FWRJ Emergence of Forward Osmosis and Pressure-Retarded Osmotic Processes for Drinking Water Treatment Steven J. Duranceau Description of Emerging Processes from a solution of a lower concentration to a so - lution with a higher concentration. These three Steven J. Duranceau is associate professor of Approximately 97 percent of the Earth’s technologies (RO, FO, and PRO) are common environmental engineering in the civil, water takes the form of salt water in oceans, seas, in that they use semi-permeable membranes to environmental, and construction engineering and lakes. Because of a worldwide water short - separate dissolved solutes from water. The semi- department at the University of Central age, a need exists for alternative desalination permeable membrane acts as a barrier that al - Florida in Orlando. technologies that can produce inexpensive, reli - lows small molecules such as water to pass able, and sustainable sources of water for the through, while rejecting larger molecules like world’s growing population, as well as to meet salts, organics, and proteins, as well as viruses, its industrial and agricultural needs. Green en - bacteria, and other pathogenic material. Both solution on the permeate side of the mem - ergy is available wherever one finds a river that FO and PRO exploit the osmotic pressure dif - brane is the driving force in the FO process. flows into a sea, equivalent to the energy con - ference that develops when a semi-permeable The flux direction of the permeating water in tained in a 900-ft-high waterfall[1]. Desalina - membrane separates two solutions of different FO, PRO, and RO is demonstrated in Figure tion technologies, such as reverse osmosis (RO) concentrations. Instead of employing hydraulic 1[2,3,4]. The governing equation describing and electrodialysis reversal (EDR), have proven pressure as the driving force for separation in the water transport through FO and PRO mem - to be effective in removing a number of dis - RO process, FO and PRO use the osmotic pres - branes, and the volumetric flux of water into solved contaminants from water and wastewater sure gradient across the membrane to induce a the draw solution compartment of osmotic supplies, but require a significant electrical de - net flow of water through the membrane into pumps, can be described by the Kedem– mand and often experience fouling problems what is referred to as the “draw” solution, thus Katchalsky equation as shown below[5]: uncommon with more conventional water and efficiently separating the freshwater from its wastewater treatment methods[2]. Forward os - solutes. Driven by an osmotic pressure gradient, Fw = k w(∆p - ∆ ) mosis (FO) and pressure-retarded osmosis FO and PRO do not require significant energy (PRO) are emerging membrane separations input as in the comparable case of RO. Where is the water flux, is the water technologies that have the potential to be inno - In osmosis, the osmotic pressure itself is mass transfer coefficient, is the applied pres - vative, sustainable, and affordable alternatives to the driving force for mass transport. The os - sure, and is the osmotic pressure. For FO, is RO and EDR because of their ability to utilize motic potential depends on the molar con - zero; for PRO . the green energy available in natural systems[3]. centration, not the weight of dissolved species. In FO application for desalination and Both FO and PRO rely on osmotically-dri - Both FO and PRO use the osmotic pressure water treatment, the active layer of the mem - ven water flux across a semi-permeable dis - differential across the membrane, rather brane faces the feed solution and the porous solved solids rejecting membrane. The term than hydrauΔlic pressure differential p (as in support layer faces the draw solution. How - “osmosis” describes the natural diffusion of RO). The FO process results in dilu∆tion of a ever, in PRO application, the porous support water through a semi-permeable membrane highly concentrated stream. The concentrated Continued on page 34 Figure 1. Solvent flows in FO, PRO, and RO. 32 July 2012 • Florida Water Resources Journal Continued from page 32 systems. The limitations of plate-and-frame el - Tofte, Norway[11,12]. It is estimated that, each layer faces the feed solution and the active layer ements are lack of adequate membrane support year, 12 TWh could be generated in Norway, of the membrane faces the draw solution. The and low packing density. Spiral-wound config - sufficient to meet 10 percent of the country’s chemical potential, which is determined by the urations have been successfully modified for FO total demand for electricity, and 1600 TWh osmotic pressure difference across the mem - mode by placement of an additional glue line could be generated worldwide[13]. brane, is the effective driving force in making at the center of the membrane envelope that In one study, the use of a custom-made energy and fresh water. This potential is usu - provides a