire system had stabilized. Because the Exchanger had taken place of our concentrate control valve (and was oversized for the application) the feed pressure to the primary bank of membranes was only 855 PSI. Using the installed valve on the Exchanger we only increased the back-pressure to allow the primary bank to operate at 940 feed inlet.
The secondary set of membranes, powered only by the Exchanger, was then adjusted by slowing closing the concentrate control valve.
David Laker, also with Seven Seas Water, and well-known as a pioneering reverse osmosis engineer with over 30 years of experience in the field, described the results as being as close as he had ever seen to "a perpetual motion machine."
Within the limitations of the instrumentation we were now making 8,900 gallons per day of "free" water. The secondary set of membranes was operating at a calculated 23 GPM at 935-940 PSI and producing 6.2 GPM at 400 TDS. It was later determined that cross leakage within the Exchanger was less than 2% and the actual overall efficiency of the Exchanger was 96% allowing for feed water pressure.
PROBLEMS IN PARADISE
After a short time running, it was noted that the seawater break tank was going down. The seawater feed pump was not large enough to continually feed the plant with the additional feed requirement.
A large pump was purchased and installed, but the feed flow did not increase sufficiently to allow the plant to run without interruption. It became clear that marine growth in the intake pipe was the root problem. The plant continued to operate in a "batch" mode for a short period of time. The plant was subjected to repeated starts and stops, without incident. All starts and stops were automatic and very smooth in nature.
Within a few days, the product water storage tank was at capacity and the plant shut down. Due to the difficulty and expense of installing in a new seawater intake system, it was concluded that long term testing of the Pressure Exchanger was not practical at this location.
During the week of running, the Exchanger membrane system produced 62,000 gallons of water. Overall recovery was 20%, including multiple shutdowns. The only kilowatts consumed were associated with the seawater feed pump and increased pumping requirement of the post-treatment of the permeate.
SUMMARY
The testing carried out indicates that utilizing the Pressure Exchanger for plant expansion is not only feasible, but is extremely practical.
A 40,000 GPD plant without energy recovery would produce sufficient waste energy to allow easy expansion to 60,000 GPD using the Pressure Exchanger. If boosting the membrane feed pressure became a necessity, utilizing a small energy recovery turbine and the waste energy of the secondary membrane system is certainly possible, and should be explored. Seven Seas Water currently has such a system designed and installed, waiting for the client to order start-up.
This seems to be truly amazing device that allows several potential applications in our industry. We have no doubts of the viability of the Pressure Exchanger in the seawater reverse osmosis industry. We only have one question:
How fast can an eight-inch model of the Pressure Exchanger be available for use on very large-scale plants?
A paper dealing with the performance of the device was delivered by Mr. Leif J. Hauge, President of Energy Recovery Inc. at the workshop. Tours were conducted at the test facility where the Pressure Exchanger was in operation.
TESTING PARAMETERS
It was decide, in discussions with Mr. Hauge, to deviate from the standard system design and determine if the Pressure Exchanger could be used to increase the capacity of an existing plant by utilizing the available waste energy. The pressure boost pump would also be eliminated to further simplify installation and evaluation. The Exchanger would act as a stand-alone pump using only waste energy to operate a separate bank of membranes. The Exchanger would be supplied with a separate source of filtered seawater.
INSTALLATION
The first impression of the Pressure Exchanger is the compact size and the simplicity of external design. The simplest way of mounting the system was to fabricate a mounting system that would allow a horizontal, wall mount of the device. This was completed and piping fabricated to meet installation requirements. Instrumentation was added to allow evaluation of the performance.
START-UP OF THE SYSTEM
The pre-existing membrane system was being operated at 940 PSI with a feed flow of 39 GPM. Recovery rate was at 39%. Pre-treated seawater feed pressure was 25-32 PSI depending upon state of filters.
