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fossa alterna


“alternating pit”



A method of recycling human waste


Volume II











Peter Morgan





The first manual describing the Fossa alterna was written in June 1999 based on the experience of a single prototype built at Woodhall Road in Harare. Since that time more experience has been gained both from the Woodhall unit and also many other units placed at Hatcliffe near to Harare by the Mvuramanzi Trust. In addition further units have been built at the Friend Foundation near to Harare and these are being closely monitored.


The Fossa alterna, or alternating pit latrine, uses a single latrine slab and superstructure which are both designed to be portable and are alternately placed on two permanently sited shallow pits constructed close to each other. 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, as our knowledge increases it is becoming apparent that the Fossa alterna has potential also for use in the rural areas. Also when  Arborloos are planted with trees like paw paw, they have a potential for use in peri -urban areas, since the trees do not occupy much lateral space.


During construction of the Fossa alterna, the single slab and superstructure  are mounted over one of the two pits which may be either partly lined with bricks to form a “ring beam” of varying depth or fully lined with bricks. The second brick lined pit can be left empty and covered with a wooden lid or it can be slowly filled up with compostable materials like kitchen scraps and fertile soil and topped up with good topsoil, watered and left. Alternatively it can be filled up with good topsoil and compost and planted with vegetables like tomatoes or pepper or flowers. The pit under the structure is put to use as a pit latrine. However there is a major difference in the way in which the Fossa alterna is used as compared to the standard pit latrine.  This is the frequent addition of a soil/wood ash mix to the pit - ideally added after every visit made. In addition other ingredients like kitchen scraps, leaves and other compostable materials should be added quite regularly. It is these additions which affect the fate of the excreta in the pit.


In standard pit latrines the excreta may form a solid, badly drained mass of excreta into which air cannot pass. Under these conditions the breakdown of excreta is very slow. However, if the excreta contained in the pit is mixed with other ingredients such as soil, ash, leaves and other compostable materials, the excreta is able to break down much more readily. In fact the presence of these “additions” help to promote the conversion of excreta in to a friable soil-like humus, which, after a period of months, becomes safe, easily excavated and suitable for use on the garden.


When the used pit is almost full, the contents of the second  pit (if filled on the first cycle with soil and compost) are emptied out and used on the garden. The slab and structure are then moved over on to the second pit which is put into use. The contents of the almost full pit are topped up and levelled off with a good layer of fertile soil, leaf litter and/or compost. This can be taken from the recently excavated pit or from elsewhere in the garden. It also helps to ram some of the soil into the pit contents with a pole to raise the soil content of the mix. The topped up pit with at least 150mm of topsoil covering the excreta layer can either be left and covered (with a wooden lid) or can be planted with vegetables or flowers and watered. In the full pit, the excreta changes its form into a soil-like humus over a period of a few months. The time will vary but may be as little of 3.5 months, when the conditions are right. These conditions include a pit which is well drained (ie not waterlogged), and also aerated (ie not filled with a mass of solid excreta). The time of conversion will also depend on the ratio of soil to excreta, the type of soil added and presence of other compostable materials. Also the type of wall lining will influence the efficiency of conversion. Drainage will be faster in a pit which is partly lined with bricks compared to a pit which is fully lined. Since the conversion of excreta into humus is partly dependent on well drained conditions, this may effect the rate of conversion of the excreta. Also the excreta/soil mix contained in a pit where the side walls are fully brick lined may take longer to convert into humus compared to the same ingredients contained in a pit which has a partial brick lining because there is a greater area of soil exposed to the excreta. This will not only improve drainage but also substantially increase the area of the active interface between the soil and the excreta. Microbes in the soil break down excreta. Thus excreta held against the ground soil which lines a shallow pit will start converting before the excreta further away from the pit wall. This conversion can at first be seen as a darkening of colour of the excreta next the pit wall. Also plant root invasions take place in partly lined shallow pits and this may also assist in the conversion. Plant roots bring oxygen into the mass and also help to break up the excreta, although they may also disturb the pit lining.


