Planning horizon in wastewater projects: The typical design period adopted for sewage treatment works (process units and appurtenances) is generally in which range?

Introduction / Context:
Infrastructure planning uses a rational design period (planning horizon) that balances capital cost, population growth, technology change, and O&M. For sewage treatment plants (STPs), civil works may last longer, but process units are typically sized for medium-term projections with modular expansion capability.

Given Data / Assumptions:

• Urban STP with staged expansion possible.
• Population and flow projections carry uncertainty beyond two decades.
• Equipment life cycles are often shorter than civil structures.

Concept / Approach:

Design periods around 15–20 years are widely adopted for STPs, allowing cost-effective phasing, incorporation of improved treatment technologies, and adaptation to regulatory changes. Trunk sewers and outfalls may use longer horizons; pumping and process equipment often have shorter replacement cycles.

Step-by-Step Solution:

Verification / Alternative check:

Compare with national/state design manuals for recommended horizons; run sensitivity analyses for growth scenarios.

Why Other Options Are Wrong:

(a) 5–10 years is too short for major civil investments; (c) and (d) are long and risk overbuilding or technological obsolescence.

Common Pitfalls:

Ignoring modularity; not allowing for stricter effluent norms; neglecting infiltration/inflow growth.

Final Answer:

15 to 20 years