Domestic water supply; septic tank and soak pit

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.

 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).

  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.

 WC/bath -> inspection chamber -> septic tank -> soak pit