The principle of reverse osmosis
Reverse osmosis - principle, function and use
Reverse osmosis (RO) is one of the most effective methods of water filtration that removes unwanted substances down to the molecular level. The technology is used for the treatment of drinking water, purification of process and waste water or desalination of seawater. RO is the ideal solution for home filtration of drinking water and ensuring clean water not only from wells but also from rainwater. This article describes in detail the principle of operation, construction and possible applications of RO.
1. What is reverse osmosis?
Reverse osmosis is a process in which water passes through a semi-permeable membrane under pressure. This separates unwanted substances such as salts, heavy metals, bacteria and viruses. This method is used not only for filtration of well water but also for demineralisation of water, thus obtaining demi-water suitable for various technical applications.
Classification of filtration methods according to particle fineness:
- Microfiltration (0.1-10 µm) - captures bacteria and solid particles.
- Ultrafiltration (0.01-0.1 µm) - removes viruses and large organic molecules.
- Nanofiltration (0.001-0.01 µm) - partially removes salts and heavy metals.
- Reverse osmosis (0.0001-0.001 µm) - effectively removes up to 99% of all dissolved substances.
2. The principle of reverse osmosis
Osmosis is a natural physical process in which a solvent (e.g. water) flows spontaneously through a semi-permeable membrane into a region of higher solute concentration.
In reverse osmosis, we reverse this flow by using a pressure higher than the osmotic pressure. The water is forced to pass through the membrane in the opposite direction, with impurities and salts remaining on the inlet side. This process is commonly used for filtration of well water, rainwater and to achieve osmotic water of high quality.
3. Water quality after RO treatment
Reverse osmosis can remove a wide range of undesirable substances. The filtration efficiency depends on the membrane properties and operating conditions.
Table: Removal efficiency of different substances by RO
| Látka | Efektivita odstranění (%) |
| Bakterie, viry | 99+ % |
| Těžké kovy (olovo, kadmium) | 96–99 % |
| Dusičnany, chloridy | 93–96 % |
| Vápník, hořčík (tvrdost vody) | 93–97 % |
| Sodík, draslík | 92–98 % |
In addition to removing contaminants from the water, it is also important to consider remineralisation as RO treated water can be depleted of some minerals. This treatment is especially essential when osmotic water is consumed over a long period of time.
4. Components and design of RO units
RO systems consist of several stages of filtration to ensure maximum water purification efficiency.
A standard five-stage RO unit includes:
- Sediment filter - removes mechanical contaminants (sand, rust, silt).
- Carbon filter (GAC) - absorbs chlorine and organic matter.
- Block carbon filter - removes finer organic matter and improves the taste of the water.
- RO membrane - the main filtration stage, separates most unwanted substances.
- Post-carbon filter - improves water taste and quality.
Specialised filters are available for different types of water, such as a filter for ferrous well water, a filter for rainwater or a hard water filter to help reduce hardness.
5. Maintenance and lifetime of membranes
For optimum performance, the membranes must be cleaned and regenerated regularly. Special solutions are used to remove mineral and organic deposits. The lifetime of the membrane varies between 3 and 5 years depending on the quality of the incoming water.
6. Use of reverse osmosis
Reverse osmosis finds a wide range of applications:
- Home filtration of drinking water - provides clean water free of impurities and pollutants.
- Rainwater filtration - allows rainwater to be used for domestic purposes.
- Well water filtration - suitable for households that draw water from wells.
- Industrial applications - production of demi water for technological processes.
- Food industry - use in the production of beverages and processing of raw materials.
7. Disadvantages of RO treated water
Although reverse osmosis provides very pure water, it can have some negative characteristics:
- Low mineral content - water after RO can have lower calcium and magnesium content.
- Possible health effects - long-term consumption of demineralised water can affect electrolyte balance.
- Higher water acidity - demineralised water may be slightly acidic and potentially corrosive.
8. And what is good to know
Many people incorrectly use the term *reverse osmosis* when searching for information about this technology, when the correct term is *reverse osmosis*, which refers to the process of purifying water using a semi-permeable membrane.
The reverse osmosis process does not remove dissolved gases from the water. Furthermore, gases can also be re-dissolved in demineralised water. Due to the absence of other minerals, this increases the acidity of the water (lowers the pH value because the dissolved gases are acidic) and the water then has corrosive effects. Also, demineralised water does not form passivating scale coatings, which protect to some extent against corrosion.
Unsuitable materials for demineralised water distribution
Bronze, brass, carbon steel, galvanised steel, chrome-plated steel and a combination of aluminium and copper components are not recommended for demineralised water pipes.
Applicable materials are plastic (PP), stainless steel, copper or aluminium.
Legislative requirements for drinking water
Czech legislation requires a minimum of 30 mg/l calcium and 10 mg/l magnesium in drinking water. Other parameters are not limited by the lower limit. This requirement can be met by using a bypass or by installing a semi-burnt dolomite (PVD) filter, which adds calcium and magnesium to the water by gradual dissolution. However, PVD dissolution cannot reach the levels required by legislation.
Reverse osmosis is a highly efficient water treatment technology that finds application in domestic, industrial and food processing applications. Proper maintenance of membranes and possible remineralisation of water can eliminate its drawbacks and ensure quality drinking water.
