What is essential to connect waste plumbing fixtures to a sewage system?

To connect waste plumbing fixtures to a sewage system, a building sewer is essential. This component serves as the primary pipeline that transports wastewater away from a building to the municipal sewage system or to a septic tank.

The building sewer plays a critical role in ensuring that all waste from various plumbing fixtures within the structure is properly channeled away. It is typically located underground and connects to the lateral sewer line which leads to the public sewer system. The building sewer must be properly sized, installed with appropriate materials, and meet local codes to ensure efficient drainage and prevent issues such as backflow or leaks.

While the other options serve important functions within the plumbing system — such as allowing for venting of gases (branch vent), providing access for cleaning (cleanout), and preventing contamination (air gap) — they do not perform the fundamental role of connecting fixtures directly to the sewage system like the building sewer does.