Large diameter reverse osmosis (RO) membrane elements were originally designed as a cost-saving solution for very large plants producing ten million gallons (1,600-m3/h) of water or more per day. However, market acceptance of 18-in diameter RO element shows that large diameter elements are also efficient and economical solutions for brackish water RO systems with a capacity beginning at 400,000-gpd (60-m3/h), and for seawater desalination systems producing as little as 100,000-gpd (16-m3/h).
The 18-in diameter elements are an alternative to the 8-in diameter elements that have been widely used since their introduction in the mid-1970s. As water treatment engineers plan and construct larger and larger plants, the limitations of the 8-in diameter elements have become apparent. For example, a 60-mgd (9,500-m3/h) brackish water RO system utilizing 8-in x 40-in elements may require 12,600 individual elements and 1,800 pressure vessels, as well as a correspondingly large number of pipe connections and O-rings.
Simpler, Space-Efficient Design
A larger diameter element increases membrane packing density and radically reduces the number of necessary components and required floor space.
A typical 8-in x 40-in element has 400-ft2 of membrane area, whereas an 18-in x 61-in element has 2,800-ft2 to 3,050-ft2 of membrane area. If both elements are operated on the same water, at the same pressure and in the same position within an RO pressure vessel, the 18-in element will produce more than seven times as much permeate as the 8-in element.
Furthermore, up to five 18-in x 61-in elements can be installed in a single RO pressure vessel, and a single 18-in pressure vessel can produce five times as much permeate as a single 8-in pressure vessel. This significantly reduces the number of pipe connections, O-rings and the size of a standard RO pressure vessel rack, which is traditionally equipped with seven 8-in x 40-in elements. Additionally, instead of requiring 12,600 of the 8-in elements, the 60-mgd (9,500-m3/h) plant would only require 1,620 of the 18-in elements, and the number of pressure vessels would also be reduced from 1,800 to 324.
Cost Savings
A smaller footprint and reduction in the number of components can result in significant savings in construction and installation costs. Shipping costs are also reduced because larger diameter element systems require fewer shipping containers. The simpler system design and reduction in quantity also allows for an expedited order and delivery schedule. Finally, maintenance costs are reduced as there are significantly fewer vessels, pipe connections and O-ring seals that could be potential leak points in the system.
A standard 8-in element containing 400-ft2 of membrane surface area can be difficult for a single person to install and remove unassisted. To facilitate installation and removal of larger diameter elements, a simple procedure using a hand-powered tool has been developed which accommodates the loading and unloading of elements for each pressure vessel from the same end. Using this system, a five element long 18-in diameter pressure vessel can be loaded in less time than a seven element long 8-in diameter vessel.
Not Just for Large Systems
Although the development of large diameter elements was originally driven by the needs of large municipal water treatment plants, the larger diameter element has also proven to be a good solution for small and mid-sized municipal and industrial water treatment plants.
The city of Goodyear in Arizona selected larger elements for a 0.5-mgd (80-m3/h) brackish water RO system designed to be transportable for use in outlying sites close to wellheads. A system of this capacity utilizing 8-in elements would not fit on the back of the trailer, but because the larger elements can be loaded and removed from the same side of the pressure vessel, the design is well-suited to mobile trailer RO systems.
The water recycling plant at Joe White Maltings Pty Ltd in Perth, Australia, utilizes large diameter Ultra Low Pressure (ULP) spiral RO elements to polish membrane bioreactor (MBR) effluent and produce water for reuse in the water-intensive malting process. The large diameter elements enable the plant to achieve its required capacity within the limited available space. Now, fifteen 18-in x 61-in elements in three pressure vessels process 330,000- (50-m3/h) to 360,000-gpd (60-m3/h) of permeate from the MBR process, with a recovery of 80 percent.
Optimal System Design for Brackish Water RO
Designing an 18-in diameter system for optimal efficiency and performance requires an understanding of the special opportunities and constraints of higher capacity elements. Some of the assumptions commonly used in designing 8-in diameter systems do not apply.
One consideration is the system's minimum flow requirement. If the system has a normal recovery in the range of 75 to 85 percent, as is typical for brackish water RO, then the total process flow path will need to be about 50-ft (15-m). This means that the system will use two (or three) banks. A balanced flow is achieved by a 2:1 array wherein the concentrate stream from the first two bank tubes is blended and becomes the feed of the second bank tube.
A 1:1 array will have too high a flow to the first bank, and a 3:1 array will have too low a flow in the first bank tubes. A 2:1 array of 2,800-ft2 large diameter elements will contain a total of 42,000-ft2 of active membrane area. At a conservative flux of 12-gfd (20-L/m2/h), this installed area will provide 0.5-mgd (80-m3/h), which is the basic nominal minimum size for a high recovery system. At different design fluxes, the production capacity will change. For example, at 15-gfd (26-L/m2/h), the capacity is 630,000-gpd (100-m3/h) and at 10-gfd (17-L/m2/h), the capacity is 420,000-gfd (66-m3/h).
Multiples of the 2:1 array will work equally well. For example, the city of Waupun in Wisconsin, and the Tate-Monroe Water Association of Ohio have both installed large diameter systems that have two trains, with each train arranged in a 4:2 array. Each 4:2 array contains six pressure vessels, for a total of 30 elements per train. The individual trains have a capacity of 1.0-mgd (160-m3/h), for a total capacity of 2.0-mgd (320-m3/h) at both locations.
