--- title: "Rapid sand filter construction and working" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:rsf-construction-working" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/rsf-construction-working site: BCT Engineering --- # Rapid sand filter construction and working Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2081 Baisakh, 4+4 marks; 2080 Bhadra, 4+4 marks; 2074 Chaitra, 8 marks; 2073 Shrawan, 8 marks; 2068 Chaitra, 8 marks (asked 5 times) - Video: [Water Supply| Wtp Filterbed step by step detail | How filter bed construct](https://www.youtube.com/watch?v=bA1-QfktkJw) — C i v i l - S i t e Motivation - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/rsf-construction-working - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question Describe, with the help of a neat sketch, the construction (filter media, base material, under-drainage) and working of a rapid sand filter, including its design considerations, operation, maintenance and the method of cleaning (backwashing). ## Short answer A rapid sand filter has a 0.6 to 0.75 m sand bed, 0.45 m graded gravel, and an underdrainage system of laterals and manifold; water flows down at 3 to 6 m/h. When head loss reaches 2.5 to 3 m, it is backwashed upward at 0.6 to 1.0 m/min for 10 to 15 minutes, expanding the bed about 50 percent. ## Answer A **rapid sand filter (RSF)** is a gravity or pressure filter in which water, normally after coagulation and sedimentation, passes down through a sand bed at a high rate (3 to 6 m/h). The bed is cleaned by reversing the flow (backwashing). #### Construction ```text Inlet (coagulated water) Wash water trough | | | ______v______________________v___v____ top | water depth 1.0-1.5 m | |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| | sand 0.6-0.75 m (ES 0.45-0.7 mm) | |---------------------------------------| | gravel 0.45 m (graded, 4 layers) | |----+---+---+---+---+---+---+---+------| | laterals with orifices / manifold |-> filtered |_______________________________________| water out wash water in <- wash waste out -> ``` - **Tank**: an open RCC or masonry tank, rectangular in plan with length : width = 1.25 to 1.33, depth about 2.5 to 3.5 m. - **Filter media (sand)**: 0.6 to 0.75 m depth, effective size 0.45 to 0.7 mm, uniformity coefficient 1.3 to 1.7. - **Base material (gravel)**: 0.45 m in 4 layers, graded from 2 mm at the top to 50 mm at the bottom, supporting the sand and spreading the flow. - **Under-drainage system**: a central manifold with lateral perforated pipes (orifices 6 to 12 mm, area about 0.2 to 0.5 % of bed area). It collects filtered water uniformly and distributes the backwash water. Other types: Wheeler bottom, porous plates, strainers. - **Wash-water troughs** above the sand, **inlet and outlet pipes** with valves, and **wash-water** and **wash-waste** pipes. #### Working Coagulated and settled water enters at the top; it flows down through the sand, where particles are removed by straining, sedimentation, and adsorption within the pores (not on a skin like the slow filter). The filtered water leaves via the lateral pipes and manifold to the clear water tank. As impurities collect, the head loss rises from about 0.3 m to the limit of 2.5 to 3.0 m (or the effluent turbidity rises), and the filter must be washed. #### Design considerations - Filtration rate 3000 to 6000 l/h/m² (about 5 m/h) and at least two units (N = 1.22√Q, Q in MLD). - Area = Q / rate, plus 2 to 4 % of the water for backwashing. - Depth of water over the sand 1.0 to 1.5 m (prevents negative head), plus 0.5 m free board. - Terminal head loss 2.5 to 3.0 m. #### Operation and maintenance - Filter runs 24 to 72 h between washes. Record loss of head, turbidity and the rate of filtration; adjust the rate with the rate controller. - Avoid **negative head** (which causes air binding) by keeping enough water above the sand. - Check for **mud balls**, cracks and sand loss; top up and replace sand every few years. Disinfect the filter after repairs. #### Cleaning by backwashing 1. Close the inlet and let water drop to about 0.2 m above the sand. 2. Optionally scour the bed with compressed air (about 0.6 m³/m²·min for 2 to 3 min). 3. Pass clean water upward at 0.6 to 1.0 m/min (600 to 1000 l/min/m²) for 10 to 15 minutes. The bed expands by about 50 %, the sand grains rub together and the dirt is carried out by the wash-water troughs to waste. 4. Stop the wash, close the wash valve, and then reopen the inlet. Run the first filtrate to waste. Backwash uses 2 to 4 % of the filtered water. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the two functions of the under-drainage system of a rapid sand filter? A. Support the water cover and remove scum B. Collect filtered water uniformly and distribute the backwash water C. Supply coagulant and chlorine D. Hold the sand in place and produce head Answer: B. The manifold and perforated laterals collect filtrate uniformly and also spread wash water evenly upward. (Key point: Under-drainage role) 2. How is a rapid sand filter cleaned by backwashing? A. Scraping the top few cm of sand B. Draining the filter and leaving it to dry C. Passing water downward at a higher rate D. Passing clean water upward at 0.6 to 1.0 m/min so the bed expands and dirt is carried to the wash troughs Answer: D. Upward flow expands the bed by about 50 %, grains rub together and dirt is washed out through the troughs. (Key point: Backwashing) 3. Why is a water depth of 1.0 to 1.5 m kept over the sand? A. To prevent negative head (air binding) B. To increase the filtration rate C. To kill bacteria by sunlight D. To reduce the sand depth needed Answer: A. Adequate water cover keeps pressure in the bed positive so air does not come out of solution and block pores. (Key point: Negative head) 4. Which set of values matches a rapid sand filter? A. Sand 0.3 m, rate 0.1 m/h, schmutzdecke cleaning B. Sand 0.6 to 0.75 m, effective size 0.45 to 0.7 mm, rate about 3 to 6 m/h C. Sand 2 m, rate 50 m/h, no backwash D. Sand 0.1 m, effective size 5 mm Answer: B. A rapid filter uses a deeper bed of coarser sand than a slow filter and runs at 3 to 6 m/h, usually after coagulation. 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