---
title: "Design discharge of rural combined sewer"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:design-discharge-combined-rural"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/design-discharge-combined-rural
site: BCT Engineering
---

# Design discharge of rural combined sewer

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2072 Magh, 4 marks
- Video: [design of sewer, Quantity of sewage in combined sewer Problem, sewer size and velocity calculation](https://www.youtube.com/watch?v=oCX5F2VnK7Q) — Civil Engineering Exam
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/design-discharge-combined-rural
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

A population of 30000 is residing in a rural area of 60 hectares. If the average coefficient of runoff for this area is 0.60, time required to the entry port is 25 minutes and the time of travel from the entry port to the section of sewer under consideration for design is 5 minutes. What will be the design discharge for a combined sewer at the section to be considered if the average flow of sewage in rural is 45 lpcd and peak factor is 2.7?

## Short answer

Peak sewage is 0.0422 m3/s using 45 lpcd and peak factor 2.7. With tc=30 min, intensity is 19 mm/hr; storm runoff is 1.900 m3/s. Combined design discharge is 1.942 m3/s.

## Answer

Design discharge of a combined sewer = peak dry weather sewage flow + storm runoff.

#### Step 1: Peak sewage flow
Average flow = 45 lpcd (given as the sewage flow), peak factor 2.7.

$$
Q_{sewage}=\frac{30000\times45\times2.7}{86400\times1000}=0.0422\ \text{m}^3/\text{s}
$$

#### Step 2: Time of concentration and intensity
$$
t_c=t_e+t_f=25+5=30\ \text{min}
$$

No rainfall formula is given, so I use the common empirical form $i=\dfrac{760}{t+10}$ mm/hr (assumed):

$$
i=\frac{760}{30+10}=19.0\ \text{mm/hr}
$$

#### Step 3: Storm water (Rational formula)
$$
Q_{storm}=\frac{C\,i\,A}{360}=\frac{0.60\times19.0\times60}{360}=1.900\ \text{m}^3/\text{s}
$$

#### Step 4: Design discharge
$$
Q=0.0422+1.900=1.942\ \text{m}^3/\text{s}
$$

**Answer: Design discharge = 1.942 m³/s** (the sewage is only about 2% of it).

## Check yourself

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

1. The time of concentration is
   A. 25 min
   B. 5 min
   C. 30 min
   D. 20 min
   Answer: C. te + tf = 25 + 5. (Key point: Time of concentration)

2. Peak sewage flow for 30000 people at 45 lpcd, PF 2.7 is
   A. 0.0422 m3/s
   B. 0.422 m3/s
   C. 0.0156 m3/s
   D. 1.9 m3/s
   Answer: A. 30000 x 45 x 2.7 / 86400000 = 0.0422 m3/s. (Key point: Sewage flow)

3. Storm runoff using i = 19 mm/hr, C = 0.6, A = 60 ha is
   A. 0.19 m3/s
   B. 19 m3/s
   C. 3.8 m3/s
   D. 1.900 m3/s
   Answer: D. 0.6 x 19 x 60 / 360 = 1.9 m3/s. (Key point: Storm flow)

4. The combined design discharge is about
   A. 0.0422 m3/s
   B. 1.942 m3/s
   C. 1.900 m3/s
   D. 19.4 m3/s
   Answer: B. Q = 0.0422 + 1.900 = 1.942 m3/s. (Key point: Design discharge)

## Related in Sanitary Engineering

- [Circular combined sewer, 60 ha](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-60ha) — Sanitary Engineering, chapter 3
- [Combined sewer section, 85 ha area](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-85ha) — Sanitary Engineering, chapter 3
- [Design discharge, separate and combined](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adesign-discharge-separate-combined-45000) — Sanitary Engineering, chapter 2
- [Wet weather discharge, combined sewer](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Awet-weather-discharge-small-town) — Sanitary Engineering, chapter 2
- [Sewer diameter for a 2.5 sq km area](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-diameter-2p5-sqkm) — Sanitary Engineering, chapter 3
- [Combined circular sewer, 35 ha](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-35ha) — Sanitary Engineering, chapter 3

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