fossa alterna


“alternating pit”



A method for recycling

human waste







Peter Morgan






The words fosas migratorias roughly translated from the Portuguese mean “migrating pit” and refer to a pit latrine system used in Brazil where the latrine slab and superstructure is portable and the contents of the pit remain. Trees can be planted in the pits that remain which are topped up with soil and vegetable matter. The system where the latrine is portable and the old pits are planted with trees is known in Zimbabwe as the “Arborloo” and a manual has been written on this subject. The end result after years of use is a series of trees which may form an orchard.


There is a slightly different concept which can be called a fossa alterna (from the Latin - alternating pit), where two more permanently sited pits are used alternately. In this case, the pit which has become full is not planted with a tree but left so that the contents are allowed to turn into compost over a period of 6 – 12  months. After this period the “humus” like material is withdrawn from the pit, which is then put back into use. The advantage here is that just two pits are used and these can occupy a very small area and thus this method becomes suitable for peri-urban settlements where space is limited. The slab and structure simply move back and forth from one pit to another.


When human faeces together with urine are added to a pit, they will eventually decompose if left untouched. This process is accelerated and enhanced if other ingredients are added to the pit so that a true compost or humus-like material is formed which is valuable in the garden. These additional ingredients include soil and wood ash, vegetable matter like leaves or grass, kitchen scraps, and extra animal manure. Ideally wood ash and soil should be added after every visit made to the latrine. The wood ash and soil cover the latest deposits and help to reduce  fly and smell problems. The ash adds potash to the mix. The addition of leaves, grass, and kitchen scraps like vegetable peelings and other organic matter helps also to promote the composting process and gives the final compost a loose texture like “humus.” Plastic bags should not be added as these do not decompose well.


The concept described in this manual may be suitable for both rural and peri-urban settlements where the space available may be quite small. The twin pits are quite small and generally between 0.5m and 1.0m deep. One is covered with a concrete slab which is portable and fitted with handles. The slab may be fitted with a pedestal or may be of the squat type. A suitable portable superstructure which may be made of many materials, is mounted on or around the slab. The superstructure described in this manual is made of wood and has a door.


Because the intention is to constantly re-use the two pits and alternate them, it is wise to partly line the upper section of the pit with bricks to form an upper pit lining and ring beam. This will avoid the upper end of the pit collapsing. In the example cited here, two pits are dug, spaced about 30cm apart, and the upper pit linings and rings beams for both can be built at the same time. Both pits are dug down within the brick lining to a depth of between 0.5 - 1.0m. The single portable slab and superstructure is mounted on one of the pits and the other pit is covered with a wooden or concrete cover for safety. The latrine is then put to use. The rate of filling will depend on the number of people using the unit and the size of the pit. The latrine is used like a normal pit latrine but it is important that the various other ingredients are added as well - particularly the soil and ash. Whilst they will increase the rate of filling of the pit, they will promote the formation of compost which can easily be removed later. At the end of a 6 to 12 month period, or when the pit has become three quarters full, the slab and superstructure are then transferred from the first pit to the second pit. The original pit is then topped up with soil mixed with leaves and grass - whatever is available, and left to decompose. It is kept moist with occasional watering.  The second pit now begins to fill up with the various ingredients.


After a period of about 6 - 12 months the original pit is dug out. The contents will not look like the original ingredients. They will have changed their form completely to look like humus. This material which is excellent for the garden, can be dug out and stored in bags or placed directly into the bases of trenches for vegetable growing or into pits dug for trees. The latrine slab and superstructure are now moved back onto the site of the original pit and the  latrine put into use again. The second pit is then topped up with soil mixed with leaves and grass - whatever is available, and left to decompose. The original pit now begins to fill up for the second time with the various ingredients. The process is repeated about once or twice a year with the latrine alternating between the two pits. Three or four pits can also be used with the latrine moving around them. However two pits should be adequate unless the owner decides to put more pits into use. He may decide to plant fruit trees. However where space is limited a two pit system may be best. This is the type described in this manual. This method is very simple and relatively low cost.


Peter Morgan

Harare, June 1999

Updated for Website August 2001.


The concept




The concept of this method of recycling human by-products is simple enough. There are two quite small shallow pits, close to one another. Both are lined at their heads to make them stable. Each has a brick “ring beam” surrounding the pit head, above ground level. A single concrete cover slab is made and placed on one of the ring beams. The slab is made so it is relatively light and is equipped with handles. A pedestal can be fitted to the slab - either directly, so that the slab and pedestal are one unit, or as separate units so that the pedestal can be removed and cleaned. A portable structure is built so that it can either surround or sit on top of the slab. The structure can be made of wood, cement on hessian, reeds or grass and poles etc. The pit which is not in use is fitted with a cover made of wood or cement.


