What is the primary function of a backwater valve?

The primary function of a backwater valve is to prevent backflow of sewage. This is critical in plumbing systems to protect against the situation where wastewater from the municipal sewer system flows back into the building's plumbing. Such backflow can occur during heavy rain events or when the municipal lines are overwhelmed, leading to potential damage and health hazards due to sewage entering habitable spaces. The backwater valve acts as a one-way gate, allowing sewage to exit the home while preventing it from re-entering, thereby maintaining the integrity of the plumbing system and safeguarding public health.

The other choices do not accurately reflect the purpose of a backwater valve. For example, controlling airflow or preventing airflow does not pertain to the function of sewage drainage, while the idea of facilitating drainage misrepresents the valve's role, as it is primarily focused on protecting against the reverse flow rather than promoting drainage itself.