---
title: "Evapo-transpiration mound"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:evapotranspiration-mound"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/evapotranspiration-mound
site: BCT Engineering
---

# Evapo-transpiration mound

Sanitary Engineering · Chapter 9: Disposal of Sewage from Isolated Buildings

- Asked in: 2071 Magh, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/evapotranspiration-mound
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Describe the purpose and construction of an evapo-transpiration mound.

## Short answer

An ET mound disposes effluent where groundwater is high or soil impermeable. A 0.5–1 m high sand mound with perforated pipes in gravel, 0.3 m topsoil and plants loses water by evapotranspiration; area sized from flow rate divided by infiltration plus evapotranspiration rate.

## Answer

#### 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.

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. When is an ET mound used?
   A. Deep sandy soil
   B. Where sewers exist
   C. For raw sewage
   D. Shallow soil, high water table, rocky or slowly permeable ground
   Answer: D. It is used where soak pits or trenches cannot work. (Key point: Purpose)

2. How is effluent mainly lost?
   A. Evaporation and plant transpiration
   B. Fast percolation
   C. Biogas
   D. Chemical treatment
   Answer: A. Evaporation from surface and transpiration by plants remove most effluent. (Key point: Disposal mechanism)

3. Which is the correct arrangement from bottom to top?
   A. Topsoil, sand, gravel
   B. Sand fill, gravel with perforated pipes, geotextile/straw, topsoil with plants
   C. Gravel, concrete, soil
   D. Clay, plastic, sand
   Answer: B. Pipes sit in gravel inside the sand, covered by geotextile and 0.3 m topsoil. (Key point: Layers)

4. Why is a diversion drain provided?
   A. Collect effluent
   B. Cool the mound
   C. Keep surface runoff off the mound
   D. Water the plants
   Answer: C. Runoff would saturate the mound and reduce its capacity. (Key point: Surface runoff)

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