Reverse osmosis for water treatment

Reverse osmosis systems in the industry for the treatment of process water have steadily gained in importance in the past. Water is an increasingly scarce commodity in many places. At the same time, the demand for pure and ultrapure water for industrial production is growing, especially in the high-tech sector, e.g. in solar cell production or chip manufacturing. Adequate treatment, effective use and reuse in the form of water recycling are therefore decisive economic factors for many manufacturing companies. The use of reverse osmosis systems reliably ensures optimal results here.

If you are looking for a solution for industrial water treatment by means of reverse osmosis, WEIL Wasseraufbereitung is your competent partner. In addition to the construction and supply of reverse osmosis systems for industrial water treatment, our core competencies also include upstream planning services, including the preparation of studies and the execution of pilot tests. In the following, we explain the functioning of reverse osmosis, its advantages and the various possible uses.

Reverse osmosis systems make use of a principle that we know from nature: Osmosis.

Osmosis regulates the electrolyte and water balance of cells that make up humans, animals and plants. The process of osmosis is the diffusion of a substance, in the case of water treatment H2O, through a semipermeable (semi-permeable) membrane. Such a membrane is permeable only to certain substances. Water can pass through the pores of the membrane, but the substances dissolved in the water, such as salts or organic compounds, are retained.

In the natural process of osmosis, water flows through the membrane from one side to the other when there are more dissolved substances, such as salts, on one side than on the other side. The aspiration of nature is a state of equalization, in which the concentration of dissolved particles in the water is the same on both sides of the membrane. This can be observed using the example of fully ripe cherries: the outer skin of the cherries is a semi-permeable membrane. Inside the ripe cherries there is water with a high content of sugar and other dissolved substances. If rain falls on the outer skin of the cherries, the rainwater reaches the interior of the cherry through the so-called “osmotic pressure”, because the concentration of dissolved particles in the cherry is much higher than in the rainwater. The skin of the cherry can not expand indefinitely, and therefore the cherry eventually bursts.

In the treatment of water in the industrial sector, the principle of osmosis is used in reverse osmosis.

In the process of reverse osmosis, the content of dissolved salts such as sodium, potassium or nitrate and pollutants such as drug residues, pesticides and others in the water is reduced or removed. The principle of reverse osmosis is to reverse the natural process of osmosis. As already mentioned above, osmosis is a concentration equalization via a semipermeable membrane. If there is water with different salt concentrations on the two sides of the membrane, the water molecules migrate from the side with less salt through the membrane to the side with more salt. The more saline solution is thereby diluted. This water flow is based on the osmotic pressure caused by the different salt concentrations on the two sides of the membrane. The osmotic pressure thus conveys the liquid with the lower particle concentration in the direction of the liquid with the higher particle concentration.

If pressure is now built up on the side of the higher salt concentration, which is higher than the osmotic pressure, the process of osmosis is reversed. The water molecules from the more saline side are pushed through the membrane and flow to the side with a lower salt concentration. The low-salt water flowing through the membranes is called permeate and represents the desired product of the reversal. The portion of water with an increased salt content is called a concentrate and is discarded.

The average pore diameter of reverse osmosis membranes is about 0.5 nm to 1 nm and is therefore only slightly larger than the diameter of a water molecule. This can also be used to separate ions and dissolved salts. The required operating pressure of a reverse osmosis system can vary within a wide range of 5 – 100 bar, depending on the application.

Commercial and industrial use of reverse osmosis

The fields of application of reverse osmosis systems are diverse. The largest and most efficient systems can be found in industry and in the commercial sector, e.g. in solar cell production and chip manufacturing, in dialysis clinics and analysis laboratories as well as at colleges and universities.

In municipal drinking water treatment, the reverse osmosis process is used where there is not a sufficient supply of fresh water. On touristic islands such as the Canary Islands, for example, there are seawater desalination plants with capacities of several thousand cubic meters per hour. The technology is also used on cruise ships to provide guests and crew with clean drinking water, both land-based and on ships or offshore installations.

Use in the private sector

In addition to industrial and municipal applications, the process is also used in the private sector, e.g. to supply single- or multi-family houses with pollutant-free drinking water. Smaller systems are also increasingly being used in kitchens in private households.

Industrial reverse osmosis systems differ depending on the performance (pressure, throughput, energy requirement), type of membranes, number of filter stages and the associated quality of the permeate. The applications are divided into low pressure systems, high-pressure systems, systems with high retention, permeate-graded systems (2 stage), back-pressure systems and combined systems with upstream softening.

For the production of pure and ultrapure water with conductivities of < 0.2µS/cm, the process is used as a precursor of mixed bed filters and electrodeionization systems (EDI).

The need for pure and ultrapure water

Ultrapure water is required for the production of semiconductors or in solar cell production, in biotechnology and in pharmaceutical production.  This is water from which almost all dissolved salts and other ingredients have been removed to avoid impurities in the production process. Reverse osmosis systems are an essential part of the process combination in order to achieve the desired purity. As a preliminary stage of e.g. mixed bed filters or an electrodeionization, the reverse osmosis system ensures the perfect operation of the fine cleaning. With permeate-graded systems, very low conductivities of a few µS / cm are already achieved here.

Pollutant-free drinking water supply

Due to intensive use of groundwater and increasing dryness in more and more areas, there is a change in the water supply and water quality. At the same time, with increased knowledge due to new or more sensitive analysis methods and increased requirements for drinking water quality, more and more complex processes for drinking water treatment are becoming necessary. Reverse osmosis systems for the desalination of salty groundwater and for the treatment of seawater make a decisive contribution to securing the drinking water supply.