How do reverse osmosis systems work

What is reverse osmosis?

Reverse osmosis is a filtration process that enables substances in the molecular range to be filtered out (nanofiltration). Similar to normal filtration, z. B. contaminated water pressed against a filter material. The impurities remain in front of the filter and the water molecules penetrate.

A semi-permeable membrane serves as the filter medium. Semi-permeable or semi-permeable is the name given to a substance which is not actually permeable to water, but which can absorb water in its molecular structure. The water molecules diffuse through the membrane.
("This principle prevails in dense membranes without real pores,
how they are used in reverse osmosis and gas separation. "

The resulting passage size (in the case of filters one speaks of pore size) correspondsless than a ten-thousandth of a micrometer.
If you try to push water through such a membrane,
so the counter-pressure that develops through osmosis plays an important role.

What is osmosis

Almost all metabolic processes between our body cells and the extracellular fluid surrounding them work according to the natural principle of osmosis.
If you are interested in this, please refer to the technical article in Wikipedia: (see many other links there,
z. B.

If two liquids of the same type are separated by a semi-permeable membrane, liquid molecules migrate alternately through the membrane from both sides (according to the principle of Braun's molecular motion).
In this case the pressure on both sides would be balanced.
There would be no pressure difference. The osmotic pressure would be zero.

If you separate z. B. a one percent salt solution through a semi-permeable membrane of pure, distilled water, so here too osmosis (molecular movement) takes place. On the salty ("contaminated") side, the available salt ions reduce the space free for the water molecules.

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As a result, more water molecules can migrate from the side with the pure (distilled) water to the salty side than vice versa. The pressure on the "contaminated" side would increase until the number of molecule changes equalized on both sides, see:
Is reverse osmosis water suitable for human consumption?

How does reverse osmosis work?

The aim of reverse osmosis, to convey the purest possible water through this membrane, is achieved by adding on the contaminated side (raw water / tap water ...) Back pressure is generated, which is much higher than the (natural) osmotic pressure. The principle of osmosis is thus reversed. Hence the name reverse osmosis (RO for short),
(see also

However, the experimental set-up would only work very briefly. Since mainly only water molecules are pressed through the membrane, the solution would concentrate more and more in front of the membrane. The osmotic pressure would rise and the system would come to a standstill. (In addition, the membrane would clog or become too salty.)

For this reason, in a reverse osmosis system, the concentrated (contaminated or heavily mineralized) water is drained along the membrane. Depending on the amount drained (which is regulated by a flow limiter), a certain concentration can be set. The set amount of concentrate or the set ratio of waste water volume: permeate (= purified / filtered drinking water) should also prevent limescale precipitating in the membrane.

The resulting concentrate contains all the impurities that were supplied by the raw water (mostly tap water), while the permeate, i.e. the purified water, mainly contains water molecules.

The following applies in principle:

The higher the print on the tap water side, the better the filtered water is demineralized (5 bar is more effective than 3 bar).

The higher the Amount of waste water for flushing the membrane, the better the filtered water is demineralized and the longer the membrane remains protected from micro-damage caused by crystallized salts (3: 1 is more effective than 1: 1).

Therefore, responsible manufacturers of high-quality household systems pay attention to a balance between wastewater costs and filter effect. (Saving water in the wrong place damages the membrane and the purity of the filtered water.)

The necessary one Work pressure is as a rule in German households present due to the water pressure from the water pipe or is generated in the application (e.g. on boats, in mobile homes or mountain huts) by a pressure-increasing pump ("booster pump").
Booster pumps are generally only required in households in Germany, Austria and Switzerland if the water pressure is below 3 bar unless you want to fill up as much filtered drinking water as possible in a short time and forego the convenience of a tank that stores pure water, as is the case with direct flow systems, e.g. B. the 600 GPD Direct-Flow - The highest flow rate).

In practice, reverse osmosis membranes are arranged in such a way that the main flow direction is perpendicular to the membrane. With this structure, the outflow of concentrate means that only a very small filter cake can build up in front of the membrane.

Anyone who drives away for days or even weeks (business trip, vacation, expedition) or temporarily lives in a second home, absolutely needs a reverse osmosis system with individually time-controlled, automatic rinsing functionin order to avoid contamination of the membrane by standing water.
Anyone who does not have someone to regularly operate the RO system during their absence are well taken care of with these reverse osmosis systems:
Water-helps-permeate S - the economical one with a time-controlled flushing function or
Water-helps-Booster S - The powerful one with a time-controlled flushing function.
Both filter systems can easily be controlled via a timer so that they z. B. Flush for a few minutes every 24 hours during your vacation days and thereby prevent
that algae form a biofilm on the membrane and grow through it.

The substances contained in the raw water (tap water) (e.g. lime, lead, uranium, spray and fertilizers, drug residues, microplastics, etc.) bounce off the membrane.
They are not collected, but immediately discharged into the wastewater so that the membrane does not clog and the "transmembrane flow" flux is maintained.

(The only exception: mobile tabletop systems without a water connection.
In these, the wastewater is not automatically drained off immediately, but is repeatedly pressed against the membrane in a concentrated form, which is why this technology cannot provide retention values ​​that are as good as all other RO systems.)

Content created by Jens-Peter Kleinhenz
revised and supplemented by Rudolf Schnappauf