Membrane processes are physical diffusion processes of particles in water. They function because certain types of membranes allow particles with particular characteristics to pass through them, while blocking the passage of particles that do not possess the same characteristics.
Today there are many different types of membranes that allow the passage of solutes and others depending on their nature, ionic charge or size. The main ones are:
• Reversal Electrodialysis
• Reverse Osmosis
Electrodialysis consists of the elimination of electrically charged ions, those of which are dissolved in water. To carry out this elimination phase, a pair of distinctly charged electrodes are introduced into the feed water so that the dissolved ions will be attracted to the electrodes of opposite sign to their own. This achieves moving the ions from one place to another in the dissolution.
The alternating use of anion and cation selective membranes is essential for the feed water to continue losing negative and positive ions after passing through the separation zone.
Most interestingly is alternately placing the membranes so that in some of the channels solutes are concentrated in water called concentrate. In other channels the feed water circulates, gradually losing its contaminants until it finishes the process with a very low salt concentration.
In this case, the polarities of the electrodes are periodically altered so that water flows temporarily change direction receiving purified water from those conduits carrying the concentrate and vice versa.
This method eliminates the risk of formation of precipitates, fouling and clogging of the membranes thanks to the periodic change of water flow direction which facilitates cleaning pipes and membranes and avoids the emergence of slime and other deposits in the plant.
Reverse osmosis uses a procedure whereby through a semipermeable membrane the chemical potentials of two dilutions located one on each side of the membrane tend to equalize, making it operate in reverse.
Reverse osmosis consists of pumping water charged with dissolved ions to a tank where it’s subjected to pressure against a membrane. During this process, water is transferred from one side of the membrane to the other. Ions are left in the feed water thereby generating a concentrate in the water that has not yet passed through the membrane and a purified water flow if it has passed through the membrane.
The concentrate generated should be eliminated from direct contact with the membrane to prevent an increase in ion concentration that can precipitate salts on the membrane surface. This provokes a loss of effectiveness of the process in addition to increased maintenance costs. It is also important to perform pretreatment to prevent clogging.
As a standard result, reverse osmosis returns 80% purified water and 20% rejection.