Water desalination, the process of removing salt and minerals from water to produce fresh water, offers a promising solution to global water scarcity. This method turns abundant saline water into a usable resource and has seen rapid adoption worldwide. However, the technology is not a simple fix and introduces significant negative consequences, ranging from economic challenges to severe environmental harm. Pursuing desalination requires a careful evaluation of these drawbacks against the benefits of securing a new water supply.
Economic and Energy Burdens
Desalination is a resource-intensive method for producing fresh water, making it a costly alternative compared to traditional sources. The initial construction of a desalination plant (capital expenditure) is substantial, often involving complex intake and discharge systems. These high upfront costs are compounded by significant operational costs.
The primary driver of high operational costs is the immense energy requirement, particularly for the widely used Seawater Reverse Osmosis (SWRO) process. Reverse osmosis forces seawater through semi-permeable membranes at extremely high pressure, requiring powerful pumps. For seawater, this pressure can range from 6,800 to over 8,000 kilopascals.
Energy consumption can account for over half of a plant’s total operating costs. This high energy demand often relies on fossil fuels, linking fresh water production to increased carbon emissions. Even with modern energy recovery devices, maintaining high-pressure operations ensures that desalination remains an energetically expensive endeavor.
The Environmental Crisis of Brine Discharge
The most damaging drawback of desalination is the disposal of brine, the highly concentrated liquid waste product. Brine is typically twice as salty as ambient seawater, sometimes reaching 80 parts per thousand, compared to normal ocean salinity of 35 to 40 parts per thousand. Desalination plants worldwide produce more brine than fresh water.
When this dense, hypersaline liquid is pumped back into the ocean, it forms a heavy plume that sinks rapidly to the seabed. This plume smothers and destroys benthic, or bottom-dwelling, ecosystems, which are sensitive to sudden changes in salinity. Organisms with low mobility, such as sea grasses and corals, cannot escape the increasing salinity, leading to impaired activities and changes in community composition.
The brine also carries residual chemicals used throughout the process, further contaminating the marine environment. These additives include antiscalants, coagulants, and antifouling agents. The accumulation of these chemicals can disrupt biogeochemical processes, like nutrient cycling. Furthermore, the dense brine plume can deplete dissolved oxygen near the bottom, creating hypoxic conditions that are lethal to marine life.
Impacts on Marine Ecosystems
Separate from the chemical impact of brine discharge, the physical process of drawing water into the plant harms marine life. The high volume of water intake required poses two major threats: impingement and entrainment. These effects are particularly pronounced with surface intake systems that draw water directly from the open ocean.
Impingement occurs when larger marine organisms, such as adult fish and crabs, are trapped against the intake screens by the force of the flowing water. Unable to escape the water velocity, they are injured or killed against the mesh.
Entrainment is the process where smaller organisms, including fish eggs, larvae, and plankton, are sucked through the intake screens and into the plant’s machinery. These microscopic organisms, which form the base of the marine food web, are killed by the physical forces, heat, and chemical treatments within the process. The combined mortality causes significant localized loss of aquatic life and biodiversity.
Infrastructure and Operational Hurdles
Beyond the environmental and economic costs, desalination plants face persistent technical difficulties that complicate long-term operation. Converting saline water into fresh water subjects the internal infrastructure to harsh conditions, primarily leading to issues of scaling and corrosion.
Scaling involves the buildup of mineral deposits, such as calcium carbonate, on membranes and piping. This restricts water flow and reduces the efficiency of the reverse osmosis process. This fouling necessitates frequent chemical cleaning, which requires the plant to be taken offline and adds to maintenance downtime and waste production.
Corrosion is an equally challenging issue, as the highly saline water is aggressive toward metal components, leading to structural failures and costly repairs. The high-pressure environment of SWRO exacerbates the vulnerability of materials. Operators must constantly manage water chemistry to balance the risk of scaling (higher pH) against the risk of corrosion (lower pH). These maintenance challenges add to the overall complexity and expense of maintaining a reliable water supply.