Water. Desalination + reuse
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PROJECTS Figure 5: UF System Figure 4: Desalination Plant 2. The overall running time of the backwash pump is reduced. This allows for more efficient backwashing: less water loss and a reduced number of backwash pumps being required. The UF system has a net capacity of approximately 3,000 m3/h (phase I) and 4,000 m3/h (phase II). During phase I, 11 membrane units will be installed. Figure 5 shows a typical skid. This number will be increased to 16 units for phase II. COmPaRiSOn The two UF plants described above use the same Seaguard membranes for pretreatment to an RO system. The goal of the UF system is identical in both case studies: proving the best possible pretreatment to an RO step at the lowest cost. The design approach in both case studies is, however, fundamentally different. The main driver for this difference in design is the size of the desalination system: one small and one large. Based on this different starting-point, the outcome of the design process looks different. Table 1 gives a comprehensive overview of the design differences and the selection criteria that drove these differences. Historically, large-scale dual-membrane plants have been designed as scaled-up versions of small-scale dual-membrane plants: larger units based on the same design principles. This does not always lead to the best design. Large-scale systems can take advantage of economies of scale: eg, a DAF pretreatment system of 2,500 m3/h is not ten times more expensive than a DAF system of 250 m3/h. When these factors are not taken into account, largescale systems will become unnecessarily expensive and might outprice themselves against alternative technologies, such as, for example, two-stage DMFs. Systems Size Small Item Design Selection Criteria Large Design Selection Criteria Pretreatment Strainer Simple and low cost Dissolved Air Flotation Flexibility Conservative design Modular design Dual membrane system operation Decoupled by means of intermediate tank UF permeate Flow modifications by changing process parameters Easier to control both systems individually Easier to control both systems individually Better control of intermediate tank level Better pretreatment to provide savings in UF Flow modifications by adding membranes and units Lower investment and operational costs Higher systems recovery Backwash water Backwash strategy Each unit individually backwashed In line operation RO concentrate Train backwash Less water loss Lower investment Table 1: Comparison of design differences between small and large systems COnCLuSiOnS A holistic approach needs to be taken for designing large scale desalination plants: l Pretreatment selection should not be based just on feedwater turbidity, as this could lead to inadequate pretreatment and shortfalls in production of the desalination plant. It is therefore proposed to base SWRO pretreatment selection on multiple criteria. l UF pretreatment has to be an integral part of the desalination plant. l Investment in UF pretreatment will be offset by savings in investment and operational costs of the total desalination plant. l Break tanks and intermediate pumping of seawater can be eliminated by installing UF in-line with the SWRO low-pressure feed line. l Use of RO concentrate (brine) will increase the overall system's recovery and can provide investment savings | 24 | Desalination & Water Reuse | August-September 2012 due to smaller intake systems and operational savings due to lower pumping requirements. l UF pretreatment is becoming the standard for SWRO pretreatment. References 1. Brantes, R A (2008) Best practices and efficient use of water in the mining industry, Cochilco (Chilean Copper Commission) 2. Petry, M, Sanz, MA, Langlais, C, Bonnellye, M, Durand, J-P, Guevara, D, Mantovani Nardes, W, Homma Saemi, C (2007) The El Coloso (Chile) reverse osmosis plant, Desalination 203, pp 141-152 3. Huehmer, R, Lozier, J,Henthorne, L, Wang, F, Lee, H, Chan, C, (2007) Evaluation of Conventional Media and Membrane SWRO Pretreatment in Hong Kong, China, Proceedings of IDA Mas Palomas, MP07-191 4. Knops, F Van Hoof, S, Zark, A (2007) Operating Experience of a new Ultrafiltration Membrane for Pre-Treatment of Seawater Reverse Osmosis, Proceedings of IDA Mas Palomas, MP07-23 5. Pearce, GK, Bartels, C Wilf, M (2007) Improving Total Cost of Desalination by Membrane PreTreatment, Proceedings of IDA Mas Palomas, MP07-57