In the Modified Aeration process, what is the most nearly percentage of returned sludge to the aeration tank?

In the Modified Aeration process, the typical percentage of returned sludge to the aeration tank is around 10%. This process is designed to balance the need for adequate aeration of the wastewater while minimizing the amount of sludge that needs to be managed.

By returning a smaller percentage of the sludge, around 10%, the system maintains sufficient biomass in the aeration tank to effectively treat the wastewater, while also ensuring that organic loading is properly balanced. This helps in achieving efficient treatment without excessively increasing the solids concentration in the aeration basin, which could hinder the overall treatment process and lead to issues such as oxygen depletion or poor settling in secondary clarifiers.

The options that present higher percentages, such as 20%, 30%, and 40%, would suggest a heavier recycling of sludge, which is generally not characteristic of the Modified Aeration process. In such scenarios, the treatment could shift towards more complex operations and could demand more energy and resources to maintain the desired treatment efficiency. Therefore, the 10% figure aligns with the operational guidelines commonly practiced in Modified Aeration systems.