When the various ingredients of the converting pit contents have changed into humus, they can be excavated, thus leaving the pit empty so that the slab and structure can be moved back from the second pit to the original pit.  The second pit is then topped up with good soil, some soil rammed into the body of the mix and then topped up again  and levelled off. The cycle is repeated time and time again. The inputs are human excreta and soil/ash/vegetable matter and the outputs are a soil-like humus (and fresh vegetables or flowers if planted).


Several experiments have now confirmed that if fertile soil and ash etc are regularly added in sufficient volume to a well drained shallow pit in combination with excreta (faeces and urine), and anal cleansing material like paper is used, the contents of the pit change over a period of about 4 - 6 months into an inoffensive humus-like material which can easily be dug out. Obviously non biodegradable articles like plastic, bottles, rags etc must not be added, as these interfere with the excavation process. The presence of a good soil, interspersed within the excreta and also placed on top of the excreta assists considerably in the conversion of excreta into humus. Fertile soil contains many beneficial bacteria, fungi and other animalcules which have a significant effect on changing the excreta. Beneficial bacteria are a potent factor in the formation of humus. These bacteria give the soil the ability to quickly digest organic matter including faeces. Wood ash adds potash and provides a slight alkalinity to the reaction which can help. Ash also helps to reduce odour. The resulting humus is easily dug out and is not offensive in any way. This humus can be mixed in with garden topsoil to grow various types of vegetable and also flowers. In all cases the plants grow well on the material extracted from the Fossa alterna pit. This is a very positive sign of the usefulness of the converted pit contents. Where flowers or vegetables are planted in the layer of topsoil above the excreta, the plants actually grow in the topsoil layer only (which should be between 150mm and 300mm deep). Whilst these plants are growing, the conversion of excreta to humus is taking place beneath them. Because of the manual levelling off of the waste products which is undertaken once the slab is removed (the initial deposits of waste material mixed with soil/ash etc may take on a conical shape), a layer of soil up to 30cm deep must be added to the pit to fill it up before planting takes place. Thus the roots of plants rarely enter the deeper organic material in the pit. Thus healthy plant growth depends on the quality of the soil used to cover the excreta which should be rich and loam-like. The constant addition of small quantities of water to the plants also helps to keep the lower pit contents moist.


However, the pit contents must never become waterlogged as this inhibits the aerobic process and may drastically slow the process down. The composting materials must be moist, but not wet. In pits which are filled with excreta only, an inefficient anaerobic process takes place which may take years to convert the excreta. This process takes place in the standard deep pit latrine. It may also take place in shallow pits if the soil beneath the pit does not allow for much seepage, and the soil added on top does not allow air to pass through it, as might be the case if clay-like soils are chosen. Such a process is undesirable in ecological sanitation.


Where dry soil and ash alone are added to the excreta (faeces and urine) and the pit is well drained, the process may be one of partial dehydration, particularly if no extra water is added to the pit contents after the topsoil is added and also where the topped up pit is exposed directly to the hot sun during the dry season. In this case the liquid fraction of the excreta (which may be up to 70%) is absorbed into the soil layers which are interspersed within the excreta if the volume of soil/ash etc is sufficient enough. There will also be some drainage from the base of the pit. This leads to a significant reduction in the volume of the pit contents. The end result after dehydration is also a friable soil.  In practice this may be the most common method of excreta conversion in the Fossa alterna.


The prototype Fossa alterna pit was excavated in Woodhall Road during the latter part of 1999. In this case, the shallow pit, partly lined with bricks, had been filled with a mixture of imported excreta, soil, ash and leaves. The pit was well drained and even invaded by some roots from surrounding plants through the ground soil. The formation of humus in this case was complete after 3 - 4 months of composting. All the soil was recycled in some way.