Without displaying the math, we had available 23.8 gallons per minute at 930 PSI (10 PSI differential) to operate a bank of three, eight-inch membranes, a very marginal feed supply. Also the Exchanger was designed for an optimum flow rate of 40 GPM. Since the Pressure Exchanger is primarily centrifugal in design, it was expected that some portion of the feed water pressure would translate to the discharge of the pump.
In starting the system, it took the Pressure Exchanger several minutes to ‘wind up’ as it does any turbine device. Within 15 minutes, the entire system had stabilized. Because the Exchanger had taken place of our concentrate control valve (and was oversized for the application) the feed pressure to the primary bank of membranes was only 855 PSI. Using the installed valve on the Exchanger we only increased the back-pressure to allow the primary bank to operate at 940 feed inlet.
The secondary set of membranes, powered only by the Exchanger, was then adjusted by slowing closing the concentrate control valve.
David Laker, also with Seven Seas Water, and well-known as a pioneering reverse osmosis engineer with over 30 years of experience in the field, described the results as being as close as he had ever seen to "a perpetual motion machine."
Within the limitations of the instrumentation we were now making 8,900 gallons per day of "free" water. The secondary set of membranes was operating at a calculated 23 GPM at 935-940 PSI and producing 6.2 GPM at 400 TDS. It was later determined that cross leakage within the Exchanger was less than 2% and the actual overall efficiency of the Exchanger was 96% allowing for feed water pressure.
PROBLEMS IN PARADISE
After a short time running, it was noted that the seawater break tank was going down. The seawater feed pump was not large enough to continually feed the plant with the additional feed requirement.
A large pump was purchased and installed, but the feed flow did not increase sufficiently to allow the plant to run without interruption. It became clear that marine growth in the intake pipe was the root problem. The plant continued to operate in a "batch" mode for a short period of time. The plant was subjected to repeated starts and stops, without incident. All starts and stops were automatic and very smooth in nature.
Within a few days, the product water storage tank was at capacity and the plant shut down. Due to the difficulty and expense of installing in a new seawater intake system, it was concluded that long term testing of the Pressure Exchanger was not practical at this location.
During the week of running, the Exchanger membrane system produced 62,000 gallons of water. Overall recovery was 20%, including multiple shutdowns. The only kilowatts consumed were associated with the seawater feed pump and increased pumping requirement of the post-treatment of the permeate.
SUMMARY
The testing carried out indicates that utilizing the Pressure Exchanger for plant expansion is not only feasible, but is extremely practical.
A 40,000 GPD plant without energy recovery would produce sufficient waste energy to allow easy expansion to 60,000 GPD using the Pressure Exchanger. If boosting the membrane feed pressure became a necessity, utilizing a small energy recovery turbine and the waste energy of the secondary membrane system is certainly possible, and should be explored. Seven Seas Water currently has such a system designed and installed, waiting for the client to order start-up.
This seems to be truly amazing device that allows several potential applications in our industry. We have no doubts of the viability of the Pressure Exchanger in the seawater reverse osmosis industry. We only have one question:
How fast can an eight-inch model of the Pressure Exchanger be available for use on very large-scale plants?
Desalination Part 4 of 4: Measuring the Impact
0 comments Posted by Energy Recovery Inc. at 10:14 PMContinued from Desalination Part 3: Getting Better All the Time.........
A bigger problem may be the leftover brine, which typically contains twice as much salt as seawater and is discharged back into the ocean. So far little scientific information exists about its long-term effects. In the past, most big seawater-desalination plants were built in places that did not conduct adequate environmental assessments, says Peter Gleick, president of the Pacific Institute, a think-tank based in California that published a report on desalination in 2006. But as plants are built in areas with tighter environmental restrictions, more information is becoming available.
Some recent measurements from Perth are encouraging. Initially scientists from the Centre for Water Research feared that the brine discharge from the plant would increase the saltiness of the coastal environment. But a monitoring study found that salinity returns to normal levels within about 500 metres of the plants’ discharge units. “The brine discharge is a problem that can be overcome with good design,” says Dr Antenucci.
