Evapo-transpiration mound

Purpose

An evapo-transpiration (ET) mound is an above-ground sand-fill bed for disposing of septic tank effluent where the soil is shallow, the ground-water table is high, the ground is rocky, or the infiltration rate is very slow, so that a soak pit or trench cannot work. The effluent is spread through the mound and is lost mainly by evaporation from the surface and transpiration by the plants growing on it; the rest percolates into the soil below.

Construction

1. The site is ploughed, and a fill of clean coarse sand (and gravel) is built up to form a mound about 0.5-1.0 m high, with side slopes of 1 V : 3 H or flatter. The sand bed is placed on the loosened natural ground.
2. Septic tank effluent is fed through a distribution box into perforated distribution pipes (75-100 mm dia) laid in a gravel bed (15-20 cm thick) inside the sand.
3. The gravel is covered with a layer of geotextile or straw, then 0.3 m of topsoil, and the surface is grassed or planted with high-transpiring plants (banana, bamboo, canna, etc.).
4. The area is sized from the effluent flow and the local evaporation: plan area $\approx Q/(\text{infiltration rate} + \text{evapotranspiration rate})$.
5. Surface runoff is kept off with a diversion drain, and the mound is fenced.

     plants / grass
    __________________
   /  topsoil 0.3 m   \
  /--------------------\
 /  gravel + distr.pipe \
/  coarse sand fill      \
=========================== natural ground
   (poor or rocky soil)