Fossa Alterna

What is a Fossa alterna?

The Fossa alterna is similar to the Arborloo but the slab and superstructure are alternately placed on one of two permanently sited shallow pits. Like the Arborloo, wood ash and soil are added regularly and other materials like organic kitchen scraps, leaves, soil etc which promote the composting process. Once the first pit is nearly full the slab and superstructure are moved on to the second pit and the first pit is topped up with fertile soil. After a period of between six and twelve months the contents of the first pit will have changed into humus. The humus is dug out and can be stored in bags, placed in a secondary compost heap, or used on the garden. The slab and superstructure are then moved back on to the first pit and the second pit is topped up with soil. This cycle is repeated time and time again.

This system was originally designed for peri-urban settlements where there is too little space to plant any trees as would be required with the Arborloo. However, this model has also proven to be very effective in rural areas, providing rich compost from a permanent toilet structure.

What is special about Fossa alternas?

Practicality:

The Fossa alterna is an excellent design for families that do not have enough space for planting trees or who would like to use the compost for other gardens that are already established. Like the Arborloo, compost from the Fossa alterna can be used to increase household nutrition and income, while the toilet provides access to sanitation and reduces the spread of disease.

Longevity:

Because the Fossa alterna is maintained by cycling between two pits, it is possible to line the pits to ensure that they can be used repeatedly. With the Fossa alterna it is not necessary to dig new holes every 6 months. Families can either have a portable structure that moves between the pits (like the Arborloo – link) or build a permanent structure around the pits that can last for many years.

Productivity:

The compost produced by a Fossa alterna is a beautiful fertile soil that will vary in texture depending on what materials are composted in the toilet pits. This compost is an organic fertilizer which can significantly enhance plant growth on nutrient poor soils where farmers cannot afford fertilizer.  The picture to the left is from an experiment done in Peter Morgan’s backyard in Harare, Zimbabwe. It shows two lettuce plants: one in local topsoil (L) and one with 50% topsoil and 50% compost (R). Productive increases like this can mean higher income for farmers and improved family nutrition.

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