Extended process has the maximum aeration period among the given options.

Explanation:
Aeration is an important process in wastewater treatment plants, as it helps in the removal of organic matter and other pollutants from the wastewater. The sludge process refers to the treatment of the solid waste generated during the wastewater treatment process.

Different sludge processes are used in wastewater treatment plants, each with its own advantages and disadvantages. Among the options given, the extended process has the maximum aeration period. Let's understand why:

1. Conventional Process:
- In the conventional process, the sludge is aerated for a specific period, usually ranging from a few hours to a day.
- The aeration period is determined based on the oxygen requirement for the breakdown of organic matter present in the sludge.
- However, the aeration period in the conventional process is generally shorter compared to other processes.

2. Extended Process:
- The extended process, also known as the extended aeration process, involves a longer aeration period compared to the conventional process.
- In this process, the sludge is continuously aerated for an extended period, usually ranging from several days to weeks.
- The extended aeration period allows for the complete breakdown of organic matter, resulting in higher removal efficiency.
- This process is commonly used in small to medium-sized wastewater treatment plants, where a longer retention time is required for effective treatment.

3. Step Aeration Process:
- In the step aeration process, the aeration period is divided into multiple steps or stages.
- The sludge is aerated in different stages, with each stage having a specific aeration period.
- This process allows for better control over the oxygen supply and helps in achieving higher treatment efficiency.
- However, the aeration period in each stage is relatively shorter compared to the extended process.

4. Tapered Aeration:
- Tapered aeration refers to gradually reducing the aeration period over time.
- This process is commonly used in activated sludge systems, where the aeration period is gradually reduced during the sludge settling phase.
- The tapered aeration helps in improving the settling characteristics of the sludge, resulting in better solids separation.
- However, the overall aeration period in the tapered aeration process is generally shorter compared to the extended process.

Based on the above explanations, it can be concluded that the extended process has the maximum aeration period among the given options.