The first Fossa alterna used by a family was excavated at Hatcliffe (a project site of Mvuramanzi Trust) on 8th April 2000, 3.5 months after it was finally topped up with soil. The excreta had been completely converted into a soil-like humus which the owner found acceptable for use in subsequent vegetable growing. In the Hatcliffe trial, wooden structures were mounted on concrete slabs supported by ring beams of three bricks depth set into the side of pits (max 1m deep).  Where two pits were dug and lined from the start (which is the recommended method), they were placed generally within a metre of each other as described in the first manual (1999).


During the year 2000, a number of experimental alternative constructional methods for the Fossa alterna were put on trial by the writer. These include a brick double vault substructure for the Fossa alterna with prototypes being built at both the Mvuramanzi Trust and the Friend Foundation. This method has the potential advantage of offering a substructure which is very secure and durable in its permanent location. The two pits are completely adjacent to each other, since the pit is initially dug as one hole and then divided by a 225mm brick dividing wall (the other walls being 112mm thick) - the total unit occupying a area of only 2m X 1.2m, which is ideal for the highly dense settlements for which the unit was designed. To avoid seepage from one vault to the other, the dividing central wall is cement plastered on both sides. This method is described in this manual. 


In twin pits which are fully lined with bricks (excluding the base), seepage of liquids from the vault is slower than for partly lined pits with greater soil area exposed to the excreta. This factor may influence the efficiency of composting and thus the time required for conversion from faeces into humus. Thus a balance must be struck between having a less stable substructure such as a brick ring beam which allows for good pit content drainage, or a more stable substructure in the form of a fully lined pit which drains less well. If the soil is very firm, the ring beam method may be adequate and also cheaper, with the two pits being placed between 1 and 2 metres apart. If the soil is looser or is more likely to collapse then the fully lined pit method is best.


Two further variants of the Fossa alterna were built at the Friend Foundation near to Harare. Both used fully lined twin brick vaults built as a single substructure. The first was dug deeper to a depth of 1.5m so the period of alternating pits would be longer. The superstructure was built with bricks so arranged that they could easily be taken apart and rebuilt. The second unit was dug with most of the substructure above ground level. The latrine slab carrying the superstructure was mounted within a steel frame which could slide up and down a larger steel frame mounted on top of the substructure. This arrangement allows the whole superstructure and slab assembly to be slid from one pit to the other in just a few minutes. In both of these units, the second pit was filled with a mix of soil and compost and initially planted with tomatoes. Individual manuals have been written on the construction and use of both these units. They are under close observation.


Various alternatives to the superstructure are possible, but the wooden unit appears to have great merit as it is adaptable and easily moved. It is normally wired down to the slab lifting handles which hold it firm in a strong wind. However, several other types of superstructure could be used including ferro-cement, plastic, fibreglass, tin, steel frames with various coverings. The primary aim of the superstructure is to provide privacy.


The Fossa alterna concept differs from other latrine models which also use a double pit (such as the double pit VIP) insofar that only one structure and concrete slab are provided (deliberately) and thus only one pit can be used and filled at one time. Where two slabs (or two pit access holes) are provided over two separate pits there is a distinct chance that both may be used simultaneously as has been shown in trials in other parts of Africa. Once the slab and structure are moved on to an empty pit, the used pit is fully exposed and easily accessed for immediate covering with soil and planting and later removal of the composted contents. It effectively becomes a miniature garden or compost pit in its own right. The waste materials deeper in the filled pit are able to decompose easily because they are (or should be) already mixed with soil and ash and vegetable matter and kept moist. If the pit enclosure is not used as a miniature garden, but is left partly exposed to the sun and not watered, the exposed location will help to dry out the pit contents and make subsequent excavation easier.


The Fossa alterna system is also adaptable. There is a link between this unit and the arborloo which is simpler but works on similar principles. All the components (slab, pedestal and superstructure) are portable and can be moved from site to site if necessary whether this is on alternating pits or on a series of pits which will be planted with trees. The same components can also be placed over deep pits where this is appropriate. This would normally be in the communal lands. Thus the same components can be used on a full range of latrine types. The addition of a screened vent pipe helps to control odour and fly breeding in any type of pit latrine. Once a vent is fitted, the latrine, whether it is an Arborloo or Fossa alterna becomes a type of VIP latrine.


