How long must mains be chlorinated before testing?

The duration for which mains must be chlorinated before testing is established to ensure effective disinfection of the new water mains. The correct answer is 24 hours, which allows sufficient time for the chlorine to act on any microbial contaminants that may be present in the system. This prescribed duration helps to achieve an effective level of chlorination, ensuring that the water is safe and free from pathogens.

Chlorination serves to disinfect the pipes and any connections where bacteria might reside, and testing after 24 hours provides an adequate window for accumulating a concentration of chlorine that can be measured. This ensures accurate testing results, allowing operators to ascertain whether the disinfection was successful and if the water quality is acceptable for service.

The other options suggest durations that are either shorter or longer than the recommended 24 hours. Chlorinating for a shorter time may not provide effective disinfection, while extending the duration excessively could lead to other issues, such as chlorine by-product formation or persistent chlorine taste and odor in the water. Hence, adhering to the 24-hour standard is critical for maintaining water quality and safety.