Membrane Processes to Address the Global Challenge of Desalination
AMY E. CHILDRESS
University of Nevada, Reno
INCREASING SALINITY: A MAJOR ENVIRONMENTAL CONCERN OF THE 21ST CENTURY
Salinity refers to the presence of soluble salts in soils and water, including surface waters and groundwater. The main chemical species contributing to salinity are sodium, calcium, magnesium, chloride, carbonate, bicarbonate, sulfate, and silica. In the oceans, which contain 97 percent of Earth’s water, salinity ranges from 35,000 mg/L to 41,000 mg/L of salts (usually referred to as total dissolved solids, or TDS). Inland waters have salinities ranging from less than 1,500 mg/L for freshwater to 1,500-20,000 mg/L for brackish water. The salinity in inland regions may be due to the presence of soils formed from marine sediments left behind by retreating seas, the weathering of rock minerals, or the deposition of oceanic salt onto the land by wind or rain. Some inland regions have naturally elevated salinity levels due to environmental features, such as mineral springs, carbonate deposits, salt deposits, and seawater intrusion. Other inland regions have anthropogenically elevated salinity levels. Like many environmental degradation processes, increasing salinity is exacerbated by human activities. Urban runoff, land use practices (clearing, irrigation, industrial farming, overextraction), septic systems, chemicals used in water treatment processes, and road de-icing can substantially increase the extent of the problem. Unlike many other environmental contaminants, salts do not naturally degrade over time. Salts that are deposited
(naturally or anthropogenically) into a body of water accumulate until they are intentionally removed.
Increasing salinity is one of the most important environmental issues of the 21st century. Rising salinity levels have both environmental and economic costs: reduction in agricultural productivity, loss of biodiversity in both terrestrial and aquatic ecosystems, and decline in water quality for drinking, irrigation, industrial reuse, and recreation purposes (Tsiourtis, 2001; Ettouney et al., 2002; USBOR and SNL, 2003).
In terms of drinking water treatment, the need for additional potable water supplies has led to increased numbers of seawater and brackish water desalination facilities being constructed worldwide. To be fit for human consumption drinking water should contain less than 1,500 mg/L of salts. In the United States the Environmental Protection Agency has set a secondary standard for TDS in drinking water at 500 mg/L. In terms of wastewater treatment, TDS discharge standards vary for different facilities but are consistently becoming more stringent for all facilities. This has substantial implications for conventional wastewater treatment facilities that were not designed for salinity removal. Many facilities must retrofit with additional processes specifically for TDS removal. Thus, desalination has developed from a concept related to seawater applications in coastal regions to a concept applying to a range of applications across all regions. Furthermore, as concerns about water scarcity and quality continue to grow, the research and engineering of economic and environmentally friendly desalination systems will continue to grow at a tremendous rate.
DESALINATION: FROM ITS ANCIENT ROOTS TO ITS CURRENT PROMINENCE
The roots of desalination can be traced back to the fourth century BC, when Aristotle and Hippocrates described the process of evaporating saltwater to produce freshwater (Service, 2006). The first actual practice of desalination, in AD 200, is attributed to sailors who distilled seawater by collecting freshwater steam from the boilers on their ships (Schirber, 2007). Modern seawater desalination began in the early 20th century; the first distillation facility was put into operation on the island of Curacao in the Netherlands Antilles in 1928. Modern reverse osmosis (RO) began in the 1950s when Reid and Breton (1959) found that a cellulose acetate film rejected sodium chloride from aqueous solution, and Loeb and Sourirajan (1962) developed a high flux modified cellulose acetate membrane. The first commercial RO facility began operation in 1965 in Coalinga, California.