What seems important is that soil and wood ash are added to the pit contents regularly and preferably after every visit made. Soil and wood ash should ideally be mixed together in the dry state in a ratio of about 4 soil to 1 ash and stored in bulk in bags in the dry state and then transferred to smaller containers for use within the latrine itself. The exact mixture is not critical but should have far more soil than ash (range 3:1 to 5:1) and is best prepared in the dry season. The volume of soil and ash added should equal at least half the volume of excreta added. Adding a bit of dust from time to time will not do. Also adding other compostable vegetable matters helps. The thing which must be avoided at all costs in the fossa alterna is to have a pit full of compact excreta in combination with little other material. This will simply not convert in time.  Aeration, good drainage, good soil and a good mix of ingredients are essential.


If there is uncertainty of the mix of ingredients within the almost full pit, it may help to ram the topsoil and compost into the pit contents at the time the pit is being topped up with soil. This can be done with the pole. It is wise to get soil and compostable materials distributed within the volume of the pit or vault. It is more logical of course to produce this mix as the pit is filling, but the new method of adding soil and ash and other ingredients day by day may not catch on at first with users who are more familiar with the use of pit latrines, where the only addition made to pit apart from excreta is anal cleansing material.  


The period of changing from one pit to the other will be infinitely variable depending on the size of the pit and the number of users and the volume of additional matter added (soil/ash/paper, leaves, kitchen wastes etc). Normally the pit will be used until it is full and then the decision will be made to move. The period of change should normally lie between 6 months and one year depending on the conditions found in the pit. The longer the period between changes, the safer will be the pit contents. It is theoretically possible to make pits deeper and thus reduce the frequency of changing pits. Where water tables are deeper it may be possible to excavate pits which are 2 - 3 metres deep and follow the same process. The system will not work so well in high water table areas if the water invades the pit, liquifies the excreta and drains away only slowly.


Where soil is regularly added to the excreta in a deeper pit, the potential of the excreta as a polluting agent for ground water may be reduced since pathogenic bacteria may not survive well in the soil-like humus formed, whereas beneficial bacteria thrive in humus. Excreta compacted in deeper pits undergo change by a slow inefficient anaerobic process which may take many years to complete. In excavations carried out in South Africa on a deeper standard pit latrine, excreta remained offensive and unpleasant to excavate even 8+ years after being deposited. Such deposits would have been easy and inoffensive to excavate had quantities of fertile soil been added throughout the life of the pit latrine. In the case cited from South Africa, a sample of the smelly 8 year old excreta taken from the deep pit, was converted within 4 months to a pleasant easily handlable humus after it had been surrounded by a humus like soil and held in a bucket. This method of adding fertile soil as a routine to pit latrines may be one way of making routine excavation easier when the time comes.


Excavating the soil from shallow pits which contain humus converted from human wastes is much easier than excavating the original pit. The soil is relatively loose although deeper down it does become more compacted. A pick or garden fork as well as a spade or shovel, or even a badza can be used to excavate and remove the soil from the pits. The addition of wood ash and soil to freshly deposited excreta does help to reduce both odour and fly nuisance, which will be greatest in the hottest and wettest part of the year which in Zimbabwe is between November and March. The addition of a screened vent pipe will also help a lot to reduce odours and fly breeding.


