Which of the following is NOT a stage of water treatment?

In water treatment processes, the stages are typically composed of primary treatment and secondary treatment, which are crucial for the removal of solids and organic material from wastewater. Primary treatment involves the physical separation of solids from liquid through processes like sedimentation. Secondary treatment focuses on biochemical processes to remove dissolved organic matter.

Ultraviolet light is a disinfection method often used to eliminate pathogens from water but is not considered a treatment stage itself. Instead, it's a technology or method applied after the primary and secondary treatments.

Lime, on the other hand, is often used as a chemical agent to precipitate certain contaminants or adjust pH levels but does not represent a distinct stage of treatment. It plays a role in processes like softening or stabilization but is not classified as a standalone treatment stage. Therefore, identifying lime as not being a treatment stage accurately reflects the structure of water treatment practices, where the focus is on established stages like primary and secondary treatments and specific disinfection methods like UV light.