The Future of Seawater Desalination: Energy, Technology, and the Environment




History
In recent years, numerous large-scale seawater desalination plants have been built in water-stressed countries to augment available water resources, and construction of new desalination plants is expected to increase in the near future. Despite major advancements in desalination technologies, seawater desalination is still more energy intensive compared to conventional technologies for the treatment of fresh water. There are also concerns about the potential environmental impacts of large-scale seawater desalination plants. Here, we review the possible reductions in energy demand by state-of-the-art seawater desalination technologies, the potential role of advanced materials and innovative technologies in improving performance, and the sustainability of desalination as a technological solution to global water shortages.

Water scarcity is one of the most serious global challenges of our time. Presently, over one-third of the world’s population lives in water-stressed countries and by 2025, this figure is predicted to rise to nearly two-thirds. The challenge of providing ample and safe drinking water is further complicated by population growth, industrialization, contamination of available freshwater resources, and climate change. At the same time, greater recognition of the broad societal and ecological benefits that stem from adequate water resources—economic vitality, public health, national security, and ecosystem health—is motivating the search for technological solutions to water shortages.

Several measures to alleviate the stresses on water supply should be implemented, including water conservation, repair of infrastructure, and improved catchment and distribution systems. However, while these measures are important, they can only improve the use of existing water resources, not increase them. The only methods to increase water supply beyond what is available from the hydrological cycle are desalination and water reuse. Of these, seawater desalination offers a seemingly unlimited, steady supply of high-quality water, without impairing natural freshwater ecosystems. Desalination of brackish groundwaters is also an option to augment water supply for inland regions; however, the management of brines from inland desalination plants is a major challenge because these plants are placed far from the coast.

There has been rapid growth in the installation of seawater desalination facilities in the past decade as a means to augment water supply in water-stressed countries. Notable examples are the large-scale seawater reverse osmosis (SWRO) desalination plants recently constructed in Spain and Israel. In 2016, the global water production by desalination is projected to exceed 38 billion m3 per year, twice the rate of global water production by desalination in 2008. Early large-scale desalination plants, mostly in the arid Gulf countries, were based on thermal desalination, where the seawater is heated and the evaporated water is condensed to produce fresh water. Such plants, still in operation in the Gulf countries, consume substantial amounts of thermal and electric energy, which result in a large emission of greenhouse gases. Excluding those in the Gulf countries, the vast majority of desalination plants constructed in the past two decades, as well as future planned facilities, are based on reverse osmosis technology, where seawater is pressurized against a semipermeable membrane that lets water pass through but retains salt. Reverse osmosis technology has improved considerably in the past two decades, and current desalination plants can desalinate seawater with much less energy than thermal desalination. At present, reverse osmosis is the most energy-efficient technology for seawater desalination and is the benchmark for comparison for any new desalination technology.


Outlook
The energy demand for seawater desalination by state-of-the-art reverse osmosis is within a factor of 2 of the theoretical minimum energy for desalination, and is only 25% higher than the practical minimum energy for desalination for an ideal reverse osmosis stage. Yet, the overall energy consumption of new SWRO plants is three to four times higher than the theoretical minimum energy due to the need for extensive pretreatment and posttreatment steps. Because thermodynamics set the limit on the energy demand for the desalination step, we argue that future research to improve the energy efficiency of desalination should focus on the pretreatment and posttreatment stages of the SWRO plant.

Eliminating the pretreatment stage or reducing the pretreatment demands would substantially reduce the energy consumption, capital cost, and environmental impact of desalination plants, but this requires the development of foulingresistant membranes with tailored surface properties, as well as membrane modules with improved hydrodynamic mixing. Accomplishing this goal is a daunting task because it requires the development of surface chemistries that resist the adhesion of a wide range of foulants while maintaining the high membrane permeability and selectivity necessary for seawater desalination. To aid in the development of such high-performance, fouling-resistant desalination membranes, it is imperative to develop detailed molecular models that establish structure-property relationships between membrane surface structure and chemistry, and membrane performance. Additionally, these models will assist in the development of oxidant-resistant membranes, which can also reduce the extent of pretreatment. Molecular simulation tools are used routinely in a variety of fields, including the development of drugs, catalysts, and chemicals, but their use for the development of water purification membranes is lagging considerably. Alternatively, developing new, energy-efficient desalination technologies that are inherently less susceptible to fouling compared to high-pressure, membrane-based desalination methods could also reduce or eliminate pretreatment.

Advances in membrane technology can also reduce the need for posttreatment in SWRO plants, thereby improving energy efficiency and reducing capital cost. Reducing boron and chloride levels in desalinated water for agricultural use to levels that crops can tolerate necessitates posttreatment. However, developing thin-film composite membranes with higher selectivity, particularly for boron, will be difficult. This is a direct consequence of the separation mechanism of thin-film composite membranes, where increasing selectivity to allow higher removal of boron and chlorides will substantially reduce the membrane permeability, which will increase energy consumption. Developing reverse osmosis membranes with higher selectivity without sacrificing water permeability will necessitate amajor paradigm shift, as it will require membranes that do not follow the solution-diffusion mechanism for desalination. Molecular simulations can aid in determining how membrane chemistry and structure can be tuned to produce high permeability and selectivity.

In the coming decades, surging population growth, urban development, and industrialization will increase worldwide demand for fresh water, requiring new sources of water. Although several options currently exist to augment freshwater sources—including the treatment of low-quality local water sources, water recycling and reuse, water conservation, regional water transfers that do not adversely impact the environment, and the implementation of smart land-use planning—these options alone will not be enough to meet this need. Seawater desalination offers the potential for an abundant and steady source of fresh water purified from the vast oceans, and although it must be considered after all other options have been implemented, it should be viewed as a crucial component in the portfolio of water supply options. For water-scarce countries that already implement all other measures for freshwater generation, desalination may serve as the only viable means to provide the water supply necessary to sustain agriculture, support population, and promote economic development.

Excerpts from the article "The Future of Seawater Desalination: Energy, Technology,and the Environment"
by Menachem Elimelech and William A. Phillip

 
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