Technology Review

PRO has seen significant progress in the past few years, mainly thanks to the development of better and better membranes. However, experimental activity has been so far mostly limited to laboratory-scale testing of commercially available RO and FO membranes [6,7, 13,14] or prototype lab-cast membranes [12,15,16]. In particular, experiments have been reported where commercial FO spiral-wound modules were used the maximum power densities reported as inferred from experimental measurements, using seawater-equivalent as the concentrated stream, are 0.5 W/m in a commercially available spiral-wound module [14] and 3.5 W/m with a prototype, lab-cast TFC membrane [12]. 

There has been significant progress made recently in the fabrication of FO membranes. While these were not specifically tested for PRO operation (e.g. permeation rates under pressurized conditions), their estimated potential performance may be calculated based on the experimentally determined characteristics, namely, the water and salt permeabilities of the filtering layer and the mass transfer resistance of the support to diffusive transport. Membrane permeability is expressed in m/s-Pa and measures the water volume flow rate permeated across a membrane area unit under a 1 Pa osmotic pressure difference between the membrane sides. High values of water permeability denote the membrane capability of avoiding resistance to water flux. 

Membrane selectivity quantifles instead the ratio between water and salt permeability. Extremely low salt permeability (and consequently high selectivity) is ultimately required in RO desalinization applications. In PRO plants for power generation, this feature is not much essential considering that salt permeation has just the secondary effect of reducing the difference in concentration on the two sides of the membrane and consequently the average flux. 

Ramon et al [17] compiled a list of commercial and prototype osmotic membranes and evaluated the resulting theoretical power densities according to their characteristics. The analysis showed that prototype lab-cast thin-film composite membranes based on a selective polyamide active layer can achieve water permeability in the range of 5-7 m/s-Pa, which result in theoretical power density in the range of 5-6 W/m with seawater feed and over 15 W/m in plant fed with I.IM brine discharged from RO-based desalinization plants. Theoretical power densities achievable with commercial membranes are below 3 W/m with seawater feed. 

Ultimately, an important characteristic of any PRO membrane is its ability to withstand the apphed hydraulic pressure in the feed stream. Since the maximum power density is achieved when this pressure is about half the osmotic pressure, the higher the concentration difference, the higher the pressure to be applied for power density maximization. For example, when RO brine (I.IM, twice the concentration of seawater) is contacted with 0.02M wastewater, the optimum hydraulic operating pressure would be around 24 bar, but it is not clear whether the currently available membranes would be able to mechanically withstand such a large pressure [17]. 

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