---
title: "Sources of sewage and city impact"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:sources-sewage-impact-city"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sources-sewage-impact-city
site: BCT Engineering
---

# Sources of sewage and city impact

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2076 Bhadra, 2+2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sources-sewage-impact-city
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

What are the sources of sanitary sewage? Briefly describe its impact based on time and development activities of the city.

## Short answer

Sources: residential, commercial/institutional buildings, industries, public places, and infiltration. With time, sewage flow varies diurnally (peak morning/evening, 2-3× average), seasonally (higher in summer/rainy), and grows with population. Development and urbanization increase per capita water use, impervious area, and wastewater volume and pollution load, demanding larger sewers.

## Answer

#### Sources of sanitary sewage
- **Residential buildings** (toilets, baths, kitchens, washing), the largest source.
- **Commercial and institutional buildings:** offices, shops, hotels, restaurants, schools, hospitals.
- **Industries:** sanitary waste of workers and process waste.
- **Public places:** public toilets, markets, bus parks.
- **Infiltration** of groundwater and **unauthorised connections**.

#### Impact of time and city development
**With time**
- *Daily:* the flow is minimum at night (about 1/3 of average), has a first peak in the morning (7-10 am) and a second peak in the evening; the maximum rate may be 2 to 3 times the average.
- *Seasonal:* flow is larger in summer (more water use) and in the rainy season (higher infiltration and illegal storm connections).
- *Long term:* as the population grows over the design period, flow increases, so sewers must be designed for the future population.

**With development activities**
- Better living standard, piped water, flush toilets, washing machines and hotels increase water use and sewage per head from, say, 60 lpcd to 135 lpcd or more.
- Growth of commercial and industrial areas, hospitals, hotels and institutions add heavy and sometimes harmful flows, and raise the pollution load.
- Urbanisation increases impervious area, so more storm water reaches sewers in a combined system.
- Densification (high-rise buildings) increases flow per hectare.
- New connections and expansion of sewer network raise infiltration if construction quality is poor.

## Check yourself

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

1. When is sewage flow usually minimum in a day?
   A. Morning peak
   B. Evening
   C. Night, about 1/3 of average
   D. Noon
   Answer: C. Peaks occur in the morning (7-10 am) and evening; the maximum may be 2-3 times the average. (Key point: Daily variation)

2. Why is flow larger in the rainy season?
   A. Higher infiltration and illegal storm connections
   B. Less water use
   C. Fewer people
   D. Sewers are bigger
   Answer: A. Rain increases infiltration and illegal connections. (Key point: Seasonal variation)

3. How does development raise sewage per head?
   A. Fewer flush toilets
   B. Piped water, flush toilets and washing machines raise it, e.g. from 60 to 135 lpcd
   C. It does not change
   D. It reduces water use
   Answer: B. A better living standard increases water use and sewage. (Key point: Development)

4. Urbanisation increases storm water reaching a combined sewer because
   A. rainfall increases
   B. impervious area increases
   C. sewers get steeper
   D. infiltration disappears
   Answer: B. More paved surfaces mean more runoff. (Key point: Urbanisation)

## Related in Sanitary Engineering

- [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×
- [Estimating sewage quantity for a city](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aquantity-sewage-nepal-city) — Sanitary Engineering, chapter 2
- [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
- [Factors affecting sewage; calculating discharge](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Afactors-sewage-calculate-discharge) — Sanitary Engineering, chapter 2

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