Submarines are marvels of modern engineering, designed to operate unseen beneath the ocean surface for weeks or months at a time.
With no access to lakes, rivers, or rainfall, submarines must use the surrounding seawater into make safe, potable water onboard. This article explores how this is done using various technologies, associated challenges, and how waste water is managed.
How Much Water Do Submarines Need?
A submarine with a crew of around 130 sailors can require anywhere from 10,000 to 40,000 gallons of fresh water per day. Water is needed not only for drinking, but also for cooking, cleaning, hygiene, equipment cooling, and battery maintenance.
U.S. Navy nuclear submarines are equipped to generate nearly all of their fresh water onboard, ensuring they can remain submerged for extended periods without the need for resupply.
Thermal Distillation: How Heat Turns Seawater into Drinking Water
The most traditional method of water production on submarines is thermal distillation. This technique relies on heating seawater until it evaporates, then condensing the vapor into fresh water.
Most submarine distillers operate at reduced pressures to lower the boiling point of water, increasing efficiency. Under these conditions, seawater typically boils at around 158 to 176 degrees Fahrenheit (70-80 degrees Celsius), rather than the standard 212 degrees Fahrenheit (100 degrees Celsius) at sea level.
The heat source for this process is the submarine’s own nuclear reactor, which generates abundant thermal energy. The design of the distillation unit includes a series of heat exchangers and flash chambers where evaporation and condensation occur repeatedly, improving water yield.
Land-based desalination plants also use thermal distillation, particularly multi-stage flash (MSF) and multi-effect distillation (MED) systems.
For example, the Shoaiba Desalination Plant in Saudi Arabia uses MSF technology to supply millions of gallons of drinking water to the population.
Brine Disposal: What Happens to the Salt Left Behind?
Thermal distillation does not remove the salt from seawater; it separates the water from the salt. The remaining brine is a dense, salty solution that must be carefully managed. On submarines, this brine is not stored, it’s discharged back into the ocean.
Because submarines operate at significant depths and remain mobile, the concentrated brine is diluted quickly by the surrounding seawater. Discharge is usually slow and continuous, minimizing any localized environmental impact. Nonetheless, protocols are in place to ensure that brine is released at depths where rapid mixing occurs.
There has been speculation over whether the brine discharge from submarines could create a detectable thermal or chemical signature, potentially exposing a vessel’s position. Some military research has investigated the tracking of subs based on thermohaline anomalies, which are subtle changes in water temperature and salinity.
However, no publicly acknowledged system currently uses brine detection as a tracking method, likely because the dilution rate and background variability in ocean conditions make this a low-confidence indicator.
Reverse Osmosis in Submarines
Newer submarines, and especially non-nuclear vessels, increasingly rely on reverse osmosis (RO) systems to produce fresh water. RO systems pressurize seawater and push it through semi-permeable membranes that block salt and impurities while allowing pure water molecules to pass.
Onboard RO units are specifically designed for compact environments and high reliability. They typically operate at pressures of 800 to 1000 psi and can include multiple membrane stages to optimize recovery rates.
These systems are favored for their lower energy consumption compared to distillation and are especially useful for diesel-electric submarines without access to nuclear heat.
RO Maintenance and Filter Lifespan
The lifespan of RO filters on submarines depends heavily on feedwater quality and pre-filtration. With adequate pre-treatment (such as sediment filters and activated carbon), RO membranes can last between 2 to 5 years under normal operation. However, periodic chemical cleaning is required to remove biofouling and scale buildup.
Cartridge filters, which protect the membranes from particulates need replacement more frequently. Because filter changeouts during deployment are difficult, submarines carry a full complement of spares and tools for maintenance while submerged.
Wastewater Onboard: What Happens After the Water is Used?
Submarines handle several types of wastewater: greywater (from sinks, showers, and galley drains), blackwater (from toilets), and oily wastewater (from machinery spaces).
Greywater and blackwater are collected in separate tanks and treated differently. Most greywater is discharged overboard after minimal treatment. Blackwater, on the other hand, is held in dedicated storage tanks and processed through marine sanitation devices (MSDs) that kill pathogens and reduce solids before discharge.
Discharge of any waste is carefully managed to avoid detection. Submarines often retain blackwater during sensitive operations and release it only at depth or when traveling at sufficient speed to ensure rapid dilution.
Submarines vs. Land-Based Systems
On land, RO and thermal systems are used extensively in municipal and industrial desalination.
Plants like the Carlsbad Desalination Plant in California use RO to produce over 50 million gallons of drinking water per day. These land systems benefit from abundant space and energy, allowing them to operate at greater scale and with easier maintenance.
Submarine systems, by contrast, are built for resilience in extreme conditions. They must be compact, vibration-resistant, and easy to maintain in confined, pressurized environments. This makes submarine water systems some of the most rugged and reliable in the world.
Smarter Systems and Autonomous Monitoring
Submarine water production systems are expected to become even more intelligent in the years ahead.
Research is currently underway on self-monitoring RO membranes that can detect early signs of fouling or mechanical failure. Predictive maintenance algorithms, informed by AI and machine learning, are being integrated into onboard systems to reduce downtime and improve efficiency.
Hybrid systems that combine thermal and RO processes are also being explored to provide greater redundancy and improve energy efficiency. By allowing submarines to dynamically shift between systems depending on energy availability and water demand, these designs aim to maximize operational flexibility.
The push toward reduced acoustic and thermal signatures is also driving innovation in brine disposal methods. New concepts include super-cooled brine discharges or dispersal via chemical neutralization to further minimize any detectable trail.