Today some form of desalination is used in approximately 130 countries; as of 2005 more than 10,000 desalting units larger than 0.3 million gallons per day (MGD) (nominal) had been installed or contracted worldwide (Cooley et al., 2006). The two processes at the core of conventional desalination facilities continue to be distillation and RO. RO generally has lower capital costs and
requires less energy than distillation. A typical seawater RO plant requires 1.5-2.5 kilowatt-hours (kWh) of electricity to produce 1 m3 of water (Service, 2006), and a typical distillation plant requires 10 times that amount (Ettouney et al., 2002; Cooley et al., 2006). On the other hand, distillation typically produces water with much lower salt content than RO systems. Distillation systems typically produce water with less than 25 mg/L, and RO systems typically produce water with less than 500 mg/L (Cooley et al., 2006; Service, 2006).
Although substantial improvements have been made to RO and distillation processes in the past decades, there is a strong need for improved desalination systems, especially systems that will achieve a greater rate of production of treated water at lower energy expenditure and cost. Several systems being investigated include dewvaporation, capacitive deionization, chemical precipitation, membrane distillation (MD), and forward osmosis (FO). This paper focuses on the viability, efficiency, and practical application of MD and FO for brackish water, seawater, and brine desalination at low energy expenditure and with minimal impact on the environment.
MEMBRANE DISTILLATION AND FORWARD OSMOSIS: INNOVATIVE TECHNOLOGIES FOR DESALINATION APPLICATIONS
MD is a temperature-driven separation process involving the transport of mass and heat through a hydrophobic, microporous membrane. The driving force for mass transfer in direct contact MD (DCMD) is the vapor pressure gradient across the membrane. In a configuration referred to as vacuum-enhanced DCMD, warmer feed water is in contact with the feed side of the membrane and a cooler water stream, under vacuum, is in direct contact with the opposite side of the membrane (Figure 1). The temperature difference between the streams and the vacuum applied on the permeate side of the membrane induce the vapor pressure gradient for mass transfer. Compared with conventional distillation methods, MD requires only small temperature differences—temperature differences achievable through the use of low-grade or waste heat sources. Compared with RO, the driving force in MD is not reduced by osmotic pressure (Figure 2) and thus, MD can provide enhanced recovery through brine desalination (Cath et al., 2004). For these reasons MD may have substantial energy and recovery advantages. MD has shown to be an effective process for desalination of brackish water and seawater (Lawson and Lloyd, 1997; El-Bourawi et al., 2006), removal of urea and endocrine-disrupting chemicals (Cartinella et al., 2006), and concentration of brines (Martinez-Diez and Florido-Diez, 2001; Childress and Cath, 2006).
FO is an osmotically driven separation process involving the diffusion of water through a semipermeable membrane. Water diffuses from a stream of low solute concentration to a draw solution (DS) stream of high osmotic pressure (Figure 3). The driving force for mass transport is the difference in osmotic pres-
sure between the DS and the feed solution. The main advantages of using FO are that it operates at low or no hydraulic pressures, it has high rejection of a wide range of contaminants, and it may have a lower membrane-fouling propensity than pressure-driven membrane processes (Cath et al., 2006). Because the only pressure involved in the FO process is due to flow resistance in the membrane module (a few bars), the equipment used is very simple and membrane support is less of a problem (Cath et al., 2006). FO has been shown to be an effective pretreatment process for RO (Figure 4) and has been used to desalinate feed streams ranging from brackish water to brine (McCutcheon et al., 2005; Martinetti et al., 2007.
DIRECT POTABLE REUSE: THE FRONTIER
MD and FO have been investigated together in a hybrid system designed for direct potable reuse in long-term space missions. Long-term human missions in space require a continuous and self-sufficient supply of freshwater for consumption, hygiene, and maintenance. Long-range or long-duration missions, like
lunar missions or a human Mars exploration mission, depend on a water treatment system that recovers potable water from wastewater generated on board a spacecraft or in the planetary habitat. The three main sources of wastewater that can be reclaimed and reused in long-term space missions are hygiene wastewater, urine, and humidity condensate. The system to treat these wastewaters must be reliable, durable, capable of recovering a high percentage of the wastewater, and lightweight. Additionally, the system should operate autonomously with low maintenance, minimum power consumption, and minimal consumables (Wieland, 1994).
