Environ. Sci. Technol. 2008, 42, 8625–8629
mortality, and a necessarily associated high per capita level of income and energy use (5).
Global Challenges in Energy and Water Supply: The Promise of Engineered Osmosis
The terrible race that is now occurring, between exponential population growth and the rapidly increasing resource consumption necessary to approach the threshold of a possible demographic transition, is one with an outcome that is very much uncertain. The great, perhaps greatest challenge of this moment in human history is the need for everyonesparticularly those of the developed nationssto do whatever is necessary to help the rapidly growing populations of the developing regions reach the level of prosperity and security that will enable them to make this transition. To not do so will lead to a continued population increase that will lead to either an environmental or Malthusian catastrophe, or both. In other words, the alternative is unacceptable.
ROBERT L. MCGINNIS MENACHEM ELIMELECH* Yale University
JULIE FARRAR
Engineered processes that cleverly exploit osmosis may provide just the answer to the global need for affordable clean water and inexpensive sustainable energy.
Water scarcity and the environmental impacts of energy use are challenges of great and increasing importance to the future of human civilization. A rapidly expanding global population and an accelerating increase in the standard of living for a growing middle class have put relentless pressure on water and energy resources (1-4). We live at a critical time in human history, when technological advancement, economic globalization, and the rapid spread of consumer capitalism have led to an unprecedented increase in the world’s population and per capita resource consumption. This increase in resource consumption, however, has not yet led to the hoped-for “demographic transition” that will enable a sustainable global population level. This transition is characterized by the change from a rapidly growing population to one which is stable or shrinking, enabled by a high standard of living, low expectation of premature 10.1021/es800812m
2008 American Chemical Society
Published on Web 12/01/2008
This effort to increase the global per capita standard of living is tied inextricably to an increase in per capita water and energy use (3, 6, 7). These important metrics of life quality measure lowest in the very regions undergoing the most rapid population growth (8). If we attempt, however, to meet this enormous need for water and energy resources through fossil fuel consumption and traditional natural resource management, we are not likely to succeed. The negative environmental impacts of climate change, accelerated by the combustion of fossil fuels, have already set the world’s climate on a precipitous course (9). Increasing this type of energy use could be equally as catastrophic as doing nothing to increase sustainable energy availability to developing regions. Equally untenable would be an attempt only to extend existing freshwater resources to meet these much higher needs. Population levels have already stressed these supplies, and climate change is shifting their allocation, making dry areas drier and causing deluge in areas in which water is already abundant (9). Increased water conservation, repair of infrastructure, and smart development of improved catchment and distribution systems will help alleviate these stresses, but while it is crucial that these measures be undertaken, they can only improve use, not increase supply. Desalination and water reuse are the only avenues to increase water availability beyond that which is available from the hydrogeological cycle (10). Given the extent, however, to which these technologies require substantial energy inputs, they will serve only to worsen a vicious cycle of climate change, water scarcity, and greenhouse gas emissions from energyintensive water purification processes. In order to meet these challenges, substantial efforts in investment, regulation, and innovation will have to be made. Technologies, market structures, and economic development methods previously deemed unconventional must now be considered, if they hold potential to meet the needs of the larger social, economic, and natural system as a whole. The purpose of this article is to introduce a largely unknown set of technologies that may contribute to this effort. These technologies, described collectively as “osmotically driven membrane processes” or alternately, the tools of “engineered VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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osmosis”, show great promise in contributing to the development of sustainable water and energy resources, potentially without requiring substantial subsidies or regulatory efforts to promote their use. Engineered osmosis relies on the design of membrane-based water separation and power generation systems, which exploit the natural phenomenon of osmosis rather than treating it as a limiting constraint. Current commercial membrane separation systems rely on hydraulic pressure to drive flux of water through a semipermeable membrane, and in such systems osmotic pressure is considered an inhibiting resistive force. In engineered osmosis, natural osmotic water flux is the primary driver for membrane transport, and hydraulic pressure, when used, is an intentional resistive force used to do mechanical work. Systems of this type may be designed to produce freshwater from nonpotable sources which may be highly saline and/or have a high membrane fouling potential, to produce electrical power from naturally occurring salinity gradients, or to generate electricity from low-temperature heat sources such as reject heat from thermal processes and conventional power plants.
