Waterguide - Oxygen
What is oxygen?
Oxygen (symbol O, atomic number 6, atomic mass 16 g/mol) is the most common element found in the Earth’s crust and the geosphere (48.9 %). It is also the second most common element on the planet (30 %; iron is the most common). Oxygen is one of the elements that is essential to life on Earth, as every living thing requires it to breathe. It is also important for combustion processes, because nothing can burn without oxygen.
In what form do you find oxygen on Earth?
Molecular oxygen is found on Earth as an O2 molecule, but it also occurs as a compound with hydrogen (H) as water (H2O). This is another reason why it is vital to life, because water is also essential for all living things.
When oxygen reacts with strong UV rays and photochemical processes, it becomes the well-known chemical compound ozone (O3). The ozone layer protects humans and all other living creatures against dangerous UVB rays (the main cause of sunburn).
Combustion processes are differentiated by complete combustion and incomplete combustion. In a complete combustion process, oxygen combines with carbon (C) to form the well-known chemical compound carbon dioxide (CO2). If the combustion process is incomplete, the resulting compound is carbon monoxide (CO). Water also forms in each process.
How does oxygen get into water and what concentration of oxygen does water have?
As a gaseous substance, oxygen does not react with water, but rather naturally dissolves when it comes into contact with it (in normal conditions). But because oxygen is always coming into contact with water, there is a certain saturation point (10 mg/l at 15 °C).
The oxygen concentration varies somewhat depending on the type of water. In freshwater, the oxygen content is 88.85 %.
In seawater, the concentration is slightly lower at 86 %. This is due to the high salinity of seawater, which is higher than for freshwater.
Groundwater often does not contain any oxygen, but rainwater is saturated.
Oxygen also plays a part in lots of different reactions that produce water, such as in the combustion processes described above.
The higher the temperature of the water, the lower the oxygen content.
Oxygen use in technical applications
As previously mentioned, oxygen in used for chemical reactions in lots of industrial plants.
- Acid production (sulphuric acid production, nitric acid production, etc.)
- As pure oxygen for breathing in a medical context
- As an additive (oxygen gas) to waste water to promote the growth of bacteria cultures
- As an additive to foodstuff (for frothing cream)
Oxygen may have to be removed first depending on the application.
Oxygen in water treatment
There is no threshold for the amount of oxygen in the German Drinking Water Ordinance. All that is specified is that the saturation concentration should not be exceeded. However, drinking water should at least have a minimum concentration of oxygen to prevent pipe corrosion (pipe passivation).
In a water treatment context in the food and beverage industry, it is sometimes necessary to remove oxygen. In brewing, for example, if oxygen is not removed from beer, it causes oxidation and results in undesirable changes in taste. In such cases, oxygen can be removed from the feedwater through membrane degasification (MEG).
To remove iron, manganese and ammonium from well water, you generally need to add oxygen to act as an oxidising agent. Compressed air is fed into a tank called an “oxidator” via an injector. The oxidator should always have a ventilation valve, as this helps to prevent air bubbles from accumulating. Iron and manganese are oxidised by the oxygen into higher-order, insoluble forms. Ammonium is converted into nitrate. Finally, the iron and manganese particles can be filtered.