Desalination is the process of removing dissolved salts and minerals from seawater or brackish water to produce fresh water suitable for human consumption, agriculture, and industrial use. With over 2. 2 billion people worldwide lacking access to safe drinking water, desalination has become an increasingly important technology, particularly in arid regions such as the Middle East, Australia, and parts of the United States. The two most common methods are reverse osmosis and thermal distillation. Reverse osmosis forces seawater through semi-permeable membranes that trap salt ions, while thermal distillation heats water to create steam, which is then condensed into fresh water.
Modern desalination plants can produce millions of litres of fresh water daily, but they require significant energy input and careful management of the concentrated brine byproduct. Reverse osmosis is the dominant desalination technology today, accounting for about 60% of global capacity. In this process, seawater is first pre-treated to remove large particles, algae, and bacteria. It is then pressurised to between 55 and 85 atmospheres—enough to overcome the natural osmotic pressure that would otherwise push fresh water into the saltier side. The pressurised water is forced through thin-film composite membranes made of polyamide, which have pores only about 0.
1 nanometres in diameter—small enough to block hydrated salt ions but allow water molecules to pass. The membranes are arranged in spiral-wound modules to maximise surface area. A typical plant may contain thousands of these modules, each capable of producing 20 to 40 cubic metres of fresh water per day. Thermal distillation, while older and more energy-intensive, remains widely used in the Middle East where energy is abundant. The most common thermal method is multi-stage flash distillation. Seawater is heated to around 110°C under pressure to prevent boiling.
Modern desalination plants can produce millions of litres of fresh water daily, but they require significant energy input and careful management of the concentrated brine byproduct.
It then flows into a series of chambers at progressively lower pressures, causing it to flash into steam. The steam is condensed on cool pipes and collected as fresh water. Each stage recovers some heat from the condensation process, improving efficiency. Modern plants can have 20 to 30 stages, achieving a gain output ratio of about 8 to 12 kilograms of fresh water per kilogram of steam. However, thermal plants require large amounts of heat energy, often supplied by natural gas or waste heat from power stations. Energy consumption is the main challenge for desalination.
Reverse osmosis plants typically use 3 to 6 kilowatt-hours per cubic metre of fresh water, while thermal plants use 10 to 15 kilowatt-hours per cubic metre. This energy cost translates into higher water prices—often two to three times the cost of treated surface water. To reduce energy use, modern plants incorporate energy recovery devices such as pressure exchangers that capture the energy from the high-pressure brine stream and transfer it to the incoming feed water. Some plants are now powered by renewable energy sources like solar or wind, though intermittency remains a problem.
Research into forward osmosis and membrane distillation aims to further lower energy requirements. Another significant environmental concern is the disposal of brine—the concentrated salt solution left after desalination. For every litre of fresh water produced, reverse osmosis generates about 1. 5 litres of brine, while thermal processes produce even more. Brine has a salinity two to three times that of seawater and may contain chemicals used in pre-treatment, such as anti-scalants and biocides. If discharged directly into the ocean, it can sink to the seafloor, harming benthic organisms and reducing oxygen levels.
To mitigate this, plants often dilute the brine with cooling water or discharge it through diffusers that promote rapid mixing. Some facilities are exploring brine mining to extract valuable minerals like lithium, magnesium, and bromine. Desalination has transformed water supply in many regions. Australia, for example, built several large reverse osmosis plants during the Millennium Drought (1997–2009), including the Sydney Desalination Plant, which supplies up to 15% of Sydney's water. In Israel, desalination provides about 80% of domestic water, with the Sorek plant near Tel Aviv being one of the world's largest, producing 624,000 cubic metres per day.
These plants have helped buffer against drought and climate variability. However, desalination is not a panacea; it is expensive, energy-intensive, and has ecological impacts. It is best used as part of a diversified water portfolio that includes conservation, recycling, and stormwater capture. Looking ahead, desalination technology continues to improve. New membrane materials, such as graphene oxide and carbon nanotubes, promise higher permeability and better salt rejection, potentially reducing energy use by 30% or more. Advances in low-pressure membranes and biomimetic aquaporin-based membranes could further cut costs. Meanwhile, solar-powered desalination is gaining traction in remote and off-grid areas.
As climate change intensifies water scarcity, desalination will play an increasingly vital role. However, its sustainability depends on integrating renewable energy, improving brine management, and ensuring equitable access. For Year 12 students, understanding desalination offers insight into how engineering, chemistry, and environmental science combine to address one of humanity's most pressing challenges.