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Which way to go!

Planting trees or making humus?

Starting off

ecological sanitation

in the homestead





Peter Morgan


Introduction - understanding the concept of ecological sanitation


Ecological sanitation is seen in Zimbabwe as a system that makes use of human excreta and turns it into something useful and valuable with minimal risk of pollution of the environment and with no threat to human health.


This system includes an ecological latrine, but that is not the only component. The system also involves processing the human excreta into products which are safe and valuable in agriculture (humus production). It also involves recycling the nutrients held in the humus formed from human excreta to produce vegetables and fruit trees. The aim is to show that the nutrients held in processed human waste can be recycled in a simple, safe and effective way to enhance the production of food (both vegetables and fruit). This is what is known as “closing the loop.” That means that food is consumed, excreta formed, excreta converted into humus and that humus is used to grow more food which is consumed again. That is closing the loop!


The practical demonstration of the usefulness of the byproducts of human excreta in agriculture is seen as an important component of the ecological sanitation programme in Zimbabwe. Consequently the crucial step of linking latrines with a method of producing humus for use in agriculture (or forestry) has been emphasised. It is this very important management procedure which is vital to the success of ecological sanitation. The programme is not simply about building latrines, it is about converting the excreta into a safe and valuable product (humus) and using it in a simple and practical way which benefits the users.


There are several types of ecological latrine which are designed to recycle human waste materials. Some use a shallow pit as the receptacle of the excreta (faeces and urine combined). Others use a specially designed pedestal which separates the faeces from the urine, which are collected separately. The end result in the case of faeces is humus, and in the case of urine, when diluted with water is a liquid plant feed. 


Latrines using shallow pits


The Arborloo


The Arborloo consists of a shallow pit (usually about 1 metre deep), covered by a concrete slab and simple superstructure. Both the slab and structure are designed to be portable. The head of the pit is best made safe with a “ring beam” to reduce the chances of collapse. The Arborloo is used much like a family pit latrine, but with two important changes. First, the management includes the frequent addition of topsoil and wood ash to the pit (and also leaves and other vegetable matter like organic kitchen scraps). Second, the avoidance of throwing down plastic, rags and other wastes which will not easily convert into humus or compost. The importance of adding good quantities of soil and ash to the pit is that they assist in the conversion of human excreta into humus and also reduce odour and fly breeding in the pit.


With the Arborloo, the latrine slab and superstructure (and portable ring beam) move on a never ending journey from one pit to the next. Once the used pit is nearly full, the structure, slab and ring beam are removed and placed on a new site. The used pit is topped up with a good layer of topsoil and left to digest. A young tree is planted in the fertile soil placed on top of the pit contents. Many trees thrive in such a place, notably banana, guava, paw paw, mango and mulberry. But almost any tree will grow if planted in the right way, protected against goats and regularly watered. The end result is a plantation of trees which may be scattered or be grouped to form a “sanitary orchard” or woodlot. There is no handling of excreta and no attempt is made at urine diversion. The risk of ground water contamination is reduced because the pits are shallow and the excreta converts to humus more rapidly (normally within 6 months) because wood ash and soil are added regularly whilst the latrine is in use. Because space is required for this concept (mature trees need space) - it is used mainly in rural areas, but it can also be used in peri-urban settlements. The excellent tree growth usually seen in trees growing on these organic pits shows that human excreta when suitably processed provides an excellent medium for the growth of trees.


The Fossa alterna


The second latrine type built over a shallow pit is called the Fossa alterna. This is similar to the Arborloo insofar as the latrine structure (including slab and structure) is portable However two shallow pits are dug close to each other in a permanent location and the use of the latrine alternates between the two pits. If the latrine is heavily used, three pits may be dug, or more. The regular addition of wood ash and loose topsoil is emphasised, and, as with the Arborloo, the addition of vegetable matter like leaves and other plant materials  and even organic kitchen scraps help to improve the fertility of the final humus produced in the pit. At 12 monthly intervals or more the slab and superstructure are moved to the other pit  and the contents of the used pit are topped up with a good layer (at least 150mm thick) of fertile soil which contains the micro organisms, and small animal life that helps the conversion process. This topsoil can be planted with shallow rooted vegetables (tomato, pepper, spinach, rape, covo etc) or flowers. It can also be covered with a wooden lid which helps particularly during heavy rainfall.  The contents should be kept moist, by the addition of water from time to time - but the pit must not get flooded. Whilst the new pit is now filling with a mixture of ingredients (including human excreta) the contents of the full pit are being converted into humus. Within about 6 months the process is normally complete, provided that the pit is not flooded. For best conversion the pit should be well drained and the fertile soil and plant material should be well dispersed within the excreta, providing air spaces within the mass. After 6 - 12  months the loose friable humus can be dug out of the pit by the family and introduced into agriculture, by digging into beds or mixing with other soils. A one year cycle of alternating pits is recommended.