Water and energy are intrinsically linked Water and energy in modern society are intrinsically linked. Both are required to enable a high quality of human lifesnecessary to maintain sustainable population levels without the repressive pressure on populations caused by disease, famine, and war (11). Water allows for adequate hydration, sanitation, and hygiene; energy, for work and achievement beyond the capability of human and animal muscular force for warmth and light; and the time and leisure to learn and educate. Together, water and energy, in the form of irrigation and fertilization, allow for the abundant production of food. The green revolution of the last century, which greatly increased global agricultural output, is substantially dependent on the increased use of both of these resources (12). With sufficient water and arable land, energy may be harnessed from the sun in the form of biofuels. With sufficiently abundant energy, water may also be made abundantspumped from deep aquifers, treated from municipal discharges for potable reuse, or desalinated from brackish and seawater supplies. Today, however, energy is not yet sufficiently abundant in any form that may be used sustainably to increase water supplies. Reuse of water makes great technical sense to those who are familiar with our many achievements in water filtration and purification, and this approach must be explored and applied to its greatest practical extent. However, reuse is not favored by most potable-water users, and although reuse may reduce the demand for additional water supply, it cannot augment the existing supply.
Sustainable water supply As climate change causes dry areas to become drier and wet areas wetter, it is easy to imagine building aqueducts to counter the imbalance. This approach, however, would be more energy-intensive and expensive than that of seawater desalination in the arid regions, because of the often great distances that would be involved. Indeed, >60% of the world’s population lives within several hundred kilometers of a coast (13), and there is no question that the mass balance of water flows would work out nicely. There is an abundant supply. Desalination as it is now practiced, however, cannot sustainably augment water supplies to meet current or future needs. Reverse osmosis (RO), a tremendously robust, effective, and increasingly efficient desalination processsnow within only a factor of 2 to 3 of the minimum theoretical energy of separation of water from the ocean (14)scannot produce water sustainably if the energy required by the 8626
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process is provided from fossil fuels. Nor is it sufficient to build a wind or solar power plant to provide this energy and say it has no climate impact, when limited renewable resources might otherwise be used to reduce existing fossil fuel consumption. The issue is not whether the energy consumption of desalination is modest compared with other energy demands, as indeed in a great many cases it is (14), but rather that accepting the transformation of nonrenewable, carbon-emitting fuels into water will serve to worsen the problem, not correct it. More emissions would only drive further water scarcity, demanding further energy consumption, and so on, in an unsustainable downward spiral. There have been attempts to evade this outcome, in the use of alternative energy inputs for desalination by thermal desalting methods such as multistage flash and multieffect distillation (MSF and MED). These processes may take rejected heat from other processesse.g., industrial, powerproducing, and geothermalsand make freshwater from the ocean. In principle, this is an excellent approach, but the temperature of heat required by these processes for economic operation is too high (∼70 °C for MED and 110 °C for MSF) (15). The delivery of heat from the power plant to the desalination process reduces the power output of the plant, requiring additional fuel consumption to maintain output, and so prime energy in the form of fossil fuels must be used again (16). Added to this high-quality heat requirement is a considerable need for electrical energy as well. MSF plants may require as much electrical energy as RO plants do, and MED plants typically require two-thirds or more as much electricity as an RO plant (17). The ideal solution would be for a process to use very low quality heat as its energy input, at or below the temperature of its rejection from a power plant or similar thermal process (and below a temperature at which it would be useful for any other purpose), and use little or no electrical energy in addition to this heat. A process of this type would be capable of producing freshwater sustainably from saline waters, so long as its heat sources were themselves sustainable. One of the methods of engineered osmosis, forward osmosis (FO), is designed to fit this set of criteria. The FO desalination process may use very low temperature heat, as low as 40 °C, given an ambient temperature of 20 °C (such that a temperature difference ∼20 °C is available between the heat input and output streams), and approximately an order of magnitude less electrical energy than RO (18). The FO process is more efficient than conventional evaporative desalination processes, as it is the solute which is removed from solution by a change of phase, rather than the water itself. Furthermore, the FO process does not require the multiple stages, large heat transfer areas, and large pumping volumes required by MSF and MED. The basis of FO, as well as of the majority of technically and economically feasible engineered osmosis processes, is in the ability to create a solution of high osmotic pressure, which contains solutes that are well-rejected by semipermeable membranes and may be at any time readily, efficiently, and completely removed. This high-osmotic-pressure solution is referred to as a “draw solution”. Many draw solutions have been considered for use, but nearly all have been discarded, as they were unable to meet these criteria (19, 20). A successful draw solution has been identified, however, in the form of a concentrated solution of ammonium (NH4+) formed by the dissolution of ammonia (NH3) and carbon dioxide (CO2) in water (21). These salts are highly soluble, create high osmotic pressures, diffuse relatively rapidly in solution, are well-rejected by semipermeable membranes, and may be removed from solution by the simple addition of low-quality heat. The salts of this solution are thermolytic: when heat is supplied, the salts decompose into ammonia and carbon dioxide gases for simple stripping from solution.
