To understand why the maximum efficiency of BOD (Biochemical Oxygen Demand) removal is achieved in an oxidation ditch, let's first understand what BOD removal is and what the different treatment processes mentioned in the options entail.

BOD removal is a critical step in wastewater treatment, as it measures the amount of oxygen required by microorganisms to break down organic matter in water. The higher the BOD, the more oxygen is consumed by microorganisms, leading to oxygen depletion in water bodies, which can harm aquatic life.

Now, let's evaluate the different treatment processes mentioned in the options:

1. Oxidation ponds: Oxidation ponds, also known as stabilization ponds, are shallow basins that use sunlight and natural processes to treat wastewater. They rely on algae and other microorganisms to remove organic matter through photosynthesis and microbial degradation. While they can provide some BOD removal, their efficiency is limited due to factors like temperature fluctuations, limited control over treatment conditions, and potential odor issues.

2. Aerated lagoons: Aerated lagoons are similar to oxidation ponds but incorporate mechanical aeration systems to enhance the treatment process. The addition of oxygen through aeration promotes the growth of aerobic microorganisms, which break down organic matter more efficiently. However, their BOD removal efficiency may still be lower compared to other advanced treatment processes.

3. Trickling filter: A trickling filter is a fixed-film biological treatment process where wastewater is trickled over a bed of rocks or plastic media coated with microorganisms. As the wastewater flows over the media, the microorganisms attached to the media break down the organic matter. While trickling filters can provide effective BOD removal, their efficiency may be limited compared to other processes due to clogging issues and potential sludge accumulation.

4. Oxidation ditch: An oxidation ditch is an extended aeration activated sludge process. It involves a long, narrow channel or ditch where wastewater is continuously aerated and mixed with microbial biomass (activated sludge). The activated sludge contains a high concentration of aerobic microorganisms that efficiently break down organic matter, resulting in high BOD removal efficiency. The extended aeration process allows for a longer detention time and promotes the growth of a diverse microbial community, leading to enhanced treatment performance.

In summary, the oxidation ditch process offers the maximum efficiency of BOD removal due to the following factors:

- Continuous aeration and mixing: The continuous aeration and mixing ensure the availability of oxygen for the aerobic microorganisms, allowing them to efficiently degrade the organic matter.

- Extended detention time: The long, narrow channel in the oxidation ditch provides an extended detention time, allowing sufficient contact between the wastewater and the activated sludge, which enhances the BOD removal efficiency.

- Diverse microbial community: The extended aeration process promotes the growth of a diverse microbial community, including different types of bacteria and fungi, which collectively contribute to the breakdown of organic matter.

- Control over treatment conditions: Unlike natural systems like oxidation ponds, oxidation ditches offer better control over treatment conditions, such as temperature, pH, and dissolved oxygen levels, which can optimize the performance of the microorganisms involved in BOD removal.

Therefore, the oxidation ditch process is the most suitable option for achieving maximum efficiency in BOD removal.