What is Desalination?

What is Desalination?

Desalination is the process of removing dissolved salts, minerals, and other impurities from seawater or brackish water to produce freshwater suitable for drinking, agriculture, or industrial use. The two dominant technologies are reverse osmosis (RO), which forces water through semi-permeable membranes under high pressure, and thermal distillation, which evaporates and recondenses water. Reverse osmosis accounts for approximately 70% of global desalination capacity due to its superior energy efficiency, while thermal methods remain prevalent in the energy-rich Persian Gulf states.

Why It Matters

Freshwater scarcity is among the defining challenges of the 21st century. The World Resources Institute estimates that 25 countries—home to one-quarter of the global population—face extremely high water stress annually, and that number is projected to grow as climate change disrupts precipitation patterns and glacial melt accelerates. Desalination offers something no other water supply technology can: independence from the hydrological cycle. Seawater is abundant, and desalination output is unaffected by drought, reduced snowpack, or shifting rainfall.

Global desalination capacity has grown rapidly, reaching approximately 130 million cubic meters per day across nearly 22,000 plants in 2025. The Middle East and North Africa account for roughly 45% of installed capacity, but growth is accelerating in water-stressed regions worldwide. Australia built six major desalination plants during the Millennium Drought (2001–2009), and Southern California's Carlsbad facility—the largest in the Western Hemisphere at 50 million gallons per day—has operated since 2015.

The technology has improved dramatically. Energy consumption for seawater RO has fallen from over 20 kWh per cubic meter in the 1970s to 2.5–3.5 kWh today, approaching the thermodynamic minimum of approximately 1.1 kWh. Costs have followed: the Carlsbad plant produces water at roughly $2,100 per acre-foot, and newer Middle Eastern facilities have achieved costs below $0.50 per cubic meter through competitive procurement and scale. While still more expensive than conventional freshwater sources in most regions, desalination is increasingly cost-competitive with alternatives like long-distance water transfers and new dam construction.

However, desalination carries environmental costs that must be managed. Brine discharge—concentrated salt solution at roughly twice seawater salinity—can harm marine ecosystems if not properly diffused. Intake systems can impinge and entrain marine organisms. And the energy intensity, though declining, means desalination carries a carbon footprint unless powered by renewable energy. These challenges are solvable but require deliberate engineering and regulatory attention.

How It Works / Key Components

In reverse osmosis desalination, seawater is first pretreated to remove suspended solids, organic matter, and biological contaminants that would foul the membranes. Pretreatment typically involves coagulation, media filtration, and increasingly ultrafiltration membranes. The pretreated water is then pressurized to 55–70 bar (800–1,000 psi) and forced through spiral-wound polyamide membranes with pore sizes of approximately 0.1 nanometers—small enough to reject salt ions while allowing water molecules to pass. Recovery rates for seawater RO typically range from 40–50%, meaning roughly half the intake volume becomes product water.

Energy recovery devices have been the single most important innovation in reducing desalination costs. Modern pressure exchangers, such as those manufactured by Energy Recovery Inc., capture the hydraulic energy in the pressurized brine stream and transfer it to incoming feedwater, recovering up to 98% of the energy that would otherwise be lost. This innovation alone reduced energy consumption by approximately 60% compared to early RO systems.

Post-treatment stabilizes the product water, which emerges from RO membranes as nearly pure H₂O—too aggressive (corrosive) for distribution without remineralization. Lime and carbon dioxide addition raises pH and alkalinity to appropriate levels. The finished water is then disinfected and blended with conventional supplies before entering the distribution system.

Brine management remains the primary environmental challenge. Best practices include diffuser systems that rapidly dilute brine discharge, reducing salinity impacts on benthic communities. Some facilities co-locate with power plants, blending brine with cooling water discharge for additional dilution. Emerging technologies—including zero-liquid discharge systems, mineral extraction from brine, and brine-to-salt crystallization—aim to transform waste brine into a resource, though commercial viability at scale remains limited.

Council Fire's Approach

Council Fire evaluates desalination as one component of diversified water supply portfolios, helping clients balance supply security against energy costs, environmental impacts, and financial sustainability. Our ocean systems expertise gives us particular depth in assessing marine environmental impacts of intake and discharge systems, and we work with clients to ensure desalination projects meet or exceed environmental standards while contributing to broader climate resilience strategies. We advocate for renewable energy integration and brine management innovation as essential elements of responsible desalination planning.

Frequently Asked Questions

How much does desalinated water cost compared to other sources?

Desalinated seawater typically costs $1.50–$3.00 per cubic meter, compared to $0.10–$0.50 for conventional surface water treatment and $0.50–$1.50 for groundwater. However, these comparisons can be misleading. In water-scarce regions, the marginal cost of the next unit of conventional supply—a new dam, longer pipeline, or deeper wells—often exceeds desalination costs. Brackish water desalination is significantly cheaper at $0.50–$1.00 per cubic meter due to lower salinity and pressure requirements. The cost trajectory continues downward: recent large-scale projects in the Middle East have achieved costs of $0.40–$0.55 per cubic meter through competitive procurement and economies of scale.

What is the environmental impact of brine discharge?

Brine discharge elevates salinity in the immediate vicinity of the outfall, which can stress or kill salt-sensitive marine organisms including seagrass beds, coral communities, and benthic invertebrates. The severity depends on the discharge method, local oceanographic conditions, and ecosystem sensitivity. Well-designed diffuser systems can reduce salinity to background levels within 50–100 meters of the outfall. Environmental impact assessments, ongoing monitoring, and adaptive management are standard practice for modern facilities. Some regions, including the Persian Gulf where desalination is concentrated, face cumulative impacts from multiple plants that require regional-scale management approaches.

Can desalination be powered by renewable energy?

Absolutely, and this is increasingly common. Solar-powered desalination is particularly synergistic because peak solar generation coincides with peak water demand. Saudi Arabia's NEOM project includes a 500,000 cubic meter per day solar-powered desalination facility. Australia's Perth and Sydney desalination plants are offset by wind energy purchases. The technology also pairs well with energy storage—desalination can operate flexibly, ramping production up when renewable generation is abundant and down when it is scarce, effectively providing demand response services to the grid. As renewable energy costs continue to fall, the carbon footprint argument against desalination weakens significantly.