Which of the following is NOT a type of preliminary treatment?

Preliminary treatment is a crucial phase in wastewater treatment that focuses on removing large solids, debris, and other materials that could hinder subsequent treatment processes. Screens, grinders, and grit channels are all essential components of this initial stage.

Screens function to filter out large objects like sticks, leaves, and plastics from the wastewater, preventing damage to machinery in later processes. Grinders help in reducing the size of larger solids to facilitate their removal or processing. Grit channels are designed to settle and remove heavier particles such as sand and gravel from the wastewater stream.

Flocculators, on the other hand, are typically associated with the secondary treatment stage, where they aid in the aggregation of fine particles into larger flocs that can be more easily settled or filtered out. This technique is critical for enhancing the efficiency of biological treatment methods rather than for initial solid removal. Thus, flocculators do not fall under the category of preliminary treatment, making it the correct answer in the context of identifying what is NOT a type of preliminary treatment.