The maximum depth of an aeration tank used in the activated sludge process is 4.5m.

Explanation:
The activated sludge process is a widely used biological wastewater treatment method that utilizes microorganisms to break down organic matter in wastewater. The process takes place in an aeration tank, where oxygen is supplied to the microorganisms to support their growth and metabolic activities.

The depth of the aeration tank plays a crucial role in the efficiency and effectiveness of the activated sludge process. Here's why the maximum depth of the aeration tank is 4.5m:

1. Oxygen Transfer:
The depth of the aeration tank is directly related to the efficiency of oxygen transfer to the microorganisms. Oxygen is necessary for the microorganisms to degrade the organic pollutants in the wastewater. If the tank is too shallow, there may not be enough time for oxygen transfer, resulting in inadequate treatment. On the other hand, if the tank is too deep, the oxygen transfer efficiency may decrease due to increased hydrostatic pressure. A depth of 4.5m strikes a balance between efficient oxygen transfer and hydrostatic pressure.

2. Mixing and Suspended Solids:
The depth of the aeration tank also affects the mixing and suspension of the activated sludge. Adequate mixing is crucial to ensure the contact between the microorganisms and the organic matter in the wastewater. If the tank is too shallow, the mixing may be insufficient, leading to poor treatment efficiency. Conversely, if the tank is too deep, the energy required for mixing may increase significantly. A depth of 4.5m allows for effective mixing and suspension of the activated sludge.

3. Sludge Settling:
After the aeration process, the activated sludge containing the microorganisms and treated wastewater needs to settle. The depth of the aeration tank influences the settling characteristics of the sludge. If the tank is too shallow, the settling process may be hindered, resulting in carryover of sludge particles into the effluent. On the other hand, if the tank is too deep, the settling efficiency may decrease, leading to higher sludge losses. A depth of 4.5m provides optimal settling conditions for the activated sludge.

In conclusion, the maximum depth of an aeration tank used in the activated sludge process is 4.5m to ensure efficient oxygen transfer, effective mixing and suspension of sludge, and optimal sludge settling.