Fossa alterna

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The Fossa Alterna is a shallow alternating double pit system, collecting and compos ting excreta from a dry toilet. Two shallow pits are dug next to each other (0.5 ­ 1.5 metre deep). Dry leaves are added to the pit base before use, and thereafter soil (and ash) are added after each defecation. When a pit is 3/4 full, the concrete slab and portable superstructure are placed on the second pit. Excreta is not only collected, but also (pre-) treated by filling the original pit with soil and leaving it to compost. By the time the second pit is full, the first pit is emptied after which the slab and superstructure is put back on top of it and the pit is reused. After composting, the content of the first pit can be used as fertiliser. The best compost time is more than 12 months. The material can be excavated earlier at 6 months, but is best transferred at this stage to a pit in which a tree will be planted.

Fossa alterna, in Epworth (for credits, click the picture)
Advantages Disadvantages
The design reduces the need to handle fresh faecal material.

Shallow depth of pit required.

No full recovery of nutrients.

Space required.
Cleaning of the toilet must be done with small amounts of water.


  • Small / medium sized family - Capacity: about 0.5-0.75 m³ - Filling time: 6-12 months
  • Family with 10 users - Capacity: about 1 m³ - Filling time: 6 months


  • Construction of total system, including material & labour - About US$ 20 - 30 (Mozambique).

Applying conditions

  • The conversion of excreta into humus will not take place if the pit is flooded with water. Therefore, the pit should not be sealed and water used for anal cleaning should not enter the pit.
  • Not applicable in areas with a high water table, with very loose soil (which could collapse), or very solid soil which would prevent drainage.
  • In order to prevent water tables penetrating the pits and contaminating the water, pits should be shallow.
  • Applicable in rural and peri-urban areas, where compost can be taken away and disposed of or, better still, used on agricultural fields.
  • As with many sanitation systems, household members need to understand the key principles for efficient operation and maintenance and discipline is required for adding soil after each defecation.

Fossa Alterna manual

This document contains a lot of information about building, maintaining a fossa alterna.

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