Reverse osmosis post-treatment steps

Fig. 10. Flow diagram for the setup of a desalination plant employing reverse osmosis, including pre- and post-treatment steps.

Sea water usually contains total dissolved solids (TDS) at the level of 35 000 ppm around the Middle East, this can go up to 40 000-50 000 ppm. Brackish waters can have TDS as low as 700-1000 ppm, going all the way up to 10 000 ppm plus. Desalination of sea water and brackish water involves a significant number of pretreatment steps, which are essential to the successful long-term performance of the reverse osmosis (RO) membranes incretisingly and invariably employed in new desalination plants. There are also a few post-treatment steps afl er desalination. The exact nature and details of the pretreatment steps are influenced by the source water for the plant As shown in Figure 11.2.1, typical pretreatment steps consist of a variety of separation... 

Post-treatment of the desalted permeate from the reverse osmosis unit (Section 7.2.1.2) consists of the addition of CI2 and Ume for disinfection and corrosion protection. If H2S is present, it is eliminated by air stripping. For brackish-water feeds containing hydrocarbons, an activated carbon adsorber is used to remove dissolved hydrocarbons prior to microfiltration, which is followed by steps needed during the pretreatment and post-treatment processes dechlorination is required as a pretreatment if the RO membrane for desalination cannot tolerate residual chlorine dissolved otcygen is often removed to avoid damaging the RO membrane via vacuum based deaeration or addition of sodium bisuMte. An introduction to the pretreatment and post-treatment processes for membrane based sea-water and brackish-water desalination has been provided by Williams et al. (2001). The scale of such desalination plants is quite large, as much as 87 million gallons per day at Ashkelon, Israel, for example. 

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