The Fossa Alterna - the double pit compost toilet

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The Fossa alterna (alternating ecological pit toilet) was researched by Peter Morgan of Aquamor and introduced in his book “Ecological Sanitation in Zimbabwe – A compilation of manuals and experiences (Volume 1. 1999).

It evolved from a study made of the conversion of human faecal matter into a form of compost simply by the addition of an equal volume of soil. The end product was improved by the addition of ash and leaves.

The Fossa alterna differs from other alternating pit systems in that soil, ash and leaves are added regularly to hasten the conversion of excreta in to a medium which can be handled.

Two version of the Fossa alterna were developed, the short cycle version, in which the use of the two pits alternates at yearly intervals and a long cycle version with larger pits, where the use of pits alternates at about 5 years.

In Zimbabwe, lightweight portable superstructures have been used, whilst in countries like Malawi and Mozambique the two pits were enclosed in a single, more permanent superstructure.

The technique involved using one pit at a time, and adding soil and other ingredients regularly whilst in use. When nearly full the used pit is closed off and the second pit is brought into use.

The contents of the first pit are filled with soil and closed off. After a year, the contents of the composting pit are dug out, and this empty pit is used again. The excavated material can be placed in the base of tree pits (pits filled with soil and compost to which trees will be planted) or mixed with top soil which can be dug into maize fields or vegetable beds.

The method has been used in Malawi, Mozambique, Kenya, Zimbabwe and several other countries. The concept mimics the use of soil as an addition to human excreta in the Earth Closet, the forerunner of the Flush Toilet. The change in the form of excreta when combined with soil is a process found widely in Nature.


Pictures & downloadable resources

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The Fossa Alterna - The Double Pit Compost Toilet

The Garden Version Fossa Alterna

The Fossa alterna. The toilet that makes humus (ppt)

A portable steel frame structure for Arborloo & Fossa alterna (ppt)

The Long Cycle Fossa alterna (ppt)

How to make simple pedestals (ppt)

Additional resources on main resource page here


© 2022 Dr Peter Morgan