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The advancement of membrane technology has led to its growing acceptance as a reliable treatment process for large-scale water purification plants. This is best demonstrated by the number of plants that have been built and are providing communities and industries with increasing proportions of their water supply.
The number of these large plants has been growing significantly in the past five years, and the plants are getting larger. Wastewater plants as large as 320,000 m3/day (Sulaibiya, Kuwait) and 280,000 m3/day (Orange County, U.S.) are operating and seawater plants as large as 200,000 m3/day (Hamma and Beni Saf, Algeria) and 326,000 m3/day (Ashkelon and Hadera, Israel) are also in use. Even larger plants such as the 600,000 m3/day Melbourne SWRO are planned.
A consequence of these large RO plants is that they require many RO elements, pressure vessels and interconnecting pipes. Engineers have been improving designs to reduce the capital cost of supplying and assembling so many parts. One important advancement is the development of longer pressure vessels. For many years, the six-element vessel was the standard size. Today, these large-scale plants are built with seven-element or eight-element vessels. This reduces the number of vessels by 14% or 25%, respectively.
Although the longer vessels are more expensive, the relative increase is not proportional, which results in significant savings on a large plant. A second improvement is the use of side-ported pressure vessels with port-to-port fittings. These allow the pressure vessels to be mated up directly with one another and eliminate the need for expensive piping and labor to connect each vessel. The incremental reduction of capital cost from such developments has helped make large SWRO systems more economical.
A further improvement is the use of high-area RO membranes. Through various assembly and material improvements by RO suppliers, the amount of area in a standard 8-by-40-in spiral-wound element has been increased over the years. Initially, most low-pressure, brackish water spiral elements had around 365 sq ft of area, which was later increased to 400 sq ft. In the past seven years, these low-pressure elements were also made with 440 sq ft. In contrast, seawater elements, which ran at two to three times higher pressure, initially had anywhere from 320 to 380 sq ft of surface area. In the early 2000s, this also was increased to 400 sq ft. Today, seawater elements are available in a 440-sq-ft configuration, which can help further reduce RO system capital costs.
Spiral Elements & Process RO Design
Design of RO systems has benefited from years of experience and determination of optimized operating conditions. Some of the key factors include flux rate, cross-flow rates and pressure drop across the elements from feed to brine side. The flux rate is one of the most critical parameters that determine the key features of an RO system. An RO system uses a crossflow-based process in which a small fraction of water permeates through the membrane and the remainder flows across the surface (see Figure 1). The high flow of water over the surface acts to flush the surface of the salts and other dissolved species that do not permeate through the membrane.
Once the flux rate is defined as shown in Table 1 (page 12), the area of membrane can be calculated from the feed flow. For example, if the Beni Saf plant with 200,000 m3/day of permeate flow uses a flux of 13.4 lmh, then the plant will require at least 609,000 sq m (6,560,000 sq ft) of membrane. If a conventional 400-sq-ft membrane is used, the plant will need approximately 16,400 spiral wound elements. However, the use of new high-area, 440-sq-ft elements will reduce this to approximately 14,900 elements. The savings of 1,500 elements simplifies the plant design and management, but it also eliminates about 186 pressure vessels (assuming eight elements per vessel). In addition, it will reduce the amount of piping and potentially make the train somewhat smaller and lighter. All of these factors can play a role in the civil construction work and maintenance of the system.
High-area elements can also be used to optimize RO system designs in other ways. A second alternative benefit is especially beneficial for plant retrofits. If a system has additional pump capacity and sufficient piping and pretreatment for higher flows, the replacement of 400-sq-ft elements with 440-sq-ft elements can result in 10% greater water production. In those cases, 10% more water can be available for sale at a small, additional operating cost increase. Generally, the return on this investment can be quite rapid.
A third option is minimize operating cost, not capital cost. In this scenario, the design with 440-sq-ft elements will have the same number of pressure vessels as the design with 400-sq-ft elements. The presence of 10% more area, however, will result in 10% lower flux rate, assuming the same amount of product water is made in both cases. Operation at lower flux rate will allow the use of 10% less net driving pressure. For a system such as the one shown in Table 1, the 440-sq-ft element would operate at 12.8 lmh.
This would result in a savings of about 1 bar of operating pressure.
Performance of High-Area Elements
High-area elements offer substantial incremental savings over traditional 400-sq-ft elements. For years, these high-area elements have found frequent use in the second pass of a two-pass RO system. They have also been used for brackish well water applications. More recently, they have been used to treat wastewater, such as at the Ulu Pandan plant in Singapore.
The Ulu Pandan NEWater plant is currently the largest operating membrane-based reclamation plant in Singapore. This plant consists of pressurized microfiltration modules which treat clarified active sludge at the wastewater treatment plant. The RO system consists of 13 trains (12 in production, one in standby) producing 12,300 m3/day (3.25 mgd) each, for a total of 148,000 m3/day (38 mgd). The trains are configured as a 64:36 two-stage array with seven elements per vessel and an average system flux of 18 lmh (10.6 gal per sq ft per day). This is a typical flux for wastewater treatment and is similar to the flux at the other NEWater plants that use 400-sq-ft elements.
The RO membrane is the high-area ESPA2+, which has 440 sq ft of surface area and is the first Singapore plant to use high-area elements. The additional area of this element reduces the total number of pressure vessels by 10%. Additionally, the reduction of pressure vessels essentially allows the number of trains to be reduced by one. This results in a substantial savings of the capital cost because the associated gauges, valves, piping and controls will also be saved.
As mentioned earlier, high-area seawater elements have not been previously available due to the more difficult design of spiral wound elements which run at high pressure. However, new 440-sq-ft elements have now been commercialized. The 440-sq-ft SWC4+ Max and SWC5 Max have been designed with the same membrane, feed spacer and permeate spacer, but with increased number of leaves and more efficient use of the active area in the element.
Pilot testing has been done to show that the performance of these new membranes will provide the same quality as the 400-sq-ft versions, but 10% more flow. The results of recent Pacific Seawater tests confirm that the performance is improved over the 400-sq-ft configuration. The trial was done using six 440-sq-ft SWC4+ Max elements in series. Samples of water were taken at the start of the test and then compared to the predicted performance using Hydranautics’ IMSDesign software.
Based on this trial, performance in full-scale plants is also expected to show the benefits of the high area, while ensuring the same stable operation that is seen with 400-sq-ft seawater elements.
The benefits of using high-area elements are readily apparent. Based on the successful use of high-area elements for large wastewater plants, it is expected that even more plants will take advantage of these new elements. The introduction of high-area seawater elements will now extend this benefit to large seawater projects as well. The system designer has a number of options to take advantage of the 10% higher area, including fewer pressure vessels which lowers capital cost, the same number of vessels and pressure, but 10% increased water production or the same number of vessels and lower flux to lower operating pressure.
The exact design condition should be selected depending on the constraints of the project. The high-area elements have demonstrated stable performance on large projects such as the 148,000 m3/day Ulu Pandan wastewater project, and new 440-sq-ft elements have demonstrated stable performance during trials with Pacific Seawater. In all of these, the performance has met expected values that are extrapolated from 400-sq-ft element performance, which greatly minimizes any risk associated with using this new technology.