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The use of the

Fossa alterna

in a

peri-urban settlement



Peter Morgan



The Fossa alterna is a type of pit latrine where the concrete cover slab and the superstructure are moved between two permanently-sited shallow pits. Normally the slab and superstructure move once a year, from one pit to the other and then back again the following year. An alternative method is to build an enlarged single superstructure around both pits. The overall aim of the Fossa alterna is to make humus from the human excreta (faeces and urine) which enters the pit. The change from excreta to humus in the pit is encouraged by the regular addition of soil, wood ash, leaves and other compostable materials as well as excreta into the shallow pit. This humus can later be used on the garden to enrich the soil for growing vegetables, flowers and trees. Since the two pits occupy a relatively small area and are used alternately from year to year, the method is ideally suited to peri-urban settlements where space may be limited.


When human excreta falls into a deep pit (as with the Blair VIP Latrine) the contents of the pit may take several years to form compost. In the Fossa alterna, however, the latrine is managed in a different way to encourage the formation of humus in a relatively short space of time. Humus is the material formed in nature following the decay of both plants and animals and their products in combination with the soil. In this case it is the material formed from the natural decay of human excreta mixed with soil, wood ash and some vegetable matter. The main difference between the use of the normal pit latrine like the Blair Latrine and the Fossa alterna, is that in the Fossa alterna other materials are also added to the pit regularly, in about equal proportions. These materials include soil (which should be topsoil), wood ash, leaves, kitchen scraps, weeds etc. These materials help the composting process. Materials which should not be added to the pit are plastic papers, rags, bottles and other things which will not change into humus. Also it is best that the Fossa alterna is not used as a bathroom, since the conversion of human excreta into humus will not take place if the pit is flooded or very wet. Very often a pedestal for sitting is fitted to a Fossa alterna, which makes the latrine more comfortable to use but also prevents bathing water going down into the pit, which may flood it.


Also it is essential that the two shallow pits are built right from the start with the Fossa alterna. These are normally placed quite close to one another, about 0.5m to 1.0m apart. The pits can be protected with a ring beam made of concrete or bricks in firm soil, or partly or fully lined with bricks in less firm or sandy soil. About 500 bricks will be used for lining two shallow pits at a depth of about 1.2m and a bag of cement is used for making the mortar for brickwork. If only one pit is dug, there will be a problem later on when the used pit become full and the owner may not be able to find the materials or have the will to build another pit. It is best therefore, that the whole job is done at one time right from the start. This means digging and lining two pits at the same time - from the beginning. Then the concept of the Fossa alterna can work properly.


The superstructure and concrete base slab are both portable so they can be moved from one pit to the other at yearly intervals. It is best to make the concrete slab with handles fitted for ease of movement. The superstructure is best made so it can easily be transferred from one pit to the other. It can be made of poles, bamboo, or a wooden  or steel frame with grass cover. Several types of superstructure are suitable. The move from one pit to the other is best undertaken during the dry months - sometime between September and November is a good time to move in Zimbabwe.


Setting up the Fossa alterna


Once the two pits have been dug and lined, with the brickwork being built up a course or two above ground level, the top of the brickwork should be plastered with a strong cement mortar to make the bricks very secure. Also all the cement mortar which has fallen on the floor of the pit during bricking should be cleaned out so that liquids like water and urine can drain away through the base of the pit.


The conversion of excreta into humus within the yearly recycling time for the Fossa alterna depends on the regular addition of fertile soil & leaves or compost to the pit as it is being used.  The more the better. Thus it helps greatly to add a sack full of fertile soil, compost or leaf litter to the base of the pit before the slab is placed over the pit. This fertile soil will contain many micro-organisms like bacteria and fungi which help to start the conversion of excreta into humus. The slab is then fitted over the pit and the superstructure mounted on this. The pedestal and a screened vent pipe are now fitted. The vent pipe draws out odour from the pit and traps flies if it is fitted with a fly screen (stainless steel or aluminium screens are best), leaving the latrine almost odourless and fly free like the Blair Latrine. It is essential to form a good seal between pit lining and slab and some weak cement mortar (20:1 sand/cement) should be applied around the joint between the slab and the pit as this seals the pit and helps the pipe to work properly. Flies can enter, breed and leave a pit through any space left between the slab and pit lining. The Fossa alterna can now be put to use.



