This concept has a value for use in peri-urban areas since the area required for the two permanently sited pits is relatively small - and makes available a source of “humus” which serves as a soil conditioner, which can enhance the production of vegetables on small family plots. If there is no immediate use by the householder, the humus can be stored in bags and may even generate an income. The humus is excavated from the pit once a year (about 0.5 cu.m.) and the latrine components are moved once a year. It is quite simple. 


Urine diversion


In a second range of latrine designs, the concept of urine diversion is used. These use a special pedestal in which the urine enters the front  part of pedestal and is then diverted through a pipe and is thus separated from the faeces. The faeces fall down directly into a brick lined vault beneath. Some soil and wood ash are added to cover the faeces after every visit. This partly dries out the faeces and makes them easier to handle and transfer. In a latrine called the “skyloo” faeces are collected in a vault built above the ground. Urine is normally taken to a seepage area near the latrine but can be directed into a tank for later use. Faeces can accumulate directly in the single vault for periodic removal or can accumulate in plastic buckets, bags  or other containers for onward passage to a secondary processing site. The bucket contents may be introduced into shallow pits or trenches, or placed into jars or other containers together with fertile soil and leaf mould in which vegetables, trees or flowers can grow. In each case the mixture of soil and faeces is converted into a sweet smelling humus like soil within a few months.


In each case, the basic aims of ecological sanitation are followed. The recycling of human excreta is made as simple and convenient as possible. Natural processes are involved in a way that retains a simplicity of method and flexibility of design. Health issues have always been considered. For instance the arborloo is designed to minimise human contact with faecal matter, and urine diversion systems attempt to sanitise faecal material more quickly and make it easier to handle. These aspects of design help to reduce the health risks associated with the practice of ecological sanitation. Also the provision of a reliable hand washing facility is essential if a latrine system is to be associated with an improvement in health.


The need for improved soils in a world which lacks food and where soils are often leached out and infertile is also an important consideration. Ecological sanitation can greatly assist this problem. The humus resulting from composted human faeces makes an excellent soil conditioner. The aim is to mix it with infertile and worked out soil together with garden compost and leaf litters which are also produced in the garden. Thus those practising ecological sanitation should also be familiar with the methods of making garden compost and leaf mould so that all these fertile soils can be mixed to form a good humus. Such humus when properly used in agriculture helps to improve food yields and also food quality and hence provides more food security and improves the nutritional status of the beneficiaries.


In the case of this manual links have been made to Permaculture where methods  associated with the best organic farming are emphasised. This manual describes two of the simplest possible procedures for starting off with ecological sanitation in the homestead. One way using the Arborloo  encourages the planting of trees. The other way using the Fossa alterna encourages the formation of humus. In both cases costs have been reduced by using a ring beam for pit protection as opposed to bricks. This beam is normally sufficient to support the lightweight structures. The ring beam is placed on level firm ground and the shallow pit dug within it. The slab design depends on whether a squat hole is used or a pedestal. Superstructures can be made of the simplest type from poles and grass or from wood or steel frames covered with a variety of materials including grass. A very acceptable latrine can be made at low cost (the main imported ingredient being a single bag of cement). Locally available materials can be used to build the superstructure. The recycling procedures are simple and easily understood by urban and rural folk.


Most of the photos in this manual were taken at Fambidzanai, the Permaculture Association of Zimbabwe, where ecological sanitation is being introduced. Thanks are due to Moses Nyapokoto, Vickers Marongedza, and Kizito Dewura. Thanks are also due to Annie Kanyemba who made the ring beams and slabs. The photos of trees growing on “organic pits” are taken from the Friend Foundation -thanks to Christine Dean and Baidon Matambura. Emphasis has been placed on using natural methods, where organic  materials of all types are used to make valuable humus. The miraculous change of human excreta into humus is one of Nature’s marvels. Without this natural process of “building up” and “breaking down” no animal or plant life could exist on Earth.   


Peter Morgan


February 2002.



How the Arborloo works


The Arborloo is a eco-latrine which is equipped with a portable concrete slab and superstructure which moves on a never ending journey from one shallow pit to the next. In this case each shallow pit is protected by a concrete ring beam, which is also moved on. As the pit is used it is filled with a combination of excreta (faeces and urine) and also soil, wood ash and other compostable materials like leaves and organic kitchen scraps. The pit is used until it is nearly full, which for a family should be about one year. Once the pit is nearly full, the ring beam, is moved to another site and a new pit dug about one metre deep within the beam. The slab and superstructure are now added on the beam and the latrine is put to use. The used pit is topped up with a good layer of fertile soil about 150mm deep. This can be left for a month or two or more to settle or can be planted with a young tree straight away. If planted straight away, the young tree roots must not come into contact with the excreta and should be allowed to grow freely into the soil. It is often best to allow the pit to stabilise before planting. Planting is best carried out during the rains if water is not freely available. It is important to protect the young tree from attack by goats and other animals. It should be watered regularly. Covering the surrounding soil with mulch helps to retain water.