A separate problem may be that some metals or chemicals leach into the brine. Thermal-desalination plants are prone to corrosion, and may shed traces of heavy metals, such as copper, into the waste stream. Reverse-osmosis plants, for their part, use chemicals during the pre-treatment and cleaning of the membranes, some of which may end up in the brine. Modern plants, however, remove most of the chemicals from the water before it is discharged. And new approaches to pre-treatment may reduce or eliminate the need for some chemicals.
Based on the limited evidence available to date, it appears that desalination may actually be less environmentally harmful than some other water-supply options, such as diverting large amounts of fresh water from rivers, for example, which can lead to severe reductions in local fish populations. But uncertainties over the environmental impacts of desalination make it hard to draw definite conclusions, the National Research Council concluded. Its report suggested that further research on the environmental impacts of desalination, and how to mitigate them, should be a high priority.
The reverse-osmosis process is increasingly being used not just for desalination, but to recycle wastewater, too. In Orange County, California, reclaimed water is being used to replenish groundwater, and in Singapore, it is pumped into local reservoirs, which are used as a source for drinking water. In both cases, the treated water is also available for tasting at local water-recycling facilities. This “toilet-to-tap” approach may leave some people feeling queasy, but wastewater is a valuable resource, says Sabine Lattemann, a researcher at the University of Oldenburg, Germany, who studies the environmental impacts of desalination. “Energy demand is lower compared to desalination,” she explains, “and you can produce high-quality drinking water.”
As water becomes more scarce, people will want to find several ways to secure their supplies. Many parts of the world also have enormous scope to use water more efficiently, argues Dr Gleick—and that would be cheaper than desalination. But sometimes, making desalination part of the approach to water management may be the only way to ensure a steady supply of drinking water.
In drought-ridden Western Australia, which ordered conservation years ago, the Water Corporation has adopted what it calls “security through diversity”, otherwise known in the industry as the “portfolio” approach. At the moment, Perth’s residents receive about 17% of their drinking water from seawater desalination. Desalination makes sense as one of several water sources along with conservation, agrees Dr Antenucci. But, he adds, “to say it is the silver bullet is wrong.”
A bigger problem may be the leftover brine, which typically contains twice as much salt as seawater and is discharged back into the ocean. So far little scientific information exists about its long-term effects. In the past, most big seawater-desalination plants were built in places that did not conduct adequate environmental assessments, says Peter Gleick, president of the Pacific Institute, a think-tank based in California that published a report on desalination in 2006. But as plants are built in areas with tighter environmental restrictions, more information is becoming available.
Some recent measurements from Perth are encouraging. Initially scientists from the Centre for Water Research feared that the brine discharge from the plant would increase the saltiness of the coastal environment. But a monitoring study found that salinity returns to normal levels within about 500 metres of the plants’ discharge units. “The brine discharge is a problem that can be overcome with good design,” says Dr Antenucci.
A separate problem may be that some metals or chemicals leach into the brine. Thermal-desalination plants are prone to corrosion, and may shed traces of heavy metals, such as copper, into the waste stream. Reverse-osmosis plants, for their part, use chemicals during the pre-treatment and cleaning of the membranes, some of which may end up in the brine. Modern plants, however, remove most of the chemicals from the water before it is discharged. And new approaches to pre-treatment may reduce or eliminate the need for some chemicals.
Based on the limited evidence available to date, it appears that desalination may actually be less environmentally harmful than some other water-supply options, such as diverting large amounts of fresh water from rivers, for example, which can lead to severe reductions in local fish populations. But uncertainties over the environmental impacts of desalination make it hard to draw definite conclusions, the National Research Council concluded. Its report suggested that further research on the environmental impacts of desalination, and how to mitigate them, should be a high priority.
