--- title: "Automatic flushing device" subject: "Sanitary Engineering" chapter: 4 key: "semester_6/sanitary_engineering:automatic-flushing-device" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/automatic-flushing-device site: BCT Engineering --- # Automatic flushing device Sanitary Engineering · Chapter 4: Sewer Appurtenances - Asked in: 2072 Asoj, 4 marks - Video: [Flushing Tanks | Manual Flushing Tanks | Automatic Flushing tanks | objects of Flushing Tanks](https://www.youtube.com/watch?v=UbruGBmArsE) — EASY CIVIL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/automatic-flushing-device - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4 ## Question With a neat sketch, describe the purpose, features and construction of the automatic flushing device used in a sewerage system. ## Short answer An automatic flushing device stores 1-2 m³ water in a tank with a siphon to periodically release high-velocity flow, scouring sewers at dead ends or flat gradients where velocity is below 0.6 m/s. Water enters slowly via a supply valve; when the level reaches the siphon crown, it discharges rapidly until air breaks the siphon. Modern practice prefers proper slopes over flushing tanks. ## Answer #### Purpose An **automatic flushing device** (flushing tank) is a storage tank that releases a large quantity of water at intervals, to flush the sewer. It is provided at the upper dead ends of sewers and in flat sewers, where the sewage flow is small and the velocity is below the self-cleansing value (0.6 m/s), so solids tend to settle. Flushing avoids blockages and removes deposits. #### Features - Works **automatically** without manual operation or attention. - Discharges a fixed quantity of water at set intervals (e.g. once every 24 hours, or every 6 h) with a velocity high enough to scour the sewer for a distance of up to 150-200 m. - Water is taken from the public water supply (or a tank, a river, a stream) by a small supply pipe with a control valve. - It consists of a siphon, so there is no moving part. #### Construction ```text supply pipe (valve) | ______v______ manhole cover | Water | | level ___|___ bell mouth | | cap | | U-siphon | |_____|__________|---> sewer ``` 1. A **flush tank** (brick or concrete) of about 1-2 m³ (1,000-2,000 litres) capacity, built in a special manhole at the head of the sewer. 2. A **siphon pipe** (inverted U) with the long leg connected to the sewer and a bell (cap) fitted over the short leg. 3. A **supply pipe** with a stop valve admits water slowly; the supply rate sets the interval of flushing. #### Working Water enters slowly and the level rises in the tank and in the siphon. When it reaches the crown of the siphon, the siphon starts, the air is expelled and the full tank flows out rapidly into the sewer. The siphonic action stops when the water level has dropped to the bell mouth and air enters. The cycle then repeats. Modern practice avoids flushing tanks by giving proper slope, because the cost of water and the risk of cross connection with water supply are high. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where is an automatic flushing tank provided? A. Upper dead ends of sewers and flat sewers with low flow B. Steep trunk sewers C. At the outfall D. In treatment plants Answer: A. Low velocity there lets solids settle. (Key point: Purpose) 2. What makes the tank discharge automatically? A. A pump B. A siphon starts when water reaches its crown C. A float valve operated by hand D. A timer motor Answer: B. There is no moving part: the siphon starts at the crown. (Key point: Mechanism) 3. What sets the flushing interval? A. The siphon colour B. The sewer length C. The rate of water supply through the supply pipe valve D. The ground level Answer: C. A slow supply sets the filling time. (Key point: Interval) 4. When does siphonic action stop? A. When the tank is full B. When valve closes C. When sewer is dry D. When the water level drops to the bell mouth and air enters Answer: D. Air entry breaks the siphon and the cycle repeats. 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