The use of the latrine alternates from one pit to the other (hence the name fossa alterna). The latrine is first used on one pit so that it becomes about 3/4 full of waste matter and additional material. At this stage the superstructure is moved to one side, the slab moved from the first to the second pit and the superstructure put in place again over the slab. The first pit is topped up with soil and left and the second pit put into use. The cycle is repeated at about 6 monthly intervals.


The latrine is not used in quite the same way as a normal pit latrine. Whilst both faeces and urine are added together with anal cleansing material, which should be some sort of paper, it is very important to regularly add other materials, which will aid the composting process. After each visit, some soil and wood ash should be added. This helps to reduce flies and smells. Also from time to time scraps from the kitchen, leaves, grass, green vegetable matter and some animal manure should be added into the pit. Thus the pit gains a mix of ingredients suitable for making compost. After the pit has become 3/4 full, and the slab and the structure have been moved on to the second pit, the first pit is topped up with good soil and left to decompose for a minimum of 6 months and preferably 12 months. When the second pit is 3/4 full, the contents of the first pit are dug out with a shovel, and transferred to bags or placed in pits dug for trees or trenches dug for vegetables. The material, which is best mixed with topsoil, can also be used in pots or placed in the garden compost heap ready for processing.


Thus the operation of this unit has the advantage that a regular supply of good soil-like humus will be available for use on the garden or in pots. But more work is required compared to the normal pit latrine. It is important that the extra ingredients are added regularly and also that the slab and superstructure should be moved at least once or twice a year. However, if the slab and superstructure are correctly designed, their movement from one pit to the second should only take a minute or two. If the pits are well supported by the upper brickwork and ring beam, they should last for many years. The soil-like humus/compost which is formed in the pit, is loose and easy to dig out, compared to the original soil which may be hard. Thus excavation will be much easier than digging out the original pit. The humus is also safe and pleasant to handle, unlike the original waste matter. The area of land required for this unit is little more than 2.5m X 1.5m.


According to literature written by Feachem et al, in 1983, Faecal coliforms survive for less than 90 days in faeces, with Salmonella surviving for less than 60 days, Shigella for less than 30 days and Vibiro cholerae for less than 30 days. Entamoeba histolytica cysts survive for less than 30 days. Enteroviruses survive for less than 100 days and usually less than 20 days. Ascaris lumbricoides eggs may last for many months (Feachem et al., 1983. Sanitation & Disease: Health Aspects of Excreta & Wastewater Management. London: John Wiley & Sons). Thus even after 6 months of composting the waste matter is made quite safe and even more so after 12 months, when the risks of any disease being transmitted by the composted matter are very small indeed.


            The Sequence



*Use latrine over first vault adding soil, ash etc as well as urine and faeces.

*When almost full move structure, pedestal and slab over to second vault.


*Top up first vault with soil and cover with lid. Water from time to time.

*Use latrine over second vault adding soil, ash etc as well as urine and faeces.



*After between 6 - 12 months remove lid on first vault and transfer compost to bags or to gardens.

*Move structure, pedestal and slab back to first vault.

*Top up second vault with soil.

*Repeat the process









Stages of construction


1. Siting and digging the double pit


The size of the pit is related to the size of the slab, so it is best to decide on the slab size first. In this manual a slab size of 1m long and 0.9m wide has been chosen. In this method two pits are used. They should be sited in a convenient place in the homestead on level ground and dug about 30cm apart. Thus each hole is dug slightly longer than 1m from back to front and slightly longer than 0.9m from side to side. The two pits are dug down to 20cm depth initially.




Marking the two pits with string. Each pit is dug just over 0.9m (90cm) across and just

over 1m (100cm) long. The pits are dug 0.3m (30cm) apart.





Each pit is dug down to a depth of about 20cm at first. Thus two courses of bricks can be

bonded with cement mortar up to the level of the ground. A third course raises the pit lining

above ground - this upper course acts as the “ring beam” on which the slab rests.  



2. Lining the pits and making “ring beam”


Where a pit will be reused several times, it is wise to make a pit lining and “ring beam” which is a cemented “ring” (round or square) of bricks mounted around the rim of the pit. This is in fact an extension of the pit lining. The brick lining and “ring beam” will stabilise the head of the pit. It is best to dig down each pit about 20cm into the soil and then lay two courses of bricks within the excavation first, bringing the lining up to ground level. A third course of bricks is then laid which stands above ground level (this stops flood water entered the pit). Once the mortar has been given a day to start curing each pit can be dug down further inside the bricks to the required depth  which will be between 0.5m and 1m.





