In plumbing, what does a 'drainage system' primarily consist of?

The primary function of a drainage system in plumbing is to transport waste away from a building to a designated disposal location, such as a municipal sewer system or a septic tank. This system is composed of various pipes, fittings, and fixtures that are specifically designed to handle wastewater and ensure proper sanitation.

A drainage system is essential for maintaining hygiene and preventing contamination within living spaces. It includes not only the pipes that carry the waste but also includes traps and vents that prevent sewer gases from entering the building interior. The design and function of the drainage system emphasize the importance of managing waste effectively to protect public health and the environment.

While the other choices involve elements related to plumbing, they do not define the core purpose of a drainage system. Pipes that deliver water are more characteristic of a supply system, air ventilation ducts relate to HVAC systems, and water treatment tanks pertain to the treatment of wastewater rather than its transport away from the source. Thus, the accurate understanding of a drainage system focuses on its role in carrying away waste, making this the correct answer.