Every multiple of the 2:1 array adds 0.5-mgd (80-m3/h) of capacity. This can be a limitation of the larger elements since accommodating intermediate sizes is not always ideal. The individual pressure vessels produce so much water that it is hard to optimally cover every size. For example, a plant requiring 750,000-gpd (120-m3/h) could not add 1.5 pressure vessels. Instead, a 4:2 array which would normally produce 2.0-mgd (320-m3/h) can be installed, and the pressure can be reduced to produce the desired lower flow.
Not every brackish water RO design fits nicely into a 2:1 array. The Bundamba Advanced Water Treatment Plant in Queensland, Australia, the country's largest recycled water project, employs large diameter elements arranged in a 7:4:2 array. Stage 1A of the project was commissioned in August 2007, with four trains. Each 7:4:2 train has a capacity of 1.9 -mgd (310-m3/h), and contains a total of 65 elements. The total capacity of stage 1A is 7.9-mgd (1,250-m3/h). Stage 1B of the project added an additional five trains and was started-up in April of 2008, for an additional capacity of 9.5-mgd (1,460-m3/h).
Very Large Capacity Systems
Very large systems, on the order of 50-mgd (8,000-m3/h) are currently under consideration at several water authorities. The optimal configuration for such systems will likely be an array of racks, with each rack holding 12 pressure vessels in two vertical columns six vessels high (see Figure 1). This is known as a pressure center design because a single pressure center can be used to pump water to all the racks, rather than a train design where each train has its own pump.
Figure 1. Optimal size rack of large diameter pressure vessels for 50-mgd or more
In a pressure center design, all of the tubes within the rack would be in parallel, and with 12 pressure vessels (60 elements), the nominal capacity of each rack would be 2.0-mgd (320-m3/h). The simplest array is a set of three racks, arranged in a 2:1 configuration, meaning that the smallest RO system that could be built in this fashion is a 6.0-mgd system (950-m3/h). However, the larger the capacity, the greater the benefit from this type of RO system layout. Such a design would enable a compact system arrangement and also minimize interconnecting pipe runs between the component racks. The space savings is significant. The footprint required for skids and membrane loading for the system would be less than 8,000-ft2. In comparison, if a typical 8-in rack design was used that is similar to the design used in many U.S. municipal RO plants today, it would have a footprint of about 20,000-ft2.
This arrangement would also permit more creative process control through variable arraying, as racks can be selectively rotated out of service for capacity control or cleaning and maintenance. For example, in the case of the 60-mgd (9,500-m3/h) system discussed above, a typical 8-in design would have 12 trains, each with a 100:50 array, with 12 high pressure pumps, and maybe even another train on standby. The large diameter system design would use one pressure center with two in-service pumps and one standby. Then there would be 27 identical 12 vessel racks in an 18:9 array. An additional standby rack would not be needed, as the plant is large enough to accommodate flow variation when one rack is off-line for cleaning or maintenance (Figure 2).
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Comparison of 8-in and 18-in Diameter RO Systems for 60-mgd (9,500-m3/h) Capacity
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8-in Diameter Element
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18-in Diameter Element
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Element Area (ft2)
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400
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3,050
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Number of Elements Per Vessel
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7
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5
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Total Number of Elements
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12,600 + 1050 standby
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1,620
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Total Number of Vessels
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1,800 + 150 standby
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324
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# Trains
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12 + 1 standby
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1 integrated pressure center
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Array
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100:50
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18:9 x 12 vessel racks
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Number of High Pressure Pumps
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12 + 1 standby
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2 + 1 standby
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Standby Membrane Area (ft2)
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420,000
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Not required
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Footprint Required (ft2)
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20,000
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8,000
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Optimal System Design for Seawater RO
For low recovery applications, such as seawater desalination, the design of large diameter RO systems is more straightforward. When recovery is unlikely to exceed 50 percent, only a single-bank design is required, and every pressure vessel can be arrayed in parallel.
The minimum system size consists of a single tube of 15,250-ft2 of active membrane (five elements, each containing 3,050-ft2 of membrane). At 8-gfd (13.6-L/m2/h), this works out to be about 85-gpm (322-L/min) of permeate per tube, or 120,000-gpd (19-m3/h).
Summary
Thousands of 18-in RO elements are now in operation around the world in a wide variety of sizes ranging from 0.5-mgd (80-m3/h) to over 10.0-mgd (1,600-m3/h). Advantages of larger diameter elements include increased membrane packing density and radically reduced component counts with the resulting reduction in the amount of plant floor space needed. The smaller footprint and the reduction in the number of components can also result in significant savings in equipment, shipping, construction, installation and maintenance costs. For larger capacities, a pressure center design of arrayed racks makes for a simple RO system that is easier to manage and maintain.
MegaMagnum is a registered trademark of Koch Membrane Systems, Inc. in the USA and other countries.
With numerous years of experience in the water and wastewater industries for industrial and municipal applications, Antonia has served as an integral member of the Koch Membrane Systems team. Recently retired from the company, Antonia has presented numerous papers on the industry and her technical articles can be found in a wide variety of top-tier industry trade publications.
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