Water. desalination + reuse

RESEARCH | 36 | Desalination & Water Reuse | November-December 2014 _________ Seung-Hyun Kim, director of GMVP, professor at Kyungnam University, and Dong-Ik Kim senior vice president of GS Engineering and Construction ___ Editor's note: when Statkraft abandoned its pilot pressure-retarded osmosis project it took much of the momentum out of the industry's then accelerating progress. This was despite the reported successes of Japan's giant Mega-ton project. The authors here outline the chief challenges and the prevalent views of the future among leading figures in the industry. PRESSURE-RETARDED osmosis (PRO) is a relatively novel technology that harvests the energy created by a salinity gradient across a semi-permeable membrane separating a low-saline (feed) solution and a high-saline (draw) solution). Electricity can be generated when the osmotic pressure of the transported water is used to drive a hydro-turbine. So PRO is a membrane process focused on energy production or recovery hence it is sometimes referred to as osmotic power. The concept of harvesting energy from the mixing of freshwater and saltwater was first proposed 60 years ago (Pattle, 1954), but it did not receive much attention from the scientific or engineering communities. The few studies conducted on the subject were only carried out in the laboratory and there have been no pilot-scale studies until this millennium. Further scaling-up pilot operations are the essential next step for PRO to be accepted by the engineering community and to progress to commercialization. There remains a need too for large-scale studies to evaluate the feasibility and to demonstrate the performance of PRO. StAtKRAft AnD MEgA-ton The breakthrough for a large-scale PRO study came after the importance of the pressure exchanger had been recognized (Loeb, 2002). Subsequently, following the configuration proposed by Loeb, Norwegian energy company, Statkraft, unveiled the first prototype PRO installation in 2009. Japan followed the Norwegian lead with a research and development project on seawater Scaling-up and piloting of pressure-retarded osmosis desalination called Mega-ton, which raised the profile of the seawater reverse osmosis (SWRO-PRO) pilot plant operation (see box, PRO's and cons). The Mega-ton project kicked-off in 2010 with research and development for a 1,000,000 m 3 /d desalination plant (1 Mt/d – hence its name). It had three missions: to reduce the SWRO energy demand; to maintain a low environmental impact; and achieve a low water-production cost. There were eight research themes including PRO. The Statkraft and Mega-ton pilot plants were different in many aspects with three chief distinctions: • the purpose of PRO within the project; • the feed and draw solutions used; and • the membrane and module used. DIffERIng AIMS The primary purpose of Statkraft's prototype was to generate electricity, while the main aim for the Mega-ton pilot plant was to reduce the environmental impact of brines generated from SWRO plants. The Statkraft prototype used seawater and river water, while the Mega-ton pilot plant used SWRO brines and treated effluent from a wastewater treatment plant. Sixty-six elements of spiral-wound, eight- inch modules were installed at the Statkraft plant, while eight elements of hollow-fibre, ten-inch modules were installed at the Mega-ton pilot plant to accommodate the flows of 460 m 3 /d of SWRO brine and 420 m 3 /d of treated sewage. Statkraft anticipated that PRO could be Positive view: a schematic of a large-scale SWRO PRO plant under development by the Korean-funded GMVP project. IntAKE outfAll SwRo pRE- tREAtMEnt SwRo ERD tuRbInE pRo pRo pREtREAtMEnt wAStEwAtER tREAtMEnt plAnt

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