How the Fossa alterna works


The Fossa alterna is a eco-latrine which is equipped with a portable concrete slab and superstructure which alternate between two shallow pits which are permanently sited. In this case, each shallow pit is protected by a concrete ring beam. As the pit is used it is filled with a combination of excreta (faeces and urine) and also soil, wood ash and other compostable materials like leaves and organic kitchen scraps. The pit is used until it is nearly full, which for a family should be about one year.


Meanwhile the second pit may have been covered and left empty. Alternatively it is filled with compostable materials from the garden like leaves, cut grass and other vegetable matter, animal droppings, soil, wood ash and kitchen scraps and kept watered so that compost is formed. It acts like a pit composter for the first year. It is up to the user to decide which way to use the second pit for the first year


When the pit being filled with excreta and other materials is nearly full, the second pit filled with compost is emptied and the superstructure and slab are moved on to this. The first pit is topped up with good soil and left to form humus. It is watered from time to time to keep the contents moist. It is advisable to cover with a wooden lid during periods of heavy rain to avoid flooding.


After another year the second pit will be full. The first pit is then emptied out - it will be entirely made of humus which is good for the garden. The superstructure and slab are then returned to the original pit. The process is repeated.



Two concrete ring beams are laid down and levelled on slightly raised ground about one metre apart. 

A pit is dug down within each ring beam to about one metre depth. If possible a layer of leaves or

fertile soil is placed in the base of the pits. Some weak cement (or termite) mortar is laid over the

ring  beam and the slab is placed on top. The portable superstructure is then placed over the slab. The latrine is put to use with regular additions of soil/ash and leaves etc being added as well as excreta.

The Fossa alterna after one year







The components of a simple ecological latrine


Whilst there are several types of eco-latrine, it is best to start simple and build an Arborloo or a Fossa alterna. Both of these systems can be upgraded to a full urine diverting type at a later date.


This manual describes how to construct an Arborloo and a Fossa alterna using  a concrete ring beam to protect the shallow pit and a simple concrete slab to cover the pit. The superstructure can be built from a great variety of materials, and this can also be upgraded as time passes. So one is able to start in the simplest way. Because of their long life, the construction of basic units like a concrete ring beam and latrine slab lies at the centre of the hardware. Once well made, the concrete ring beam and slab will last for decades.  The parts are listed below


1. The concrete ring beam


With the simpler types of eco-latrine the excreta falls into a shallow pit about 1 metre deep. The strength of the pit walls depends on the nature of the soil. In firm ground a hole which is simply dug into the ground without any support may be good enough. But it is wise to protect the shallow pit in some way to avoid the effects of rainwater erosion which may flood the pit and also make the walls of the pit collapse. Humus does not form in flooded pits. A simple way of doing this is to make a ring beam, that is a square shaped ring of bricks or concrete which is placed at the top of the pit and is raised above the ground level. The concrete slab will be placed on top of this.


Ring beams can be built in bricks and cement mortar. If made of bricks they are normally started off beneath ground level, by cutting back the upper part of the pit and building up the brickwork to at least one course above ground level. Normally they are one brick width, but a stronger upper pit lining can be made with a double brick width. These have been described in Vol. III of Ecological Sanitation in Zimbabwe. In the case of the Arborloo, the brick ring beam is taken apart and rebuilt on the new pit site. In the case of the Fossa alterna two permanently sited pits are dug and the upper part of each pit is lined with bricks. In loose or sandy soil, it may be necessary to line the pit of the Fossa alterna to the bottom to avoid collapse.


In most situations the soil is moderately firm and a concrete ring beam may be all that is required for both the Arborloo and the Fossa alterna. This manual describes this method.  The ring beam is cheap to make and if well constructed from good materials and suitably reinforced with wire can be used for many years. It is a good investment. Also the portable structures used with the Arborloo and Fossa alterna are light and put less strain on the pit structure compared to brick superstructures. The concrete ring beam is placed on firm ground in a slightly raised location. It is made level. The pit is then dug inside down to about 1 metre depth. The soil taken from the pit is placed around the beam and rammed in place. Grass may also be planted in this soil, so that the roots will help to consolidate the soil around the beam. The soil will compact around the beam and help to make it stable enough to mount the slab and a portable structure above. Note that if the superstructure is made of bricks it is essential to brick line the pit to the base of the pit. Ecological latrines of this type do not normally use brick structures.


2. The concrete slab.


This can also be made simply and cheaply. If made well it will last for many years. The slabs can be made with a single squat hole or a larger hole for a pedestal. The slab can also be cast with a hole for a screened vent pipe, which will help to reduce odours and flies from the latrine. Since the concrete slabs used on these eco-latrines are moved from time to time, it helps to fit them with steel handles. Best use steel of 8mm thickness or more.


The concrete slabs can be made flat and smoothed down with a steel float to make the floor smooth. If they are flat and exposed to the rain, much of the rainwater falling on them will not enter the squat hole and will run away around the sides (about one third will go down the squat hole). If the slab is slightly dished towards to the squat hole, it will act as a rainwater harvester if the slab is exposed, and most of the water falling on the slab will enter the pit during the rains and may waterlog the pit. This is very undesirable as humus cannot form in wet pits. Pits used in eco-latrines must be well drained so the contents, whilst being moist should never be allowed to get waterlogged.  So in the case of the simplest superstructure, which may not have a roof, the slab should be flat. Where a roof is fitted, which will be the case in most structures built, the concrete slab can have a slight slope on it. A slope on the slab will help in washing down the slab, but the amount of water added should be minimal.


