Septic tank and 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).

  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.

  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.