Osmosis system - function and structure

This article explains in detail how a reverse osmosis system works, what its structure is, and why so much water is consumed during operation in a reverse osmosis system.

principle of operation

During the osmosis process in nature, concentrations between a semipermeable membrane tend to balance the concentration gradient. If, on the one hand, there is a low-concentration solution and, on the other hand, a solution with a higher concentration, two things can happen, depending on which side the membrane is permeable to:

  • Water flows from the lower concentration side to the higher one to dilute it
  • Ions flow from the solution from the higher concentration to the lower concentration

Between the two sides osmotic pressure prevails, which is dependent on the concentration of the solution.

In nature, transport processes are accomplished in the cells in this way. In reverse osmosis this principle is reversed.

At a pressure higher than the osmotic pressure, the water is pressed against the membrane, leaving all ions of the higher concentration behind. Due to the very small pore size (up to 0.0001 micron) of the membrane, all molecules that are larger than the water molecules remain at the membrane. That's about 99.9 percent of all contaminants.

Prevention of clogging of the membrane

As there are many dissolved substances in the water (pollutants, lime, minerals), the membrane becomes very fast with the so-called concentrate. The osmotic pressure would be insurmountable at this concentration gradient between concentrate on the one hand and permeate on the other hand, the process would come to a standstill.

Therefore, it is repeatedly rinsed with nachkommendem water to rinse the forming "filter cake" of the membrane. That's why most reverse osmosis systems use so much water to clean the water. Individual plants need nearly 10 liters of water to produce one liter of clean water.

Tips & Tricks

Systems with cross-flow filtration work on a more complex, but water-saving principle. But more energy is needed.

Video Board: Anatomy and physiology of the respiratory system