Sewerage design practice: For which component is a design period of 40–50 years typically adopted in municipal sewer planning?

Introduction / Context:
In municipal sewerage planning, different components are assigned different design periods based on cost, durability, accessibility, and ease of augmentation. Understanding which assets warrant the longest horizon is essential for reliable, cost-effective infrastructure.

Given Data / Assumptions:

• The question refers to common practice in urban sewer system planning.
• Design period means the planning horizon over which the capacity is expected to remain adequate with reasonable growth assumptions.
• System elements considered: branch (lateral) sewers, main sewers, and trunk sewers.

Concept / Approach:
Large, deep, and strategically located conduits are expensive and disruptive to replace or upsize. Therefore, longer design periods are adopted for such elements. Smaller, more accessible elements that can be replicated or paralleled may use shorter horizons.

Step-by-Step Solution:
1) Identify components by hierarchy: branch (local), main (collectors), trunk (primary interceptors).2) Consider replacement/augmentation difficulty: trunk sewers are deepest, largest, and most disruptive to upgrade.3) Assign typical horizons: laterals often 15–20 years; mains about 25–30 years; trunk/interceptor sewers 40–50 years.4) Select the component matching 40–50 years: trunk sewers.

Verification / Alternative check:
Design manuals commonly recommend the longest period for trunk or interceptor sewers for lifecycle economy and to minimize repeated large-scale urban disruptions.

Why Other Options Are Wrong:
Branch sewers: Typically shorter horizons; easier to relay or parallel.Main sewers: Intermediate horizons; still easier to augment than trunk lines.All of the above / Pumping mains only: Do not reflect the differentiated practice by hierarchy.

Common Pitfalls:

• Assuming a single universal design period for an entire network.
• Ignoring practical constraints like depth, traffic, and urban density.

Final Answer:
Trunk sewers