--- title: "Grit chamber: purpose, construction, design" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:grit-chamber-purpose-design" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/grit-chamber-purpose-design site: BCT Engineering --- # Grit chamber: purpose, construction, design Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2080 Chaitra, 8 marks; 2075 Baisakh, 1+2+5 marks; 2069 Bhadra, 8 marks; 2068 Bhadra (old course), 6 marks (asked 4 times) - Video: [design for Grit chamber, grit chamber design, grit chamber dimensions, wastewater treatment, water](https://www.youtube.com/watch?v=YkoL0evzffY) — Civil Engineering Exam - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/grit-chamber-purpose-design - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question What is a grit chamber? Why does a sewage treatment plant need a grit chamber? Describe with the help of neat sketches the purpose, construction and design criteria of a grit chamber. ## Short answer A grit chamber is a velocity-controlled channel that settles sand and gravel while organic matter stays in suspension. It protects pumps and pipes from abrasion, prevents silting in tanks, and yields clean inert grit. Design velocity is 0.2–0.3 m/s with a 30–60 second detention time. ## Answer A **grit chamber** is a narrow, long channel or tank placed after the screens (and before the primary sedimentation tank) in which the velocity of sewage is controlled so that heavy inorganic particles (sand, gravel, cinders, eggshell fragments of size about 0.15 mm and above, specific gravity about 2.65) settle, while the lighter organic matter stays in suspension and passes on. #### Why a grit chamber is needed - Grit is abrasive and wears pumps, pipes and sludge-handling equipment. - It settles in channels, tanks and digesters, reducing their capacity and causing clogging. - If grit mixes with organic sludge in the primary tank, the sludge becomes heavy and difficult to digest and dewater. - Removing grit separately gives clean, easily disposable inert material. #### Purpose To remove grit without removing the organic solids, and so protect the downstream mechanical equipment and save tank capacity. #### Construction (horizontal-flow type) ```text Plan Section ---------------------- ~~~~~~~~~~~~~~~~~~~~~~~ water level inlet -> | | -> v = 0.2-0.3 m/s -> zone -> | channel | -> \_ grit settles _/ ----------------------- grit storage hopper proportional weir / Parshall flume at outlet ``` - A rectangular channel, normally two or more units in parallel so that one can be cleaned while the other works; inlet and outlet have tapered transitions. - The outlet has a **proportional (Sutro) weir** or a **Parshall flume** to keep the horizontal velocity almost constant at about 0.3 m/s even when the flow varies. - A hopper at the bottom (or extra depth of 0.2-0.3 m) stores the grit; it is removed by hand-scraping in small plants or by mechanical scrapers, screw conveyors or air-lift in large plants. - Aerated and vortex-type grit chambers are other variants. #### Design criteria 1. **Flow-through velocity:** about 0.2-0.3 m/s. It must be below the scour velocity of grit, $v_c=\sqrt{\dfrac{8k}{f}(G-1)g\,d}$ ($k=0.04$-$0.06$, $f=0.02$-$0.03$), but high enough to carry organic matter ($v \ge$ about 0.15 m/s). 2. **Detention time:** 30-60 seconds (usually about 1 minute) at maximum flow. 3. **Settling velocity** for 0.2 mm grit of specific gravity 2.65 is about 0.02-0.04 m/s (Hazen: $v_s=418(G-1)d^2\dfrac{3T+70}{100}$ mm/s, $d$ in mm). 4. **Length** $L=\dfrac{v\,H}{v_s}$, increased by 20-50% for inlet and outlet turbulence. 5. **Cross-section** $A=Q/v$, with depth $H$ chosen (0.6-1.5 m) so that width $B=A/H$ is practical; allow 0.3 m free board and 0.2-0.3 m grit storage depth. 6. **Surface loading** (overflow rate) about 800-1200 m³/m²/day. 7. Grit removed is about 0.03-0.1 m³ per 1000 m³ of sewage. **Example:** $Q=0.2$ m³/s, $v=0.25$ m/s gives $A=0.8$ m²; with $H=0.8$ m, $B=1.0$ m; if $v_s=0.025$ m/s then $L=0.25\times 0.8/0.025 = 8$ m, plus 25% = 10 m. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a grit chamber needed in a sewage treatment plant? A. To disinfect the sewage B. Grit abrades pumps and equipment, and settles in channels and digesters reducing capacity C. To remove floating oil only D. To remove dissolved BOD Answer: B. Grit is abrasive and clogs tanks and digesters, so it is removed separately early. (Key point: Purpose) 2. How does a grit chamber remove grit but not organic matter? A. By filtration through sand B. By chemical coagulation C. By very low velocity so everything settles D. Flow velocity is controlled (about 0.2-0.3 m/s) so heavy inorganic particles settle while lighter organics stay suspended Answer: D. Velocity is set low enough for grit to settle but high enough to carry organic matter on. (Key point: Selective settling) 3. What is the function of the proportional (Sutro) weir or Parshall flume at the outlet? A. Keeps horizontal velocity nearly constant when flow varies B. Aerates the sewage C. Collects floating scum D. Measures BOD Answer: A. A proportional weir changes discharge in proportion to depth so the velocity stays about 0.3 m/s. (Key point: Constant velocity) 4. How is the chamber length obtained? A. L = v H / vs, increased by 20-50% for turbulence B. L = detention time x vs C. L = vs H / v D. L = Q / v Answer: A. A particle falling vs while moving at v across depth H needs L = vH/vs; an allowance is added for inlet/outlet turbulence. 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