Which step in the disinfection process is used to verify the effectiveness before the main is returned to service?

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

Answer: (D)

The key idea is that you must prove the disinfection worked before returning the system to service, and this proof comes from sampling and analysis. After the disinfection steps are complete, representative water samples are collected and tested for indicators of microbial contamination and for the presence of a safe disinfectant residual. This testing shows whether the disinfection achieved the required microbial safety and that the residual disinfectant is at protective levels throughout the system. Only if the samples meet the established standards should the system be put back into service. The other steps—pre-flushing to clear debris, applying the disinfectant, and flushing to remove excess residuals—prepare and execute the disinfection, but they don’t by themselves demonstrate that it was effective. Sampling/analysis provides the actual verification.