Which of the following is NOT a main stage of wastewater treatment?

In the context of wastewater treatment, the process is typically divided into three main stages: primary treatment, secondary treatment, and tertiary treatment. Each of these plays a vital role in removing contaminants and improving water quality before discharge or reuse.

Primary treatment involves the physical separation of solids from wastewater, primarily through processes such as sedimentation. Secondary treatment primarily focuses on the biological degradation of dissolved organic matter, where microorganisms break down remaining pollutants. Tertiary treatment, which can include advanced filtration and disinfection, is aimed at further improving water quality and removing specific contaminants that may not have been adequately addressed in the previous stages.

Filtration is a technique that is often used as part of tertiary treatment, but it is not classified as a stand-alone main stage of the wastewater treatment process. Therefore, it does not belong among the recognized stages of treatment, which is why it is the correct choice in this question. This distinction is crucial for understanding the overall structure and process of wastewater treatment.