Introduction:
An oxidation pond is a type of wastewater treatment system that utilizes natural processes to remove organic matter and pollutants from wastewater. It is primarily used in rural areas or small communities where conventional treatment methods may not be feasible. The efficiency of a wastewater treatment system is often measured by its ability to remove biochemical oxygen demand (BOD), which is a measure of the amount of oxygen consumed by microorganisms during the decomposition of organic matter in the water.

Explanation:
The maximum BOD removal efficiency of an oxidation pond is 90%. This means that the pond is capable of removing up to 90% of the organic matter present in the wastewater. The remaining 10% may still contain some organic pollutants that could be harmful to the environment if discharged without further treatment.

Factors Affecting BOD Removal Efficiency:
The efficiency of BOD removal in an oxidation pond can be influenced by various factors, including:

1. Retention Time: The length of time that the wastewater remains in the pond affects the extent of organic matter degradation. Longer retention times allow for more complete decomposition of organic matter and higher BOD removal efficiency.

2. Oxygen Transfer: Adequate oxygen supply is necessary for the growth and activity of aerobic microorganisms that decompose organic matter. Oxygen can be supplied through natural means, such as diffusion from the atmosphere or photosynthesis by algae, or through mechanical aeration systems.

3. Temperature: The rate of microbial activity and organic matter decomposition is influenced by temperature. Higher temperatures generally result in faster degradation and higher BOD removal efficiency.

4. Algae and Macrophytes: Algae and macrophytes play a crucial role in an oxidation pond by producing oxygen through photosynthesis and absorbing nutrients from the wastewater. They help create an environment conducive to the growth of aerobic microorganisms and enhance BOD removal efficiency.

5. pH and Nutrient Availability: Optimal pH conditions and the availability of essential nutrients, such as nitrogen and phosphorus, are necessary for the growth and activity of microorganisms involved in BOD removal.

Conclusion:
The maximum BOD removal efficiency of an oxidation pond is 90%. This efficiency can be achieved by optimizing factors such as retention time, oxygen transfer, temperature, presence of algae and macrophytes, pH, and nutrient availability. It is important to note that while oxidation ponds can be effective in removing a significant portion of organic matter, they may not be suitable for treating highly polluted or industrial wastewater. In such cases, additional treatment processes may be required to meet the required effluent quality standards.