The pictures in this series were taken in the Epworth Peri-urban settlement near to Harare. Here both pits have been dug 1.2m deep and lined with fired bricks mortared with cement mortar. 13 courses of bricks were used with 16 bricks being used per course. One course of bricks was laid above ground level. The outer measurement of the pit lining in the case was 1.2m X 0.9m the same as the concrete slab that was made for the job. A layer of strong mortar is being laid above the uppermost layer of bricks. Note in this case the soil was very sandy and liable to collapse - so a full brick lining was used. In firmer soils a partial lining may be adequate. 





After the pit has been fully lined with bricks, it is very important to scrape the pit floor and remove all wasted mortar used for brickwork. In order for the excreta to turn into compost, the pile must be moist but not wet. If the base of the pit does not allow for adequate drainage, it is possible that the contents may become partially flooded. It helps therefore to clean up the base of the pit of mortar which may restrict the drainage of fluids from the pit.


The higher the proportion of soil/ash and vegetable matter (eg kitchen scraps/leaves) added to the excreta, the more efficient the conversion of excreta into humus will be. In this case a bagful of fertile humus-like soil taken from a fertile part of the garden has been introduced into the base of the pit. This soil contains micro-organisms which will assist in the conversion of excreta to humus.








Before the concrete slab is added to the pit, a very weak mix of cement and pit sand is made up and spread on top of the brickwork (about 20:1).  The main effect will be to form a good airtight seal between the pit and the slab. This is important if the full effects of the vent pipe in controlling odours and fly breeding are to be realised.




The concrete slab has been made beforehand. In this case the vent pipe is being placed within the structure and the slab size measures 0.9m X 1.2m (slab shown at rear). The slab is about 40mm thick and is made with a mix of 5 parts sharp river sand and 1 part cement. 3mm wire is used for reinforcing. A bowl has been used to make the hole for pedestal which will be added later. A hole has also been made for the vent pipe. The slab has steel carrying handles cast in it which help in moving. The slab is allowed to cure for at least one week under wet conditions.







Moving the slab over the pit and laying in the weak cement mortar.


Adding the superstructure





Several types of superstructure can be used with the Fossa alterna.

In this case a steel frame has been used on to which grass has been attached. The roof is covered with a layer of chicken wire and plastic sheet which is protected from the sun with more grass. The steel door frame is attached to the main frame with hinges cut from a car tyre. These are very durable. The door frame has been covered with a sheet of hessian.






Once the superstructure has been fitted in the right place, the base of the slab and upper

brickwork are covered with weak cement mortar for neatness and protection. 


The ventilation pipe


Odours and flies are controlled with a vent pipe fitted with a corrosion resistant screen like aluminium. In this case the pipe is “home made.” Annie Kanyemba shows how the pipe is made over a PVC pipe acting as a mould, round which hessian soaked with a slurry of pit sand and cement (1:1) is added. Two layers are applied to the pipe mould which is covered with plastic. Once covered the pipe is covered with plastic sheet and allowed to cure for at least one week and kept wet at all times.



The cured pipe has been fitted with an aluminium flyscreen and is fitted within the structure through a hole made in the roof. The base of the pipe is fitted over the  hole made in the base slab and  secured with cement mortar.




Preparing the latrine for introduction of home made pedestal.




The final interior of the Fossa alterna latrine.

Home made pedestal (see other manual), home made vent pipe (see other manual)

and 5 litre container of soil and woodash (4:1) for adding to the pit after every visit.

A mug is used as a soil/ash dispenser.

Management of the Fossa alterna


It is important that fertile soil is added regularly to the pit and it is even better if this is mixed with wood ash. This means that after every visit made to the toilet some soil (with ash added) should be added to the pit. The mix of soil and wood ash should ideally be prepared first (about 4 parts soil to one of ash). This can be stored in a sack and added to a smaller container for use inside the latrine. A mug can be used to add the soil/ash to the pit. In addition Leaves can also be added and also organic kitchen scraps like vegetable peelings, fruit skins like bananas etc should be added to the pit through the pedestal periodically. From time to time, additional soil should be added, say every month.  If all else fails, the soil which was dug out of the pits can be stored in bags and reintroduced back into the pit together with the excreta. What is important is that there is a mix of ingredients in the pit, not just excreta. The final quality of the humus will depend on what ingredients have been added.


