Seawater desalination

Out of an estimated worldwide overall capacity of 40 million cubic metres per day, Veolia Water occupies top position in this market, with 12% of installed capacity, or 5.24 million cubic metres per day.

Veolia Water Solutions & Technologies consequently has over 800 references worldwide, for all technologies combined. The two processes it uses, capable of producing drinking water of better quality than that from groundwater, are thermal desalination and membrane desalination.

Distillation or thermal desalination is the oldest seawater desalination process. By vaporising the water in distillation facilities, it is separated from the salts that it contains.

Irrespective of the technique used, the principle behind distillation is to reproduce the natural phenomenon of rain by concentrating this phenomenon in space and time. In order to reduce the significant energy consumption related to the heat needed to vaporise water, "multiple effect" processes (MED) have been developed. These allow the energy released during condensation to be re-used.

Thermal desalination is a solution that is fully justified when the facility is paired with a fossil-fuel electricity power plant, the by-product energy from which it uses. If water alone is being produced, the membrane process proves to be a more competitive solution.

In terms of seawater desalination using thermal processes, VWS to date has more than 370 units built by its SIDEM subsidiary in this area.

SIDEM has had a long history in Saudi Arabia and other Gulf states since the installation of its first desalination plant in the 1970s. To date, 46 desalination plants have been completed by SIDEM in Saudi Arabia, including several with capacity of over 100,000 cubic metres per day, one of 267,000 cubic meters per day and a dozen of over 20,000 cubic metres per day.

Membrane desalination: This technology, known as "reverse osmosis", has resulted in the construction of many mains water production plants over the last ten years. This technique, which is more recent than thermal desalination, is generating market share forecasts of the order of 70% by 2020, compared to 30% now.

Seawater desalination by reverse osmosis consists of passing water under pressure through a membrane which lets water through but retains salts and impurities.

Osmosis is a natural process. If two aqueous solutions with different saline concentrations are separated by a membrane, water spontaneously passes from the solution with the lower salt concentration to the more concentrated solution. Reverse osmosis is based on the opposite principle. It consists of applying significant pressure to the salt water, which requires high energy consumption, to push it through a membrane. At the end of the process, only water molecules have passed through the membrane, thus providing fresh water.

The membrane consists of a flat surface with selective permeability. It removes salt and undesirable constituents in water (micro algae, bacteria, certain viruses, micro-organisms, micropollutants) and enables purified water to be produced.

Essentially focused on the process of reverse osmosis, Veolia Environnement's research aims to optimize the operation of desalination plants utilizing membrane processes.

The research is focused on two main aspects:

• Pretreatment of the seawater to limit membrane clogging further down in the treatment process;
• Reducing the energy expenditure to cut the cost of desalination and improve the environmental outcomes.