Biological ETP 
The water is sent to the biological oxidation tank through gravity that guarantees a homogenous flow to the biological oxidation treatment. The biological oxidation is the most important phase of the treatment; here the main activity is the biological oxidation, in order to achieve the degradation of the pollutants that are present in the water flow, which takes place in the following order:
1. Bacterial cells assimilation
2. Total biodegradation or mineralization
3. Partial biodegradation
4. Co-metabolisms
5. Bio-absorption by solid matrixes
The oxygen transfer from the gaseous phase to the liquid phase (water) is a crucial part of the wastewater treatment, especially when it is performed by the activated sludge. Since the solubility of oxygen in water is low, and thus the natural oxygen transfer is low, the oxygen that is necessary for the biological process oxidation does not enter the water naturally, also because the air-water interface is very limited, thus additional interfaces need to be created in order to carry out the process. Oxygen can be supplied as pure bubbles or as air. The air bubbles are generally chosen instead of the pure oxygen bubbles since the cost is much lower. The air is provided through blowers; once the air supply has been sent through the air piping, it arrives to the air diffusers.

Biological ETP 
The water is sent to the biological oxidation tank through gravity that guarantees a homogenous flow to the biological oxidation treatment. The biological oxidation is the most important phase of the treatment; here the main activity is the biological oxidation, in order to achieve the degradation of the pollutants that are present in the water flow, which takes place in the following order:
1. Bacterial cells assimilation
2. Total biodegradation or mineralization
3. Partial biodegradation
4. Co-metabolisms
5. Bio-absorption by solid matrixes
The oxygen transfer from the gaseous phase to the liquid phase (water) is a crucial part of the wastewater treatment, especially when it is performed by the activated sludge. Since the solubility of oxygen in water is low, and thus the natural oxygen transfer is low, the oxygen that is necessary for the biological process oxidation does not enter the water naturally, also because the air-water interface is very limited, thus additional interfaces need to be created in order to carry out the process. Oxygen can be supplied as pure bubbles or as air. The air bubbles are generally chosen instead of the pure oxygen bubbles since the cost is much lower. The air is provided through blowers; once the air supply has been sent through the air piping, it arrives to the air diffusers.