---
title: "Septic tank and soak pit design in Nepal"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:septic-tank-design-considerations-nepal"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-design-considerations-nepal
site: BCT Engineering
---

# Septic tank and soak pit design in Nepal

Sanitary Engineering · Chapter 9: Disposal of Sewage from Isolated Buildings

- Asked in: 2076 Bhadra, 8 marks; 2070 Bhadra, 8 marks (asked 2 times)
- Video: [Design of septic tank and soak pit Part 1 in Nepali. DCE sanitary engineering](https://www.youtube.com/watch?v=sPhBArhdHt8) — Er Dipak Sir
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-design-considerations-nepal
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

What are the design considerations adopted for the design of septic tank and soak pit in Nepal (state the procedure for designing). Briefly describe them in neat sketch showing the zones and their purposes.

## Short answer

Septic tank zones: scum, settling, digestion, storage. Design based on 24 h detention, per capita sludge volumes (0.0425 m³ digestion, 0.085 m³ storage for 3 yr), two chambers, T-pipes, watertight structure, min 6 m from water sources. Soak pit area from percolation test, circular pit, 15 m from well.

## Answer

A **septic tank** is a watertight, single-storey sedimentation and digestion tank that gives primary treatment to the sewage of an isolated building. Its effluent is still polluted, so it is disposed of in a **soak pit** (or dispersion trench) where the soil gives further treatment.

#### Zones of a septic tank and their purposes

```text
 Inlet                       Outlet
  |   (T-pipe)  | partition   |(T-pipe)
  v             |  wall       v
 ___|____________|_____________|___  <- cover slabs
 |  ^ scum   |  ^ opening  | scum  |  
 |-----------|-------------|-------|  <- water level
 |  Sewage zone (settling)  | zone  |
 |..........................|.......|
 |  Sludge digestion zone   |       |
 |  Digested sludge storage |       |
 |___________________________|______|
   Chamber 1 (2/3 L)      Chamber 2 (1/3 L)
```

- **Scum zone (top):** grease, oil and light solids float here; the dipped T-pipes keep scum from leaving.
- **Sewage (settling) zone:** sewage is held about 24 h so that suspended solids settle; designed for the daily flow.
- **Sludge digestion zone:** settled solids are digested anaerobically by bacteria, producing gas and reducing volume.
- **Digested sludge storage zone:** stabilised sludge collects between desludging periods (usually 2-3 years).

#### Design considerations (Nepal practice)

1. **Flow:** sewage = 80% of water supply (or 80-100 L per person per day); users from the building occupancy.
2. **Detention time:** 24 h (minimum 12-24 h) for the sewage zone.
3. **Sludge:** digestion zone 0.0425 m³/person; digested sludge storage about 0.0283 m³/person/year (0.085 m³/person for 3 years).
4. **Shape and size:** liquid depth 1.0-2.0 m, plus 0.3 m free board; width at least 0.75 m; length at least 1.5 m; $L:B$ = 2:1 to 4:1; usually 2 chambers, first = 2/3 length.
5. **Inlet/outlet:** T-pipes dipping 0.3 m (up to 0.45 m) below the water level, with vent pipe and manhole cover for inspection and desludging.
6. **Structure:** watertight masonry or RCC, 1:3 cement plaster, 75 mm thick floor sloping towards inlet; keep at least 6 m from water sources and 1.5 m from buildings.
7. **Effluent disposal:** soak pit, dispersion trench or evapotranspiration mound, depending on soil.

#### Procedure for design

1. Find the number of users and sewage flow $Q$.
2. Volume of sewage zone $= Q \times$ detention time.
3. Add digestion and sludge storage volumes (per-person rates $\times$ users).
4. Fix depth, then $L \times B = V/d$ with $L:B$ = 3:1; add free board.
5. Divide into two chambers; fix T-pipe, vent and manhole.

#### Soak pit design

- Area $A = Q/\text{infiltration rate}$ (L/m²/d, from percolation test).
- Circular pit of diameter $D$ and effective depth $h$: $\pi D h = A$, usually $D$ = 1.5-3 m, $h$ = 2-3 m.
- Pit is honeycomb-lined and filled with brick bats/gravel; bottom at least 1 m above ground-water; at least 15 m from a well. For larger areas use several pits or dispersion trenches.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the purpose of dipped T-pipes at the inlet and outlet?
   A. They aerate the sewage
   B. They stop scum from leaving and prevent disturbing settled sludge
   C. They add chlorine
   D. They measure the flow
   Answer: B. T-pipes dip about 0.3 m below water level so floating scum stays inside the tank. (Key point: Scum zone and T-pipes)

2. What occurs in the sludge digestion zone?
   A. Grease floats and is skimmed
   B. Oxygen oxidises BOD
   C. Settled solids are digested anaerobically, producing gas and reducing volume
   D. Sewage is held for 24 h to settle
   Answer: C. The digestion zone breaks down solids anaerobically; digested sludge is stored below until desludging every 2-3 years. (Key point: Sludge zones)

3. How is the septic tank usually divided?
   A. Three equal chambers
   B. Two equal chambers with L:B 1:1
   C. One chamber with L:B 1:3
   D. Two chambers, the first about 2/3 of the length, with L:B 2:1 to 4:1
   Answer: D. Two chambers with the first 2/3 length improve settling; depth 1-2 m plus 0.3 m free board. (Key point: Tank layout)

4. Soak pit area is calculated from
   A. A = Q / infiltration rate (from percolation test)
   B. A = Q x infiltration rate
   C. A = detention time x Q
   D. A = population / 80
   Answer: A. Required area = flow divided by the soil's infiltration rate; pi D h = A gives pit size. (Key point: Soak pit sizing)

## Related in Sanitary Engineering

- [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×
- [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×
- [Septic tank and soak pit, hostel of 400](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-soak-pit-hostel-400) — Sanitary Engineering, chapter 9
- [Septic tank and soak pits, 15 users, Terai](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-15-users-terai) — Sanitary Engineering, chapter 9

## Related in other subjects

- [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×
- [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
- [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
- [Septic tank with soak pit: excavation, floor, plaster](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aseptic-tank-soak-pit-f) — Estimating and Costing, chapter 6