Desalination
Desalination or desalinization is a process that removes minerals from saline water Salt water is desalinated to produce fresh water suitable for human consumption or irrigation. One potential by-product of desalination is salt. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use. Along with recycled wastewater, this is one of the few rainfallindependent water sources
The traditional process used in these operations is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. This is because the boiling of a liquid occurs when the vapor pressure equals the ambient pressure and vapor pressure increases with temperature. Thus, because of the reduced temperature, low-temperature "waste" heat from electrical power generation or industrial processes can be minimized. The principal competing processes use membranes to desalinate, principally applying reverse osmosis technology. Membrane processes use semipermeable membranes and pressure to separate salts from water. Reverse osmosis plant membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. Desalination remains energy intensive, however, and future costs will continue to depend on the price of both energy and desalination technology.
Schematic of a multistage flash desalinator A – steam in B – seawater in C – potable water out D – waste out E – steam out F – heat exchange G – condensation collection H – brine heater
Costs of desalinating sea water (infrastructure, energy, and maintenance) are generally higher than the alternatives (fresh water from rivers or groundwater, water recycling, and water conservation), but alternatives are not always available. Achievable costs in 2013 range from US$0.45 to $1.00/cubic metre ($US2 to 4/kgal). (1 cubic meter is about 264 gallons.) However, more than half of the total cost of desalination comes directly from energy cost, and since energy price is very volatile, any calculated cost of desalination is only good based on the energy price at the time of calculation.
Average water consumption and cost of supply by sea water desalination (Âą50%) Area
Consumption USgal/person/day
Consumption litre/person/day
Desalinated Water Cost US$/person/day
USA
100
378
0.29
Europe
50
189
0.14
Africa
15
57
0.05
UN recommended minimum
13
49
0.04