--- title: "Septic tank and soak pit" subject: "Building Technology" chapter: 12 key: "semester_6/building_technology:septic-soak-pit" source: https://www.bctengineer.com/q/semester_6/building_technology/septic-soak-pit site: BCT Engineering --- # Septic tank and soak pit Building Technology · Chapter 12: Other services in building - Asked in: 2081 Chaitra, 4 marks; 2071 Bhadra, 8 marks; 2070 Magh; 2068 Baisakh (old course), 4 marks; 2066 Bhadra (old course), 4 marks; 2064 Jestha (old course), 4 marks; 2062 Poush (old course) (asked 7 times) - Video: [How a SEPTIC TANK works and How to (build) a septic tank STEP BY STEP](https://www.youtube.com/watch?v=ubQ3IqXoqsY) — Ian Construction - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/septic-soak-pit - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12 ## Question Describe septic tank and soak pit with neat sketches. ## Short answer A septic tank is a watertight two-chamber settling and anaerobic digestion tank with T-baffles, providing 24-hour detention. Effluent flows to a soak pit: a circular honeycomb brick pit filled with brick bats that allows percolation into soil, installed at least 15 m from water sources and 3 m from foundations. ## Answer #### Septic tank A **septic tank** is a watertight, underground, single-storey sedimentation and digestion tank that treats the sewage of a household or small building where no public sewer exists. Solids settle at the bottom as sludge and are digested by anaerobic bacteria; the partly clarified liquid (effluent) flows out for further treatment in a soak pit. **Parts:** inlet pipe with a T-baffle (dip pipe), settling chamber, sludge zone, scum zone, outlet T-pipe, vent pipe, access manhole with cover. Often two chambers are provided with a dividing wall (first chamber 2/3 of length). ```text inlet manhole vent outlet | | | | ===v========v=============v=======v==== GL | T scum layer | T | |___| |___| | | liquid (effluent zone) | | | sludge digestion zone [] | | |_____________________________|_______| chamber 1 (2/3) chamber 2 ``` **Working:** sewage enters, flow velocity drops; heavy solids settle, light fats float as scum; anaerobic bacteria reduce sludge volume (about 30-50%) producing gas that escapes through the vent. Clear effluent leaves via the outlet. **Design points (IS 2470 / Nepal practice):** detention time about 24 hours; sewage 90-135 L/person/day; sludge allowance about 30 L/person/year with desludging interval 1-2 years; length : width = 2:1 to 4:1; width not less than 0.75 m; liquid depth 1.0-1.8 m; free board 0.3 m; walls of brick in cement mortar plastered inside with waterproofing; floor and cover of RCC. #### Soak pit A **soak pit** (soakaway) is a circular pit that receives effluent from the septic tank and lets it percolate into the surrounding soil. **Construction:** circular, diameter 1.0-2.5 m (typically 1.5 m) and depth 1-3 m, lined with dry brick masonry with honeycomb openings; filled with brick bats/stone aggregates (coarse at the bottom, finer above); covered with RCC slab having a manhole; the inlet pipe enters near the top. ```text from septic tank | cover slab =====v======+====== GL | honeycomb brick wall | | ooo brick bats ooo | | ooo aggregate ooo | |_______________________| | | seepage -> soil ``` **Conditions:** soil must be permeable, water table well below the pit bottom, and the pit at a safe distance (about 15 m or more) from drinking-water sources and 3 m from building foundations. Where soil is impermeable, use a leach/drain field or an absorption trench instead. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What happens in a septic tank? A. Solids settle as sludge, are digested anaerobically, and partly clarified effluent leaves B. Aerobic bacteria give drinking water C. Sewage is chlorinated and discharged D. Only fats are removed Answer: A. It is a sedimentation and anaerobic digestion tank. (Key point: Septic tank function) 2. Which design value is correct for a septic tank? A. Detention time 1 hour, length equals width B. Detention time about 24 hours, length : width 2:1 to 4:1 C. Detention 7 days, width 0.1 m D. Liquid depth 5 m Answer: B. IS 2470 practice uses about 24 h detention and 2:1 to 4:1 length:width. (Key point: Design values) 3. How is a soak pit built? A. Watertight sealed concrete box B. Open pit without lining C. Circular, dry honeycomb brick lining, filled with brick bats/aggregate (coarse at bottom), RCC cover D. Filled with clay only Answer: C. Honeycomb lining and graded filler let effluent percolate. (Key point: Soak pit construction) 4. Which condition suits a soak pit? A. Impermeable clay B. High water table C. Next to a well D. Permeable soil and low water table, about 15 m from drinking-water sources Answer: D. Otherwise use a leach field or absorption trench. (Key point: Soak pit conditions) ## Related in Building Technology - [Domestic water supply; septic tank and soak pit](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-septic-067as) — Building Technology, chapter 12 - [Building services, septic tank and soak pit](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Abuilding-services-septic) — Building Technology, chapter 12 ## Related in other subjects - [Septic tank and soak pit design in Nepal](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-design-considerations-nepal) — Sanitary Engineering, chapter 9, asked 2× - [Septic tank and soak pit, 15 users](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-soak-pit-15-users-100) — Sanitary Engineering, chapter 9, asked 2× - [Septic tank and soak pit, household of 80](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-soak-pit-80-persons) — Sanitary Engineering, chapter 9, asked 2× - [Septic tank and soak pit, 10 users, 85 l/m²/d](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-10-users-85) — Sanitary Engineering, chapter 9, asked 2×