A pilot system referred to as the direct osmotic concentration (DOC) is one of several technologies that are being evaluated by the National Aeronautics and Space Administration (NASA) (Beaudry and Herron, 1997; Beaudry et al., 1999) to reclaim wastewater. In the original DOC concept three different membrane pro-
cesses (RO, FO, and osmotic distillation [OD]) were integrated into one system. Due to poor performance of the OD process the system design was revised. In the new design (Figure 5) DCMD is used to treat urine and to pretreat humidity condensate; FO is being used as pretreatment for RO for the hygiene wastewater. Separation of the feed streams in this hybrid system prevents the surfactants from hydrophilizing the hydrophobic MD membrane. This is critical because the MD membrane is required to remove the urea that would pass through an RO membrane. One important aspect of this investigation is the removal of trace contaminants. During long space missions, crewmembers will consume water that is continuously recycled. Thus, it is important to ensure that trace contaminants and particularly endocrine-disrupting chemicals (EDCs) be removed from the treated water. EDCs (e.g., estrone and estradiol) are excreted in urine by both males and females and conventional wastewater treatment is often insufficient for removing them (Belfroid et al., 1999; Johnson et al., 2000; Johnson and Sumpter, 2001).
Preliminary testing of the new DOC concept revealed that DCMD achieves greater than 99 percent estrone and estradiol removal and greater than 99.9 percent urea removal. Furthermore, the FO membrane provides an economical means to pretreat the hygiene wastewater and protects the RO membrane from excessive fouling. These results confirm that MD and FO processes can be combined with RO in a hybrid membrane configuration to treat complex liquid streams that cannot be treated with the individual processes. These results are not only pertinent to NASA’s DOC system but they also have implications for terrestrial wastewater treatment applications.
Terrestrially, much less emphasis is currently being placed on wastewater reclamation than on seawater desalination for direct potable water treatment.
However, wastewater reclamation costs are only a fraction of seawater desalination costs. Furthermore, wastewater (sewage) is the only guaranteed abundant source of water wherever there are people. Thus, for environmental sustainability the major barriers to direct potable reuse must be overcome. Ironically most of these barriers relate to public acceptability rather than technical capability. Further studies on systems such as the DOC will bring the concept of direct potable reuse closer to a reality.
SUSTAINABILITY: A GOAL FOR DEVELOPED AND DEVELOPING COUNTRIES
According to an Association of Environmental Engineering and Science Professors Research Frontiers Conference summary (Dentel and Cannon, 1999), there are five research directions that must be pursued in order to meet the demands of a sustainable environment. One of these directions, “Targeted or Tailored Separation and Destruction Systems,” includes membrane processes. More specifically, the summary refers to the development of membrane applications that employ new advances in membrane chemistry, membrane physical properties, and flow configurations. Furthermore, the Desalination and Water Purification Technology Roadmap has identified membrane technologies as an area of research necessary to develop cost-effective technological tools that can be used to help solve the nation’s water supply challenges. The research areas identified in the roadmap were selected to both speed the evolution of existing (current-generation) desalination and water purification technologies and lay the scientific and technical foundation for the development of advanced next-generation technologies (USBOR and SNL, 2003).
Water reuse and low energy consumption are key objectives to achieving sustainability in water and wastewater treatment. Because energy and water are inextricably bound, energy usage and production must be considered when evaluating water and wastewater treatment processes. Thus, a major objective of all desalination investigations should be to reduce energy costs and to explore opportunities for energy production. Consideration of these objectives has led to the development of desalination systems powered by renewable energy. One of the more well-studied of these systems is solar-powered MD (Hogan et al., 1991; Banat et al., 2002). In a similar type of application the combination of MD with a solar pond has also been considered (Walton et al., 1999). A major goal is to make these systems economically viable for remote regions.
MD and FO offer the potential for substantial energy and resource savings over conventional desalination processes. Use of these technologies may substantially mitigate the environmental impacts associated with current desalination
practices. For developing countries a combination of these technologies with renewable energy may enable potable water production in remote, arid locations. For developed countries a combination of these technologies with existing desalination practices may bring the goal of direct potable reuse closer to a reality and make the goal of zero liquid discharge more cost-efficient.
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