--- title: "Domestic water supply; septic tank and soak pit" subject: "Building Technology" chapter: 12 key: "semester_6/building_technology:water-supply-septic-067as" source: https://www.bctengineer.com/q/semester_6/building_technology/water-supply-septic-067as site: BCT Engineering --- # Domestic water supply; septic tank and soak pit Building Technology · Chapter 12: Other services in building - Asked in: 2067 Asar (old course), 2+3+3 marks - Video: [What is a Soak Pit? Working & Types](https://www.youtube.com/watch?v=OkURVGlGgRY) — Civil Engineering Fanatics - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/water-supply-septic-067as - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12 ## Question What is domestic water supply system? Discuss with relevant sketches about septic tank soak pit system of sewage disposal at residential building site. ## Short answer A domestic water supply system delivers water from the main or source via a service pipe, sump, pump, and overhead tank to fixtures. The septic tank soak pit system provides on-site sewage treatment: the watertight septic tank settles and anaerobically digests solids, and the soak pit percolates the clarified effluent into the surrounding soil. ## Answer #### Domestic water supply system It is the arrangement of pipes, valves, tanks and pumps that delivers water from the public main or source to taps and fixtures of a house in sufficient quantity, pressure and quality. Components: service pipe and meter, sump, pump, overhead tank (at least one-third of daily demand), distribution pipes, valves and fixtures. Demand is taken as 135 L/person/day. ```text Main -> [sump] -> pump -> [roof tank] -> floors -> taps ``` #### Septic tank and soak pit system for a residential site Where there is no public sewer, toilet sewage is treated on site by a septic tank, and the effluent goes to a soak pit. #### 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. ```text WC/bath -> inspection chamber -> septic tank -> soak pit ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which are components of a domestic water supply system? A. Service pipe and meter, sump, pump, overhead tank, distribution pipes, valves and fixtures B. Septic tank, soak pit and manhole only C. Fire extinguisher and hydrant D. Rain gutters only Answer: A. The system brings water through service pipe and meter to sump, pump, overhead tank, distribution pipes and fixtures. (Key point: Domestic supply components) 2. What is the path of sewage at a residential site with no public sewer? A. Soak pit, septic tank, inspection chamber B. WC/bath, inspection chamber, septic tank, soak pit C. Septic tank, WC, soak pit D. Soak pit, then river Answer: B. Sewage passes an inspection chamber into the septic tank and the effluent then goes to the soak pit. (Key point: Sewage flow path) 3. How does a septic tank treat sewage? A. Solids settle as sludge and are digested by anaerobic bacteria; fats form scum; clear effluent leaves via the outlet B. Chlorine kills solids C. Sewage is filtered through sand D. Solids are burned Answer: A. Settling and anaerobic digestion occur in the tank, leaving a partly clarified effluent to flow out. (Key point: Septic tank working) 4. What safe distances are advised for a soak pit? A. About 15 m or more from drinking-water sources and 3 m from building foundations B. 1 m from wells and 0.5 m from foundations C. Right next to the well D. 50 m from foundations only Answer: A. Keep it about 15 m or more from drinking water sources and 3 m from foundations. (Key point: Soak pit placement) 5. What per capita demand is used, and how big should the overhead tank be? A. 135 L/person/day; at least one-third of daily demand B. 50 L/person/day; full daily demand C. 135 L/person/day; one-tenth of demand D. 300 L/person/day; no tank needed Answer: A. Demand is taken as 135 L/person/day and the overhead tank holds at least one-third of the daily demand. (Key point: Demand and tank) ## Related in Building Technology - [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 - [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 and drainage systems](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-drainage) — Building Technology, chapter 12 - [Water supply and sanitation management](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-sanitation) — Building Technology, chapter 12 - [Short note: Water supply and distribution system](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-distribution-short) — Building Technology, chapter 12, asked 2× - [Principles of laying out water supply system](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-principles) — Building Technology, chapter 12 ## Related in other subjects - [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, 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× - [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, 8 persons](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-8-persons) — Sanitary Engineering, chapter 9, asked 2×