The reverse-osmosis process is increasingly being used not just for desalination, but to recycle wastewater, too. In Orange County, California, reclaimed water is being used to replenish groundwater, and in Singapore, it is pumped into local reservoirs, which are used as a source for drinking water. In both cases, the treated water is also available for tasting at local water-recycling facilities. This “toilet-to-tap” approach may leave some people feeling queasy, but wastewater is a valuable resource, says Sabine Lattemann, a researcher at the University of Oldenburg, Germany, who studies the environmental impacts of desalination. “Energy demand is lower compared to desalination,” she explains, “and you can produce high-quality drinking water.”
As water becomes more scarce, people will want to find several ways to secure their supplies. Many parts of the world also have enormous scope to use water more efficiently, argues Dr Gleick—and that would be cheaper than desalination. But sometimes, making desalination part of the approach to water management may be the only way to ensure a steady supply of drinking water.
In drought-ridden Western Australia, which ordered conservation years ago, the Water Corporation has adopted what it calls “security through diversity”, otherwise known in the industry as the “portfolio” approach. At the moment, Perth’s residents receive about 17% of their drinking water from seawater desalination. Desalination makes sense as one of several water sources along with conservation, agrees Dr Antenucci. But, he adds, “to say it is the silver bullet is wrong.”
Desalination Part 3 of 4: Getting Better All the Time
0 comments Posted by Energy Recovery Inc. at 10:12 PMContinued from Desalination Part 2: No Salt, Please..........
In the late 1970s John Cadotte of America’s Midwest Research Institute and the FilmTec Corporation created a much-improved membrane by using a special cross-linking reaction between two chemicals atop a porous backing material. His composite membrane consisted of a very thin layer of polyamide, to perform the separation, and a sturdy support beneath it. Thanks to the membrane’s improved water flux, and its ability to tolerate pH and temperature variations, it went on to dominate the industry. At around the same time, the first reverse-osmosis plants for seawater began to appear. These early plants needed a lot of energy. The first big municipal seawater plant, which began operating in Jeddah, Saudi Arabia, in 1980, required more than 8 kilowatt hours (kWh) to produce one cubic metre of drinking water.
The energy consumption of such plants has since fallen dramatically, thanks in large part to energy-recovery devices. High-pressure pumps force seawater against a membrane, which is typically arranged in a spiral inside a tube, to increase the surface area exposed to the incoming water and optimise the flux through the membrane. About half of the water emerges as freshwater on the other side. The remaining liquid, which contains the leftover salts, shoots out of the system at high pressure. If that high-pressure waste stream is run through a turbine or rotor, energy can be recovered and used to pressurise the incoming seawater.
The energy-recovery devices in the 1980s were only about 75% efficient, but newer ones can recover about 96% of the energy from the waste stream. As a result, the energy use for reverse-osmosis seawater desalination has fallen. The Perth plant, which uses technology from Energy Recovery, a firm based in California, consumes only 3.7kWh to produce one cubic metre of drinking water, according to Gary Crisp, who helped to oversee the plant’s design for the Water Corporation, a local utility. Thermal plants suck up nearly as much electricity, but also need large amounts of steam. “A thermal plant only is practical if you can build it in such a way that it can take advantage of very low-cost or waste heat,” says Tom Pankratz, a water consultant based in Texas, who is also a board member of the International Desalination Association.
Economies of scale, better membranes and improved energy-recovery have helped to bring down the cost of reverse-osmosis seawater-desalination. Although the cost of desalination plants and their water depends on where they are, as well as the local costs of capital and operations, prices decreased from roughly $1.50 a cubic metre in the early 1990s to around 50 cents in 2003, says Mr Pankratz. As a result, reverse osmosis is preferred for most modern seawater-desalination (though rising energy and commodity prices mean the cost per cubic metre has now risen to around 75 cents). Experts reckon that further gains in energy efficiency, and hence cost reductions, will be increasingly difficult, however. According to a recent report on desalination from America’s National Research Council, energy use is unlikely to be reduced by much more than 15% below today’s levels—though that would still be worthwhile, it concludes.
