What does reverse osmosis do

How reverse osmosis works clearly explained

The natural principle of osmosis

To understand the principle of reverse osmosis, one must first look at the natural process of osmosis: Osmosis refers to the process of equalizing 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 semipermeable (semi-permeable) wall.

In nature, the osmosis principle is of the greatest physiological importance when only the solvent but not the dissolved substances can pass through the semipermeable membrane. This is because on the one hand the water balance of the cells can be regulated and on the other hand an internal pressure (turgor, osmotic pressure) can be maintained for stability.

From a physical point of view, the ion solutions - which are separated from one another by membranes - always strive to achieve a concentration balance. This means that ions want to move from the highly concentrated side to the lower concentration side.

Since the membrane represents a barrier that the ions cannot easily pass through due to their molecular size, the smaller water molecules flow instead from the lower concentration to the higher concentration. The water molecules flow until either the ion concentrations on the two sides are balanced or a pressure is built up on the highly concentrated side - the so-called osmotic pressure. The osmotic pressure of a very dilute solution obeys the laws that apply to ideal gases. It increases proportionally to the concentration of the solution and increases proportionally to the temperature.

How reverse osmosis works

In reverse osmosis technology, the previously described osmosis principle is reversed. On the side with the high ion concentrations (tap water, raw water) a pressure is applied which forces the water in the other direction, namely on the pure water side with the lower concentration.

The unwanted dissolved substances (e.g. hardness components, nitrate, silica, residues of pesticides and medicines, to name just a few) cannot get through the ultra-fine membrane due to their molecular size - on the pure water side there is almost exclusively water. Reverse osmosis technology is comparable to extremely fine filtration and is therefore also known as nanofiltration.

Waste water generation in reverse osmosis

Since tap water with the substances it contains constantly flows in during operation, the substances retained by the membrane must be continuously removed to prevent the membrane from clogging. As a result, a reverse osmosis system produces not only pure water but also wastewater (concentrate) that contains the undesirable substances in high concentrations and that are washed away. Here one of the major differences between reverse osmosis technology and techniques with accumulation filters becomes clear.

The degree of efficiency (amount of filtered water per amount of raw water from the pipe) is never one, since “waste water” is always created. The wastewater enriched with pollutants is drained off permanently, so that there can never be any accumulation of pollutants retained on the osmosis membrane. To reduce this waste water - and thus increase the efficiency - a patented permeate pump (currentless) is installed in our systems, which reduces the waste water by approx. 85%!

The membrane as the most important part of the reverse osmosis process

Membrane technology has developed significantly in recent years. While cellulose acetate membranes have been in use in recent years, the polysulfone membrane has established itself on the market in recent years. The membrane is a complex structure. The average lifespan of the membranes we use is approx. 5 to 7 years. The cleaning performance and the yield of a reverse osmosis membrane depend on many factors. For example, from the raw water pressure.

Our systems for the household usually work at a water pressure between 2.8 and 6 bar. However, if your water supply has less pressure, this is not a problem, as we also offer individual solutions. As the pressure increases, so does the amount of pure water produced.

If, for example, a system produces 30 l / day at 4 bar and 10 ° C, this system can produce twice the amount of pure water at twice the pressure. The ratio of the amount of concentrate to the amount of pure water also changes slightly at different pressures. But with small systems that are operated in the range of 3-6 bar, this slight change in the ratio can be neglected.

The temperature also changes the yield of the pure water. As the temperature rises, the mobility of the water molecules increases, and so more water can be pushed through the membrane. The pure water performance increases e.g. by 60% if the temperature is increased from 10 ° C to 25 ° C. However, an increase in performance by increasing the temperature should not be attempted, as reverse osmosis membranes are normally temperature-sensitive. A temperature of 30 ° C should not be exceeded.

NASA technology for everyone

The technology was developed in the 1960s on behalf of NASA, which needed a drinking water recycling system for manned space flights. To this day, all membranes from well-known manufacturers come from the USA. The most important field of application today is large-scale seawater desalination. Other areas of application can be found in the food industry (concentrating fruit juices), medicine (dialysis), waste water recycling (e.g. in electroplating plants). Reverse osmosis systems have long since found their way into households in the USA. A water filter with osmosis technology is now standard in a well-equipped kitchen.

If you would like to receive further information on water filtering with reverse osmosis systems, then take a look at our water information or find out more from external sites with qualitative articles.