During disinfection of a newly installed pipe, achieving a uniform free chlorine concentration at the end of the treated section is intended to be what value?

• A. 10 mg/L
• B. 25 mg/L
• C. 40 mg/L
• D. 5 mg/L

Answer: (B)

The main idea is to ensure the interior of the new pipe is disinfected evenly, which means the free chlorine remains at a level high enough at the end of the treated section to contact all surfaces and achieve effective disinfection. A target of about twenty-five milligrams per liter at the end of the treated section is used because it provides strong enough residual to overcome any chlorine-demand from the pipe materials, fittings, or residues and to sustain the required contact time across the entire length. This level helps ensure uniform disinfection throughout the section rather than just near the inlet. Values much lower would risk pockets of inadequate disinfection, while much higher levels are more challenging to manage and would need longer flushing and subsequent decay before regular service. After the disinfection process, the line is flushed and the chlorine residual is reduced to normal operating levels before customers are supplied.