Sometimes, using desalination within water management may be the only way to ensure supply.
To achieve these reductions, researchers want to find better membranes that allow water to pass through more easily and are less likely to get clogged up. Eric Hoek and his colleagues from UCLA, for example, have developed a membrane embedded with tiny particles containing narrow flow channels, producing a significant increase in water flux. The membrane’s smooth surface is also expected to make it harder for bacteria to latch onto. Depending on a plant’s design, the new membranes could reduce total energy consumption by as much as 20%, reckons Dr Hoek. The technology is being commercialised by NanoH2O, a company on UCLA’s campus.
Meanwhile, the possibility of making membranes out of carbon nanotubes, which consist of sheets of carbon atoms rolled up into tubes, has also garnered attention. A study published in the journal Science in 2006 demonstrated unexpectedly high water-flow rates. But insiders think it will be a decade before the idea is ready for commercialisation.
As desalination becomes more widespread, its environmental impacts, including the design of intake and discharge structures, are coming under increased scrutiny. Some of the damage can be mitigated fairly easily. Reducing the intake velocity enables most fish species and other mobile marine life to swim away from the intake system, though small animals, such as plankton or fish larvae, may still get caught in the intake screens or sucked into the plant.
In the late 1970s John Cadotte of America’s Midwest Research Institute and the FilmTec Corporation created a much-improved membrane by using a special cross-linking reaction between two chemicals atop a porous backing material. His composite membrane consisted of a very thin layer of polyamide, to perform the separation, and a sturdy support beneath it. Thanks to the membrane’s improved water flux, and its ability to tolerate pH and temperature variations, it went on to dominate the industry. At around the same time, the first reverse-osmosis plants for seawater began to appear. These early plants needed a lot of energy. The first big municipal seawater plant, which began operating in Jeddah, Saudi Arabia, in 1980, required more than 8 kilowatt hours (kWh) to produce one cubic metre of drinking water.
The energy consumption of such plants has since fallen dramatically, thanks in large part to energy-recovery devices. High-pressure pumps force seawater against a membrane, which is typically arranged in a spiral inside a tube, to increase the surface area exposed to the incoming water and optimise the flux through the membrane. About half of the water emerges as freshwater on the other side. The remaining liquid, which contains the leftover salts, shoots out of the system at high pressure. If that high-pressure waste stream is run through a turbine or rotor, energy can be recovered and used to pressurise the incoming seawater.
The energy-recovery devices in the 1980s were only about 75% efficient, but newer ones can recover about 96% of the energy from the waste stream. As a result, the energy use for reverse-osmosis seawater desalination has fallen. The Perth plant, which uses technology from Energy Recovery, a firm based in California, consumes only 3.7kWh to produce one cubic metre of drinking water, according to Gary Crisp, who helped to oversee the plant’s design for the Water Corporation, a local utility. Thermal plants suck up nearly as much electricity, but also need large amounts of steam. “A thermal plant only is practical if you can build it in such a way that it can take advantage of very low-cost or waste heat,” says Tom Pankratz, a water consultant based in Texas, who is also a board member of the International Desalination Association.
Economies of scale, better membranes and improved energy-recovery have helped to bring down the cost of reverse-osmosis seawater-desalination. Although the cost of desalination plants and their water depends on where they are, as well as the local costs of capital and operations, prices decreased from roughly $1.50 a cubic metre in the early 1990s to around 50 cents in 2003, says Mr Pankratz. As a result, reverse osmosis is preferred for most modern seawater-desalination (though rising energy and commodity prices mean the cost per cubic metre has now risen to around 75 cents). Experts reckon that further gains in energy efficiency, and hence cost reductions, will be increasingly difficult, however. According to a recent report on desalination from America’s National Research Council, energy use is unlikely to be reduced by much more than 15% below today’s levels—though that would still be worthwhile, it concludes.
