What is the correct sequence of steps for disinfecting a new water main?

• A. Pre-flushing, chlorination, final flushing, and sampling/analysis
• B. Pre-flushing, final flushing, chlorination, and sampling/analysis
• C. Chlorination, pre-flushing, final flushing, and sampling/analysis
• D. Pre-flushing, chlorination, sampling/analysis, final flushing

Answer: (A)

The main idea is to ensure the new main is clean, properly disinfected, and verified before serving water to customers. Start with pre-flushing to remove sediments and debris that could shield microbes or react with chlorine. This step helps the disinfectant work more effectively and gives a clearer starting condition for the pipe surface.

Next, apply chlorine to disinfect the main. The goal here is to achieve an adequate, uniform disinfectant concentration throughout the new pipeline so any potential microbes are inactivated.

Then perform final flushing to move the disinfectant through the system, remove the initial slugs of water containing higher levels of byproducts and residuals, and bring the line to a stable state that reflects normal operation conditions.

Finally, collect samples for analysis to confirm that the disinfection was effective and that the water meets regulatory criteria, including appropriate residual chlorine and absence of microbial contamination.

If sampling were done earlier, or if flushing came before disinfection, results could be misleading due to sediments, uneven chlorine contact, or high residuals. This sequence—pre-flushing, chlorination, final flushing, then sampling—best ensures the main is clean, properly disinfected, and its water quality is verifiably safe before commissioning.