A plant is designated as an Activated Sludge Plant when:

An Activated Sludge Plant is defined by its process of treating wastewater using a mixture of partly treated sewage and activated sludge, which contains microorganisms. These microorganisms are essential in breaking down organic matter in the sewage, converting it into the biomass that helps reduce pollutants. The process allows for efficient treatment of sewage, significantly improving the quality of water that is discharged or further treated.

In this context, mixing partly treated sewage with activated sludge creates an environment conducive to biological treatment. During this aeration process, microorganisms consume organic material, ultimately resulting in clearer effluent. This core principle of activated sludge systems distinguishes them from other treatment methods and ensures effective sewage treatment.

The other options pertain to aspects of sludge management or alternative uses for sludge but do not define the fundamental process that characterizes an Activated Sludge Plant. Using thickened sludge as fertilizer or burning gases from sludge digestion focuses on post-treatment processes, while the requirement for drying is a method of preparing sludge for disposal. None of these options capture the critical mixing of sewage with activated sludge that is essential for the operation of an Activated Sludge Plant.