The process of desalination removes salt and minerals from saline water, such as seawater or brackish groundwater, to produce fresh, potable water. As global populations grow and freshwater sources become increasingly strained, this technology presents a solution to water scarcity. Desalination facilities are already a reality in arid regions and coastal cities worldwide, ensuring a reliable, drought-independent water supply. Despite the growing need for new water sources, the technology has not been adopted universally. The limited spread is due to significant economic, ecological, and logistical hurdles.
High Financial and Energy Requirements
The greatest barrier to the widespread adoption of desalination is the enormous financial outlay required to build and operate the facilities. Capital expenditure (CAPEX) involves the massive upfront cost of construction, including specialized infrastructure like durable intake and outfall systems, pre-treatment facilities, and high-pressure pumping stations. Building a large-scale seawater reverse osmosis (RO) plant often requires an initial investment ranging from hundreds of millions to billions of dollars. This burden is compounded by the need for materials resistant to the corrosive nature of saltwater.
Operational expenditure (OPEX) is dominated by the sheer amount of energy required to force seawater through semi-permeable membranes in the most common method, reverse osmosis. Even with modern energy recovery devices that operate efficiently, RO still requires a substantial energy input, typically between 3 and 6 kilowatt-hours of electricity for every cubic meter of water produced. This is significantly more energy-intensive than treating conventional freshwater sources. For many desalination plants, energy costs alone account for 30% to 45% of the total operating budget, making the process highly sensitive to fluctuations in global electricity prices.
Older thermal desalination methods, which boil water to create steam, can consume up to 25.5 kWh/m³ of equivalent energy, illustrating the energy challenge. The high energy demand directly translates to a high cost for the final product water. Desalinated water is often priced two to five times higher than water from traditional sources like surface water or groundwater. For example, in Southern California, desalinated water can cost an estimated $1,900 to $2,100 per acre-foot, compared to imported water at around $1,059 per acre-foot. This prohibitive cost structure makes the water economically unviable for large-scale uses like agriculture.
Environmental Consequences of the Process
The ecological toll of desalination is a major factor limiting its expansion, stemming from the intake of source water and the discharge of concentrated waste. Drawing large volumes of seawater into a plant harms marine life at the intake structure in two ways. Impingement occurs when larger organisms, such as fish and crabs, are trapped against the intake screens by the water flow. Entrainment occurs when microscopic organisms, including plankton, eggs, and larval stages, pass through the screens and are sucked into the plant’s machinery.
Organisms entrained into the system face 100% mortality due to mechanical damage, high pressure, and chemical exposure. This continuous loss of biodiversity, particularly organisms at the base of the marine food web, can have cascading effects on the local ecosystem.
The second, and often more significant, environmental concern is the disposal of the hypersaline brine concentrate remaining after freshwater extraction. This brine is typically double the salt concentration of the original seawater and may contain chemical additives used during the treatment process, such as antiscalants and coagulants. When this dense, chemically altered effluent is discharged back into the ocean, it sinks to the sea floor, where it can spread across the seabed for several kilometers.
The resulting plume of hypersaline water alters the local salinity, temperature, and dissolved oxygen levels in the benthic zone. Organisms living on or near the seabed, including corals, seagrasses, and bottom-dwelling invertebrates, are sensitive to these abrupt environmental changes. Exposure to the brine can impair their activities and cause morphological deformations, leading to a shift in community composition.
Infrastructure and Location Constraints
The logistical requirements of desalination impose significant geographical limitations, ensuring it remains primarily a coastal solution. Plants must be sited near the source water, typically the ocean or a large brackish aquifer. This coastal necessity creates a problem for populations living far inland, where the need for new water sources is often greatest.
Transporting the water required to supply an inland city over long distances presents a major infrastructure challenge. Pumping desalinated water from the coast up to higher elevations inland requires a complex network of new pipelines and powerful pumping stations. The construction and maintenance of this new infrastructure add substantially to the overall project complexity, timeline, and final cost.
Inland desalination of brackish groundwater, while possible, faces an economically prohibitive hurdle: concentrate disposal. Unlike coastal plants, which can discharge brine into the vast ocean, inland facilities have no easy way to manage the hypersaline waste. Disposing of the brine requires expensive and complex solutions, such as deep-well injection into geological formations or using evaporation ponds. Evaporation ponds require significant land area and are subject to stringent regulations. The high cost of this specialized concentrate management often renders inland desalination economically unfeasible.