AARON QUALLS
FIGURE 1. The NH3/CO2 forward osmosis desalination process. Adapted from Refs. 20 and 21. These gases, when introduced to water again at a lower temperature, readily reconstitute the desired draw solutes. This process is inherently more efficient than a process that removes water from a solution by vaporizing it (18), as the enthalpy of vaporization of water far exceeds the enthalpy of vaporizing these dissolved salts. In the NH3/CO2 FO process (Figure 1), this draw solution is used to induce natural osmotic flux of water across a semipermeable membrane from a relatively dilute saline feedwater (21). In the case of seawater desalination, the seawater is dilute relative to the draw solution, and flux can be quite high, easily matching the rates of flow of hydraulically driven membrane separation processes like RO. As water flows across the membrane from the seawater, both the seawater salts and the draw solution solutes are rejected by the membrane, such that the seawater feed becomes concentrated to brine, and the concentrated draw solution becomes dilute. A portion of the dilute draw solution is then directed to a simple distillation column, also known as a reboiled stripper, where low-temperature heat is used to strip the NH3 and CO2 gases from the draw solution for reuse, producing a freshwater product containing 250 °C, and >100 °C for economical production) and relatively shallow wells (32). Recent research in the use of deep wells to obtain highquality heat from bedrock and water injection, in a much more widespread geographic region, is promising, but current thermal energy conversion technologies require, in many cases, wells as deep as 10 km below the surface (33). It would be highly useful to have a thermal energy conversion process that was capable of economically converting lower-temperature heat sources (50-150 °C) into power, as this would enable the use of much shallower wells. In most places in the U.S., for example, a 3.5 km well is sufficient to produce heat at 50 °C year-round, and in the Southwest, a 6.5 km deep well may readily produce heat at 150-200 °C (33). This capability may potentially be enabled by the use of another of the tools of engineered osmosis, the osmotic heat engine (OHE). This system is based on the principles of pressure-retarded osmosis (PRO), in which a solution of high osmotic pressure is placed under a (relatively lower) hydraulic pressure (34). This solution is exposed to a robust semipermeable membrane, on the other side of which is a dilute feedwater. The natural tendency of such a system is for the VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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tively inexpensive distillation column construction costs; and the use of a compact liquid turbine, smaller and less expensive than the large turbines used in low-temperature Rankine power plants, which at very low-temperature can have diameters as great as 10 m across (36). This capability for low-temperature heat conversion, allowing the utilization of deep-well, otherwise marginal geothermal resources, and the possibility of this effort producing economically competitive sustainable power make this engineered osmotic system an important subject for continued development and research.
Prospects and challenges
FIGURE 2. The NH3/CO2 osmotic heat engine. Adapted from Ref. 35. unpressurized, low-osmotic-pressure solution to transfer water through the membrane to the high-osmotic-pressure and high-hydraulic-pressure solution. As this pressurized solution expands, the additional volume may be depressurized to produce electrical power. In systems that use natural streams of differing salinity, such as seawater and river water, a system of this type is referred to as open-cycle PRO (34). In closed-cycle systems, however, both the dilute feedwater and the concentrated draw solution are recycled in the system by the use of input heat. Investigations into possible methods of designing closed-cycle PRO systems have been similar in many ways to investigations into methods of FO. Many potential draw solutions have been considered, as has the vaporization of water to reconstitute the dilute feed, in a method paralleling the separation techniques of thermal desalination systems. The most effective approach to the design of a system of this type, however, appears to be in the use of a more concentrated version of the NH3/CO2 draw solution used in FO. This is known as the NH3/CO2 OHE (35). In the NH3/CO2 OHE (Figure 2), the dilute solution is a nearly deionized working fluid, containing
