WaterWare   On-line Reference Manual
    Release Level 5.0
    Release Date 2005 05
    Revision Level 1.0

Desalination

More than 7,500 desalination plants are in operation worldwide. About 60% of them are located in the Middle East. The world's largest plant in Saudi Arabia produces 600,000,000 m³ per day of desalted water. About 12% of the world's capacity is produced in the Americas, with most of the plants located in the Caribbean and Florida.

Desalination Technologies

Desalination removes dissolved minerals from seawater, brackish water, or treated wastewater. A number of technologies have been developed for desalination, including
  1. reverse osmosis (RO),
  2. distillation,
  3. electrodialysis, and
  4. vacuum freezing.
Reverse Osmosis and distillation have the biggest market share.

A primary distinction is also the source of the water to be used: directly from the ocean or estuaries, brackish groundwater, or treated wastewater.

Reverse Osmosis

In RO, feedwater is pumped at high pressure through permeable membranes, separating salts from the water. The feedwater is pretreated (filtered) to remove particles that would clog the membranes. The quality of the water produced depends on the concentration of salts in the feedwater, the pressure (and thus energy expended) and the salt permeation constant of the membranes. Product water quality can be improved by adding a second pass of membranes, whereby product water from the first pass is fed to the second pass.

Distillation

In the distillation process, feedwater is heated and then evaporated, leaving the salts in the feedwater. The most common methods of distillation include
  1. Multistage flash (MSF): the feedwater is heated and the pressure is lowered, so the water "flashes" into steam by the combined effect. This process constitutes one stage of a number of stages in series, each of which is at a lower pressure.
  2. Multiple effect distillation (MED): the feedwater passes through a number of evaporators in series. Vapor from one series is subsequently used to evaporate water in the next series.
  3. Vapor compression (VC): involves evaporating the feedwater, compressing the vapor, then using the heated compressed vapor as a heat source to evaporate additional feedwater.
Desalination plants may use a combination of technologies.

Water Quality

Distillation plants produce a high-quality product water that ranges from 1.0 to 50 ppm TDS, while RO plants produce a product water that ranges from 10 to 500 ppm TDS. The WHO drinking water standard is at 500 ppm. For domestic, that is drinking water use, post-treatment processes like chlorination are often employed to ensure that product water meets the health standards for drinking water.

Desalination product water may be used in its pure form (e.g., for make-up water in power plant boilers) or it may be mixed with less pure water and used for drinking water, irrigation, or other uses. The desalinated product water is usually more pure than drinking water standards, so when product water is intended for municipal use, it may be mixed with water that contains higher levels of total dissolved solids. Pure desalination water is highly acidic and is thus corrosive to pipes, so it has to be mixed with other sources of water that are piped onsite or else adjusted for pH, hardness, and alkalinity before being piped offsite.

Energy use and costs

Energy use is a major consideration in desalination. Estimates range from 2 to 10 kWh per cubic meter (Wilf, 1991).

Costs therefor depend very much on energy use (and energy price), but also the size of a plant (economies of scale). Costs range very approximately from 0.5 to 5 US$ per m³ depending on location.

More unconventional schemes propose the use of solar power, wave energy, or hydropower: co-generation application of hydropower development with reverse osmosis (RO) desalination for the Aqaba-Disi water pipeline project in Jordan and the Mediterranean-Dead Sea (MDS) Conduit scheme in Israel/Jordan. The unit cost of hydropowered reverse osmosis desalination is preliminarily estimated to be US $0.4/m3 for brackish groundwater and US $0.68/m3 for seawater.

Technology kWh/m3
  min max
Multi-Stage Flash36
Multiple Effect Distillation24
Vapor Compression812
Reverse Osmosis (single pass)59
Reverse Osmosis (double pass)5.210


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