Final answer:
The disinfection of water with UV light and Chlorine occurs in the tertiary (Final) stage of water treatment, targeting remaining harmful pathogens.
Step-by-step explanation:
The step of water treatment that includes disinfecting water with UV light and Chlorine is the tertiary (Final) stage. Disinfection serves to kill any remaining harmful bacteria and other pathogens in the processed wastewater. During this stage, water is treated with disinfectants like chlorine or it can be exposed to ultraviolet (UV) light. The use of UV light is a chemical-free process and effective against resistant parasites such as Cryptosporidium or Giardia. However, UV disinfection does not remove dissolved organics, inorganic compounds or particles from the water.