All concrete slabs (as well as the concrete ring beams described above) should be reinforced with steel wire.  3mm wire is quite satisfactory. Barbed wire will also work well. In both the ring beam and the concrete slab, four wires are placed within the concrete in each direction. That makes 8 wires per unit, 4 across the slab or beam and 4 down the length of the slab or beam. In both cases the concrete is made with sharp river sand or a mix of sharp river sand and small stones. The curing is important. Once set, the concrete should be covered up and allowed to cure for about one week before lifting. It should be kept wet at all times during this period before being moved.


3. The superstructure


This is the third important part of the eco-latrine. There are many ways of making the superstructure. An important consideration in the case of the Arborloo and Fossa alterna is that the superstructure can be moved from one location to the next relatively easily. Normally the superstructure is built by erecting a frame of some sort and then attaching a walling material to the frame. A roof can also be added.


The simplest structure may be made of poles covered with grass. Gum poles can be used and even bamboo. The poles can be driven into the ground at four corners and the grass walls built around this. Alternatively, the structure can be built so that it rests directly on the slab --  in which case  contact with soil is reduced which reduces the risk of termite attack. The structure can be made with a hinged door (using car tyres as hinges) or a hanging door or even doorless  if a square spiral structure is built. Steel frames can also be used and also covered with grass. Walling material includes grass, reeds, slit bamboo, sacking, hessian, plastic material, shade cloth, wood slats, cement soaked hessian and tin sheet etc. Once again the type of walling can be upgraded as time passes. If a sturdy frame is made, various types of walling can be added over time.


It is best to fit a roof, but even this can be added during a period of upgrading. The roof can be a frame, laid over with chicken wire and then plastic sheet and covered with grass. It can be upgraded with tin sheet later.


These parts then make up the essential components of the simple co-latrine. All the concrete parts can be made with a single bag of cement, sharp river sand, (and even better if small granite chips are available) and reinforcing wire. This manual describes how it is done.




1. Making the concrete ring beam


The concrete ring beam is made with a mixture of 5 parts sharp river sand and one part cement, or a mix of 3 parts small granite chips, 2 parts sharp river sand and one part cement. In these photos the mix was made with chips, sand and cement. 10 litres of cement were mixed with 20 litres river sand and 30 litres small stones. A level piece of ground is chosen and plastic sheet laid on it. The mould for the ring beam can be made with bricks as shown in the photos. Wooden shuttering can also be used as a mould. The ring beam is made 75mm thick.


In those examples described in this manual, the external measurements of the beam are 1.25 metre X 1 metre , the inner measurements (the size of the hole) 1m X 0.7m. This ring beam is made for slab measuring 1.2m X 0.9m. Wire reinforcing is used within the concrete mix, four strands of 3mm wire down each length. With the ring beam, the corners are a potential weak point. For ease of lifting 4 steel handles can be set in the concrete. These are particularly useful for the Arborloo. In the case of the Fossa alterna the beams, once set in place may not be moved for several years. Handles are less important in this case.


Half the full mixture is made first, that is 5 litres cement, 10 litres sand and 15 litres small stones (or 5 litres cement and 25 litres sharp river sand). It is mixed with water to make a thick slurry. This is added to the lower half of the mould. Then the wire reinforcing is added. This is followed by a further mix, the same as the first. The four handles can be made from 4 steel bars about 8-10 mm diameter and about 25cm long. These are bent and set in the concrete (see photo). Smooth off the top of the concrete. Cover with a plastic sheet if possible and allow to set overnight. Water the following morning and keep covered and wet for a week before lifting.







Two concrete ring beams made for Fambidzanai showing the brick mould that was used. The

bricks have been removed from the ring beam closest to the camera. Note the handles fitted on

the edge of the beam facing slightly outwards. Each beam is 1 metre wide and 1.25 metres deep.

This allows for the 0.9m X 1.2m slab to rest on the beam with a little overlap.



2. Making the concrete cover slab


This is made with the same mix as for the ring beam. Either a 3:2:1 mix of small stone, river sand and cement or a 5:1 mix of river sand and cement. If small stones are available, then this will be stronger, but usually small stones are not available so river sand alone must do. The river sand should be clean and sharp. In the photos below a mix of cement and river sand has been used. 


