The process of purifying ocean water to make it safe for drinking, known as desalination, addresses the fundamental challenge of high salinity. Seawater contains an average of 35,000 milligrams of dissolved solids per liter, mainly sodium chloride, which is far too concentrated for human consumption. Desalination involves separating the pure water molecules from these dissolved salts and minerals to produce water with total dissolved solids (TDS) typically below 500 mg/L, making it potable. The process involves conditioning the raw source water and adjusting the final product for health and safety.
Initial Steps: Why Seawater Needs Preparation
Before purification begins, raw ocean water must undergo extensive preparation to protect the machinery downstream. Intake systems must be carefully designed to draw water while minimizing the collection of marine life, sand, and debris, often using submerged pipes or wells. Once collected, the water immediately enters a pre-treatment phase designed to remove suspended solids, organic material, and microorganisms.
Initial screening removes large debris, followed by filtration through processes like multimedia filters, which use layers of materials such as sand and anthracite to capture smaller particles. Chemical pre-treatment is then used to prevent fouling and scaling within the main desalination units. Chemicals are added to coagulate fine particles, making them easier to filter out in processes like dissolved air flotation (DAF) or clarifiers.
Antiscalants are dosed into the water to prevent the precipitation of mineral salts, which would otherwise build up on heat transfer surfaces or membrane materials. Chlorination is often used to kill bacteria and algae that could cause biological fouling. However, chlorine must be removed (dechlorination) before the water reaches certain types of purification membranes.
Pressure-Based Purification: Reverse Osmosis
The dominant modern technique for large-scale desalination is Reverse Osmosis (RO), a pressure-driven membrane process. This method works by applying immense pressure—often exceeding 80 bar (1,160 psi) for seawater—to the salty feed water. This pressure overcomes the natural osmotic pressure that would otherwise cause fresh water to flow toward the saltier side.
The force pushes water molecules through a semi-permeable membrane, typically made of a thin-film composite polymer, while dissolved ions are rejected. The membrane acts as a molecular filter, allowing water to pass through as “permeate” while concentrating the salts into a separate stream called brine. Modern RO systems are highly efficient, achieving salt rejection rates often greater than 99%.
Energy consumption is a major factor in RO, as the process requires constant high-pressure pumping to overcome the osmotic gradient. To mitigate this, modern plants incorporate energy recovery devices (ERDs). These devices capture the hydraulic energy from the high-pressure brine waste stream and transfer it back to the incoming feed water, significantly reducing the overall energy demand.
Heat-Based Purification: Distillation Methods
Thermal desalination techniques rely on heating and phase change, and were historically the most common methods. These processes involve boiling seawater and then condensing the resulting pure steam to collect fresh water, known as distillate. The two primary industrial thermal methods are Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED).
Multi-Stage Flash (MSF) works by heating the seawater and then introducing it into a series of chambers, or stages, each operating at a progressively lower pressure than the last. As the hot water enters each stage, the sudden drop in pressure causes a portion of the water to instantaneously “flash” into steam. This steam is then condensed on heat exchanger tubes to produce fresh water, simultaneously preheating the incoming feed water.
Multi-Effect Distillation (MED) uses a series of successive chambers, or effects, where each effect operates at a lower temperature and pressure than the one preceding it. The steam produced in one effect is used as the heat source to boil the seawater in the next effect, allowing the repeated reuse of latent heat. MED generally operates at lower maximum temperatures than MSF, which helps reduce scaling and corrosion.
Finalizing Potability: Making Purified Water Safe
The water produced by both RO and distillation, known as permeate or distillate, is exceptionally pure but is not yet suitable for distribution or consumption. This pure water is often slightly acidic, low in alkalinity, and lacks mineral content, making it corrosive to distribution pipes. The final step, known as post-treatment, involves adjusting the water chemistry to make it safe and non-aggressive.
The most important step is remineralization, where compounds containing calcium and magnesium are added back into the water. This process increases the water’s hardness and alkalinity, which stabilizes the water and protects the infrastructure by preventing corrosion. Limestone contactors, where the water flows over calcium carbonate rock, are a common method for this adjustment, often using carbon dioxide injection to enhance dissolution.
After remineralization, the water’s pH is adjusted to a stable, near-neutral level. A final disinfection step is performed to prevent microbial growth during storage and distribution, often involving the addition of chlorine or treatment with ultraviolet (UV) light. The finished product is a stable, non-corrosive, and mineral-balanced drinking water supply.