Reverse osmosis

Osmosis and Reverse Osmosis: What they are?

If we connect two solutions separated by a membrane, the first with low salt concentration and the second with high salt concentration, there will be a passage of the solvent (water) from the first solution to the second.

The water will then begin to migrate due to a force that is generated naturally: this force is called osmotic pressure.

In reverse osmosis, a pump system is used, which generates a pressure greater than the osmotic pressure that opposes it; the water is thus pushed against the membrane and is forced to migrate from the more concentrated to the more diluted phase.

The semi-permeable membrane allows only the water molecules to pass through, retaining the other chemicals. In other words, it is possible to isolate the solvent, i.e. pure water.

This technique, known as reverse osmosis, is much more efficient than mere filtration because the membrane does not simply function as a physical trap, but also acts at the chemical level with specific interactions between the membrane itself and the chemicals.

The chemical content of water and how it is measured

How much dissolved matter do we drink in water?

The value obtained takes the name of dry residue at 180 °C or, more simply, fixed residue.

The fixed residue therefore tells us how much matter is dissolved, but not what type of matter it is (for that, chemical analysis is necessary), however this parameter is very important becauseit gives anidea of the characteristics and properties of awater and it is used as a discriminant to classify commercial mineral waters.

  • FR less than 50 mg/l: minimally mineralised water
  • FR less than 500 mg/l: water with trace minerals
  • FR between 500 and 1000 mg/l: medium-mineral water
  • FR greater than 1000 mg/l: water rich in mineral salts

Modern science tends to consider minimally mineralised waters and waters with trace minerals better for human consumption.

Osmotised water is harmful because it is free of mineral salts. Is this statement true?

We have said that low mineral content waters are nowadays considered healthier because of their properties (diuretic waters have already been highly publicised through advertising).

However, it is also known that if the water is too demineralised, these benefits are lost: on the contrary, the water becomes harmful.

We also said that reverse osmosis produces almost pure water ( fixed residue close to 0) because osmotic membranes let only water molecules pass through.

So far, it would appear that osmotised water is to be avoided. Yet reverse osmosis systems are used successfully all over the world and produce water that is anything but harmful.

The reason is that these systems allow you to arbitrarily regulate the fixed residue of the water that you want to obtain at the outlet.

This adjustment is achieved thanks to a bypass system controlled by a valve. The bypass ensures that only a defined percentage of water is pushed against the membranes; the remaining portion is diverted (“bypassed”) and remains as it is.

The two water flows are then reconnected and the bypassed water re-mineralises the osmotised water.

By adjusting the bypass valve you can select the desired fixed residue. That is, you can choose whether to drink minimally mineralised waterwater with trace mineralsmedium-mineralised water, etc. in line with your own preferences.

Percentage abatement of substances by reverse osmosis

The following table shows the abatement percentages for different chemical entities by reverse osmosis.

Element % reduction Element % reduction
Selenium93-98%Total hardness93-97%
Silica80-90%Carbon hardness.90-95%

Reduction of fixed residue

Waters containing high amounts of salts can pose many problems for both domestic and agricultural use.

The best system for treating these waters is certainly reverse osmosis.

This technology is based on the physical capacity of semi-permeable membranes, which can retain salts and other foreign agents, when they are crossed by the water to which the correct pressure is applied.

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