The concrete slab described in this manual is 0.9m wide and 1.2m long and about 40mm deep. The mould in which the slab is cast can be made with bricks or wooden planks or a combination as shown in the photos below. Most slabs in Zimbabwe are made with squat holes as this is the preferred position, but slabs can also be prepared for fitting a pedestal (instructions on how to make a homemade low cost pedestal are available in Ecological Sanitation in Zimbabwe Vol. IV). In the case of the squat hole, a suitable hole can be made by taking a 20 litre bucket, cutting off the bottom and bringing together the base into a pear shape with wire. A squat hole size of about 30cm X 19cm is required. In the case of the hole for the pedestal - this is 30 cm across and can be formed by a plastic basin. The photos below show these arrangements. A hole can be made for a vent pipe also in the slab. This is made by inserting a short length of pipe (75mm long) in the slab at the appropriate position (see diagrams). The hole size should match the pipe. It is possible to make low cost homemade vent pipes (these are also described in Volume IV).Where generous quantities of soil and wood ash are added regularly to the pit contents, there may be little need to fit a pipe, as these additions greatly help to reduce fly and odour nuisance. Because the slabs of both the Arborloo and Fossa alterna will be moved at approximately one yearly intervals it helps greatly to fit four carrying handles to the slab. These can be made by cutting 4 lengths of 8 - 10mm steel rod each about 25cm long and bending them and inserting in the fresh concrete. The concrete is levelled off with a wooden trowel and finally smoothed down with a steel float. Once the concrete has begun to harden, the moulds for the squat or pedestal holes and the vent pipe hole can be removed. The slab should be covered with plastic sheet if possible and left for a week to cure. During this period it should be kept wet. If plastic is not available it can be covered with sand which is kept wet. For all concrete work good curing is essential.









Mould in which the 1.2m X 0.9m concrete slab made for squatting will be cast.






Mould in which the 1.2m X 0.9m concrete slab made for the pedestal is to be made. Note handles and wire reinforcing prepared beforehand. Also shaped bucket and short pipe used to make squat and vent holes.






Which way to go - plant a tree or make humus?


It is very likely that the decision as to whether to construct an Arborloo or a Fossa alterna will have been taken before the ring beam and slab were made. Normally arborloos are used where there is space for the tree to grow. Many trees become huge and there may not be the space in a small peri-urban or urban plot, to consider planting a tree. But some trees occupy less space than others. Paw paw trees for instance do not occupy much lateral space, and may be suitable for planting along borders in peri-urban plots. In the case of the paw paw, the tree wood is soft and easily cut down if the tree becomes too large. There is no handling of processed human wastes in the Arborloo and some people may prefer to start off with this option. Needless to say, the fruits grown on trees emerging from Arborloo pits are tasty. In some countries (Malawi, Mozambique, Rwanda, Kenya etc) villagers have been growing fruit trees on old latrine pits for generations - they have learned that the trees grow well and the fruit is tasty!


If there is a requirement to increase the fertility of the soil in a small peri-urban or urban plot, then the Fossa alterna may be best, as it should provide a source of humus which can be applied to vegetable and flower gardens every year. The same humus can be mixed with other soils as a medium in which to plant trees. Of course this also applies to the rural areas as well. In Niassa Province, Mozambique, for instance, the Fossa alterna is the preferred option partly because it offers a simple method of constructing a relatively permanent solution to sanitation, is cheap, helps reduce flies and odours (even in the absence of a vent) and also provides a yearly supply of  humus which can be used in a variety of ways (Ned Breslin, WaterAid, pers.comm.).


The ring beam and simple slab so far described can be used to build either an Arborloo or a Fossa alterna. In the case of the Arborloo one ring beam and one slab will be required. In the case of the Fossa alterna, two ring beams and one slab will be required. There is a lot of room for flexibility. An Arborloo can become a Fossa alterna (by the addition of an extra ring beam) or a Fossa alterna can become an Arborloo (if a tree is planted in the used pit). Some people may decide to plant a tree some years and make humus in other years. There are no hard and fast rules.


Choosing a site


The site chosen for the latrine should fulfil several requirements. It should be convenient for the users. It should also be placed on slightly raised ground to avoid flooding during the rains. It should also be sited some distance (about 30m) away from any well used for domestic purposes. If the Arborloo is chosen, the fact that a tree will be growing there in years to come should also be taken into consideration. Also thought should be given to the possibility that in many years time there may be a lot of trees growing on old pit sites which will form an orchard or wood lot. Trees do require space.


The ring beam method of protecting a shallow pit may require some experimentation in some areas. In firmer ground it will be very stable, but in looser and sandy soils more care should be taken. If there is doubt, it is best to line a pit dug in sandy soils with bricks. That method, in practical terms, may only apply to the Fossa alterna where the twin pits are permanently sited. If an Arborloo pit shows any signs of collapsing - the ring beam, slab and structure can be removed within a few minutes and relocated elsewhere. In some places the soil is so firm that even a ring beam may not be required, especially where the Arborloo method is chosen. But even in this case, the possibility of pit flooding should be avoided with the use of a ring beam.


Embedding the ring beam and digging the shallow pit.


After a suitable site has been located, the ground should be levelled off and the ring beam placed on the surface of the ground. This must not involve too much removal of soil as the aim is to make the ring beam stand above ground level. The beam must be level, stable and well seated on the ground.



