ANS:D - all the above.

The design period refers to the expected lifespan or duration for which a particular infrastructure component is planned and engineered to operate effectively without significant deterioration or obsolescence. In the context of sewer systems:

• Main sewers and trunk sewers: These are large-scale sewer pipelines designed to convey wastewater from smaller branch sewers to treatment facilities or disposal points. Due to the substantial investment and the critical role they play in wastewater management, main and trunk sewers typically have design periods ranging from 40 to 50 years. This extended design period ensures that the infrastructure can accommodate anticipated population growth and urban development over several decades.
• Branch sewers: Branch sewers are smaller pipelines that collect wastewater from individual properties or neighborhoods and feed into larger sewer lines. While they are integral parts of the overall sewer network, branch sewers may have shorter design periods compared to main and trunk sewers. The design period for branch sewers depends on factors such as local population trends, land use changes, and infrastructure planning.

In summary, the design period reflects the projected lifespan and functionality of sewer infrastructure, with main and trunk sewers typically designed for longer durations compared to branch sewers due to their larger scale and critical role in urban wastewater management.