Water obtained from marine sources (seawater) has a higher density than freshwater as a result of the high concentrations of dissolved salts. These salts work to increase the mass of the seawater by a greater proportion than they increase the volume, achieving a density of 1.0 g/ml at 4 °C. The most common ions found dissolved in seawater are sodium, chloride and magnesium, followed by sulphate and finally, calcium.
Marine Water Makers
Marine water makers offer several advantages for seagoing vessels. The most prominent of these is the reduction in weight as a result of eliminating unnecessary water cargo. This serves to improve performance and reduce fuel and emission. The range of travel is extended, and there is also the guarantee of safe water, even in drought conditions. Most marine water makers work by using reverse osmosis (RO) to remove particles and ions from the seawater feed.
Seawater Reverse Osmosis Plants
Reverse osmosis in industrial filters works by applying intense pressure (approx. 800psi) to salt water and forcing it through a semi-permeable membrane. The principles of straining and size differentiation work to ensure that unwanted particles and ions are retained on one side while the purified solute escapes to be collected for use. RO plants employ this method, together with various pre and post treatment technologies, on an industrial scale to eliminate contaminants, such as salt and bacteria from drinking water.
Seawater Reverse Osmosis Systems
Desalination in RO seawater systems results in the production of brine, which is highly concentrated solute, containing high levels of ions and contaminants, together with residues of pre-treatment and disinfection chemicals. It is important to consider the environmental impact of releasing this brine into the ocean and to ensure that only small amounts are released to allow for adequate dilution and prevent the brine from harming the aquatic environment.
Desalination has traditionally been performed using a vacuum boiling method. The use of a vacuum allows the liquid to boil at a much lower temperature, allowing for savings in energy and emissions. Energy consumption from the process of seawater desalination may be 3 kWh per m3 at lowest. This energy consumption may be compared to the transportation of freshwater over long distances. Nowadays, reverse osmosis has become the preferred method of desalination, using membrane technology to separate the components of seawater and generally using a lower amount of energy than the thermal distillation described above.