Some water should also be added from time to time to keep the contents of the pit moist. This water can also be used to wash down and clean the pedestal. In fact some moisture is essential to the process of composting. However, there must be some control of the amount of water which goes down the pit. Pit flooding must be avoided – humus cannot form in flooded or very wet conditions in the pit. This will vary from one place to another because some soils have better drainage properties than others.


When this combination of ingredients is added to the pit, the pile in the pit may start to build up directly beneath the pedestal. If this becomes pronounced it is advisable to take a strong stick or pole and pass this down the pedestal - from time to time - and spread out the pit contents. This is best done after some fertile soil has been added. This “rodding” will help to mix the pit ingredients and level them off a bit.


For an average family the pit filling process will take place over the course of a year. After about one year there should be some space left in the pit, but it is best to change sides at a certain time of the year, which can become well known in the family calendar. The hot dry months of September, October and November are best in Zimbabwe.


Management of the second pit (first time around).


Whilst the first pit is filling, it is best to add materials to the second pit which will form compost (soil, leaves, grass, garden compost, etc). The compost will be useful for the garden and can also be used to add to the used pit as it is filling. This compost can also be used to cover the used pit once it is nearly full.


Look around the garden and take leaves, weeds and other old vegetable matter and throw it in the pit. Cover with a thin layer of soil and water if possible. Then add more leaves. Continue this process regularly so that the second pit slowly fills with layers of leaves and soil which will form compost. The composting process takes place best if the materials are moist. So add water from time to time. It is also best to cover the second pit with a cover, which can be made of wood, for the safety of children.





Sources of fertile soil and compost in the garden


Sources of fertile soil can be found in the garden to add to the used pit and also to add on top of the leaves which are added to the second pit. A good place to look for fertile soil is under trees where leaves may have fallen and begun to make “leaf litter” - that is partly decomposed leaves. Look around the garden for places where compost may have been made before and vegetables grown. Often the soil is barren in dry areas. Therefore the search for fertile soil will be more difficult. It is always a good idea to start making a compost heap and start to enrich the garden soil for planting vegetables. Sometimes it may be necessary to import some compost or fertile soil (on a Scotch cart for instance) from some other place where the soil is more fertile. Once the Fossa alterna is working properly, a yearly supply of humus for the garden will be available from within the composted pits within the homestead. It is worth making the effort in the early years.







Preparing the soil wood ash mix

Here a source of wood ash has been found and is collected in a bucket




The wood ash is mixed with soil at the ratio of about 4:1. This mix is stored in a bag with

some being transferred to a smaller container within the latrine. 




Finding leaves in the garden

The second pit can be left empty and covered but it is useful to fill it with a mix of leaves

interspersed with thin layers of fertile soil to make leaf mould which can later be used on the

garden and also added to the pit which is in use. Leaves accumulate under trees.



Adding the leaves to the second pit. A layer about 30cm deep is first added and levelled off. This is followed by a layer of soil  which  is only a few cm deep. Layers of leaves are then added again. Water must be added to make the mixture moist, otherwise the formation of leaf mould will not take place. Without moisture, leaves remain almost unchanged in the dry state for years.




What to do at changeover time


After the Fossa alterna has been used for a year it is time to change sides. If the second pit has been filled with leaves and soil and other compost materials, these must be removed and put to one side first. This leaves the second pit empty.


The pedestal and superstructure are then removed from the first pit and put to one side. If the superstructure is fitted with a vent pipe, this must be moved with the superstructure in the most convenient way. Normally this will be mean pulling out the pipe from the slab and removing with or without the superstructure attached. This will depend on whether the pipe is outside the structure (with larger slab) or inside the structure (with smaller slab). Once the pedestal and superstructure are removed, the slab is then moved and placed over the second pit which is now empty. Once again it is best to introduce leaves or loose soil at the base of the pit which is just about to be put to use.


