The water molecules have an electrically positive and negative part. In the presence of an electrically active ion (positive or negative) the molecules are attracted forming a stable agglomeration of molecules of dimensions proportional to the electrostatic force of the ion.
The membranes exploit the difference in size between agglomerates and molecules to filter the water: the osmotic ones prevent the passage to monovalent ions such as sodium and chlorine and for this reason they are used in the filtration of sea water.
Osmosis is the phenomenon of diffusion of water molecules that occurs when an osmotic membrane is interposed between two solutions with different salinity: a pressure proportional to the concentration gradient that pushes the water from the solution poor in solute to the richer one.
This pressure responds to the need of the system to bring itself into equilibrium, ie to equalize the salinity of the two solutions. The osmotic flow can be interrupted by applying a pressure equal to the osmotic pressure to the more concentrated solution, if the pressure increases further the flow is reversed thus obtaining the reverse osmosis process.
The pressure, therefore the energy to be supplied to reverse the process, depends on the difference in concentration.
The process involves high energy consumption due to the compression of the water, to ensure that only part of it (about 50%) is desalted.
The brine rejected by the membrane still contains a lot of pressure energy which is recovered through special systems (turbo-pumps or pressure exchangers).
Osmosis systems usually consist of:
- Water pretreatment systems (clarification, filtration, pH correction)
- Membrane systems where real osmosis takes place
- Post-treatment systems (remineralization)
The greater the pretreatments to which the water is subjected, the less energy will be needed to complete the osmosis process.
In addition, correct use of the membranes will allow for self-maintenance as well as reduced production of brines to be disposed of or discarded at sea.
