Water. Desalination + reuse
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TECHNOLOGY based process has been developed for the preparation of make-up water from impaired water sources, ranging from seawater to treated sewage effluent. This new technique allows the economic use of water sources that otherwise would not be considered for make-up and therefore extends the applicability of evaporative cooling and, just as important, allows water substitution. This frees up valuable potable water or treated sewage effluent for more appropriate use. The process is beautifully elegant in concept. To draw in water, to replace that lost by evaporation drift and blowdown, the cooling-water chemistry is changed to increase its osmotic pressure. A portion of the high-osmotic-pressure cooling water is introduced to one side of a FO membrane with on the other side a feedwater such as seawater, brackish water or treated sewage effluent. The natural process of osmosis takes place and essentially pure water flows into the recirculating cooling water replacing that lost in the process. Figure 6 illustrates a typical arrangement. This system, because it is essentially pure FO without a regeneration step, operates with very low energy-consumption, requiring only to overcome the pumping losses on either side of the membrane system, unlike RO, which is dependent on the salinity of the feed water. The higher the salinity, the more energy is saved compared with using a conventional desalination process for make-up water, typically 50% of RO. This is discussed in more detail in Reference 2. There have been a number of spin-off benefits identified for the process, including the inhibition of Legionella pneumophila in the osmotic agent without the use of oxidising or non-oxidising biocides and significantly reducing the chemical consumption of large molecular weight cooling-water additives through the use of a blowdown recovery system. Modern Water has built a complete pilot unit including the cooling tower, which was initially trialled in Sohar, Oman, in a petrochemical plant. This unit can be moved and tested to trial sites to optimise the process for that particular location. CONCLUDING REMARKS In conclusion FO-based processes are here to stay; they have vast potential across a wide range of applications and in some cases these processes are truly disruptive. The progress that has been made primarily over the last four years is enormous, yet this Figure 5. Al Najdah 200 m3/day FO Desalination Plant Evaporation Drift Loss Cooling Tower Recirculated Osmotic Agent Heat Exchangers Concentrated Osmotic Agent Blowdown System Forward Osmosis System Reject from forward smosis system Feedwater Blowdown Figure 6. FO Based Evaporative Cooling Make-up Water System is only the beginning with much more yet to come in both process and FO membrane development. Progress can be accelerated by further vision, recognition and support by public utilities and industry as a whole. Three main applications are already a commercial reality, which are the common platform on which many more will be developed across a range of industries. REFERENCES Thompson & P Nicoll, Forward Osmosis Desalination: A Commercial Reality, Proceedings IDA World Congress, Perth, Western Australia, September 2011. •N •P Nicoll, N Thompson, V Gray, Forward Osmosis Applied to Evaporative Cooling Make-up Water, Cooling Technology Institute, February 2012. February-March 2013 | Desalination & Water Reuse | 33 |