The correct answer is D. All the above.
The design period of a sewer is the length of time for which it is expected to function without major repairs. The design period is typically based on the expected life of the materials used in the sewer, the anticipated rate of growth in the population served by the sewer, and the level of maintenance that will be provided.
For branch sewers, the design period is typically 20 to 30 years. This is because branch sewers are typically made of smaller diameter pipes that are more susceptible to damage from roots and other obstructions.
For main sewers, the design period is typically 30 to 40 years. This is because main sewers are typically made of larger diameter pipes that are less susceptible to damage from roots and other obstructions.
For trunk sewers, the design period is typically 40 to 50 years. This is because trunk sewers are typically made of the largest diameter pipes and are the most heavily used sewers.
The design period of a sewer is an important factor in determining the cost of the sewer. The longer the design period, the more expensive the sewer will be to construct. However, a longer design period can also save money in the long run by reducing the need for repairs and replacements.