Ensure that the bricks courses are level.





The two finished brick lined pits or vaults which at this stage are only 20cm deep.

They will be deepened to between 0.5m and 1m deep later



3.  Make the pedestal and concrete slab.


It is possible to make the latrine slab so that it is a squat plate or to adapt it for use with a pedestal. The construction of a low cost (non urine separating) pedestal is described below.



3a The low cost pedestal seat


Many types of pedestal can be used. This manual describes the construction of a low cost toilet seat made from concrete fitted to a 20 litre bucket which is surrounded by a chicken wire reinforced concrete sheath. The picture below shows a toilet seat mould made out of a very strong mix of concrete into which a standard toilet seat has been pushed and held in place. Once set, the toilet seat is removed and the inner surface of the mould smoothed down with fine sand paper. It is cured for at least a week in water.  The toilet seat itself is made by covering the mould surface with a layer of grease and adding some very thin plastic sheets to cover the mould area. The mix for the seat itself is made with 2 parts sharp river sand and 1 part cement. This is added to the mould and reinforced with a ring of wire. Also a series of about 8 L shaped wires are added to the concrete seat for later attachment to the “shute” of the pedestal which is made from a 20 litre bucket with the base removed. After a day or two the seat can be carefully removed from the mould. Keep the seat wet for at least one week.



The toilet seat mould has been greased prior to the application of very thin plastic sheets which fit closely to the mould. A very strong mix of river sand and cement is added to the mould to make the seat.



The completed concrete toilet seat is mounted on the 20 litre bucket. In an upturned position the seat is attached to the bucket  with strong cement mortar reinforced with chicken wire and the 8 L shaped wires

which protrude from the lower side of the seat and help to bond seat to bucket.



3b Making the concrete slab and fitting the pedestal


This should be made light and strong and be reinforced with 3mm steel rods or 12mm chicken wire. It should also be fitted with stout steel handles - one at each corner. The mixture should be 1 part fresh cement to 2 parts sharp river sand. In this case the concrete slab measures 0.9m wide and 1.0m long with a depth of about 35mm. Where the slab is designed for squatting, a hole measuring about 30cm X 15cm is made in the slab in the centre line but nearer to the back than to the front. If the slab/pedestal combination is made – the lower part of the 20 litre bucket which will be attached to the seat is inserted into the mould – in the centre line and about 22cm from the rear of the slab (the hole is 27cm across). The mix of sand and cement is made in two lots. The first layer uses 2 X 5 litre buckets of cement mixed with 4 X 5 litre buckets of sharp river sand and water and laid around the bucket within the brick mould. It is best to make the slab over a sheet of plastic. A pre-cut piece of chicken wire is then laid over the concrete and held in place with a few stones for 30 minutes. The second mix of the same cement (2:1) is then made and added on top of the wire and first mix and smoothed down. Steel handles are added. Once the concrete has set, the bucket is removed. It is at this stage that the bucket can be bonded to the seat with strong cement mortar.


The next day a thin sheet of plastic is added within and around the round hole in the slab and the bucket (with seat attached) is added to the hole and placed in the correct position. Strong cement mortar is now built up around the bucket so that it meets up with the mortar used to connect the bucket with the seat. The mortar is broadened out at the base on the plastic sheet so that it does not bond with the slab. This mortar is reinforced with chicken wire and strands of binding wire. This mortar is then left to cure overnight. Once cured, the pedestal unit can be removed from the slab. It is best that both are watered down and left to cure properly for several days. In this way both the 0.9m X 1m slab and the pedestal can each be lifted by one person.







Picture showing completed pedestal with concrete seat and concrete sheath surrounding

a 20 litre plastic bucket insert (which makes cleaning easy) and the 0.9m X 1m slab

with hole for insertion of pedestal and steel carrying handles.





Concrete slab fitted over one of the twin pit/vaults used in the fossa alterna system. The cement

 pedestal is also shown - not yet fitted on the slab.







This vault has been dug down to about 0.5m exposing more soil on the side walls as well as the bottom. The soil is an important medium in the breakdown of human faeces - it contains many organisms that assist in the production of compost and humus. Soil has also been built up around the ring beam.



4. The superstructure


There are many ways to make the structure - in hessian and cement on a steel rod frame, in poles and grass or reeds etc. In this case the superstructure is made in wood and has a door. The slab and superstructure are matched so that the wooden base of the structure sits on top of the slab.





