Which of the following is commonly used in the final treatment stage before being released into a stream?

Chlorine is widely used in the final treatment stage of wastewater before it is released into a stream. It serves as a disinfectant, effectively killing harmful bacteria, viruses, and other pathogens that may be present in the water. The use of chlorine ensures that the treated water meets health and safety standards for discharge into the environment, which is crucial for protecting aquatic ecosystems and public health.

While ozone and activated carbon are also used in some water treatment processes, they are typically employed in different contexts. Ozone is a powerful oxidant that can treat water by breaking down organic compounds and disinfecting it, but it may not always be used in the final steps due to its short-lived nature and the need for special equipment. Activated carbon is primarily used for removing organic contaminants and improving taste and odor, but it does not provide the same level of disinfection as chlorine. Hydrochloric acid is generally used to adjust pH levels rather than as a treatment for pathogens. Thus, the role of chlorine in disinfection makes it the most commonly used agent in the final treatment phase.