--- title: "Service reservoirs: purpose and construction" subject: "Water Supply Engineering" chapter: 7 key: "semester_5/water_supply_engineering:service-reservoir-purpose-construction" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/service-reservoir-purpose-construction site: BCT Engineering --- # Service reservoirs: purpose and construction Water Supply Engineering · Chapter 7: Reservoirs and Distribution System - Asked in: 2072 Chaitra, 8 marks; 2076 Asoj, 2 marks (asked 2 times) - Video: [SERVICE RESERVOIRS | CE | by Jaspal Singh (Ex. IES) MADE EASY Faculty](https://www.youtube.com/watch?v=QjYVhpaBafk) — MADE EASY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/service-reservoir-purpose-construction - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7 ## Question Describe the purpose and construction of service reservoirs (and clear water reservoirs) with neat sketches. ## Short answer Service reservoirs provide balancing storage, emergency and fire reserves, and maintain pressure. They are constructed of RCC or masonry with watertight walls, a roof, two compartments, and fittings like inlet, outlet, overflow, and washout. ## Answer Service (distribution) reservoirs are storage structures built near the distribution area, to store treated water and supply it to the consumers at the required pressure during demand variations. A clear water reservoir is the tank at the treatment plant that stores the filtered, disinfected water. #### Purposes 1. **Balancing storage:** supply is at a uniform rate, but demand varies hourly; the reservoir stores the surplus and supplies the deficit. 2. **Emergency storage:** supply continues during breakdown of pumps, power failure or repair of the main. 3. **Fire reserve:** keeps water for fire fighting. 4. **Maintains pressure** in the system (elevated or hill-top tanks give pressure by gravity). 5. Lets the treatment plant and pumps work at a steady rate, which saves cost. #### Types - Underground or ground-level (on a hill), RCC, masonry or steel - Elevated tanks (towers) for flat towns #### Construction - **Site:** as high as possible and near the centre of the demand area. - **Walls and floor:** RCC (M20 or richer) or brick/stone masonry with cement plaster; watertight, with floor sloping to the washout. - **Roof:** RCC slab or dome, covered with earth, to prevent contamination and keep the water cool. - **Compartments:** divided into two cells so one can be cleaned while the other works. - **Fittings:** inlet, outlet (above the floor), overflow, washout (scour) pipe, ventilation pipes, manhole with cover, ladder, level indicator and water-level gauge. ```text vent manhole ___ ___|___________|____ roof | |-- overflow | inlet-> water level ~~~~ | | | |__________________________|-- outlet washout floor sloped ``` #### Clear water reservoir (CWR) It is built at the treatment works after filtration, usually partly underground, with baffles. It stores the filtered water, gives chlorine contact time (30 minutes or more), supplies the pumps for back washing and for pumping to the town, and balances the difference between plant output and pumping rate. Its construction is the same as above (RCC, roof, baffles, manholes, vents, inlet/outlet, overflow, washout). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is balancing storage needed in a service reservoir? A. Supply is uniform but demand varies hourly B. To treat water C. To remove silt D. To increase source yield Answer: A. The reservoir stores surplus and supplies the deficit as demand varies. (Key point: Balancing storage) 2. Why is the roof of a service reservoir covered with earth and kept closed? A. To increase pressure B. To prevent contamination and keep the water cool C. To allow sunlight for disinfection D. To reduce storage volume Answer: B. A covered roof protects water from contamination and keeps it cool. (Key point: Roof) 3. Why is the reservoir divided into two cells? A. To double the storage B. So one cell can be cleaned while the other keeps working C. To reduce the wall thickness D. To increase the pressure Answer: B. Two cells allow cleaning without stopping the supply. (Key point: Two compartments) 4. What is a function of the clear water reservoir at the treatment plant? A. Distributes water to houses by gravity B. Gives chlorine contact time of 30 minutes or more and supplies backwash and pumps C. Settles suspended solids D. Removes hardness Answer: B. The CWR holds filtered water, provides chlorine contact time and balances plant output against pumping. 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