In general, The rate of chlorine application for disinfecting a newly installed pipe should result in a uniform free chlorine concentration of ______ at the end of the section being treated.

• A. 5 mg/L
• B. 25 mg/L
• C. 50 mg/L
• D. 15 mg/L

Answer: (B)

Disinfecting a newly installed pipe is about delivering enough chlorine so that, after the pipe’s chlorine demand and any losses during contact, the far end still has a protective residual. Targeting a uniform 25 mg/L free chlorine at the end of the treated section ensures the entire length receives an effective disinfectant dose for the required contact time, while avoiding excessively high residuals that could cause taste, odor, or corrosion concerns once the section is flushed into the system. In practice, a higher concentration is introduced upstream so that, after reacting with the pipe interior, the end of the section reaches about 25 mg/L. After the contact time, flushing and testing confirm proper residuals throughout the system. Other end concentrations listed would be either too low to guarantee disinfection at the far end or unnecessarily high for this purpose, so 25 mg/L at the end is the appropriate target.