Which sequence correctly lists the steps for disinfecting a new water main system?

• A. Pre-flushing, Chlorination, Final Flushing, Sampling/Analysis
• B. Chlorination, Pre-flushing, Final Flushing, Sampling/Analysis
• C. Pre-flushing, Final Flushing, Chlorination, Sampling/Analysis
• D. Sampling/Analysis, Pre-flushing, Chlorination, Final Flushing

Answer: (A)

Disinfection sequencing for a new water main is about cleaning first, then disinfecting, then clearing the system, and finally verifying safety with tests. Starting with pre-flushing helps remove loose debris and reduces the initial disinfectant demand, so the subsequent disinfection step is more effective and uniform throughout the mains. Introducing chlorine after pre-flushing provides the needed disinfectant contact to inactivate microorganisms as the water travels through the newly laid pipe. After the disinfectant has had time to act, final flushing clears out excess chlorine and any dislodged sediments, ensuring the water leaving the system isn’t overloaded with residual disinfectant and is visually clear. Only after this flushing should sampling and analysis be performed to verify that residual chlorine levels, bacterial indicators, and other quality parameters meet safety standards. Doing the steps in this order—pre-flushing, chlorination, final flushing, then sampling/analysis—ensures proper cleaning, effective disinfection, and reliable verification of safe water entering the distribution system.