Biochemical treatment of sewage effluents involves the use of microorganisms to break down organic matter and pollutants present in the wastewater. This process primarily involves oxidation reactions. Let's break down the process in more detail:

1. Introduction:
- Biochemical treatment is a commonly used method for treating sewage effluents to remove organic contaminants and improve water quality.
- This treatment process harnesses the natural abilities of microorganisms to degrade organic matter.

2. Oxidation:
- The primary process that takes place during biochemical treatment is oxidation.
- Oxidation involves the transfer of electrons from one molecule to another, resulting in the breakdown of organic compounds into simpler forms.
- Microorganisms, such as bacteria and fungi, play a crucial role in this process by utilizing the organic matter as a source of energy and nutrients.
- These microorganisms use enzymes to break down complex organic compounds into simpler molecules, such as carbon dioxide and water.
- The oxidation process helps in the decomposition of organic matter and the conversion of pollutants into less harmful substances.

3. Other processes:
- Carbonation and hydration are not directly involved in the biochemical treatment of sewage effluents.
- Carbonation is the process of converting carbon dioxide into carbonates, which is not a significant transformation occurring in sewage treatment.
- Hydration is the process of adding water to a compound, which is not the primary mechanism in the treatment of sewage effluents.

4. Conclusion:
- The correct answer is option 'A' - oxidation.
- During biochemical treatment of sewage effluents, microorganisms utilize the organic matter present in the wastewater as a source of energy, leading to the oxidation of organic compounds into simpler forms.
- This process helps in the degradation of organic matter and the removal of pollutants from sewage effluents.