As activated sludge process takes place in the presence of oxygen. An organism is said to be aerobic if it can only exist in the presence of free oxygen. A heterotroph is an organism that cannot fix carbon from inorganic sources (such as carbon dioxide), but uses organic carbon for growth. So, aerobic heterotrophs dominate in activated sludge process.

The dominating microorganisms in an activated sludge process reactor are aerobic heterotrophs.

Aerobic Heterotrophs:
Aerobic heterotrophs are microorganisms that require oxygen to carry out their metabolic processes and obtain energy by breaking down organic compounds. In an activated sludge process, the wastewater is mixed with microorganisms in a reactor tank, creating an environment rich in organic matter. The aerobic heterotrophs utilize the oxygen present in the reactor to decompose the organic matter and convert it into carbon dioxide, water, and energy. These microorganisms play a crucial role in the treatment of wastewater by breaking down organic pollutants.

Key Points:
1. Oxygen Requirement: Aerobic heterotrophs require oxygen to carry out their metabolic processes.
2. Decomposition of Organic Matter: These microorganisms decompose the organic matter present in the wastewater and convert it into simpler compounds.
3. Energy Production: As a result of the decomposition process, aerobic heterotrophs release energy that they use for their growth and reproduction.
4. Conversion of Pollutants: The organic pollutants present in the wastewater are converted into carbon dioxide and water, which are less harmful to the environment.
5. Role in Activated Sludge Process: The presence of aerobic heterotrophs is essential for the efficient treatment of wastewater in an activated sludge process. They help in reducing the organic load and improving the quality of the treated effluent.

Conclusion:
In an activated sludge process reactor, aerobic heterotrophs are the dominating microorganisms. They utilize the oxygen present in the reactor to decompose the organic matter in the wastewater and convert it into simpler compounds, carbon dioxide, water, and energy. Their presence is crucial for the effective treatment of wastewater and the removal of organic pollutants.