Ships and submarines cannot carry enough bottled or tanked fresh water for long missions, so they often make it from seawater. This process is called desalination, and it removes dissolved salts and other impurities to produce water for drinking, cooking, washing, cooling, and machinery. Onboard desalination is especially important for submarines because they may stay underwater for weeks or months.
It also reduces the need for ships to return to port just to refill water tanks.
Two common onboard methods are evaporation and reverse osmosis. Evaporators heat seawater so that water vapor separates from salt, then condense the vapor back into liquid fresh water. Reverse osmosis uses high pressure to push seawater through a semipermeable membrane that lets water molecules pass but blocks most salt ions.
Both systems need pumps, filters, storage tanks, sensors, and careful maintenance to keep the fresh water safe and reliable.
Understanding Ships and Submarines: Making Fresh Water at Sea
Seawater is difficult to process because it contains more than visible salt. It may carry sand, rust particles, tiny living organisms, oil traces, and chemicals from harbors. Before water reaches the main desalination unit, it passes through intake screens and filters.
These steps protect pumps and membranes from damage. Some systems add chemicals to stop scale forming inside pipes.
Scale is a hard mineral layer, similar to the deposit that can build up in a kettle. Warm water makes scale more likely, so engineers must control temperature and clean equipment regularly.
Reverse osmosis depends on pressure because dissolved salt naturally draws water toward the saltier side of a membrane. This effect is called osmosis. To make fresh water move in the intended direction, a pump must apply enough pressure to overcome that natural effect.
Producing pressure takes electrical energy. Dirty membranes need even more pressure because less water can pass through them. Crews watch pressure readings, flow rate, and salt content to spot problems early.
A falling flow rate may mean a clogged filter or membrane. A rising salt content may mean that a membrane has developed damage.
Evaporation systems need heat rather than extremely high pump pressure. On many ships, they use waste heat from engines or power equipment. This improves efficiency because the heat would otherwise be released into the sea or air.
Lowering the pressure inside an evaporator can make water boil at a lower temperature. That reduces the energy needed for vapor production. The vapor must be kept separate from droplets of salty water.
If droplets enter the fresh water line, they carry salt with them. Condensers then cool the vapor into liquid water, often using colder seawater flowing through nearby pipes.
The leftover liquid is called brine. It has a higher salt concentration than the incoming seawater. Ships discharge it carefully, usually while moving, so it mixes quickly with a large volume of ocean water.
Desalination still requires monitoring because a faulty discharge system or chemical treatment process can harm nearby marine life, especially in confined waters. Fresh water from the plant is checked before use. Sensors measure conductivity, which increases when more dissolved ions are present.
Water may then be disinfected and stored in clean tanks. Students should remember that making water is only part of the task. Keeping pipes, tanks, filters, sensors, and power supplies reliable is what makes an onboard supply safe during a long journey.
Key Facts
- Desalination removes dissolved salts from seawater to produce fresh water.
- Typical seawater salinity is about 35 g of salt per 1 kg of seawater, or 35,000 ppm.
- Reverse osmosis works when applied pressure is greater than osmotic pressure: P_applied > π.
- Osmotic pressure can be estimated by π = iMRT for dilute solutions.
- Evaporation desalination separates water from salt because water vapor leaves while most salt stays in the brine.
- Water production rate can be estimated by rate = volume produced ÷ time.
Vocabulary
- Desalination
- The process of removing dissolved salts and impurities from seawater or brackish water to make fresh water.
- Reverse osmosis
- A desalination method that uses pressure to force water through a membrane that blocks most dissolved salts.
- Evaporator
- A device that heats seawater so water becomes vapor and leaves salts behind before being condensed.
- Brine
- The extra salty leftover water produced after some fresh water has been removed from seawater.
- Semipermeable membrane
- A thin barrier that allows some particles, such as water molecules, to pass while blocking many dissolved ions and larger particles.
Common Mistakes to Avoid
- Thinking boiling seawater automatically gives drinkable water, but the vapor must be condensed and the system must prevent salt spray or contamination from entering the fresh water.
- Confusing filtration with desalination, because ordinary filters can remove sand and particles but cannot remove most dissolved salt ions.
- Ignoring brine disposal, but desalination always leaves a concentrated salty waste stream that must be managed safely and discharged properly.
- Assuming reverse osmosis needs only a membrane, but it also requires high-pressure pumps, pretreatment filters, flow control, and regular membrane cleaning.
Practice Questions
- 1 A ship produces 12,000 L of fresh water in 8 hours. What is its average water production rate in L/h?
- 2 A reverse osmosis unit takes in 5,000 L of seawater and recovers 40 percent as fresh water. How many liters of fresh water and how many liters of brine are produced?
- 3 A submarine has both an evaporator and a reverse osmosis unit. Explain one advantage and one limitation of each method for making fresh water while at sea.