Ring beam being moved on to site


Next the shallow pit is dug within the ring beam and slightly within the area of the beam (about 2.5cm within the beam). The hole is now dug down to about 1 metre depth. The maximum depth is about 1.5 metre. It is helpful to excavate using  a pick with a shortened handle. The walls should be vertical to gain the maximum volume of the pit beneath and within the ring beam.




After embedding and levelling, the pit is dug down inside the beam


The soil taken out of the pit should be placed around the ring beam and rammed in place. Even better if some grass roots can also be laid in the soil as they will help to consolidate the soil. Another method is to mix a little cement in the soil, moisten, and lay this around the beam and ram. In most cases however just compacting the soil around the beam should be sufficient to hold the beam in place. Experience with this method will increase over the years.



Soil from the pit is placed around the beam and rammed in place




The beam finally surrounded by soil



Preparing the pit before use


Before the slab is fitted it is a very good idea to add a few inches of leaves, humus or compost to the base of the pit to promote the composting process from the time fresh excreta is added. This process will take longer if the excreta falls on barren soil.



Adding leaves to the base of the pit - leaves or compost, grass cuttings and other

vegetable matter will help to get the processing of excreta into humus

underway as soon as the waste is deposited.



The base of the pit is covered with a layer of laves, humus or compost.


Adding the concrete slab


The concrete slab is now mounted on top of the ring beam. If a vent is fitted to the system it is wise to seal any gap between the beam and the slab. This will improve venting of the pit and fly control. A very weak mix of sand and cement can be used as a sealant or soil cement or termite mortar. The slab is now fitted centrally over the ring beam.


Spreading a weak mix of sand/cement on top of the beam before fitting the slab. 




Carrying the slab on to site

(Not everybody uses the handles!)


Concrete slab in place (squatting type)





Concrete slab in place (pedestal type)



Optional pedestal


In Zimbabwe the squatting position is the one most commonly used. However it is possible to fit a pedestal on to any pit or eco-latrine. Commercial pedestals are available, and it is also possible to make homemade units. As time passes, pedestals are becoming more popular in Zimbabwe and those who are more elderly greatly appreciate them. Constructional methods for homemade pedestals are described in Ecological Sanitation in Zimbabwe. Volume IV.



Home made pedestal being made by Annie Kanyemba, 15 at the time the photo was taken. These

units use a plastic bucket and concrete as ingredients. The seat is made in concrete and also the side

walls which surround the bucket which provides a smooth surface suitable for cleaning.


Home made pedestal fitted to the Fossa alterna slab in Fambidzanai.

 The base of the pedestal is bonded to the slab in weak cement mortar.




A large range of materials can be used for the construction of the superstructure. These include poles and grass, frames made from poles, reeds, bamboo, wooden planks and steel etc with a variety of coverings for privacy. Hinged or hanging doors may be fitted or the structure can be made without a door in a square spiral configuration. The type of superstructure chosen may depend on the availability of local materials.



A steel framed “doored” structure is being fitted to the Fossa alterna in Fambidzanai

A second ring beam has been installed and the pit excavated on this Fossa alterna at

Fambidzanai. The two ring beams are held in permanent positions, whilst the slab and

 structures alternate between one pit and the other at yearly intervals.


The interior of the Fossa alterna at Fambidzanai. This photo shows the pedestal within the structure.

A PVC pipe has been fitted. In this case the diameter of the pipe is 75mm. Normally vent pipes fitted

 to VIP latrines have a recommended diameter of 110mm. However in this case a 75mm pipe is

 being tested. 90mm pipes are also on trial. They work well because part of the fly and odour control in the pit is undertaken by the constant addition of soil and ash to the fresh faeces

A simple gum pole framework has been erected on this Arborloo site at Fambidzanai.

This will be covered with grass.

Variations in superstructure design

Final superstructure on the Arborloo at Fambidzanai. Grass has been used for privacy. The roof section is covered with chicken wire, then a plastic sheet followed by a layer of grass to protect the plastic sheet.


Similar simple gum pole frame/structure at Woodhall Road. In this case the door section is covered with hessian. Walling material can be made of  shadecloth, grass, reeds, bamboo, sacking, plastic sheets, wooden slats, tin sheet etc.

Superstructure made of gum poles covered with shade cloth. Note handwashing facility.


loser view of handwashing facility made from discarded plastic pill bottle and oil container.

Hand washing is an important aspect of disease control associated with sanitation. 


Using Bamboo as a structural material


Bamboo is a strong and versatile material commonly used in the Far East for constructions of all types. There are many ways of using this material to make simple and durable structures for the eco-latrine. Bamboo will also grow in organic Arborloo pits. Thus there is the opportunity to plant trees which will later be used for building future latrine structures. The gum tree also fits into this category. Gums also grow well on Arborloo pits.



The four uprights in this simple construction are made of bamboo and embedded in

holes made in the ground. The upper sections are braced with further lengths of bamboo.

Wires are placed around the uprights and various materials can be used for walling.

