Interview Questions › Guesstimate

Estimate daily water consumption for a city of 5M people

Estimate the total daily water consumption (in liters or gallons) for a metropolitan city with a population of 5 million people.

Context

Include residential domestic use, commercial and institutional use, industrial manufacturing, and distribution leakage loss.

Clarifying questions to ask first

• Are we measuring water delivered to taps or raw water treated at the reservoir including network losses?
• What is the climate/geography (tropical vs arid)?

How to approach it

1. Estimate per capita domestic daily water consumption (~135-150 liters/day per person for drinking, cooking, bathing, toilet flushing, washing)
2. Scale to 5 million residents for total domestic demand
3. Add Commercial & Institutional demand (Offices, Malls, Hospitals, Schools: ~25-30% of domestic)
4. Factor in industrial manufacturing and municipal non-revenue water (NRW) pipe leakage (~15-20%)

Framework: Total Daily Water = (Domestic Population × LPCD) + Commercial + Industrial + Distribution Pipe Losses

Sample answer

• Domestic: 5M people × 150 Liters per Capita per Day (LPCD) = 750 Million Liters (MLD)
• Commercial & Institutions: ~20% of domestic = 150 MLD
• Industrial: ~100 MLD
• Pipeline Transmission Loss (20% NRW): ~250 MLD

1. Domestic: 5,000,000 × 150L = 750M Liters = 750 MLD
2. Commercial + Industrial: 250 MLD
3. Net consumption = 1,000 MLD (1 Billion Liters/day)
4. Gross raw water demand with 20% loss = 1,250 MLD (1.25 Billion Liters/day)

Executive estimate: ~1.0 Billion Liters (1,000 MLD) of daily consumed water, requiring ~1.25 Billion Liters of municipal reservoir supply.

What interviewers look for

• Per Capita LPCD Standard
• Domestic vs Commercial Split
• Industrial & Infrastructure Inclusions
• System Distribution Losses (NRW)