--- title: "DWF and WWF in sewer design" subject: "Sanitary Engineering" chapter: 2 key: "semester_6/sanitary_engineering:dwf-wwf-importance" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/dwf-wwf-importance site: BCT Engineering --- # DWF and WWF in sewer design Sanitary Engineering · Chapter 2: Quantity of Wastewater - Asked in: 2081 Chaitra, 4 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-importance - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2 ## Question Why are wet weather flow and dry weather flow considered important parameters in designing a sewer line? ## Short answer Dry weather flow is the sewage flow without rain, used to check self-cleansing velocity and size treatment works. Wet weather flow, the sum of DWF and storm runoff, sets the maximum sewer capacity to avoid flooding. Both are needed for economical, safe design and proper functioning under all conditions. ## Answer **Dry weather flow (DWF)** is the flow in a sewer in the absence of rain, made of domestic and industrial sewage plus groundwater infiltration. **Wet weather flow (WWF)** is the total flow in a sewer in rain, DWF plus the runoff entering from the surface. #### Why both are important in sewer design 1. **Maximum capacity:** in a combined or partially separate system the sewer must carry the peak WWF, $Q = \text{peak DWF} + \dfrac{CiA}{360}$. If only DWF were used, the sewer would be too small and overflow, flooding streets. 2. **Self-cleansing velocity:** a sewer sized for WWF is almost empty in dry weather. The minimum DWF (about one-third of average) is used to check that velocity is at least 0.6-0.75 m/s, so that solids do not settle. 3. **Choice of sewer system:** the large ratio of WWF to DWF (often 20 to 100 in monsoon areas) is the reason a separate system is chosen. 4. **Sizing of treatment works:** the treatment plant is designed for about 3 x DWF, and the excess is released through storm overflows. Both flows are needed to design overflows, storm tanks and regulators. 5. **Pumping stations:** pumps are selected for DWF range; storm pumps for WWF. 6. **Infiltration and illegal connections:** the difference between DWF and expected flow shows leakage, and extra flow in rain shows illegal roof connections. 7. **Economy and safety:** a design based on both gives a sewer which is neither costly nor unsafe, and prevents pollution from overflows. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Wet weather flow is A. the infiltration only B. DWF plus the runoff entering from the surface C. the minimum flow at night D. the flow in a separate sewer in dry weather Answer: B. WWF = DWF + storm runoff. (Key point: WWF definition) 2. Why is peak WWF used for a combined sewer? A. To keep velocity low B. To reduce cost C. Using DWF only would make the sewer too small and cause overflow and flooding D. To avoid manholes Answer: C. The sewer must carry the peak wet-weather flow. (Key point: Maximum capacity) 3. What is the minimum DWF used for? A. To check that velocity is at least about 0.6-0.75 m/s so solids do not settle B. To size the storm tank C. To find the runoff coefficient D. To decide the rainfall intensity Answer: A. A sewer sized for WWF is almost empty in dry weather, so velocity at low flow must be checked. (Key point: Self-cleansing check) 4. Treatment works are usually designed for about A. 1 x WWF B. 3 x DWF, with excess released through storm overflows C. 10 x DWF D. 0.5 x DWF Answer: B. Flow above this is diverted by storm overflows. (Key point: Treatment plant) ## Related in Sanitary Engineering - [Define DWF, WWF; estimating WWF](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-quantity-wwf) — 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 - [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 - [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 - [Separate sewer, check self-cleansing](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseparate-sewer-0p5-self-cleansing) — Sanitary Engineering, chapter 3 ## Related in other subjects - [Peaking RoR: max power and wet/dry energy ratio](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apeaking-ror-reservoir-250m-q25) — Hydropower Engineering, chapter 3 - [When wet process is better than dry](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Awet-process-better-than-dry) — Civil Engineering Materials, chapter 5 - [Canal discharge for dry and wet crops](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akor-discharge-dry-wet-20000-30000) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [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×