Once the latrine slab and superstructure have been removed, the contents of the first pit are now exposed. They should be levelled off with a shovel and covered with fertile soil or some of the compost taken out of the second pit. To increase the rate of conversion of the materials, it is a good idea to take a pole and ram soil into the contents of the pit. The more soil can be rammed in the better as this drives more soil into the mixture. Once rammed in, more fertile soil can be added on top of the contents of this pit - at least 150mm deep.


This pit filled with excreta and soil etc can now be left to decompose into humus.  The pit is just left to form humus over the months that follow. In fact the conversion will normally take place in less than a year depending on temperature, pit drainage, the mix of ingredients and other conditions of the pit.  Pits which are unlined or partly lined tend to offer better conditions for conversion than fully lined pits, possibly because more soil is in contact with the converting materials. It is best to add some water from time to time to keep the contents a little moist. To test for the extent of the conversion into humus, take a steel rod and drive it into the mass of the pit. At first the material attached to the extracted rod will smell very bad, but as the conversion into humus takes place over the months, the smell on the “dip stick” will change. Eventually it will all smell good. Also the colour of the mix changes into a much darker material over time. Eventually it will look, feel and smell just like soil.


The second move.


After another year the second pit will have filled up with the mix of ingredients (faeces, urine, soil, ash, kitchen scraps, leaves etc. The time has come to excavate the converted contents of the first pit. These are dug out with a pick and shovel. It should be quite easy. When fully converted into humus, the material smells very good and is quite crumbly. Soil tests have shown that it is also very fertile. It can be placed around the garden, being dug into vegetable or flower beds. It can also be stored in bags for future use on the vegetable garden or as the soil which will go down the pit in use. 


It is best to remove all the contents of the pit full of humus. This humus can be turned with a shovel to break it up. Some of the loosened humus can then be placed back down the pit which has just been emptied.


The superstructure and slab are now removed from the second pit and placed over the first pit and the latrine put back into use. The cycle is then repeated.


Fertile soil, or some of the humus from the first pit is now added to the second pit which is now full. Some of this soil is rammed into the mixture, like the first time, to add more soil and humus into the pit contents. Like the first time, a good layer of humus or fertile soil is added on top of the pit contents.


This same procedure is followed every year. And every year a pit full of good humus should become available. It is best if rags, plastic papers or bottles and other things which do not decompose well (condoms, sanitary towels etc) are not put into the pit. Normally pit latrines are used as a dumping ground for all sorts of refuse. This must be avoided in the Fossa alterna. Pit excavations are much easier and more acceptable if it is just humus which is being removed.


The use of models


Models can also be used to demonstrate the way in which the Fossa alterna works and is used. It is often difficult for people to visualise what goes on before they see it with their own eyes. This can be done “ahead of time” by doing a demonstration with small models which are small replicas of the full size Fossa alterna and its double pit. Different coloured soils and sands can be used to depict the different ingredients added to the pit. People can start to understand if they see the process with their own eyes. Seeing is believing.



Use of models



Annie Kanyema describes how the Fossa alterna works using a model. It is sometimes difficult for a family to visualise how the system works. By using soils of different colours, which represent excreta, soil, leaves etc the method of management can be carried out in miniature “ahead of time.” This method helps greatly in the understanding of how the Fossa alterna works. The same applies to the use and

management of the Arborloo and urine diverting systems. 



Another model which is also a Fossa alterna, but one made of bricks. Here the cycle of change is longer and the pits therefore deeper. However it is possible using special techniques to take down a brick structure, remove the concrete slab and transfer to the second pit and then  rebuild the structure within a single day. The cycle may be as long as 3 or 5 years or even up to 10 years. This method is described in another manual.



Demonstration of conversion of excreta into humus in split cement jars


Very few people are convinced that human faeces can turn into humus or something which smells good and is useful. Sometimes it is good to do a demonstration on the site of a new programme to show people what can happen in the pit if the right ingredients are added. This demonstration can be undertaken on a small scale in split cement jars, although many other containers can be used. Details of how to make split cement jars are given in another manual.


