Desalination is the process of removing dissolved salts and minerals from seawater or brackish water to produce fresh, potable water. Although the technology offers a drought-proof source of water, its widespread adoption is significantly limited by high costs, making it generally more expensive than sourcing water from rivers, reservoirs, or groundwater. This expense stems from the technologically intensive nature of the process, particularly the immense energy required to separate water molecules from a high concentration of dissolved solids.
Energy Intensity Requirements
The single largest driver of operational cost in modern desalination is the substantial energy needed to overcome the natural osmotic pressure of saltwater. In the most common method, Reverse Osmosis (RO), high-pressure pumps must force water through semipermeable membranes. This requires a continuous application of pressure significantly greater than the osmotic pressure—the force that naturally drives water to equalize salt concentrations. For seawater, this pressure can be up to 1,000 pounds per square inch (psi), or about 68 atmospheres.
This high-pressure requirement means that the energy cost can account for up to 45% of the total cost of producing desalinated water. While the Specific Energy Consumption (SEC) for seawater RO has dramatically improved, the sheer volume of water processed makes the total energy bill immense. This improvement is largely due to the integration of Energy Recovery Devices (ERDs), which recapture energy from the highly pressurized brine stream before it is discharged.
Thermal distillation methods, which involve heating and evaporating water, are also used but generally consume two to ten times more power than RO plants equipped with ERDs. Even with improved efficiency from ERDs, the fundamental physics of separating water from salt dictates a high energy floor. This makes the operational expense prohibitive compared to simply treating surface water.
Initial Capital Investment
The high price of desalinated water begins with the massive upfront expenditure required to construct the facility, known as Capital Expenditure (CAPEX). Building a large-scale seawater desalination plant can cost hundreds of millions to over a billion dollars, depending on the capacity and location. Key elements of this investment include the costs associated with specialized land acquisition, often needed near coastlines or existing infrastructure.
A significant portion of the CAPEX is dedicated to the complex intake and outfall infrastructure. These systems involve building long pipelines, sometimes underground or far into the sea, to draw in large volumes of raw water and safely discharge the brine. The core desalination technology, including the high-pressure pumps, piping, and membrane vessels, also consists of specialized, robust components engineered to withstand the corrosive nature of seawater and extreme operating pressures. These facilities are designed for long operational lifespans, which results in substantial financing, interest, and depreciation costs that are embedded in the final price of the water.
Specialized Operational Maintenance
Beyond the daily energy bill, specialized operational maintenance introduces a significant recurring expense for desalination plants. The most costly maintenance item is the periodic replacement of the Reverse Osmosis membranes, which are highly susceptible to fouling and scaling. Fouling is the accumulation of organic matter, bacteria, or suspended solids on the membrane surface, while scaling is the precipitation of minerals like calcium carbonate.
Membrane performance degradation necessitates replacement every five to seven years in a typical seawater RO plant. A complete replacement of the membrane modules can cost millions of dollars, representing a large portion of the long-term maintenance budget. To prevent premature fouling, extensive chemical pre-treatment is necessary, using substances like antiscalants and acids, which adds to the operational cost. Finally, the desalinated water requires post-treatment, such as adding back mineral salts to make it potable and non-corrosive, which introduces another layer of chemical and process expense.
Brine Disposal and Environmental Compliance
The final cost element is the logistical and environmental expense of managing the concentrated waste product, known as brine or concentrate. Desalination produces a highly saline effluent, often polluted with residual chemicals used in pre-treatment. Disposing of this hypersaline and potentially toxic brine without harming marine ecosystems requires strict environmental compliance and costly infrastructure.
Many plants must construct long, specialized diffuser pipelines that extend far from the shore to ensure the brine is rapidly and widely mixed with the receiving seawater. This necessary dilution infrastructure can add significantly to both the initial capital investment and the ongoing energy consumption, as additional pumping may be required. Environmental regulations mandate continuous monitoring to ensure the brine discharge does not exceed permitted salinity and chemical limits in the mixing zone, adding to the recurring operational costs.