Estimating demand for a municipal water-supply project Project cost estimates depend on per-capita rate of supply (LPCD) and the probable population at the end of a chosen design period. For typical Indian urban systems, what design-period range is commonly adopted for water-supply components?

Introduction / Context:
While planning a city water-supply system, engineers must forecast demand over a realistic life span of the assets. The “design period” is the future time horizon used for sizing components based on predicted per-capita consumption and projected population. Selecting an appropriate design period is crucial for avoiding both under-design and costly oversizing.

Given Data / Assumptions:

• The question refers to typical practice for urban water-supply in civil engineering.
• Per-capita consumption is assumed as per code or norms (for example, 135 LPCD for residential use, unless otherwise specified).
• Population projection is carried to the end of the design period.

Concept / Approach:
Different components have different economic lives (intake, transmission main, treatment plant, reservoirs, distribution). Planning guidelines commonly use longer design periods for water-supply works than for rapidly changing urban facilities. For most core water-supply components, a 20–30 year range is widely accepted for capacity planning, with allowance for future duplication or augmentation.

Step-by-Step Solution:

Verification / Alternative check:
Planning manuals and standard textbooks on water-supply engineering routinely recommend around three decades for the core network and major facilities, often with staged construction to manage capital expenditure.

Why Other Options Are Wrong:

• 5–10 years and 10–15 years: too short, leading to early saturation and repeated upgrades.
• 15–20 years: used for some parts but not preferred for the entire system capacity.
• 25 years (single value): a single point within the correct band but not a general “range” as asked.

Common Pitfalls:
Confusing asset economic life with design period, and ignoring possibilities of staged expansion within a 20–30 year planning horizon.

Final Answer:
20 to 30 years