What is next for PRO?

What is next for PRO?

PRO membrane development

Compared to FO membranes, the development of PRO membranes has evolved more slowly because of the high pressure applied on the draw solution [3], [6] and [52]. Most FO membranes may collapse or be severely deform during PRO operations [53]. Spacers are needed for PRO flat-sheet membrane modules to maintain flow channels and improve mass transfer. The feed spacers not only cause hydraulic pressure losses along the flow channels but also inevitably deform PRO membranes under high pressure operations [54] and [55]. As a consequence, the reverse salt fluxes and internal concentration polarization (ICP) are drastically increased and result in substantial reductions in both water flux and power density. Moreover, the burst pressure of flat sheet membranes is highly dependent on the spacer design [56]. Therefore, identification of spacers such as tricot fabric feed spacers, compatible with PRO membranes, is of paramount importance for the development of effective flat-sheet PRO membrane modules [56]. In contrast, no spacers are needed in PRO hollow fiber modules due to the mechanically self-supported nature of hollow fibers. However, deformation of hollow fibers also happens in PRO hollow fiber modules and the deformations of inner-selective and outer-selective hollow fibers under high PRO pressures are different. Hence, different strategies on material selection, membrane morphology formation and macrovoid distribution must be incorporated in the design in order to produce highly robust PRO hollow fiber membranes [57], [58] and [59].

Today, high performance PRO flat-sheet membranes that can withstand hydraulic pressures up to 22 bar with corresponding power density of 18 W/m², and hollow fiber membranes that can withstand hydraulic pressures up to 20 bar with corresponding power density of 27 W/m² using seawater brine (1.0 M NaCl) and deionized water as feeds, have been developed [60] and [61]. These PRO performances are superior to others reported in the literature. Moreover, outer-selective PRO hollow fiber membranes, which may have a less pressure drop along the fiber, have been demonstrated[59] and [62]. Fouling in PRO membranes is more complicated than that in FO because the feed stream faces the porous substrates in PRO operations. In addition, the reverse salt flux may facilitate fouling and complicate fouling mechanisms [63], [64], [65] and [66]. Accordingly, hollow fiber membranes with anti-fouling properties for osmotic power generation have been designed by grafting hyper-branched polyglycerol and zwitterionic polymers on polyethersulfone hollow fiber membranes [67] and [68]; even though, PRO membranes with higher power density, capacity to withstand greater pressures and better anti-fouling properties are urgently needed.

3.2. Feed streams: seawater vs. RO brine

For the Statkraft PRO pilot, it required two long pipes to transport seawater and river water to the PRO pilot and extensive pre-treatments were conducted to remove foulants and scalants from both feeds [10]. A high pressure pump was employed to pressurize the seawater. Since the salinity gradient between seawater and river water is relatively low, there is no substantial gain in economic and energy when balancing the energy produced from the PRO plant and the energy consumed in pretreatments, pumping the feeds and pressurizing the seawater compartment.

Therefore, future PRO studies should focus on the use of either (1) retentates from both SWRO and wastewater reuse RO (WWRO) or (2) SWRO retentate and river water as feeds. Not only can these feed streams create much greater osmotic energy, but also save some of the pre-treatment costs. This is because RO retentate has been well pre-treated in its previous processes, which will significantly reduce the membrane fouling in the PRO step. In addition, since the RO retentate is already under a high pressure, it is unnecessary to have an additional pump to pressurize the high pressure compartment as in the case of using seawater and river water as the feed pair for PRO.

3.3. Integration of RO, PRO and pressure exchanger

If SWRO retentate is used as the draw solution, then the salinity gradient between the SWRO retentate and river water is much greater than that between seawater and river water (about 7.9–8.5 vs. 3.5 wt% NaCl). The PRO plant may be able to raise its operational pressure from 13.5 bar for the feed pair of seawater and river water to 20–35 bar depending on RO brine concentration [6] and [52]. This will significantly increase the production of osmotic energy, but also bring tremendous challenges for the design of PRO membranes with very high mechanical strength to withstand the higher operational pressure. Nevertheless, the integration of osmotic power generators and SWRO plants can (1) make seawater desalination less energy dependent and more sustainable and (2) alleviate the disposal and environmental issues of waste RO brine.

Fig. 2 illustrates the integration of pressure exchangers (PX) with a SWRO plant and a PRO unit. The 1st pressure exchanger not only transmits energy from the highly pressurized RO retentate to the feed seawater but also discharges the highly pressurized and concentrated RO retentate as the draw solution for the subsequent PRO operation. The 2nd pressure exchanger takes advantages of the diluted and pressurized RO brine to pressurize the feed seawater. As a result, the high pressure pump for SWRO requires less energy to pressurize the seawater feed.

 

 

 

Many experiments on integrated RO–PRO–PX systems [69] and [70] and theoretical calculations on their energy consumption have been reported [71], [72], [73] and [74]. Various design strategies for pressure exchanger devices were also proposed [73], [74],[75], [76] and [77]. Once the osmotic power generator is fully integrated with the SWRO plant with the aid of pressure exchangers, it is envisioned that seawater desalination will become much more energy-efficient and cost-effective and this integration will entirely revolutionize the desalination industry and future osmotic energy production.

Acknowledgements

This research was funded under the project entitled “Membrane development for osmotic power generation, Part 1. Materials development and membrane fabrication” (1102-IRIS-11-01) and NUS Grant Number of R-279-000-381-279. This research grant is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB. Special thanks are due to Dr. S. Zhang, Dr. X. Li and Dr. G. Han for their useful comments and assistance.