Water. Desalination + reuse
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technology flow. In a similar way to RO, we talk about salt passage in the direction of forward flow, but ion mass transfer also takes place in the reverse direction, which is often termed as back diffusion. The process has considerable potential across a wide variety of applications; • Emergency drinks • Power generation • Enhanced oil recovery • Produced water treatment • Fluid concentration • Thermal desalination feedwater softening • Desalination • Water substitution. However only a few of these applications have been currently commercialised; emergency drinks, produced water treatment, desalination and water substitution. It is the latter two applications that are outlined in this article and in particular the progress that Modern Water has made to develop and commercialise them. Fo DeSAlinAtion The basic principles of this process, in all its variations, is an initial FO step followed by a regeneration step to reconcentrate the osmotic agent / draw solution for reuse and to separate the 'desalinated' water, prior to any post-treatment. The regeneration step is generally either thermal or membrane based. Modern Water has successfully used a RO regeneration step, with a simplified process illustrated in Figure 2. At first sight this might seem a trivial problem to have solved, however what also must be considered is: • The selection of the osmotic agent / draw solution, which by necessity must be non-toxic • Maintaining a constant osmotic pressure for the concentrated osmotic agent • Managing the contamination of the osmotic agent with salts from the feed solution • Minimising back diffusion of the osmotic agent to the feed solution • A robust FO membrane that can adequately deal with flow on both sides • Operating on a continuous and economic basis. MAin ADvAntAgeS oF Fo Given the above challenges, why then bother to solve them? The answers again are not so straightforward and, like all membrane processes, can be site-specific. Briefly, the two main advantages over conventional RO are lower fouling propensity and lower energy consumption. The significantly lower fouling potential of the FO process compared with RO membranes, operating under the same feed conditions, has been shown by a number of academic researchers. More importantly under real-world conditions on challenging feedwaters, Modern Water has demonstrated that, on one particular site in Oman, no chemical cleaning was required over several years operation, whereas the conventional Reject from forward osmosis system Feedwater Forward Osmosis System Diluted Osmotic Agent Concentrated Osmotic Agent Regeneratiom Membranes Product Water Figure 2. The Forward Osmosis Desalination Process process required cleaning every few weeks with several membrane changes 1. This of course also means a reduction in the use of membrane cleaning chemicals and improved availability. The potential for a lower energy consumption, which is sometimes not so readily understood and requires careful explanation, comes about in a number of ways. Consider that a state-of-the-art RO system has a similar energy consumption to a state-of-the-art FO system in the clean state. Then, given that FO fouls at a lower rate than RO, then it is clear that to maintain the same output the RO-based process will require more energy. The degree of energy saving depends on the degree of fouling. Now consider the regeneration step, which is where the bulk of the energy is consumed for this FO desalination process. The osmotic agent is free of all particulates and large molecular weight organics from the feedwater, as the solution is made from permeate. As such, a parallel should be drawn with the normal design criteria for secondpass RO systems. This means that the recovery rate, the membrane selection and configuration can be fully optimised, and, of course, conventional energy-recovery systems can be applied to the high-pressure concentrated osmotic agent stream. The regeneration step, again because it is fed with a 'perfect' solution, has improved salt rejection compared to conventional RO. This may eliminate the use of a second pass and as such, energy is saved. Boron rejection Boron in desalinated water produced by RO has long presented a challenge to the membrane industry with very poor rejection and the necessity for special high pH second passes or the use of ion-exchange columns, to meet the relevant in-country standards. These add not just complexity, capital cost and operational cost but increased energy consumption. The advantage of having a recirculating osmotic agent is that its properties can be controlled and, in combination with the FO membranes, it has improved boron rejection characteristics. The desalination process, as it has currently been implemented by Modern Water, may be considered as an advanced pretreatment to RO and, as such, the process could be retrofitted to existing conventional RO plants, in circumstances where the economics of the process make sense February-March 2013 | Desalination & Water Reuse | 31 |