If the facility is to become permanent as it was designed to be, it is important that the substructure is well built with good bricks and strong cement mortar right from the start. In the brick double vault unit, the 225mm thick dividing wall between the two pits should be cement plastered to reduce any leakage of the used chamber contents into the unused chamber. Such a situation might only arise if the used chamber had too much water added and became waterlogged. This should be avoided. Where the pits are between 0.75 and 1 metre deep, the chances of underground water pollution are reduced compared to much deeper pits. In most situations the soil is a very good filter and pathogenic bacteria do not travel far. However, it must be remembered that the natural breakdown of faeces into humus, although it requires moisture, cannot cope with flooding. If flooding occurs, or the excreta becomes liquified with limited chance of drainage, the natural process of conversion from excreta into humus is much reduced. The process of changing pits and emptying them should ideally coincide with the driest parts of the year so that in the preceding months the contents of the pits have their lowest moisture content. Pits should not be alternated at less than 6 month intervals and preferably at 9 or 12 month intervals.


To avoid pit flooding during the rains, it is essential to raise the ring beam or pit lining at least one brick height above ground level and to build the unit on slightly higher ground. Grass can be planted around the pit head to consolidate soil and improve appearance. The roof is important as it diverts rainwater away from the latrine area. Latrine slabs which are exposed to the atmosphere act as rainwater harvesters and water can find its way into the pit. Pit type latrine systems used in ecological sanitation work at their best with only limited quantities of water being added to the pit. The pit contents should be moist but never wet. Also lime should not be added to the products, at least not in any quantity, as this may destroy the beneficial bacteria present in the soil which helps to change faeces into humus. What is important is that the best conditions are established for converting the waste products into a valuable humus-like material which can be used to enhance agriculture. The process should following the principles found in Nature.


The management of the Fossa alterna is more complex than that of a standard pit latrine. In the pit latrine, excreta is just added until the pit is full. Then a new location is sought, which may be after a period of 10 years. In the Fossa alterna, a stricter system of management is required. There must be a balance between the rate of filling of the pits and the period of changing sides. If the filling rate is too fast, due to overuse etc, the period between changes will be too short. This will mean that the excreta will not be fully converted to humus. Also if the pits become flooded, or are not well drained, the rate of conversion from excreta to humus will be slowed down. Thus humus formation will take longer. Humus formation will also depend on the ratio of excreta to the additional materials added. If there is just a very small amount of soil added and a large amount of excreta, the soil organisms will struggle to cope - and the conversion process will take much longer. And soils also vary a great deal in their properties. Inert soil or sand will not have the same effect as fertile humus-like soil with a resident content of micro-organisms present within it. Also soil which is loose and friable and humus-like will hold more air than solid clay like soil. So there are many variables and these need to be understood.


The success of this system thus partly depends on the users understanding the concepts involved and the users being willing to put the management principles into practice. A good educational programme is therefore essential. Ideally the unit should be used by a family of about 6 - 10 persons. The pit may then take about one year to fill. If the number of family members (and visitors) remain the same, then the second pit should also take about one year to fill, which means that the pit of converting contents will have about one year to convert from excreta into humus, which should be ample time. In order to work well in practice, the system of management must be made as simple as possible.  The aim should be to get a pit which fills up in about a year (or more) - this includes the volume of additional ingredients like soil, ash and vegetable matter. Pit depth can vary a little but should not go much beyond two metres. The area of the pit will depend on the slab size and thus the size of the ring beam. As the pit is filling, it will rise in a cone shape, particularly if soil and wood ash are added regularly, which tends to make the additions less mobile. It is a wise practice for the owner to level off the pit contents from time to time with a stick or pole placed through the squat hole or pedestal. Thus the pit volume will be fully occupied and there will be little wastage of space resulting in longer pit life.


To repeat, the excreta in the pit must be accompanied by meaningful quantities of soil and also wood ash. These items must be added regularly. Additional volumes of fertile soil can be added periodically. Also the addition of other compostable materials helps a great deal. Leaves, compost, kitchen wastes and other materials which can break down to form compost should be added. The pit must be filled with a mix of ingredients, not just excreta. Also the pit should be well drained. The liquid fraction of the pit, if it builds up, must be allowed to seep away. To reduce rainwater entering the pit from above, a roof should be used. To avoid the flooding of the pit during the rains, a raised ring beam should be used to divert flood water away from the pit site. The latrine should not be used as a washroom. It may help to put a pedestal in the latrine rather than a squatting hole.


