Skip to main content

Pit privy; septic effluent disposal, rocky area

Short answer

A pit privy contains and decomposes excreta to prevent disease; design: 0.04–0.06 m³/person/year, bottom ≥1.5 m above groundwater, lined pit, raised slab with squat hole. For rocky areas with high water table, use an evapotranspiration mound: effluent in perforated pipes over sand/gravel bed, planted, base ≥1 m above water table.

Describe in brief about the purpose and design criteria of pit privy. Also, with a neat sketch, discuss the suitable disposal of septic tank effluent for the rocks area with high ground water table.

Pit privy: purpose

A pit privy is a simple on-site latrine that holds human excreta in a pit, keeping it away from people, flies and water so that it decomposes safely, which prevents faecal-oral disease. It is used in rural and low-income areas without sewers or piped water.

Design criteria

  • Location at least 15 m from a well/spring and 6 m from a house; pit bottom at least 1.5 m above the ground-water level.
  • Pit size: 0.06 m³/person/year for dry pits and 0.04 for wet pits, designed for 5-10 years; usually 1.0-1.2 m wide and 2-3 m deep, plus 0.5 m free space.
  • Pit lined with open-jointed brick/stone (or bamboo/wood), with the top 0.3 m watertight and raised above ground against surface runoff.
  • Slab of RCC/wood, 1.2 m × 1.2 m, with a 0.2-0.25 m squat hole and a cover lid.
  • Superstructure about 1.0 × 1.2 × 2.0 m with a door and roof; when the pit fills to 0.5 m below the slab, it is covered with soil and a new pit is dug.

Disposal of septic tank effluent for a rocky area with a high water table

Soak pits and trenches cannot be used, because the rock prevents absorption and the water table is high. The suitable method is an evapotranspiration (ET) mound / sand mound:

  1. Effluent from the septic tank goes to a distribution box and perforated pipes (75-100 mm) laid in a gravel bed inside a mound of clean coarse sand, built on the rock or ground above the high water table.
  2. The mound is 0.5-1.0 m high with side slopes of 1:3, and is covered with 0.3 m of topsoil and planted with grass or high-transpiring plants (banana, bamboo).
  3. Effluent moves upward and through the sand by capillary action; most of it evaporates or is taken up by the plants, and the rest drains slowly.
  4. Surface water is diverted away, and the base of the mound is kept at least 1 m above the water table.
   plants / grass cover
   ___________________________
  /    topsoil + straw layer   \
 /-------------------------------\
 / gravel + perforated pipe <- from \
/        coarse sand bed       dist.box
================================ rock / natural ground
   ^ at least 1 m  GWT (water table)

Alternatively, a sand filter or constructed wetland can treat the effluent before it is released into a drain or stream.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Sanitary Engineering chapter 9 questions →