path for the feed to flow inside the and laboratory-scale membrane module en - ally lower than the potential generated by the envelope. The feed flows into the first half of the abled the collection of experimental PRO data. osmotic pressure difference between the bulk perforated central pipe, is then forced to flow Results obtained with a flat-sheet cellulose tri - feed and bulk draw solution due to external into the envelope, and then flows out through acetate (CTA) FO membrane, and NaCl feed concentration polarization[6]. the second half of the perforated central pipe. and draw solutions, closely matched model The main advantages of using FO and PRO The limitation of spiral-wound FO configura - predictions[4]. Maximum power densities of are that they can be operated at low or no hy - tion is the difficulty to induce flow into the per - 2.7 and 5.1W/m 2 were observed for 35 and 60 draulic pressure and they possess a high rejection meate channels for the purpose of cleaning or g/L NaCl draw solutions, respectively, at 970 of a wide range of contaminants. Since the only backwashing[7]. In practical applications, hol - kPa of hydraulic pressure[4]. The authors re - pressure involved in the FO and PRO processes is low fibers for continuously operated FO and port that “power density was substantially re - due to flow resistance in the membrane module, PRO processes may have some advantage as duced due to internal concentration it may have a lower membrane fouling tendency compared to the other configuration. This is be - polarization in the asymmetric CTA mem - than pressure-driven membrane processes. The cause they can support high hydraulic pressure branes, and to a lesser degree, to salt passage. ideal material of membranes for FO and PRO without deformation and can be easily packed External concentration polarization was found would have a high active-layer density and result in bundles directly inside a pressure vessel. Also, to exhibit a relatively small effect on reducing in high solute rejection; the thin membrane film, hollow fiber membranes do not need a thick the osmotic pressure driving force.” having minimum porosity of the support layer, support layer as in flat sheet RO membranes, would minimize internal concentration polar - thereby reduce internal concentration polariza - Applications ization, and therefore, provide a higher water tion and enhance membrane performance. flux. In addition, it would be desired that the With PRO, the water potential between The FO application has thus drawn much membrane be hydrophilic for enhanced flux and fresh water and sea water corresponds to a attention, with applications developed in var - reduced membrane fouling; an important aspect pressure of 26 bars. This pressure is equivalent ious fields, such as desalination, wastewater of these alternative membranes would be high to a column of water (hydraulic head) of 885 treatment, and pharmaceutical and juice con - mechanical strength to sustain hydraulic pres - feet (270 meters) high[8]; however, the opti - centration, and even power generation and sure when used for PRO. mal working pressure is only half of this, rang - potable-water reuse in space. Recent studies Different module configurations for these ing from 11 to 15 bar[9]. This method of of applications of FO in the field of wastewater membrane technologies include plate-and- generating power was invented in 1973 by Pro - treatment and water purification include frame, spiral-wound, tubular, and bag configu - fessor Sidney Loeb at the Ben-Gurion Univer - treatment of landfill leachate, concentration of ration[2]. Plate-and-frame modules are the sity of the Negev, Beersheba, Israel[10]. This dilute industrial wastewater, potable reuse of simplest devices for packing flat sheet mem - concept was not realized until the world's first wastewater in advanced life support systems, branes and can be constructed in different sizes osmotic plant, with capacity of 4 kW, was and concentration of liquids from anaerobic and shapes, ranging from lab-scale to full-scale opened by Statkraft in November 2009, in sludge digestion at a domestic wastewater treatment facility. Desalination and Water Purification To date, the only viable commercial drinking water application for FO has been developed by Hydration Technology Innova - tions, which has developed a small water pu - rification bag (hydration bag), used mostly by the military[14]. The bag is made of a semi- permeable FO membrane and contains a solid glucose draw solution. When immersed in contaminated water from puddles and ponds, purified water diffuses into the bag, slowly di - luting the solid draw solution to produce a potable drink. With sufficient contact time, such water will permeate the membrane bag into the draw solution, leaving the undesirable feed constituents behind. The diluted draw so - lution may then be ingested directly. Typically, the draw solutes are sugars, such as glucose or Figure 2.