Desalination, the process of removing salt and minerals from saline water sources like the ocean, represents a crucial technology for water security in an increasingly drought-prone world. It offers a reliable, climate-independent source of fresh water, which is particularly appealing for coastal communities facing water scarcity. However, the widespread adoption of desalination is significantly limited by complex technological, economic, and environmental problems.
High Energy Use and Financial Barriers
The largest economic hurdle for desalination is the immense energy required to operate facilities, which translates directly into high financial costs. The dominant technology, reverse osmosis, functions by pushing seawater through semi-permeable membranes at extremely high pressure to overcome the natural osmotic pressure. This force is the primary driver of the high energy demand.
Modern seawater reverse osmosis (SWRO) plants typically require between 3 to 4 kilowatt-hours of electricity for every cubic meter of fresh water produced, though some highly efficient plants have achieved consumption closer to 2 kilowatt-hours per cubic meter. This high energy consumption makes desalinated water significantly more expensive than traditional freshwater sources, which may cost as little as $0.25 per cubic meter to treat. Desalinated water costs generally fall in the range of $0.43 to $0.65 per cubic meter, with energy accounting for up to half of the total operating costs.
These substantial input costs create a significant barrier to entry for developing nations and directly impact municipal water bills in established economies. High energy demand also places a strain on local power grids, and if sourced from fossil fuels, it negates some environmental benefits of securing a water supply. While advances in energy recovery devices have helped reduce consumption by recycling energy from the pressurized waste stream, the process remains fundamentally energy-intensive.
Managing Hypersaline Brine Discharge
The primary environmental challenge of desalination is managing hypersaline brine, the highly concentrated saltwater waste product. A substantial volume of brine is created for every gallon of fresh water produced, typically discharged back into the ocean near the plant. This brine is denser and saltier than ambient seawater, often exhibiting salinities in the range of 65 to 90 practical salinity units (PSU) compared to the ocean’s average of 33 to 39 PSU.
The concentrated effluent often sinks to the seabed, where it can spread along the bottom and form a plume that increases local salinity and reduces oxygen levels. This altered environment severely impacts benthic ecosystems (organisms living on the seafloor), including seagrasses, polychaetes, and corals. Ecological damage can include impaired activities, morphological deformations in organisms, and significant changes in community composition near the discharge point.
Beyond the salt, the brine also contains chemical additives used during the pre-treatment phase of desalination, which can intensify the ecological impact. These chemicals include anti-scalants (such as polyphosphonate-based compounds) and coagulants (like ferric-based salts). The accumulation of these additives in the seabed’s porewater can alter the sediment’s chemical composition and redox zones, disrupting natural biogeochemical processes.
Operational Hurdles and Infrastructure Constraints
Desalination facilities face technical and logistical challenges related to operation and physical location. Operating a reverse osmosis plant requires constant vigilance against membrane fouling, which occurs when materials accumulate on the semi-permeable surfaces. This accumulation can be due to scaling from inorganic solutes or biofouling from the growth of microbes.
Membrane fouling necessitates expensive and complex pre-treatment processes, such as ultrafiltration, to clean the source water before it enters the main system. If fouling is not controlled, it reduces the membrane’s permeability, forcing operators to increase pressure to maintain water production, which in turn increases energy consumption and maintenance costs. Furthermore, the high salinity of the source water and the concentrated brine exposes metal components to severe corrosion, requiring the use of costly, specialized materials like super austenitic stainless steel for high-pressure piping and pumps.
Siting desalination plants also presents complex infrastructure constraints, particularly for facilities near densely populated coastal areas. The location must minimize environmental impact from the intake and brine discharge while remaining close to both a reliable energy source and the existing municipal water distribution network. Finding a suitable coastal plot that balances these logistical needs with local political and environmental sensitivities often proves to be a significant hurdle.