By Kaitlyn Longstaff
Desalination is a key process used in residential, municipal, commercial, and industrial water treatment. The desalination process removes salts and minerals from a specific substance such as soil or saltwater. Saltwater goes through the desalination process to produce water that is suitable for human consumption or irrigation. Desalination processes are also used aboard many seagoing vessels, such as naval ships and submarines.
Distilling Plant Operations
Having fresh water available is necessary for long journeys at sea, such as those taken by cruise ships or freight-laden vessels. The United States Navy fleet comprises all sorts of vessels, but water processing is similar on most ships, with the two most common methods being flash-type evaporators or reverse osmosis (RO) units.
There are several variations of these plants in use, all of which serve vessels based on the anticipated water demands for both the crew and services that support the ship. The capacity for water production from the abundant seawater is usually dependent on the design of the ship, for example whether the ship is boiler powered, the size of the crew it carries, and anticipated operations for the vessel. Distilling plants aboard naval vessels are used to generate fresh water for shipboard applications, such as producing potable water for drinking, food services, and showers. They are also used to produce high-purity feed water for boilers and reactor systems.
The common practice for surface vessels involves a constant influx of seawater that is boiled to produce usable water, with the byproduct—the brine—being discharged back to the sea. Units that achieve this can process up to about 100,000 gallons per day.
The Desalination Process
The flash-type method of distillation draws seawater into a unit with multiple heat exchangers that serve to both preheat the incoming water, as well as cool the fresh water during its production. As the water reaches the first stage of the process, it encounters a vacuum, which allows operation at temperatures well below the normal 212 degrees Fahrenheit required to create steam. The steam generated goes through a mechanical separator and is collected at the top of the unit, while any water that did not “flash” to steam gets additional chances to do so in follow-on stages at increasing levels of vacuum.
Biological deposits on the unit can be inhibited while still maintaining water purity thanks to the process of vacuum formation. The final stage of processing results in brackish water, with a higher concentration of impurities found in seawater discharged back to the ocean. This whole process is constant while the units are operational.
The RO process used for naval vessels is almost identical to that used by civilian watercraft. Multiple stages of semipermeable filters are used at increasing pressures to remove impurities from seawater until it is suitable for use in potable water systems and follow-on filtration for steam-plant applications. The advanced RO unit used aboard navy vessels is a pressurized membrane that pushes saltwater through to filter into fresh water. This technology reduces the amount of energy required to produce fresh water by replacing the pump and motor system, incorporating a device that captures energy from the high-pressure discharge water created by the RO process.
Uses for Treated Water
While personnel water consumption is certainly a priority, over the last 15 years, it has become more apparent that distillation processes on carriers are intrinsically more important for the overall operation of such large vessels. Similar to the way water is treated on boiler-powered sister ships, a steam plant is operated in tandem with the reactor systems, all of which require reserve water inventories of varying purity.
Potable water purity from the distillation process results in clean drinking water, which is treated with chlorination solutions to ensure purity and safety for consumption. The chemical additive process varies from ship to ship, but the Navy Bureau of Medicine and Surgery provides very clear water-processing guidelines to ensure personnel safety and health. Treated water is stored in large tanks designated solely for potable water and is supplied to the ship’s services via pumps operated by highly trained sailors.
Boiler water, as it’s referred to on petroleum-based vessels, is what is used for makeup water for the steam plant. This water begins its journey from the same units potable water is generated from, but instead of ending in the potable water tanks, it undergoes another process to strip it of any remaining impurities prior to its application as part of the steam plant. This process ensures that high-purity steam is used, which prolongs the life of turbines, and that the piping for the system does not incur corrosion that would result from using less-pure water.
Lastly, the reactor-plant water is similar to that of the steam plant’s, but with additional restrictions and filtration processes. For the purpose of this article, the processes to maintain the steam-plant water and reactor-plant water cannot be discussed in depth; however, naval sailors are also trained in commercial applications for deionized water.
Not So Different from Civilian Uses
Desalination aboard naval vessels is used in ways that are very similar to the civilian uses of desalination plants. Keeping vessels stocked with fresh water has a long history that has led to the use of desalination and RO units being added to navy vessels. From early seafarers realizing rainwater could be funneled from their sails into storage, to using barrels laced with alcohol to prevent algae from growing in the collected water, to the first distilling plants operating in the 1700s, the process of providing clean water to sailors has been of utmost importance for hundreds of years.
About the authors
Michael Longstaff is an active-duty machinist’s mate in the United States Navy. He has served as a nuclear-trained sailor on the USS Enterprise, USS Ronald Reagan, and currently on the USS Abraham Lincoln. His career has focused on the systems operated to produce fresh water and makeup water for steamand reactor-plant systems, as well as electric power generation, propulsion turbines, and the catapult systems that are used on aircraft carriers.
Kaitlyn R. Longstaff is associate editor at Water Conditioning & Purification International magazine. She studied English at Southern New Hampshire University and 
Publishing at The George Washington University. She can be reached at [email protected].