---
title: "Define DWF, WWF; estimating WWF"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:dwf-wwf-quantity-wwf"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/dwf-wwf-quantity-wwf
site: BCT Engineering
---

# Define DWF, WWF; estimating WWF

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2079 Chaitra, 2+2 marks
- Video: [2. Dry weather flow and wet weather flow || Waste water Engineering](https://www.youtube.com/watch?v=CgHmPXrBeCM) — NITIANS ACADEMY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/dwf-wwf-quantity-wwf
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

Define dry weather flow (DWF) and wet weather flow (WWF). Discuss in brief the quantity estimation of WWF.

## Short answer

Dry weather flow is sewer flow in dry periods (domestic, industrial sewage and infiltration). Wet weather flow is DWF plus storm runoff. WWF estimation: compute DWF from design population, per capita water supply (80% return), peak factor and infiltration; then add storm runoff by rational method Qs=CiA/360 with intensity for duration equal to time of concentration.

## Answer

#### Dry weather flow (DWF)
DWF is the flow in a sewer when there is no storm runoff. It consists of domestic sewage, industrial wastewater and groundwater infiltration. It is taken as the average flow during a dry period, usually seven consecutive days without rain.

#### Wet weather flow (WWF)
WWF is the flow in a sewer during and after rain. It is the sum of the DWF and the storm water that enters the sewer:

$$
\text{WWF} = \text{DWF} + \text{storm runoff}
$$

#### Estimation of WWF
1. Find the **DWF** from the design population, the water supply rate (sewage = 75-80% of supply) and a peak factor, plus infiltration.
2. Find the **storm runoff** by the rational method:

$$
Q_s = \frac{C\,i\,A}{360}\ \ (\text{m}^3/\text{s})
$$

   where $C$ is the weighted runoff coefficient, $i$ the rainfall intensity (mm/hr) for a duration $t_c = t_e + t_f$, and $A$ the area in hectares.
3. Add them: $Q_{WWF} = Q_{DWF}+Q_s$.

For a combined system the whole storm runoff is taken. In a partially separate system only the runoff from roofs and courtyards (say one-third to one-half of the area) is used. As a rough rule, a sewer may be designed for about 6 x DWF, but the rational method is preferred.

## Check yourself

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

1. Dry weather flow consists of
   A. domestic and industrial sewage plus groundwater infiltration
   B. storm water only
   C. domestic sewage and surface runoff
   D. industrial waste only
   Answer: A. DWF has no storm runoff. (Key point: DWF composition)

2. Wet weather flow is estimated as
   A. DWF minus runoff
   B. DWF x runoff
   C. runoff only
   D. DWF plus storm runoff
   Answer: D. WWF = DWF + storm runoff. (Key point: WWF formula)

3. The storm runoff added to find WWF is calculated by
   A. Manning's formula
   B. the rational method Q = CiA/360
   C. Harman's formula
   D. the arithmetic increase method
   Answer: B. i is for duration tc = te + tf. (Key point: Storm runoff estimate)

4. For a partially separate system, which runoff is added to DWF?
   A. Only roofs and courtyards (part of the area)
   B. All the runoff
   C. None
   D. Only street runoff
   Answer: A. Only roof and courtyard runoff enters the sewer. (Key point: Partially separate system)

## Related in Sanitary Engineering

- [DWF and WWF in sewer design](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-importance) — Sanitary Engineering, chapter 2
- [DWF, WWF and storm water factors](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-storm-factors) — Sanitary Engineering, chapter 2
- [Differentiate DWF and WWF; sources](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-sources-sewage) — 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
- [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
- [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×

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- [Canal discharge for dry and wet crops (30000/40000 ha)](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akor-discharge-dry-wet-30000-40000) — Irrigation and Drainage Engineering, chapter 2, asked 2×
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