A door can be made by wiring together four lengths of bamboo as shown in this photo

and bracing with wire. The door hinge can be made of car tyre as shown in this photo,

wired to the main “king post.” A roof can also be added by making a frame, and covering

with a layer of chicken wire and plastic sheet, then finally covering with grass. Walling for

the structure can be made from hessian, grass, plastic sheet, shade cloth, reeds, split bamboo,

wooden slats etc. Old engine oil, ash or ant killers can be added around the poles in the

ground to reduce attack by ants. Greater use should be made of this versatile material.



Management of simple eco-latrines


Two of the simpler eco-latrines, the Arborloo and the Fossa alterna have been described in this manual. Day to day use and management of the two systems is very similar. In both systems it is desirable to add a layer of leaves, humus, good topsoil or compost at the base of the pit, before the pit is put to use. This will help to start off a new process of humus formation within the pit and will also assist in drainage of the pit.


Daily management of the Arborloo and Fossa alterna.


Both the Arborloo and Fossa alterna are used like a normal pit latrine in that urine, faeces, anal cleansing materials (preferably paper) are added to the pit on a daily basis. In order to build up the mix of ingredients which assist in the conversion of faeces into humus, it is important to regularly add soil and wood ash to the pit, preferably after every visit made. This material is best made up beforehand in the dry state, stored in sacks or bags and then added to a smaller container within the latrine. About 4 parts sifted soil and 1 part wood ash are mixed.  A mugful of the soil/ash mix should be added after every visit made to deposit faeces. It also helps to add leaves and/or organic kitchen scraps to the pit from time to time. These additional materials help to improve the final texture and quality of the humus formed in the pit. It is also desirable to add a small sackful of humus/leaves etc to the pit at 3 or 6 monthly intervals to increase the proportion of “living soils” to the pit contents. The proportion of addition materials placed in the pit can be 50% by volume of the excreta deposited or even more.  In fact much of the bulk of the excreta will be absorbed into the “soils” added.


It is also important to avoid placing non - biodegradable materials down the pit. These include rags, plastic sheets and bags, bottles and all manner of other objects that are often put down standard pit latrines.


The conversion of excreta into humus will not take place if the pit is flooded with water. This means that only limited amounts of water should be added to the pit. Good pit drainage is very much dependent on soil type and area of soil in the pit available for drainage. Where the ring beam method of pit protection is used a large surface area of soil will be available for pit drainage. Where the walls of shallow pits are lined with bricks to the base  - only the base will be available for drainage. There will be a lot of variation depending on soil type, pit volume, pit protection type and the material content of the pit.


For the best results, it pays for the user to look down the pit from time to time. Since dry soil and ash added to the pit tend to make deposits of excreta less fluid, there is a tendency for the pit contents to rise up centrally within the pit under the squat or pedestal hole (called turreting). Thus it is advisable from time to time for the user to take a stick and try to level off the contents so that more of the available pit space can be used.


Management of the second pit in the Fossa alterna


It is very advisable to dig and protect both Fossa alterna pits during the initial building stage. Whilst the second (and as yet unused) pit can be left empty and covered with wooden planking, it can also be used to the best advantage, whilst the first pit is filling.


The second pit can be used to make compost by the addition of leaves, grass cuttings, weeds, kitchen scraps etc with some fertile soil or compost (and even some animal droppings) also being added. Since ecological sanitation concerns recycling in all its aspects and the development of a recycling habit within the homestead, it makes good sense for the concept of “pit composting” to be introduced. The aim is to develop the interest of composting as a sound gardening practice in combination with the use and management of the eco-latrine.  The Fossa alterna system makes this possible. At times of heavy rain it may be best to cover a composting to avoid flooding.










Leaves falling under trees offer an excellent source of leaf litter and fertile soil. This should be gathered up and added to both the pit in use and also the second pit in which compost is being made.







The second pit of the Fossa alterna being filled with a combination of ingredients which can include

fertile soil, leaves, compost, chicken manure, organic kitchen scraps, grass etc.




Time to move!


After about 9 - 12 months the time will come to move the family eco-latrine to another site. What now happens depends on the technology used. One year is a good period as this is easy to remember.


“Changing pits” on the Fossa alterna


Once the used Fossa alterna pit is nearly full the time has come to change pits. For a family this should be about one year. If the latrine is heavily used, and the pit filling time is less than 6 months, it is best to make an additional ring beam and use 3 pits which are filled in rotation. The time to convert to humus is dependent on several factors which include moisture content, temperature, mixture within the pit etc - the more topsoil the better. The pit must not be flooded, neither should it be very dry. Temperature will depend on altitude and season. In warm/hot areas under the right conditions, humus can be formed in 2 - 3 months. 6 months may be required in cooler areas. The higher the ratio of soil/ash/leaves in the pit the more effective the conversion will be. Also the more varied the ingredients - the higher the fertility of the humus will be.


If the second pit has been filled with compost during the past year, this should be emptied - thus making available the pit. The excavated compost can be used on the garden, but it is also usefully added to top up the pit filled with excreta and soil etc. The same material can be used to add to the new pit as it fills up. 


