Which type of valve must be installed to prevent sewer contaminants from backing up into a building?

The type of valve that must be installed to prevent sewer contaminants from backing up into a building is the backwater valve. This valve is specifically designed to allow wastewater to flow out of a building while preventing the reverse flow of sewage from the municipal sewer system back into the building's plumbing.

When there is a blockage or an increase in water levels within the sewer system, the backwater valve closes automatically to prevent backflow. This is critical in protecting the interior of the building from potential sewage overflow, which can cause health hazards and property damage.

In contrast, a check valve is generally used to prevent backflow in systems where fluid is being pumped and does not specifically target sewage systems. A gate valve is normally utilized for isolation purposes and does not have the functionality required to prevent backflow during a sewer backup. A relief valve, on the other hand, is used to relieve pressure in a system rather than preventing backflow. Thus, for sewage protection, the backwater valve is the appropriate choice.