Sometimes, using desalination within water management may be the only way to ensure supply.
To achieve these reductions, researchers want to find better membranes that allow water to pass through more easily and are less likely to get clogged up. Eric Hoek and his colleagues from UCLA, for example, have developed a membrane embedded with tiny particles containing narrow flow channels, producing a significant increase in water flux. The membrane’s smooth surface is also expected to make it harder for bacteria to latch onto. Depending on a plant’s design, the new membranes could reduce total energy consumption by as much as 20%, reckons Dr Hoek. The technology is being commercialised by NanoH2O, a company on UCLA’s campus.
Meanwhile, the possibility of making membranes out of carbon nanotubes, which consist of sheets of carbon atoms rolled up into tubes, has also garnered attention. A study published in the journal Science in 2006 demonstrated unexpectedly high water-flow rates. But insiders think it will be a decade before the idea is ready for commercialisation.
As desalination becomes more widespread, its environmental impacts, including the design of intake and discharge structures, are coming under increased scrutiny. Some of the damage can be mitigated fairly easily. Reducing the intake velocity enables most fish species and other mobile marine life to swim away from the intake system, though small animals, such as plankton or fish larvae, may still get caught in the intake screens or sucked into the plant.
Continued from Desalination Part 1: Tapping the Ocean
This reduced the energy consumption of sugar refining by up to 80%, says James Birkett of West Neck Strategies, a desalination consultancy based in Nobleboro, Maine. But it took about 50 years for the idea to make its way from one industry to another. Only in the late 19th century did multi-effect evaporators for desalination begin to appear on steamships and in arid countries such as Yemen and Sudan.
A few multi-effect distillation plants were built in the first half of the 20th century, but a flaw in the system hampered its widespread adoption. Mineral deposits tended to build up on heat-exchange surfaces, and this inhibited the transfer of energy. In the 1950s a new type of thermal-desalination process, called multi-stage flash, reduced this problem. In this, seawater is heated under high pressure and then passed through a series of chambers, each at a lower pressure than the one before, causing some of the water to evaporate or “flash” at each step. Concentrated seawater is left at the bottom of the chambers, and freshwater vapour condenses above. Because evaporation does not happen on the heat-exchange surfaces, fewer minerals are deposited.
Countries in the Middle East with a lot of oil and a little water soon adopted multi-stage flash. Because it needs hot steam, many desalination facilities were put next to power stations, which generate excess heat. For a time, the cogeneration of electricity and water dominated the desalination industry.
Research into new ways to remove salt from water picked up in the 1950s. The American government set up the Office of Saline Water to support the search for desalination technology. And scientists at the University of Florida and the University of California, Los Angeles (UCLA) began to investigate membranes that are permeable to water, but restrict the passage of dissolved salts.
Such membranes are common in nature. When there is a salty solution on one side of a semi-permeable membrane (such as a cell wall), and a less salty solution on the other, water diffuses through the membrane from the less concentrated side to the more concentrated side. This process, which tends to equalise the saltiness of the two solutions, is called osmosis. Researchers wondered whether osmosis could be reversed by applying pressure to the more concentrated solution, causing water molecules to diffuse through the membrane and leave behind even more highly concentrated brine.
Initial efforts showed only limited success, producing tiny amounts of fresh water. That changed in 1960, when Sidney Loeb and Srinivasa Sourirajan of UCLA hand-cast their own membranes from cellulose acetate, a polymer used in photographic film. Their new membranes boasted a dramatically improved flux (the rate at which water molecules diffuse through a membrane of a given size) leading, in 1965, to a small “reverse osmosis” plant for desalting brackish water in Coalinga, California.
