Septic tank and soak pit design in Nepal

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

 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.