Recovery systems SWRO plants

The RO system consists of nine first-pass and three second-pass skids. The RO membranes are 20 cm spiral-wound TFC polyoxide type. The membrane pressure vessels have side-entry front/rear permeate ports. The high-pressure first-pass SWRO unit is supplied by a multi-pump system (three on-Hne, one on-standby). The SWRO feed pressure is 60 bar g TDH (total dehvery head). The second-pass RO unit treats the first-pass permeate and, therefore, operates at much lower feed pressure. The SWRO unit deploys dual work exchanger energy recovery devices. The plant is among the most energy efficient in the world, with an average efficiency of 3.58 kWh/m. The ERDs recover about 97% of the energy required to convert seawater to product permeate. 

To justify the assumptions, the estimated cost in power consumption, labor, and membrane replacement was compared with that in a seawater reverse osmosis desalination (SWRO) plant (Atikol et al., 2005). For SWRO, the reported cost for power was 0.04 US m, we estimated 0.022 US m for our system. The lower cost in energy consumption is mainly due to the low operating pressure and significantly higher water recovery in this system. The cost for pre-treatment was assumed to be lower than seawater plant due to the significantly much better water quality in the drinking water sources. The maintenance cost was adopted from the... 

An example of a two-pass/two-stage SWRO plant is the 110,000-m /day Point Lisas facihty in Trinidad. This plant produces high-quality desalinated seawater of TDS concentration of 85 mg/L or less, which is predominantly used for industrial applications. The first pass of the Point Lisas SWRO system consists of six two-stage RO units. Each of the first-stage RO trains uses SWRO membranes and is coupled with energy recovery devices. The second-stage trains of the first pass are equipped with brackish water RO elements. 

Salt content of the desalinated water depends on the SWRO plant recovery ratio and the quality of the membranes. The product may contain NaCl at a level below 100 mg/L and up to 1000 mg/L. That can be reduced by a second-stage membrane treatment of permeate as explained in the previous sections of this chapter. Using two- or three-stage SWRO systems to remove boron to lower levels will further reduce the sodium chloride salt content as weU. As the water quality is improved, however, the produced water would have very low mineral content (especially calcium hardness) and therefore would be more aggressive and may dissolve matter from the piping system and increase pipe material corrosion rate. 

As previously mentioned, the key SWRO project construction expenditures are associated with building the plant intake, the pretreatment system, procurement and installation of the plant pumps and piping, the SWRO membranes and pressure vessels, the energy recovery system, the water posttreatment facilities, and the concentrate disposal system. It is difficult to compare the investment and construction costs of existing desalination projects because projects may differ significantiy in one or more of the cost-related parameters listed above. Based on previous experience, however, it can be estimated, for example, that the seawater pretreatment costs are in the range of 6-8 US cents/m and the costs of water conditioning and boron and chloride removal are between 4 and 8 US cents/m. ... 

More complex multi-pass SWRO systems include the system used in Ashkelon, Israel, which uses four RO passes in series to treat seawater from an open water intake in the Mediterranean Sea (40,700 mg/1 TDS). The permeate must be produced with less than 0.4 mg/1 boron and 20 mg/1 chloride. Thus, a series of passes with changes in pH was necessary to obtain the required permeate water quaHty. The first pass has a recovery of 45% and is operated at neutral pH. Permeate from the feed end is collected as product, while permeate from the concentrate end is collected and flows to the second pass, which operates at 85% recovery and pH > 8.5 to achieve greater boron removal. The concentrate firom the second pass continues to the third stage, also operated at 85%, but at low pH. The objective of the third pass is to achieve higher recovery without salt precipitation. However, the boron removal in the third pass is minimal at low pH, and a fourth pass (high pH, 90% recovery) treats the third pass permeate for boron removal. Overall, the recovery is approximately 44%, and the plant uses 25,600 SWRO membranes and 15,100 BWRO membranes [3,51]. 

Membrane processes for this analysis include SWRO, BWRO, low-pressure RO (LPRO), brine recovery RO (BRO), pressurised MF/UF (pMF/UF), immersed membrane bioreactor (iMBR), cross-flow membrane filtration (XMF) and electrodeionisation (EDI). Membrane process characteristics for water treatment are detailed in Table 5.1. Typical process flow schematics of RO membrane plants are shown in Figures 5.1 and 5.2. RO/NF systems are typically multi-stage and single-pass or multi-stage and double-pass, as shown in Figures 2.21-2.23. 

The first pass of the Fujairah plant consists of 17 duty and 1 standby RO trains using standard rejection SWRO membranes producing permeate of TDS concentration of 400— 500 mg/L. The overall recovery of the first-pass SWRO system is 43%. The second pass has 8 BWRO trains with a total recovery rate of 90%. The second-pass BWRO trains have two stages and treat approximately 80% of the first-pass permeate to TDS concentration of 10-20 mg/L. The rest (i.e., 20%) of the first-pass permeate is blended with the second-pass permeate to produce finished water of TDS concentration of less than 120 mg/L. The concentrate produced by the second-pass RO system has salinity lower than that of the source seawater and is recycled to the feed of the first-pass RO system... 

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