The energy requirements for thermal desalination do not much depend on the saltiness of the source water, but the energy needed for reverse osmosis is directly related to the concentration of dissolved salts. The saltier the water, the higher the pressure it takes (and hence the more energy you need) to push water through a membrane in order to leave behind the salt. Seawater generally contains 33-37 grams of dissolved solids per litre. To turn it into drinking water, nearly 99% of these salts must be removed. Because brackish water contains less salt than seawater, it is less energy-intensive, and thus less expensive, to process. As a result, reverse osmosis first became established as a way to treat brackish water.
Another important distinction is that reverse osmosis, unlike thermal desalination, calls for extensive pre-treatment of the feed water. Reverse-osmosis plants use filters and chemicals to remove particles that could clog up the membranes, and the membranes must also be washed periodically to reduce scaling and fouling.
This reduced the energy consumption of sugar refining by up to 80%, says James Birkett of West Neck Strategies, a desalination consultancy based in Nobleboro, Maine. But it took about 50 years for the idea to make its way from one industry to another. Only in the late 19th century did multi-effect evaporators for desalination begin to appear on steamships and in arid countries such as Yemen and Sudan.
A few multi-effect distillation plants were built in the first half of the 20th century, but a flaw in the system hampered its widespread adoption. Mineral deposits tended to build up on heat-exchange surfaces, and this inhibited the transfer of energy. In the 1950s a new type of thermal-desalination process, called multi-stage flash, reduced this problem. In this, seawater is heated under high pressure and then passed through a series of chambers, each at a lower pressure than the one before, causing some of the water to evaporate or “flash” at each step. Concentrated seawater is left at the bottom of the chambers, and freshwater vapour condenses above. Because evaporation does not happen on the heat-exchange surfaces, fewer minerals are deposited.
Countries in the Middle East with a lot of oil and a little water soon adopted multi-stage flash. Because it needs hot steam, many desalination facilities were put next to power stations, which generate excess heat. For a time, the cogeneration of electricity and water dominated the desalination industry.
Research into new ways to remove salt from water picked up in the 1950s. The American government set up the Office of Saline Water to support the search for desalination technology. And scientists at the University of Florida and the University of California, Los Angeles (UCLA) began to investigate membranes that are permeable to water, but restrict the passage of dissolved salts.
Such membranes are common in nature. When there is a salty solution on one side of a semi-permeable membrane (such as a cell wall), and a less salty solution on the other, water diffuses through the membrane from the less concentrated side to the more concentrated side. This process, which tends to equalise the saltiness of the two solutions, is called osmosis. Researchers wondered whether osmosis could be reversed by applying pressure to the more concentrated solution, causing water molecules to diffuse through the membrane and leave behind even more highly concentrated brine.
Initial efforts showed only limited success, producing tiny amounts of fresh water. That changed in 1960, when Sidney Loeb and Srinivasa Sourirajan of UCLA hand-cast their own membranes from cellulose acetate, a polymer used in photographic film. Their new membranes boasted a dramatically improved flux (the rate at which water molecules diffuse through a membrane of a given size) leading, in 1965, to a small “reverse osmosis” plant for desalting brackish water in Coalinga, California.
The energy requirements for thermal desalination do not much depend on the saltiness of the source water, but the energy needed for reverse osmosis is directly related to the concentration of dissolved salts. The saltier the water, the higher the pressure it takes (and hence the more energy you need) to push water through a membrane in order to leave behind the salt. Seawater generally contains 33-37 grams of dissolved solids per litre. To turn it into drinking water, nearly 99% of these salts must be removed. Because brackish water contains less salt than seawater, it is less energy-intensive, and thus less expensive, to process. As a result, reverse osmosis first became established as a way to treat brackish water.
Another important distinction is that reverse osmosis, unlike thermal desalination, calls for extensive pre-treatment of the feed water. Reverse-osmosis plants use filters and chemicals to remove particles that could clog up the membranes, and the membranes must also be washed periodically to reduce scaling and fouling.
Subscribe to:
Posts (Atom)