Detention period refers to the amount of time wastewater stays in a treatment system. In the case of oxidation ponds, the detention period is the time it takes for wastewater to flow through the pond before being discharged. The correct answer to the given question is option D, which states that the detention period for oxidation ponds is typically 3 months.

The purpose of the detention period is to allow sufficient time for the wastewater to undergo biological and chemical processes, leading to the reduction of pollutants and improvement in water quality. During this period, various mechanisms occur within the oxidation pond that contribute to the treatment of the wastewater.

Below is a detailed explanation of why the detention period for oxidation ponds is usually kept as 3 months:

1. Bacterial Growth and Activity:
- Oxidation ponds rely on the growth and activity of microorganisms, particularly bacteria, to break down organic matter in the wastewater.
- The detention period of 3 months allows sufficient time for the bacteria to multiply and establish a stable population, ensuring efficient treatment.

2. Biological Processes:
- During the detention period, biological processes such as aerobic and anaerobic decomposition take place.
- Aerobic bacteria break down organic matter in the presence of oxygen, while anaerobic bacteria break down organic matter in an oxygen-deficient environment.
- Both processes contribute to the removal of organic pollutants and the stabilization of organic compounds.

3. Algae Growth and Photosynthesis:
- Oxidation ponds also promote the growth of algae due to the availability of sunlight and nutrients in the wastewater.
- Algae perform photosynthesis, which helps in oxygenation and the removal of carbon dioxide from the water.
- The detention period of 3 months allows algae to reach a balanced growth, contributing to the overall treatment efficiency.

4. Settling of Solids:
- During the detention period, larger particles and solids in the wastewater settle at the bottom of the pond.
- This sedimentation process helps in the removal of suspended solids, reducing turbidity and improving water clarity.

5. Pathogen Removal:
- The 3-month detention period is also necessary for the inactivation and removal of pathogens, including bacteria, viruses, and parasites.
- Pathogens are naturally reduced through predation, exposure to sunlight, and competition with other microorganisms.

In conclusion, the detention period for oxidation ponds is typically kept as 3 months to ensure sufficient time for the biological, chemical, and physical processes to occur, resulting in effective treatment of wastewater.