What is Chlorine (as CI2) primarily used for in water treatment?

Chlorine, in the form of chlorine gas (Cl2), is primarily used in water treatment for controlling microbes. The primary function of chlorine is to disinfect water by killing or inactivating harmful microorganisms, such as bacteria, viruses, and some parasites. This disinfection process is crucial in ensuring that the water is safe to drink and meets public health standards. Chlorine is effective because it can penetrate the cellular structures of microorganisms, leading to their destruction.

While other processes in water treatment may aim to improve taste, reduce hardness, or address temperature, these are not the primary roles of chlorine. For instance, chlorine does not reduce hardness, which relates to the concentration of calcium and magnesium ions in water. Additionally, it does not influence water temperature nor is it typically used to enhance the taste of water. Hence, the most accurate description of chlorine's primary function in water treatment is its role in controlling microbial populations.