--- title: "Recommend a suitable sewerage system" subject: "Sanitary Engineering" chapter: 1 key: "semester_6/sanitary_engineering:recommend-sewerage-system" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/recommend-sewerage-system site: BCT Engineering --- # Recommend a suitable sewerage system Sanitary Engineering · Chapter 1: Introduction - Asked in: 2078 Chaitra, 4 marks; 2075 Bhadra, 4 marks; 2073 Magh, 4 marks; 2070 Bhadra, 4 marks (asked 4 times) - Video: [3.10 Sewer systems](https://www.youtube.com/watch?v=Badhfm0TY2M) — Citywide inclusive sanitation (CWIS) channel - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/recommend-sewerage-system - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-1 ## Question Define sewerage system and recommend a suitable type of sewerage system for your locality / a highly populated urban area with justifications. ## Short answer A sewerage system is the complete infrastructure of sewers, appurtenances, pumping stations, and treatment works for collecting, conveying, treating, and disposing of wastewater and storm water. For a highly populated urban area with heavy monsoon rainfall, the separate system is recommended because it uses smaller sewers that maintain self-cleansing velocity, reduces treatment plant size and cost, prevents sewage overflow during storms, allows direct storm water discharge, and simplifies maintenance in congested streets. ## Answer A **sewerage system** is the complete arrangement of sewers, appurtenances, pumping stations, treatment works and disposal works used to collect, convey, treat and dispose of wastewater and, when included, storm water from a community. #### Types - **Separate system:** one set of sewers for sanitary sewage and a different set (drains) for storm water. - **Combined system:** one sewer carries both sewage and storm water. - **Partially separate system:** one sewer carries sewage plus a part of storm water (roof and courtyard runoff), and the remaining storm water goes in surface drains. #### Recommendation for a highly populated urban area I recommend the **separate system** (with the storm drains laid as open or covered drains along roads). Justification: 1. **Rainfall in Nepal is intense and seasonal (monsoon).** A combined sewer would be very large for rare peak flows, and would run almost empty in the dry season, with silting problems. In a separate system, the sewage sewer is small and always flows at reasonable velocity. 2. **Treatment plant economy:** only sewage reaches the plant, so the plant is smaller and works at nearly constant load. In a combined system the plant would be overloaded in rain or the overflow would carry raw sewage to rivers. 3. **No overflow pollution:** sewage is not discharged into rivers during storms. 4. **Storm water needs no treatment** and can be discharged directly into the nearest stream; its drains can be built cheaply and in stages. 5. **Easier maintenance in narrow congested streets:** smaller sewer pipes, easier to lay, and working conditions inside are better. 6. **Pumping is cheaper** where the sewage must be lifted. 7. **Reuse:** sewage of constant quality can be reused for irrigation or biogas. The only disadvantage is higher total cost because two systems are laid, and illegal connection of roof drains into sewers must be controlled. For a dense urban area with heavy monsoon rain, such as Kathmandu valley, the separate system is more suitable. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A sewerage system is A. only the pipes that carry storm water B. the complete arrangement of sewers, appurtenances, pumping, treatment and disposal works for wastewater (and storm water if included) C. only the treatment plant D. a system of water supply pipes Answer: B. It covers collection, conveyance, treatment and disposal of wastewater. (Key point: Definition of sewerage system) 2. What does the partially separate system carry in one sewer? A. Sewage only B. All storm water only C. Sewage plus a part of storm water (roof and courtyard runoff) D. Industrial waste only Answer: C. The rest of the storm water goes through surface drains. (Key point: Types of system) 3. Which system is recommended for a dense urban area with heavy monsoon, and why mainly? A. Combined, because it is cheapest in all cases B. Separate, because peak monsoon flow would make a combined sewer huge and the plant overloaded C. Combined, because storm water must be treated D. Partially separate, because it needs no drains Answer: B. A separate system keeps the sewage sewer small and flowing at good velocity, and the plant gets a steady small load. (Key point: Recommended system) 4. How does a separate system benefit the treatment plant? A. Only sewage reaches the plant, so it is smaller and works at nearly constant load B. The plant receives all the storm water C. It needs a storm overflow at the plant inlet D. The plant is unnecessary Answer: A. Storm water needs no treatment and goes directly to a stream. (Key point: Treatment plant economy) ## Related in Sanitary Engineering - [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× - [Storm water estimation and its limitations](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Astorm-water-limitations) — Sanitary Engineering, chapter 2 ## Related in other subjects - [T/F: loop arrangement in urban area economical](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-loop-urban-economical) — Transmission and Distribution System Design, chapter 5 - [Consensus; highly available service](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aconsensus-high-availability) — Distributed System, chapter 6 - [T/F: thermal limit design urban, losses rural](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-thermal-limit-urban-loss-rural) — Transmission and Distribution System Design, chapter 7, asked 3× - [Underground feeder economical in urban areas](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ajustify-underground-urban-economical) — Transmission and Distribution System Design, chapter 5, asked 4×