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Which of the following is NOT a secondary treatment process?
Sequencing Batch Reactor
Aerated lagoons
Primary sedimentation
Activated sludge
The correct answer is: Primary sedimentation
The correct choice identifies that primary sedimentation is not a secondary treatment process. Secondary treatment processes are designed primarily to remove dissolved and suspended organic matter from wastewater, typically after primary treatment has already taken place. Primary sedimentation, on the other hand, is part of the primary treatment phase. It focuses on the physical removal of large solids and sediments from wastewater through settling. This process is essential for reducing the load that will be processed in secondary treatment but does not actively contribute to the biological processes that characterize secondary treatment. The remaining options, such as Sequencing Batch Reactors, Aerated Lagoons, and Activated Sludge systems, all refer to biological treatment methods that rely on organisms to break down organic matter in the wastewater. These methods are integral to secondary treatment processes, which aim to further purify the effluent following the removal of larger particulates during primary treatment.