Nanofiltration and reverse osmosis systems
While ultrafiltration systems are limited to the removal of suspended solids of every kind,
with nanofiltration and reverse osmosis systems we can also separate the dissolved salts from the water. Generally speaking, nanofiltration could be considered as a reverse osmosis with a milder separation level, up to divalent ions, while full-fledged osmosis, even if there are a few differences concerning the type of membrane used, is able to retain even monovalent ions of smaller dimensions (Cl-, Na+).
Osmosis is a natural phenomenon whereby, between two solutions with different saline
concentrations separated by a semipermeable membrane, water tends to pass from the most
diluted solution to the most concentrated one. When this happens, it decreases the pressure of the side at lower concentration until it reaches an equilibrium point that stops the flow of water. The pressure difference between the two solutions under equilibrium conditions is called OSMOTIC PRESSURE. REVERSE OSMOSIS is a process which reverses the natural one. It is sufficient, in fact, to apply to the concentrated solution a pressure higher than the osmotic one in order to provoke an inverse flow through the semipermeable membrane obtaining the separation of the dissolved salts in the solution. A semipermeable membrane is a structure that allows the water flow and the retention of dissolved ions, colloids and bacteria through the phenomenon of osmosis. The membranes normally applied in our plants are spiral wound type (TFC), with saline rejection and different operating pressure according to the customer’s needs and the deionization quality desired. From the osmotic membranes we get a share of deionized water (PERMEATE) and a share of waste water with influential salts (CONCENTRATE).