First remove the superstructure and slab and place to one side. Now place the slab on the ring beam above the empty pit and also seal this off. Now add the superstructure back on the slab (and any pedestal if used). The procedure for latrine management is now started on the second pit, just as before.


The pit filled with excreta/soil/ash etc is now levelled off and topped up with loose soil. A layer of about 150mm is best. The soil is best fertile and can be taken from the compost pit or from a layer of good topsoil if available. Leaves can also be added as a mulch. The pit is watered to make the contents moist. The pit can now be left to convert. It can also be planted with flowers or shallow rooted vegetables. Occasional watering is required, even if plants are not added to keep the contents moist. The pit must never be flooded.


In a year’s time the change will take place again. Repeat the process. In this case dig out the humus formed from the mix of human excreta/soil/ash/leaves etc. Move the latrine across and top up the newly filled pit. The humus taken from the pit can be dug into garden soil, mixed with other soils in vegetable beds or used like normal compost. The “loop” is closed if the material is used to grow fruit trees or vegetables which are consumed on the homestead, and the latrine used again. 


Relocating the Arborloo


In the case of the Arborloo, the superstructure and slab are removed and put on one side. The ring beam is dug out and removed from the old site and placed on a new site nearby. A new hole is dug down as before and the ring beam surrounded by some of the soil taken out of the hole.


The slab and superstructure are remounted on the ring beam. A seal is made between ring beam and slab. The new location is chosen with care, taking note that the trees once planted will often grow large. Pits should not be closer that 3 metres apart. For trees which are known to grow large, 5 metres may be a better distance.




Preparations for tree planting


The used pit (filled with excreta, soil/ash/leaves etc) is now topped up with fertile soil, at least 150mm deep. This soil can come from old compost heaps, fertile soil/leaf litter found under trees or any other place where the soil looks good. The aim is to plant the young tree in the topsoil so the roots come nowhere near the excreta layer.


Young trees can be obtained from a nursery, or in some cases can be taken from cuttings from existing trees (mulberry, banana) or can be grown from seeds (guava, paw paw, mango, avocado etc). Mulberry, banana, gum, mango, guava, paw paw & avocado do well. In fact most trees will thrive if given the right opportunity.


At least three things are important.


1. Keep the young tree roots well away from the excreta layer.

2. Protect the young tree from goats etc

3. Water regularly. A mulch helps to retain water in the soil.


If the pit is filled up and topped up during the dry season, it may be better to leave the pit to compost for a few months first. Then add the young tree later. Also if the trees are very young it may better to allow them to establish themselves in buckets or pots first so the root system is more established and extensive. Experimentation will be required. There will be a variation in local conditions - soils, climate, best tree type will vary with the area. Also the owner will choose some trees in preference to others. Some may choose trees for fruit, others for fuel, others for shade etc. The banana is perhaps the most widely grown fruit tree on latrine pits.


In some cases the tree may not grow fast at first, a pattern known as “hesitancy.” Obviously a pit full of richly organic material may not be the most ideal environment in which young trees can grow. But with time, the conditions of the pit become favourable. Some trees are more tolerant of the richly organic conditions than others.




The importance of trees


The tree is one of nature’s marvels. It can be the provider of food, fuel, building materials, and medicine. It helps to stabilise the soil and offers shade. It provides leaf litter and thus provides additional fertility to the soil. It also provides beauty and richness to our environment.


Every means possible should be taken to grow more trees of all sorts, exotic trees and also indigenous trees. The links made between sanitation and the propagation of more trees is an important one. The value of the nutrients available in excreta, even if they are below ground level, can be realised and witnessed with ease. Fruit trees grow very healthily on such organic pits if planted correctly, watered and protected. The fruits when tasted are delicious. What simpler and better way of demonstrating the concept of ecological sanitation and “closing the loop!”


Various trees growing on organic pits

(Photos taken at the Friend Foundation 10.2.2002)


Banana growing in Arborloo pit which was first located within a ring beam at ground level. An

 experimental latrine built above ground level (for high water table areas) has been constructed next

to the original pit. Bananas do exceptionally well on old latrine pits and also arborloo pits.



Mulberry growing on organic pit filled with dog manure and soil. Note: Most trees planted in the sanitary orchard at the Friend Foundation are growing in a mixture of  dog  manure and soil. The Foundation looks after animals and the orchard solved the problem of manure disposal.


Paw paw growing on organic pit filled with dog manure and soil.



Gum tree growing on organic pit filled with dog manure and soil.


Avocado growing on organic pit filled with dog manure and soil.



Lemon tree growing on organic pit filled with dog manure and soil.


Indigenous trees also grow well on organic pits



Swartzia madagascariensis growing on organic pit filled with dog manure and soil



Brachystegia glaucescens growing on organic pit filled with dog manure and soil


Other trees grown successfully on these organic pits include Guava, Lemon, Mango, Lime, Orange, Naartjie, Genoa fig, Quinine tree, Maroela, Wild mango, African thorn bush, Peacock flower,

Jacaranda, Frangipani, Leucaena & Bamboo.