Texas Commission on Environmental Quality (TCEQ). Class C Water License Exam

What is considered an acceptable disinfection residual at the ends of a water distribution system?

1. 0.1 mg/L free chlorine residual

2. 0.5 mg/L free chlorine residual

3. 0.8 mg/L chloramine residual

4. All of the above

The correct answer is: All of the above

In water distribution systems, maintaining acceptable levels of disinfectant residual is crucial for ensuring the microbiological safety of the water supply as it travels to consumers. Different disinfectants have varying acceptable residual levels, which can depend on several factors, including the specific compound used and water quality. A free chlorine residual of 0.1 mg/L is the minimum level to ensure that chlorine is effectively working as a disinfectant throughout the distribution system. Higher levels, like 0.5 mg/L, provide a more robust residual that offers greater protection against microbial contamination. For chloramines, a residual of 0.8 mg/L is generally considered effective. This level helps ensure that chloramines, which are less reactive than chlorine, still provide adequate disinfection while minimizing the formation of disinfection byproducts. Given these different disinfectants and their acceptable residual levels, it is valid to consider all these figures as acceptable in their respective contexts. Therefore, the answer reflects the idea that multiple acceptable residual levels exist depending on the chemical used, thus encompassing all options presented.