|Fig. 1: Desalination by reverse osmosis. (Source: Wikimedia Commons)|
Fresh water supplies are diminishing due to the effects of climate change, desertification, and overpopulation. It is predicted that in 2025, more than 3.5 billion people will live in areas where there will be severe water shortages.  The obvious answer is two-fold: minimize water usage and recycle wastewater. However, with states like California only recycling 9% of waste water while depleting groundwater stores for crop irrigation, a water shortage looms overhead. In order to address this need, it is crucial to find an efficient way to remove salt from seawater.
One of the main problems with desalinating seawater is the enormous amount of energy required. To generate 100 gallons of purified water, 1kWh of electricity is needed.  One major concern with desalination at the moment is its potential for negative impacts on the environment. A principal issue is that desalination plants primarily use thermoelectric energy. This is the energy source for Salt Water Reverse Osmosis (SWRO) and its use emits greenhouse gases. An example of a SWRO reactor is shown in Fig. 1. Presently, these plants consume somewhere between three and four kilowatt hour per meter cubed of electricity which translates to emission of 1.4 to 1.8 carbon monoxide per cubic meter of water generated.  Therefore, it will be critical to mitigate the carbon footprint of SWRO desalination plants by optimizing their energy usage by using waste heat from nuclear power plants, or through renewable energy sources.
Desalination is a very energy intensive process. Nuclear desalination offers a dual purpose for plant that produces both electricity and purified water. Currently 10% of power reactors have dedicated desalination purposes.  Waste heat is a byproduct of energy production that cannot be used to make electricity or as an energy source for evaporative desalination.  However, it is hypothesized that this waste heat can be used to develop systems that can supplement SWRO. The technology that couples waste heat to desalination uses more total energy than traditional means, but the electricity usage is decreased. Although this seems ecologically unfavorable, it is actually positive because it saves the waste heat from being discharged into the environment and instead leverages it for desalination. 
Another technology that can use waste heat to desalinate seawater water is forward osmosis by using a solution with lower chemical potential than seawater to draw a water across a membrane for filtration. Forward osmosis will require different membranes for filtration than reverse osmosis, yet these requirements are also what might allow scientists to leverage waste heat as an energy source for membrane distillation. The main reason for this is the low vapor pressure of water can be the membrane driver. 
In nuclear power plants, fuel cannot be completely "burnt" in the primary reactor. Thus, it exits the reactor incredibly hot, with a lot of energy potential remaining. However, there is no secondary cycle to recapture this available energy. This once-through cycle harnesses 5% of the energy; whereas Plutonium recycling only adds another 1%. A recycling process called polymetallurgical reprocessing has the theoretical capability of harnessing 99% of energy; yet public fear and reduced funding has halted this to idea to the prototype stage. 
But, another source of waste heat is the beta and gamma decay of fuel rods years after they have been removed from the reactor. This heat could therefore be harness to be a driver for desalination processes.
A recent development that could promote rapid implantation of nuclear desalination is the development of a floating reactor. This suggestion could produce energy with little environmental pollution by siting them offshore high density populations to generate electricity. This clean electricity could then be coupled to desalination processes.
Another notable development is Low-Temperature Evaporation (LTE) desalination technology. This process utilizes low-quality hot water waste-heat or low pressure steam from a nuclear plant to purify seawater. Experts have validated the feasibility, safety, and cost-benefit of LTE technology and feel it has the potential to be a large-scale solution. Conversely, solar, wind, and wave-power, at present, incapable of supplying cost-effective energy options to support desalination. 
© Alison Jahansouz. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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