---
title: "Wet weather discharge, combined sewer"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:wet-weather-discharge-small-town"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/wet-weather-discharge-small-town
site: BCT Engineering
---

# Wet weather discharge, combined sewer

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2077 Chaitra, 4 marks
- Video: [TYPE: -1 DESIGN DISCHARGE FOR A COMBINED SEWER](https://www.youtube.com/watch?v=bVTt74aHswQ) — Engineersnepal
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/wet-weather-discharge-small-town
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

A small town of population 500 with water supply of 160 lpcd. Town covers about 1.5 hectares and has a maximum rainfall intensity of 50 mm/hr. Roughly 40% is occupied by roofs and pavements of C = 0.8 and 60% of lawn and gardens of C = 0.2. Calculate the discharge of sewage out of the town in wet weather conditions if combined sewer is adopted. (C is coefficient of runoff)

## Short answer

Weighted runoff coefficient is 0.44. Storm runoff = 0.0917 m3/s and dry weather sewage = 0.00074 m3/s, so wet weather combined discharge is 0.0924 m3/s (92.4 L/s).

## Answer

In a **combined system**, the wet weather discharge is the dry weather sewage flow plus the storm runoff.

#### Given
Population = 500, water supply = 160 lpcd, area $A$ = 1.5 ha, rainfall intensity $i$ = 50 mm/hr. Assume that 80% of water supplied becomes sewage.

#### Step 1: Weighted coefficient of runoff
$$
C=\frac{0.4(0.8)+0.6(0.2)}{1.0}=0.44
$$

#### Step 2: Storm runoff (Rational method)
$$
Q_s=\frac{C\,i\,A}{360}=\frac{0.44\times 50\times 1.5}{360}=0.0917\ \text{m}^3/\text{s}
$$

#### Step 3: Sanitary sewage (dry weather flow)
$$
Q_d=\frac{500\times160\times0.8}{86400\times1000}=0.00074\ \text{m}^3/\text{s}
$$

#### Step 4: Wet weather discharge
$$
Q=Q_s+Q_d=0.0917+0.00074=0.0924\ \text{m}^3/\text{s}
$$

**Answer: Wet weather discharge is about 0.0924 m³/s (92.4 l/s)**; storm water is 99% of it, so the sewage part is almost negligible.

## Check yourself

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

1. In a combined system, wet weather discharge is
   A. storm runoff only
   B. dry weather sewage plus storm runoff
   C. sewage minus runoff
   D. infiltration only
   Answer: B. Q = Qs + Qd. (Key point: Combined sewer discharge)

2. The weighted C for 40% at 0.8 and 60% at 0.2 is
   A. 0.50
   B. 0.44
   C. 0.32
   D. 1.00
   Answer: B. 0.4 x 0.8 + 0.6 x 0.2 = 0.44. (Key point: Weighted C)

3. Q_s for C = 0.44, i = 50 mm/hr, A = 1.5 ha is
   A. 0.0917 m3/s
   B. 0.917 m3/s
   C. 9.17 m3/s
   D. 0.00917 m3/s
   Answer: A. 0.44 x 50 x 1.5 / 360 = 0.0917 m3/s. (Key point: Storm runoff)

4. Why is the sewage part almost negligible in the total?
   A. Because C is small
   B. Storm runoff (about 0.0917) is far larger than sewage (about 0.00074 m3/s)
   C. Sewage is not included
   D. The population is large
   Answer: B. Storm water forms about 99% of the 0.0924 m3/s total. (Key point: Sewage part)

## Related in Sanitary Engineering

- [Combined circular sewer, 35 ha](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-35ha) — 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
- [Design discharge of rural combined sewer](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adesign-discharge-combined-rural) — Sanitary Engineering, chapter 2
- [Minimum sewer diameter, 80000 population](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-min-diameter-80000) — Sanitary Engineering, chapter 3
- [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

## Related in other subjects

- [Maximum rainfall intensity for 1, 3, 5 hr](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Amax-intensity-1-3-5hr-7am) — Engineering Hydrology, chapter 2
- [24-hr maximum rainfall for 50-year return period](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Agumbel-24hr-max-50yr-semilog) — Engineering Hydrology, chapter 6
- [Design pipes AB, BC; 160 lpcd, peak factor 3](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apd-c71) — Water Supply Engineering, chapter 7
- [Balancing reservoir: village 600 population, 65 lpcd](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-as70) — Water Supply Engineering, chapter 7