Water. desalination + reuse

February/March 2013

Water. Desalination + reuse

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technology Forward osmosis is not to be ignored _________ Peter Nicoll, technical director, Modern Water plc ___ ∆ TT Osmotic Agent (High Osmotic Pressure) ∆ TT Feed Water P2-P1< ∆TT P1 > P2 P2 P1 Forward Osmosis (FO) Pressure Enhanced Osmosis (PEO) Figure 1. Osmotic Processes | 30 | Desalination & Water Reuse | February-March 2013 P2 P1 Pressure Retarded Osmosis (PRO) P2-P1 > ∆TT ∆ TT ReADeRS oF this article may occasionally read an article or see a reference about forward osmosis (FO) in D&WR and, whilst the process may not currently be 'mainstream', it is increasingly becoming a topic of some interest. At the last World Congress of the International Desalination Association in Perth, Australia, in September 2011, six papers were published on this subject. In the Journal of Membrane Science, the number of papers published has seen a very significant increase over the last three years (24 in 2012), showing the increasing level of academic interest. We have also seen the emergence of a number of commercial organisations with significant funding to develop and exploit the technology, such as Modern Water plc, Oasys Water Inc and Hydration Technology Innovations Inc. This brief article discusses two processes that have been commercialised by Modern Water plc. FO, or more simply just osmosis, has been used in nature for rather a long time by plants, trees, sharks and human cells to name just a few. All biological cells have a semipermeable membrane which causes osmosis to occur in the presence of liquids of differing solute concentrations. It also takes place as drawback when a reverse-osmosis (RO) plant shuts down and the permeate flows back across the membrane to dilute the feed solution, so this should give some clue as to its potential. The process, just like RO, requires a selectively permeable membrane separating two fluids with different osmotic pressures. If the solvent is water then effectively almost pure water flows from the fluid of lower osmotic pressure to dilute the fluid of higher osmotic pressure. The process in its pure form takes place at atmospheric pressure, with variations such as pressure-enhanced osmosis and pressure-retarded osmosis. These are simply illustrated in Figure 1. It is worth just reminding ourselves just what FO can do: • It can dilute a solution of higher osmotic pressure with a solution of lower osmotic pressure. • It can concentrate a solution of lower osmotic pressure with a solution of higher osmotic pressure. So why might this be useful in a water treatment application? One key element is that the dilution/ concentration process takes place across a selectively permeable membrane, at low pressure and the ions are rejected in both the direction of forward flow and reverse ∆ TT editor's note: For about 3 or 4 years now, D&WR has annually requested a feature from Modern Water about the company's development of forward osmosis and its desalination plants in gibraltar and oman. So it was with pleasure that we finally received the article below which shows the consistent strides the firm has been making with the process. P2 P1 ReverseOsmosis (RO)

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