What is dissolved oxygen?

Dissolved oxygen (DO) has the same structure as the oxygen we breathe in. The simple difference is that it is dissolved in water. This means that free oxygen molecules are available to be consumed by living organisms and play a very important role in many chemical processes.
Oxygen can enter water bodies in various ways, e.g. through gas exchange with the atmosphere, through photosynthesis by aquatic plants and through aeration systems. 
The typical units to express the amount of oxygen dissolved in water are milligrams per litre (mg/l) or parts per million (ppm) for concentration and percentage (%) for saturation.

Where is dissolved oxygen measured?
The control of dissolved oxygen is essential in many applications to make processes safe, efficient and cost-effective. 
In biological wastewater treatment plants, nitrification and denitrification play an important role in removing nitrogen from municipal waste water. While no oxygen is desired for the anaerobic process of denitrification, just the right amount of dissolved oxygen is needed for nitrification. In the aeration process of nitrification, the DO control assists the oxygen delivery to the aerobic bacteria in order to control the exact amount needed for organic waste conversion. The precise regulation of the oxygen concentration enables targeted cost control and cost savings.
Another important parameter that can be determined with the help of oxygen sensors is the biological oxygen demand after 5 days (BOD5). BOD5 describes the amount of oxygen consumed by bacteria and other microorganisms to decompose organic material in 5 days at a given temperature. In addition to the respirometric method, BOD5 can also be determined using the dilution method. Here, the concentration of dissolved oxygen in a sample is determined using an oxygen sensor at the beginning of an experiment and after 5 days. Afterwards, the BOD5 value is derived from the experiment.
Particularly low oxygen values are desirable in industrial water systems, for example to protect metallic components from corrosion and thus prevent enormous material and maintenance costs.
Since oxygen is especially important for the ecological balance in natural waters and fish farms, the concentration of dissolved oxygen must be controlled in order to create the right conditions for aquatic life. 
In the food and beverage industry, dissolved oxygen plays a significant role in ensuring the consistent quality of products. This includes product properties such as taste and colour.

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