When collecting fecal coliform samples from chlorinated effluents, which practice must be applied?

• A. Dechlorinate the sample.
• B. Do not dechlorinate the sample.
• C. Add chlorine to the sample to enhance detection.
• D. Use deionized water to preserve chlorine.

Answer: (A)

Residual chlorine in chlorinated effluents can kill or inhibit fecal coliforms, leading to falsely low counts in culture-based tests. Dechlorinating the sample neutralizes that residual disinfectant, allowing viable bacteria to grow and be detected during analysis. The dechlorinating agent (commonly sodium thiosulfate) is added in amounts specified by the testing method so that chlorine is neutralized without harming the bacteria or the test reagents. If chlorine is left in the sample, it can continue to inactivate organisms and skew results downward. Conversely, adding chlorine to the sample would worsen the inhibition, and using deionized water does not neutralize chlorine, so it would not correct the interference.