The aim is to add some raw excreta into the jar together with some fertile soil, ash, kitchen scraps and leaves. Once the small jar is placed somewhere near the toilet the various ingredients can be located. Finding the faecal matter may be unpleasant. A badza may be dipped into a nearly full pit latrine to withdraw some of the excreta. It is accepted that this will be a most unpleasant and unacceptable task to most, if not all people, but the fact that it is so unacceptable and unpleasant makes the later removal of humus from the jar even more remarkable to the observers. The raw excreta should be carried in a bucket to the site of the jar. Some fertile soil should be added first to the base of the jar, then some excreta added on top. Add a few scraps from the kitchen, then add soil and some wood ash on top. Add some more excreta and some leaves and more soil. Thus the jar contains a mix of ingredients. Finally, cover the contents of the jar with a final layer of soil about 3 inches (75mm) thick. This can serve as a demonstration, on a small scale, of what goes on in the larger pit. A flower or even a vegetable like a young tomato plant or spinach can be added into the topsoil in the jar. This can be watered from time to time, or the jar can just be left. It is good to water from time to time, to keep the contents slightly moist.


The entire process of filling the jar, offensive as it may be at the time, should be undertaken in front of an audience - even if they do stand back at a distance. They will serve as “observers” who can testify that raw excreta did enter the jar on a certain date.


After about 4 – 6 months the audience should be gathered back and the jar opened up in their presence. The humus in the jar should be excavated and removed. What they will all witness is a most pleasant humus-like material which has been formed in the jar. It is crumbly, smells good and one by one, each observer should be happy to smell the soil and pick it up and feel it in his hands. It is an experience he or she will not forget. The same process takes place in the pit but will take longer because the ingredients will not be as well mixed as in the jar and the volume of the ingredients is considerably larger. Thus the conditions for conversion in the pit will not be as ideal as those in the jar, where the ratio of soil to excreta is high (ie high soil content), the jar is well aerated and well drained and has an ideal mix of ingredients. The presence of a plant will also help as the roots will help to break up the soil/excreta and also take oxygen into the contents of the jar.






Demonstration of the conversion of excreta into humus in split cement jars.



People cannot easily believe that excreta can turn into a pleasant smelling humus. To be convincing it must be seen. In this case a demonstration has been set up at the house where the Fossa alterna has been installed. The various ingredients are added to a 30 litre split cement jar (building method in another manual).  These include excreta (taken from a pit latrine or eco-latrine), fertile soil, leaves, organic kitchen wastes etc. A layer of topsoil is added above all the ingredients and a flower or vegetable is planted. This is watered regularly. After 4 - 6 months the jar is opened up to reveal a pleasant humus-like material  which can be used on the garden.






Annie Kanyemba empties half bucket of human excreta (with toilet paper) into a jar to which some fertile soil has already been added.  Kitchen scraps and more soil are also added.







The jar is topped up with  fertile soil and planted in this case with a young tomato plant.
This is watered. The jar is re-inspected in 4 - 6 months, revealing humus








If the Fossa alterna is managed properly, it should supply good humus for use in the garden every year.  This can be mixed with existing soils to improve the soil fertility. The Fossa alterna works best where the pit is well drained. Decomposition does not take place well in pits which are constantly flooded. However, in this country very few places are flooded at all during the rains and when flooding does occurs in some places,  it may be for a short period only.  The Fossa alterna does not take much space in the homestead and is therefore useful in peri-urban areas, where space may be limited. It can be seen as a potential solution both to the disposal of human excreta and the production of useful humus for use in the garden. As a latrine system, it can make a strong link to agriculture and helps to increase the users’ knowledge of the usefulness of humus in organic farming. Whilst the Fossa alterna pits fill up more quickly that the normal pit latrine, the system has been designed so the pits can be emptied easily. This overcomes the problem faced in normal deep pit latrines, like the Blair Latrine, where the deep pit fills up – is very difficult to empty and where the latrine is normally abandoned. However, there are ways of recycling even the Blair Latrine. These are described in other manuals.


I thank Annie Kanyemba who constructed most of the components of the Epworth demonstration and performed the educational component which so convinced the user family. I also thank Gift Chasasa for making the steel frame superstructure used in this unit.


Peter Morgan


September 2001

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