The Fossa alterna technology is still evolving. Much still needs to be learned about how it can function at its best in a family setting. Certainly it is important that the users fully understand the basic principles involved so they can maintain and manage the unit correctly. If properly managed under the right conditions, the end result should be a never-ending source of humus for the garden. This is something worth aiming for in a world crying out for increased fertility of the soil and where the disposal of excreta is a major problem, particularly in urban and peri-urban areas. 



Peter Morgan

Harare. April. 2000, updated December 2000 and April 2001.

Updated for website August 2001,



Stages in the construction of a fully lined substructure for the Fossa alterna


Volume I of the Manual for the Fossa alterna described a method of lining the upper part of each of the two pits with a ring beam. This is a simple and effective method for stable soils. Ring beam stability can be increased by increasing the wall thickness of the brickwork from 11cm to 24cm. Ideally the two pits should be relatively close together, for convenience, but can be dug in separate locations on the plot if necessary. In the method described below, a single larger pit is dug and this is brick lined from the bottom with a strong dividing wall built to form the two pits. This will be a very stable structure and should last for many years.



Stage 1. Location and digging the pit.


The pit should be sited in a convenient place near the home and on slightly raised ground if possible. The outside dimensions of the brickwork which will line the pit are 1.8m X 1.2m, and thus the pit should be dug slightly larger than this. The final depth of the lined pit should be about one metre, but since 0.1m of this will be formed by the single brick layer above ground level the actual dug pit depth should be around 0.9m.





Dimensions of the pit





Digging the pit for the fully lined fossa alterna substructure at the Friend Foundation


Stage 2. Lining the pit with bricks


The pit should be lined with fired bricks and strong cement mortar (about 5:1 sand and cement). The outer brick lining should be made as a single layer of bricks, with the central dividing wall a double layer as shown in the photo. The external dimensions of the brickwork are 1.2m X 1.8m. The pit depth can vary between 0.75 and 2.0m deep. It is desirable to add a strong mortar to cover the uppermost course of bricks to strengthen this section as the latrine slab will be moved many times on this course. The uppermost course of bricks must lie above ground level to avoid flooding of the pit during the rains. It is also desirable to cement plaster both sides of the dividing wall, to reduce seepage of materials from one vault to the other. The pit base should expose soil to allow for drainage of the liquid fraction of the excreta. Cement falling on the base of the pit should be cleaned out. Where the two pits are located in different places, it is also desirable to brick line to the base unless the soil is very firm and able to cope with repeated emptying and refilling. In this case a sturdy ring beam of mortared bricks may be adequate.   





     The completed brickwork. The next stage is to cement plaster both sides of the dividing wall.






Photo showing the cement plastered dividing wall of the twin vault. This helps to reduce seepage

from one vault to the other in cases where the liquid fraction of the used vault is elevated.

However the dry soil and ash which should be added regularly to the pit filling with excreta

should help to reduce the liquid content of the excreta, reducing its mobility and potential

for seepage both from vault to vault and from vault into the soil beneath. The soil, ash and

other added compostable ingredients also helps to hasten the conversion of excreta into humus.




Stage 3 Making and adding the latrine slab


The slab measures 0.9m X 1.2m X 50mm thick and is made with a 5:1 mix of sharp river sand and cement. It is fitted with four strong steel handles to assist lifting and moving from one pit to the other. Two holes are cast in the slab, one for a vent pipe, the other being the pit access hole. The pit access hole can be shaped and used as a squat hole or shaped and fitted with a non urine-separating pedestal. The slab can be cement mortared to the head of one of the two pits with a weak mortar mix (15:1sand and cement).





          Mounting latrine slab on pit lining



Stage 4. Adding the superstructure


In this case the superstructure is made from wood and is commercially made. It is designed to match the slab. Several types of superstructure can be used, including steel frames lined with low cost material like wood or grass.    




