All PWS in Texas are required to use ________ as the final disinfectant.

The requirement for all public water systems (PWS) in Texas to use chlorine as the final disinfectant stems from its efficacy in killing pathogens and its residual properties. Chlorine is widely used because it not only effectively disinfects water by ensuring the inactivation of bacteria, viruses, and other microorganisms, but it also provides a lingering residual that helps prevent recontamination as water travels through pipelines to consumers.

Chlorine's ability to form chloramines, when combined with ammonia, enhances water stability and reduces the formation of disinfection by-products. This makes it a practical choice for maintaining water quality throughout distribution systems. While there are other disinfection methods such as ozone, ultraviolet light, and hydrogen peroxide—each with its benefits and specific applications—chlorine's combination of effectiveness, residual protection, and cost efficiency makes it the standard disinfectant mandated by regulations for final disinfection in Texas public water systems.