Urban water demand planning: A city with a population of 50,000 has an average daily demand of 20 million litres per day (20 mld). Using the conventional peak-factor approach, what is the expected maximum daily demand for design purposes?

Introduction / Context:
In water supply engineering, systems are sized not only for the average day but also for peak demands. A commonly used guideline is to estimate the maximum daily demand by applying a peak factor to the average daily demand so that storage, pumps, and mains can handle short-term surges without service deficits.

Given Data / Assumptions:

• Population = 50,000 (used only to give context).
• Average daily demand = 20 mld (million litres per day).
• Typical design practice: maximum daily demand ≈ 1.8 * average daily demand.
• Water losses and diurnal variations are implicitly contained in the peak factor.

Concept / Approach:

Design handbooks widely adopt a peak factor of about 1.8 for the maximum day relative to the average day in municipal systems, unless local data justify another value. This captures diurnal peaks and seasonal effects while remaining conservative for sizing facilities.

Step-by-Step Solution:

Verification / Alternative check:

Many utility design standards also apply a maximum hourly factor to the maximum day. Our result focuses only on the maximum day, consistent with the question. Sensitivity checks with k_md = 1.6–2.0 would give 32–40 mld; 36 mld sits near the center of common practice.

Why Other Options Are Wrong:

• 24 mld and 30 mld: Too low; factors of 1.2–1.5 underestimate typical peak-day conditions.
• 54 mld: Implies a factor of 2.7, which is excessive for most municipal systems.
• 18 mld: Below average demand; not a peak.

Common Pitfalls:

• Confusing maximum day with maximum hour.
• Ignoring local demand data; use local peaking where available.

Final Answer:

36 mld.