Mounting superstructure on latrine slab at the Friend Foundation


Stage 5 Adding the pedestal


The pedestal and seat shown here are homemade and very satisfactory (see manual on construction method). It is a strong unit made of concrete with smooth wall lining made from a bucket. This pedestal is mounted directly over the hole in the slab. It is also advisable to fit a screened vent pipe to the structure and a hole should be made in the slab for this.  Both PVC and asbestos vents are available - the asbestos unit being more durable. A PVC pipe has been used in this example built at the Friend Foundation.




New pedestal added to the latrine.







Pedestal and pipe in position. Also note bucket of soil ready for adding to the pit after each visit.




Stage 6.  What to do with the second pit - first time around.


There are several ways of dealing with the second empty pit once the latrine has been mounted over the first pit. The empty pit can be covered with a wooden lid and simply left until the first vault is full. The second pit can also be covered with the wooden lid and then slowly filled with scraps from the kitchen, other vegetable matter and loose fertile soil which will turn into compost. The second pit can also be filled with leaves and soil to form a leaf mould. Alternatively the second vault can be filled up with fertile soil and compost and planted with vegetables or flowers.






Adding a wooden cover to the second pit which has not yet been put to use.







Alternative method of adding soil and compost to fill the second pit in preparation for

 planting vegetables or flowers





          Leaves and soil have been added to this second Fossa alterna pit to make leave mould.







Organic kitchen scraps have been added to the second Fossa alterna pit together

with soil to make compost.








Management of the Fossa alterna


The primary aim of the Fossa alterna is to be able to convert human excreta into humus, on site, and make this available at regular intervals for enriching the vegetable garden. The minimum period of alternating the pits should be 6 months. Normally the superstructure will be moved from one pit to the next at 6 - 12 monthly intervals depending on the rate filling of the pit. The family should aim to change pits once a year. The rate of filling depends on the number of users, the volume of the pit and the quantity of other materials (soil, ash, leaves, kitchen scraps, vegetable matter etc) added to the pit. It is very important that both soil and wood ash are added to the pit regularly. The mix of dry soil and wood ash can be mixed and stored in bulk and placed in a smaller container within the latrine for regular use. A mug-full is added after every visit. The regular addition of dry soil and ash to the excreta stiffens the consistency of excreta and leads to a piling up of the pit contents directly under the pedestal - into a cone shaped pile - a phenomenon known as “turreting.” It may be necessary to flatten this off from time to time with a rod or pole passed through the pedestal, pour in a bucket of water (which can be used for cleaning down the pedestal) and add more soil/ash - to gain greater use of the available pit volume. Vegetable matter should also be added. Rags, plastic, bottles, glass and other non compostable materials must not be added to the pit.  They make subsequent excavation much more difficult.



Sequence of changing pits


The following stages are followed once the first pit is thought to be full of the excreta/soil/ash mixture. This must be judged by the user. The following photos were taken at the Friend Animal Foundation, Harare.




1.      Remove the pedestal





Pedestal being removed from latrine. Note second empty pit to the left.



2.      Move superstructure to one side




Superstructure being moved to one side






Slab being moved from used pit to new pit. Note the full brick lining of

the pit in conditions where soil stability is in doubt. 



3.      Mount the latrine slab on to new pit





Latrine slab being moved on to unused pit and mounted in weak cement mortar




4.      Move the latrine superstructure on latrine slab and replace pedestal.






Latrine structure being moved on to latrine slab



5. Level off contents of pit and add good topsoil


The pit contents are levelled off and a good layer of fertile topsoil added. This can be rammed into the pit contents with a pole and more soil added to cover the excreta layer by at least 150mm. Once topped up with soil, the pit can be left to develop humus by itself. The addition of water from time to time helps to keep the contents moist, but the pit most not be allowed to flood with water. Alternatively the pit with its contents of soil and excreta can be allowed to dry out. However, the pit can be used as a miniature garden for growing flowers or vegetables.





