--- title: "Building services, septic tank and soak pit" subject: "Building Technology" chapter: 12 key: "semester_6/building_technology:building-services-septic" source: https://www.bctengineer.com/q/semester_6/building_technology/building-services-septic site: BCT Engineering --- # Building services, septic tank and soak pit Building Technology · Chapter 12: Other services in building - Asked in: 2078 Chaitra, 5 marks - 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/building-services-septic - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12 ## Question Explain the term Building Services, Septic tank and Soak pit. ## Short answer Building services are functional installations like water, drainage, electrical, fire, and lifts. A septic tank is a watertight underground tank for sewage settling and anaerobic digestion; effluent goes to a soak pit. A soak pit is a percolation pit (1-2.5 m dia, dry brick lining, filled with aggregates) discharging effluent into soil. ## Answer #### Building services **Building services** are the installations that make a building functional, safe and comfortable apart from its structure. They include: water supply, drainage and sanitation, electrical supply and lighting, fire protection, ventilation and air conditioning, lifts and escalators, gas supply, telecom/IT, and waste disposal. They are planned along with architectural and structural design so that shafts, ducts and spaces are provided early. #### 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 are building services? A. Installations making a building functional, safe and comfortable apart from structure B. Only the structural frame C. Only painting D. Only furniture Answer: A. Water, electricity, fire, HVAC, lifts, etc. (Key point: Building services) 2. A septic tank is: A. Watertight underground sedimentation and digestion tank B. Open pit C. Aerobic aeration plant D. Water storage tank Answer: A. Anaerobic bacteria digest sludge. (Key point: Septic tank) 3. Anaerobic action reduces sludge volume by about: A. 90% B. 30-50% C. 0% D. 5% Answer: B. Gas escapes through the vent. (Key point: Sludge reduction) 4. Soak pit receives: A. Raw rainwater only B. Drinking water C. Effluent from the septic tank D. Sludge Answer: C. Effluent percolates into soil. (Key point: Soak pit) ## Related in Building Technology - [Septic tank and soak pit](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aseptic-soak-pit) — Building Technology, chapter 12, asked 7× - [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 ## 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×