---
title: "Factors affecting sewage; calculating discharge"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:factors-sewage-calculate-discharge"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/factors-sewage-calculate-discharge
site: BCT Engineering
---

# Factors affecting sewage; calculating discharge

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2073 Bhadra, 4 marks
- Video: [Estimation of Sewage flow and Factors affecting Dry Weather Flow हिंदी में#environmentalengineering](https://www.youtube.com/watch?v=oqrI4Z87cxY) — Engineering with Mindfulness
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/factors-sewage-calculate-discharge
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

What are the various factors affecting the discharge of sanitary sewage? How do you calculate sanitary sewage discharge?

## Answer

#### Factors affecting discharge
1. Population and its density, and the future growth over the design period.
2. Rate of water supply (sewage is 75-80% of supply).
3. Type of area (residential, commercial, industrial) and standard of living.
4. Private and public water supplies, and water metering.
5. Groundwater infiltration, depending on water table and condition of joints.
6. Unauthorised connections (roof, yard drains).
7. Climate, season and hour of the day.

#### Calculation of sanitary sewage discharge
1. **Design population** $P$ at the end of the design period (20-30 years), by arithmetic, geometric or incremental increase method.
2. **Average sewage flow:**

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

   where $q$ = water supply (lpcd) and 0.8 = fraction becoming sewage.
3. **Peak (maximum) flow:** $Q_{max}=\text{PF}\times Q_{avg}$. The peak factor is 2 to 3 (about 3 for small, 2 for big towns; or by Harman's formula $PF = 1+\dfrac{14}{4+\sqrt{P/1000}}$).
4. **Minimum flow** is about $\tfrac13 Q_{avg}$ (used to check the self-cleansing velocity).
5. **Add infiltration** (e.g. 5,000-15,000 l/day/km per cm diameter of sewer, or a percentage) and industrial flows if present.

**Design discharge** = peak sewage flow + infiltration (+ storm runoff $CiA/360$ for combined systems).

## Check yourself

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

1. In Q_avg = P x q x 0.8 / 86400 l/s, the factor 0.8 represents
   A. the peak factor
   B. the infiltration allowance
   C. the fraction of water supply becoming sewage
   D. the fraction of population served
   Answer: C. About 80% of the supply returns as sewage. (Key point: Average flow formula)

2. Peak sewage flow is found by
   A. dividing Q_avg by the peak factor
   B. multiplying Q_avg by the peak factor (2 to 3)
   C. adding 1/3 Q_avg
   D. using Q_avg directly
   Answer: B. Q_max = PF x Q_avg; smaller towns use a higher factor. (Key point: Peak flow)

3. Minimum flow, used for the self-cleansing check, is about
   A. 3 x Q_avg
   B. Q_avg
   C. one-third of Q_avg
   D. zero
   Answer: C. It is used to check the velocity. (Key point: Minimum flow)

4. For a combined system the design discharge is
   A. peak sewage + infiltration + storm runoff CiA/360
   B. average sewage only
   C. storm runoff only
   D. minimum sewage + infiltration
   Answer: A. Storm runoff is added for combined systems. (Key point: Design discharge)

## 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×
- [Factors affecting self-purification](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aself-purification-factors) — Sanitary Engineering, chapter 6
- [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 sewage quantity for a city](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aquantity-sewage-nepal-city) — Sanitary Engineering, chapter 2
- [Sewage sampling and BOD dilution method](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asampling-bod-dilution) — Sanitary Engineering, chapter 5
- [Methods of sewage disposal](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Amethods-sewage-disposal) — Sanitary Engineering, chapter 6

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