The structure should be portable. One medium sized person can move this one

on a set of plough wheels!






The wooden structure mounted over the slab which is fitted over the used vault.

The second vault has yet to be covered with a lid.



Use of Vent pipe: A screened vent pipe can be fitted to the Fossa alterna and this will assist in the control of flies and odours. In this case holes are made in the slab and roof as in the case of the “skyloo” - see manual. A seal must be made between slab and ring beam - best made rainproof of clay or low strength cement mortar.







Some painting



The concrete seat should be smoothed down and if necessary a filler added and painted with

at least two coats of enamel paint. Blue looks good. The external sides can be painted with

black PVA. The wooden structure must also be protected with a wood preservative

such as carbolineum to protect against termites etc.




The Pedestal is fitted and a cover mounted over the pit/vault which is not in use.

This is made of treated timber. A thin concrete cover made like the main slab

can also be made. A cover is useful for the safety of children.



Using and managing the fossa alterna latrine.


The latrine is used like a normal pit latrine but extra ingredients must be added regularly. Soil and wood ash should be added after every deposit of faeces. Leaves, cut grass, animal manure, kitchen scraps, and other vegetable matter which will rot down should be added quite often but not on a daily basis. Thus the pile that accumulates within the pit/vault is a mixture of ingredients which are suitable for the composting process. Water can also be added to wash down the shute of the pedestal - and this tends to distribute the contents of the pit. Composting cannot take place if the pit contents are very dry.


What should be added:


Soil: This adds a medium which helps to break down the faeces. It is filled with micro-organisms. The soil which covers fresh faeces reduces odours and fly breeding.

Wood ash: This helps to reduce odours and fly breeding and adds potash to the mix.

Leaves and grass: These add vegetation to the pile which is very important to the composting  process. Do not add too much at one time as the layers may take time to rot down.

Kitchen scraps: These add a variety of vegetable and other matter to the contents of the vault and also moisture.

Other vegetable matter: Chopped up old fruit and plants can be added.

Animal manure: Manure from goats, cattle, dogs etc can also be added to the pile.

What else: Earthworms perform miracles in such a mix. They like living in such organic conditions and thrive. They mix up the contents and produce a valuable by-product themselves - worm droppings. Add a small tin full of earthworms if you can find them!




The ingredients that can be added to a  fossa alterna latrine. Sample buckets of:

Wood ash, kitchen scraps, grass, soil, leaves, human manure, urine, animal manure, water.



Time to change sides


When the latrine pit/vault is nearly full, the time has come to move the latrine slab and superstructure on to the second pit/vault. This should be performed after about 6 months of use, but of course the period will depend on the number of users and what extra ingredients have been added to the vault. The filling time may be more than 6 months. In warm conditions the ingredients may well be broken down after 3 months. If a 6 month period is chosen the change should ideally take place in about May and November during the dry season. The depth of the pit can also vary depending on the number of users. A pit of between 0.5 - 0.75m depth may be suitable for small to medium size families and a 1m deep pit for larger families. A pit of 1m depth using the dimensions given here will offer about 0.5 cu.m. of vault space when full. A family of 10 may almost fill a 1m deep pit/vault in about 6 months including the extra additions. However the only test is by experience.



What to do?


Once the time has come to change sides, the latrine superstructure should be moved to one side and the pedestal removed and also put on one side. The slab can then be transferred on to the empty second pit/vault - then the pedestal and superstructure are refitted. The contents of the first pit/vault should be levelled off with a shovel and fertile soil added to top up the vault. If little soil/ash/leaves/grass/vegetable matter etc has been added during the period of use, this should be added now and mixed into the pile. After the pile has been mixed and topped up with soil, it should be given a good watering and the cover added. The heap can be raised slightly above ground as the contents will rot down and reduce in volume as a result of the composting process. The pile should be watered from time to time to keep the contents moist.



When to remove the compost?


Normally this will be dug out at the next change over of pit vaults. After 6 months, the composted contents of the vault should be quite unlike the faeces and unlike the original soil that was dug out of the ground. It should be loose and friable and have a dark humus-like texture. It should smell good. Before the next move, the contents of the composting vault are removed with a shovel and either stored in bags or dug into trenches or placed into pits prepared for tree planting. It can also be added to the garden compost heap. Once the original pit has been emptied of the new compost/humus material, the slab and superstructure can be moved back onto the original pit/vault. This process is repeated every 6 or 12 months.