Water Management Using Membranes: Forward Osmosis and Pressure Retarded Osmosis as Technologies of Future
Arijit Chakraborty
Department of Chemical Engineering
BITS Pilani Goa

Ridhish Kumar
Department of Chemical Engineering
BITS Pilani Goa

Anirban Roy
Assistant Professor
Department of Chemical Engineering
BITS Pilani Goa

Nobel Laureate Richard Smalley listed the Top 10 global challenges which he predicted will haunt the human race in the coming decades. Energy and water topped the list with ranks 1 and 2, respectively. All the other cited challenges, e.g., food, environment, poverty, terrorism, etc either can directly or indirectly be related to either energy or water. The recent crisis in Syria is an example of water crisis (drought) affecting an already unstable region. Thus, it is imperative to understand the effect of water crisis and its impact on human civilization. In this regard, the present article focuses on two technologies - forward osmosis and pressure retarded osmosis. Whereas FO is being vouched as low energy intensive process for water treatment (industrial or drinking water), PRO is being looked into a green and sustainable technology for power generation from water mixing .

Forward osmosis is one of the most promising technologies for seawater desalination with less energy consumption. In a scenario of rising population and limited energy resources forward osmosis is being "tipped" as the technology which can help diminish the offset between the supply and demand of drinking water requirement. Forward osmosis (FO) is essentially an osmotic pressure driven process in which a semipermeable membrane is used which separates dissolved solutes and pure water (Fig. 1). In principle, a "draw" solution having higher concentration relative to feed solution is selected which generates a osmotic pressure gradient across membrane. The pure water permeates through the membrane while retaining the dissolved solutes osmotic pressure gradient vanishes. This process is in contrast to reverse osmosis (RO) technology which uses hydraulic pressure as the driving force (working against the osmotic pressure) for separation of solute and fresh water. Thus, RO energy requirement is significantly higher than FO process. Since FO is driven by osmotic pressure difference, thereby it is less susceptible to membrane fouling compared to traditional hydraulic pressure driven membranes like RO.

FO is enjoying great popularity and has garnered lot of attention in food processing industry, power generation sector and waste water treatment plants. The significant player which influences the efficiency in FO process is membrane selection and draw solution. There have been a lot of recent advancements in improving the flux across membrane including development of FO-ultrafiltration system and development of FO hybrid membranes using hydrophilic nano-sized particle with polyacrylic acid as draw solute. Generally FO membranes can be categorized into three classification on the basis of synthesis, e.g., cellulosic, thin-film composite (TFC) and chemically reformed membranes. There are currently two types of commercially available TFC membranes in market which is extensively used in FO study and research. A recent nanofiber TFC-FO membrane has been developed by Electrospinning and interfacial polymerization. This has increased the FO performance due to high porosity and low tortuosity. The support layers of commercially available TFC membranes can be modified by Polydopamine (PDA) for engineered osmosis application.

Figure 1: FO process

Enormous research and publications on FO for desalination have been carried out in past 20 years (general schematic in Fig. 2). But development of economically viable membrane from experimental studies is crucial to attain sustainable FO applications. For optimization of desalination efficiency the FO membranes should be chemically and mechanically stable with minimal internal concentration polarization and negligible reverse solute flux. The draw solutes for future desalination application should be easily regenerated with minimum energy requirement.

Figure 2: FO assisted desalination

Thus looking ahead, there is huge potential in FO technology in coming future .The overall energy requirement of FO can further be minimized by integrating both FO and RO for water purification thereby designing a hybrid system.

Harvesting Salinity Gradient For Renewable Energy Generation

Since time immemorial, the debate of renewable energy methods has been troubling academics and industry likewise. There have been the clichéd methods such as solar, wind, geothermal and such, but one that has not been given its due share of academic investigation is that of Pressure Retarded Osmosis (PRO). It is an oft overlooked method that caught the academia off guard when Statkraft, Europe's largest supplier of renewable energy and Norway's leading power company shut down their PRO operation in 2012, reaching to the conclusion that it wasn't economically viable to operate the plant.

PRO was first developed in the early 70s as a clean way to utilise osmotic pressure for the generation of electrical power (Fig. 3). It involves using a very large semipermeable membrane to allow two streams, one being fresh river water, to flow into a more concentrated and pressurized solution, such as sea water. Salt molecules pull the fresh water through the one-way membrane as a result of osmotic pressure gradient that developed across the membrane, and cause an increase in pressure, which is harvested by a turbine to generate power. It has been estimated that utilization of osmotic potentials between rivers and oceans across the globe represents an energy potential of 1600-2000 TWh(corresponding to approximately 1% of the World energy consumption). This process has no carbon footprint, and the full -scale process plant will have only brackish water as the by-product.
It is well known, that energy and water are two of the top concerns in the near future. The goal needs to be to manage this resource in the most efficient and sustainable way possible. Obtaining usable work from a renewable and clean energy generation process is a step in this direction - that hits both birds with one stone! For an energy deficient country like India, looking towards novel and alternate options and tapping such sources is one of our best bets towards solving the energy crisis. The Indian peninsula is strategically located in the Asian subcontinent with Himalayas to the North and Oceans surrounding the other three directions with annual monsoons taking care of water supply to the perennial rivers. The total river water discharge into the Bay of Bengal and Arabian Sea is 628 km3/year and 274 km3/year, respectively. Thus, huge volumes of fresh water meet saline water and this mixing of two streams of dissimilar salinity gives rise to colossal mixing energies which can be harvested for usable work for various purposes like energy generation using the novel method of Pressure Retarded Osmosis. Harvesting energy from such an option is certainly new in the Indian subcontinent. However, it has been significantly researched upon and eventually implemented, if not successfully

Figure 3: Pressure Retarded Osmosis Power Generation

Theoretical studies confirm that higher the salinity difference among the two streams better is the rate of energy production. The concerns however, for PRO have been raised when in 2012, Statkraft concluded that one of the major challenges to having an economically viable full-scale PRO plant was the unavailability of the advancements in the domain of membrane technology to enable manufacturing of high performing and inexpensive bulk membranes. Another concern that has caught the attention of researchers and environmentalists alike, is that concerning the future largescale operations of PRO and their effect on salinity levels or how pre-treatment processes might impact local marine life.

Interest in PRO has been growing internationally, such that researchers at NASA have been studying use of such process that can purify water and generate energy at the same time. With the need for renewable and clean energy alternatives greater than ever before, this could be a topic that would get picked in the near future with tremendous application that can benefit all.

Keeping in mind India's economic growth and recent initiatives like "Make in India", it is important to start concentrating on striking a balance between conserving and harvesting an important resource like water. It is of utmost importance that while recycle and reuse of water should be encouraged, however compensating the offset between supply and demand of water should be addressed with potentially sustainable technologies like Forward Osmosis. Also research into harvesting energy from mixing energies of sea-water and river water should also be initiated as India is geographically well positioned to benefit from such technologies.