Topsoil being added to fill used pit in Hatcliffe, a project of Mvuramanzi Trust.

 The richer the soil, the better the plant growth






Tomato plants growing in topped-up Fossa alterna pit. Tomatoes like rich soil.  The growth of

plants in the Fossa alterna pits is dependent on the quality of the soil added on top of the

decomposing excreta below. In this system, plants roots will rarely penetrate deep enough to

reach the decomposing layer. The addition of water used to keep the plants healthy assists  in

keeping the decomposing materials moist. Excreta is converted to humus within about 4 – 6 months.




6. Excavating and using the converted wastes





Ephraim Chimbunde (of Mvuramanzi Trust) digging out the contents of the Fossa alterna pit in

Hatcliffe in April 2000, 3.5 months after the pit contents were covered with topsoil. The topsoil layer

is first removed and deeper down a darker friable humus-like material is found which is the decomposed excreta. The addition of soil and ash to the raw excreta (both urine and faeces) during the filling stage of the pit greatly assists the breakdown of the material. The humus should be entirely suitable for use in the garden for growing vegetables, trees and flowers. It is best mixed with other humus-like soils like leaf mould or good topsoil and compost. By mixing, the converted excreta comes into contact with the fertile living soil to provide the best medium for plant growth.

Picture taken on Mvuramanzi Trust project.






Here the soil excavated from the Fossa alterna pit is being bagged for use elsewhere.




The Fossa alterna at the Friend Foundation, Harare




Fossa alterna mounted on an elevated double vault brick lined substructure at the Friend Foundation. 

The structure is mounted on a base slab cast within a steel frame which runs up and down within a larger steel frame mounted on the substructure. The entire superstructure can be moved without dismantling the unit from one pit to the other in a minute or two. Note tomato growing in unused pit which has been topped up with soil and compost.




Fossa alterna made of brick mounted on deeper double vault brick lined substructure at

the Friend Foundation. Being deeper, each pit takes longer to fill up and the period between changeovers is extended. In this case the superstructure is built from bricks, but in such a way that it is easily dismantled and rebuilt using the same materials (slab, roof, pipe, bricks, door and frame). 


The Fossa alterna in Mozambique.





Family Fossa alterna being built in Lichinga, Mozambique, as part of a WaterAid funded Programme.

Thanks to Ned Breslin and his staff.







Large Fossa alterna being built in Mandimba, Mozambique, as part of a WaterAid funded Programme.


The Fossa alterna in Malawi






Fossa alterna being built in Lilongwe, Malawi.

Two brick ring beams are being finished off and a single slab has been made.

Both pits are one metre deep.

Thanks to Steven Sugden and WaterAid, Malawi.





Starting the construction of portable superstructure for Fossa alterna in Lilongwe, Malawi.

The soil is firm and three brick courses have been used to stabilise the pits, two courses below ground level and one course above. The bricks sit on a ledge cut back in the pit. Bricks are bonded with cement mortar.








The soil from Fossa alterna pits can be used not only for growing vegetables but also flowers.

In the cases shown here the Fossa alterna soil has been mixed with an equal volume of topsoil

and flowers planted. The results are impressive.  







The writer acknowledges the contributions made by the staff of Mvuramanzi Trust in the implementation and uptake of the Fossa alterna method in Hatcliffe, Harare. In particular, Ephraim Chimbunde, Cleophas Musara, Edward Guzha and David Proudfoot are thanked for their efforts. In addition many thanks are due to Christine Dean, Managing Director of the Friend Foundation and her builder Baidon Matambura and their staff for every possible support. The assistance of those Friend Foundation staff who constructed the various experimental & demonstration Fossa alterna units, which are illustrated in this manual, is also acknowledged. The contributions made by Ned Breslin (Mozambique) and Steven Sugden (Malawi) and the staff of WaterAid in these two countries in demonstrating and promoting the

Fossa alterna is also much appreciated.