What RO removes from the water

Reverse osmosis

Reverse osmosis, often also called UO or RO (from the English Reverse Osmosis), is a process in which water is pressed through a semi-permeable membrane under high pressure, thereby demineralizing the water.

To understand the purpose and process of reverse osmosis, it is necessary to first explain the term osmosis. Osmosis is a naturally occurring phenomenon in which a weaker salt solution tends to migrate to a strong salt solution in order to dilute it or to equalize both concentrations in the liquids. The force with which the low-concentration solution or the solvent is attracted by the higher-concentration solution is called osmotic pressure designated.


Figure 1 shows the osmosis phenomenon. A semipermeable membrane is arranged between 2 chambers which are filled with two differently concentrated liquids, pure water on one side and salty water on the other. Semi-permeable means semi-permeable, which means that the membrane is used for some things, e.g. certain molecules, is permeable and not for others. In osmosis (as in reverse osmosis), the membrane is mainly permeable to water molecules, but not to most dissolved ions (salts) or other impurities.

In order to achieve equilibrium (the same salt concentration on both sides of the membrane), the water migrates from the pure water compartment into the saline compartment in order to dilute the salt solution.

In the figure you can see a different height of the two solutions. This height is due to the fact that the water can only flow through the membrane in one direction in order to balance the concentration on both sides, since the dissolved salts cannot pass through the membrane. It does this until the pressure of the water column is as high as the osmotic pressure and the flow of water stops. The height of the water column or the water pressure against the membrane corresponds to the osmotic pressure of the liquids at the equilibrium point.

Figure 1 - Phenomenon of osmosis

Reverse osmosis

In order to generate pure water with the help of this phenomenon, a "force" (pressure) must be applied to the side with the salty water in order to make the water flow in the opposite direction and to transport the water molecules through the semipermeable membrane to the fresh water compartment .This is the basis of the Reverse osmosis process (Figure 2). The pressure applied must exceed the osmotic pressure.

Figure 2 - Reverse osmosis principle

When pressure is applied to the concentrate solution, the water molecules are pushed through the semipermeable membrane, but not the larger and heavier molecules, such as most salts and other impurities. A few small dissolved salts are pressed through the membrane with them. Depending on the membrane, the composition of the water and the temperature, the water can be desalinated up to 99%.

How does reverse osmosis work?

The reverse osmosis process is a cross-flow filtration process in which the "salty" feed water flows across the membrane surface after it has been pressurized with the aid of a high pressure pump. The water molecules are pressed through the membrane and the water is divided into a permeate and a concentrate stream. The concentrate stream (with approx. 95-99% of the remaining dissolved salts) remains on the side of the feed water and leaves the module on the other side, the permeate is the desalinated pure water that has permeated through the membrane.

In Figure 3 you can see a simplified diagram of a membrane system. The feed water is divided into a desalinated permeate or pure water and a concentrated concentrate or brine or reject stream.

Figure 3 - Simplified reverse osmosis scheme

What does reverse osmosis remove from the water?

With the reverse osmosis membrane, dissolved salts (ions), particles, colloids, organic substances, bacteria and pyrogens can be removed from the water.

An RO membrane repels contaminants based on their size and charge. Any contamination with one Molecular weight more than 180 Da is most likely rejected by an RO system. Likewise, the larger the Ionic chargethe more likely it is that it cannot pass through the RO membrane.

Remove the RO membranes no gases like CO2 or O2. These gases are not highly ionized (charged) in solution and have a very low molecular weight.

Water is always electrically neutral, which means that the sum of positively charged cations and negatively charged anions (expressed in equivalents per liter, meq / L) must be the same. This therefore also applies to RO permeate; for every cation that permeates through the membrane, an anion must also follow.

Permeate is always electrically neutral in the ion balance.


Applications of reverse osmosis

Reverse osmosis is very suitable for the treatment of ground and tap water, sea water and also well pre-cleaned surface water. Generally speaking, brackish water is often used, i.e. bodies of water with a high salt content, although no clear limit for the salt concentration is specified and this varies per language.

Some application examples:

  • Municipal drinking water
  • Food and beverage industry
  • Agricultural irrigation
  • Industrial ultrapure water
  • Industrial process water
  • Wastewater reuse
  • Energy industry (boiler feed water, cooling towers)
  • Municipal / service water reuse
  • households

Lenntech offers sustainable complete solutions with reverse mosquito technology and the associated necessary pre- and post-treatment steps.

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