How must new mains be disinfected before being put into service?

The correct approach for disinfecting new mains before they are put into service is to use a dosage of 50 mg/l of Cl2 for a duration of 24 hours. This method is designed to ensure effective disinfection of the entire length of the new piping system.

Disinfecting mains is a critical step to eliminate pathogens and prevent contamination when the system is first installed or after maintenance activities. The specific dosage and contact time are based on guidelines that aim to achieve a sufficient level of disinfection throughout the system.

Using a dosage of 50 mg/l of Cl2 strikes a balance between effectiveness and safety. It is high enough to ensure that any potential harmful microorganisms are effectively killed while not exceeding levels that could pose health risks to consumers if residues remain.

Other dosages listed in the options either do not align with the standards typically recommended for new mains or exceed what is considered necessary for effective disinfection. Thus, the choice of 50 mg/l over a 24-hour period provides the assurance needed for public health safety during the commissioning of new water mains.