Autotroph comes from the Greek language and means self-nourishing. This means that organisms are able to metabolise purely inorganic substances with an energy source (light, chemical) and generate biomass growth.

There are different types of autotrophy:

- Photoautotrophy: Use of light for energy production and metabolism by means of photosynthesis
- Chemoautotrophy: Use of chemical energy and CO₂ for metabolism and biomass development

The main bacteria used in wastewater treatment for nitrogen decomposition are chemoautotrophic bacteria which are able to metabolize by the oxidation of ammonium and CO₂. The energy gained from the chemical reaction is sufficient to build up biomass from the carbon dioxide and to ensure the growth of these bacteria. However, these bacteria grow very slowly - therefore certain boundary conditions such as sufficient aerobic sludge age (depending on the temperature) must be guaranteed to keep this type of bacteria in the system.


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