---
title: "Estimating sewage quantity for a city"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:quantity-sewage-nepal-city"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/quantity-sewage-nepal-city
site: BCT Engineering
---

# Estimating sewage quantity for a city

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2075 Baisakh, 6 marks
- Video: [2 | Sewage Flow Estimation for Design | Per Capita Demand, Return Flow Factor & MLD Calculation](https://www.youtube.com/watch?v=jIwuI_pZLqc) — Five Percent Imperfect 🅥︎
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/quantity-sewage-nepal-city
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

Describe the method of estimating the quantity of sewage for a city in Nepal in detail.

## Short answer

Project design population by arithmetic or geometric method. Compute average sewage flow as 0.8×P×q/86400 l/s with q=45-135 lpcd. Apply peak factor 2-3 for max flow, add infiltration (5-10% of DWF) and industrial waste. For combined systems, add storm runoff from rational method with DHM intensity. Example: P=60,000, q=135 lpcd, PF=3 yields Qmax=225 l/s.

## Answer

The quantity of sewage is estimated from the future population, the water supply per capita, and the factors that raise or reduce the flow. For a city in Nepal (e.g. Pokhara, Biratnagar) the method is as follows.

#### Step 1: Design population
Collect census data (CBS, Nepal) and project the population for the design period (about 20-25 years after completion) using the arithmetic, geometric or incremental increase method:
- Arithmetic: $P_n = P_0 + n\bar{x}$
- Geometric: $P_n = P_0(1+r/100)^n$

Add floating population (tourists, commuters, students) if large.

#### Step 2: Water supply rate
Use the design water demand of the city (Nepal Water Supply Corporation/NWSC design guidelines): 45 lpcd for rural and 85-135 lpcd for urban areas, with additions for institutional, commercial and industrial demand and leakage.

#### Step 3: Average sewage flow
About 80% of supply returns as sewage (the rest is lost in gardening, evaporation, leakage and consumption):

$$
Q_{avg}=\frac{0.8\,P\,q}{86400}\ \ \text{l/s}
$$

#### Step 4: Peak and minimum flow
- Peak factor 2 to 3 (higher for small towns); $Q_{max}=PF\cdot Q_{avg}$ for sewer design.
- Minimum flow about $\tfrac13 Q_{avg}$ for velocity check at start of the design period.

#### Step 5: Additions
- **Infiltration** depending on groundwater table and pipe quality (about 5-10% of DWF in Nepali cities, or 5,000-10,000 l/km/day/cm diameter).
- **Industrial wastewater** from large industries (from their own records).
- **Storm water** with $Q_s=\dfrac{CiA}{360}$ if the system is combined or partially separate. The intensity for the Nepal city is taken from the DHM intensity-duration-frequency curves, with $t_c=t_e+t_f$.

#### Step 6: Design discharge
$$
Q_{design} = Q_{max} + Q_{infiltration} + Q_{industrial}\ (+Q_{storm})
$$

The sewers are designed for $Q_{design}$ at the end of the design period and checked for minimum flow at the beginning (velocity at least 0.6 m/s).

#### Example
For $P=60{,}000$, $q=135$ lpcd, PF = 3: $Q_{avg}=0.8\times60000\times135/86400=75$ l/s and $Q_{max}=225$ l/s.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the design population estimated?
   A. From the present census only
   B. By projecting census data with arithmetic, geometric or incremental methods for 20-25 years
   C. From water meter readings
   D. By assuming no growth
   Answer: B. Add floating population if large. (Key point: Design population)

2. For P = 60,000, q = 135 lpcd, what is Q_avg?
   A. 75 l/s
   B. 225 l/s
   C. 90 l/s
   D. 7.5 l/s
   Answer: A. 0.8 x 60000 x 135 / 86400 = 75 l/s. (Key point: Average flow)

3. With PF = 3 for the same city, Q_max is
   A. 75 l/s
   B. 25 l/s
   C. 150 l/s
   D. 225 l/s
   Answer: D. 3 x 75 = 225 l/s. (Key point: Peak flow)

4. The design discharge is
   A. Q_max + infiltration + industrial (+ storm for combined)
   B. Q_avg only
   C. Q_min + infiltration
   D. storm water only
   Answer: A. Checked for minimum flow velocity (at least 0.6 m/s) at the start. (Key point: Design discharge)

## Related in Sanitary Engineering

- [Sources of sewage and city impact](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asources-sewage-impact-city) — Sanitary Engineering, chapter 2
- [Estimating storm water quantity](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Astorm-water-quantity-estimation) — Sanitary Engineering, chapter 2, asked 3×
- [Sources and factors of sanitary sewage](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asources-factors-sanitary-sewage) — Sanitary Engineering, chapter 2, asked 3×
- [Time of critical DO deficit, 50 l/s city](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acritical-do-time-city-50ls) — Sanitary Engineering, chapter 6, asked 2×
- [Critical DO deficit, city 100 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acritical-do-city-100ls) — Sanitary Engineering, chapter 6, asked 2×
- [Degree of treatment, city 1200 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adegree-treatment-city-1200ls) — Sanitary Engineering, chapter 6

## Related in other subjects

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- [Components of detail estimate report (road)](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Adetail-estimate-report-road) — Estimating and Costing, chapter 5
- [Distribution layout for haphazardly growing city](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Alayout-haphazard-city) — Water Supply Engineering, chapter 7, asked 2×
- [Compare and contrast pollution in a city](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Acompare-contrast-pollution-city-nepal) — Communication English, chapter 1