Reverse osmosis technology is comparable to extremely fine filtration and is therefore also called hyperfiltration. This method uses the reversal of a process that has elementary significance in nature: osmosis refers to the process of equalising the concentration of two liquids through a semi-permeable membrane. This process always occurs when two aqueous solutions with different ion concentrations are separated by a semi-permeable wall.
In reverse osmosis, water is pressed against a synthetic semi-permeable membrane that is permeable to water molecules but impermeable to impurities in the raw water.
On the side with high ion concentration (tap water, raw water) pressure is applied (water pipe pressure) that forces the water in the other direction, namely to the pure water side with lower concentration.
The undesirable dissolved substances (e.g. hardness components, salts, lime, nitrate, residues of pesticides, hormones and medicines, microorganisms, to name but a few) cannot pass through the ultra-fine membrane due to their molecular size - consequently, on the other side of the membrane, the pure water side, almost exclusively clean water without ions can be found.
Since tap water containing its substances constantly flows in during operation, substances retained by the membrane must continuously be removed to prevent the membrane from clogging.
As a result, in addition to pure water, a reverse osmosis system also produces waste water (concentrate), which contains the undesirable substances in increased concentration and which must be flushed away. This immediately reveals one of the crucial differences between reverse osmosis technology and techniques with accumulation filters. The efficiency (amount of filtered water per amount of raw water from the pipe) can never be one, as some "waste water" is always produced. However, waste water enriched with pollutants is always discharged, so that an accumulation of retained pollutants on the osmosis membrane, which could considerably reduce the service life of the membranes, never occurs.