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The ‘Arborloo’


Leave the contents -- move the Loo


A Method for Recycling Human Waste













Peter Morgan






Compost latrines, which are now being promoted on a small scale in Zimbabwe, have the primary aim of disposing and recycling of human waste formed in pit and Blair Latrines in a safe and, if possible, a valuable way. Manuals for the Blair compost latrine have been produced and these describe the construction of both twin pit and single pit latrines. In both cases it is recommended that soil and ash together with leaves and other compostable materials are added regularly to the pit together with human wastes and resulting mixture is allowed to mature in the pit to form compost. After a period of time, which depends largely on the rate of filling of the pit, the contents are removed. They can then be added to an existing garden compost heap, stored in sacks for future use, placed deep in trenches which will be planted with vegetables or placed in the lower half of pits dug for planting fruit trees. In all these cases it is assumed that the contents of the compost latrine will be removed from the pit in which they have accumulated and moved to another site for further processing and recycling.


This manual describes a different approach, but one that has the same aim - that of disposing and recycling human waste formed in a latrine. In this case the latrine itself is moved and the pit contents remain in place - the pit becoming a hole for planting a tree. Wood ash and soil are added regularly to reduce fly breeding and odour and leaves and kitchen wastes are also added to assist in the breakdown of the materials to form a humus material suitable for planting. The latrine is used until the pit is about two thirds full. Then the latrine structure is moved and the pit topped up with well- composted soil. The young tree is then planted within the hole and watered and allowed to grow and mature. The most suitable trees for this are paw paw, guava, mango, avocado and mulberry - with the “passion fruit” - granadilla also growing well in such conditions. Orange, naartjie, “neem” and Moringa are also being examined. Because the latrine is linked with a tree we call it an ArborLoo.


The concept fulfils most of the aims of both ecological sanitation and organic farming - it is simple, cheap, easy to manage and sustainable, organic wastes being efficiently recycled with little effort. There is minimal pollution of either the ground or the environment and the end result is a supply of vitamin rich fruit. 


The latrine itself must, of course, be light enough to move by hand. This means that the concrete slab on which the latrine is built over the pit, whilst retaining strength, must not be too thick and heavy. Ideally it should have carrying handles cast into it. In order to remain portable, the slab should not be too large and this means that pit area, but not depth is limited also. However a slab measuring between  1 metre square to 1m X 1.2m and made about 4cm deep can be strong enough and also portable enough to move easily. Materials used for the superstructure must also be light. Steel bars and hessian, or poles and reeds or grass can be used - or reed mats.


The concrete latrine base is made with a central hole which can be used either as a squatting hole or hole for insertion of a pedestal. If the latrine is to be built directly on top of the slab - it helps to insert four steel “corner posts” made from angle iron into the corners of latrine slab for later attachment of the superstructure. If preferred, the structure can be built in poles and grass outside the slab area in which case this must be rebuilt once the slab has been repositioned. This type does not need corner posts. A concrete cover fitted with a handle should also be made to cover the squat hole - this will help to reduce flies. Round slabs can also be made and used in the same way.


Once completed, the portable latrine is lifted and mounted over a hole dug in the ground which will normally be between 0.5m and 1m deep and measure about 0.8m square so that the slab overlaps by at least 0.1m on each side. For round slabs a round hole can be dug. If the soil is relatively firm there will be no need for a pit lining. It is desirable to mount the slab on a “ring beam” of bricks or stones to raise it off the ground.


The latrine is used as normal with soil, ash, leaves and even kitchen wastes being added regularly together with the human waste. The squat hole cover is kept in place when the unit is not in use. When the pit is about 2/3rds full, a  new pit is  dug at a site nearby and the latrine slab lifted up and moved on to the new site. Four people are able to perform this task in a few minutes. If the structure is separate - this needs to be rebuilt on the new site The pit is then topped up with garden compost and soil and well watered. The next day a suitable young tree is planted and well watered. If there is not much space available, paw paw or granadilla are most suitable. If more space is available, the mango,  avocado or guava will be suitable. The cycle is repeated again once the second pit has filled up. The end result is a garden filled with fruit trees. A grove of trees called a “sanitary orchard” may result.




Peter Morgan

Harare, August 1998, updated May 1999.


Stage 1. Making the concrete latrine slab


 1a. Making a rectangular slab with corner posts.


If the superstructure is to be built directly over the slab it is best to make it rectangular with corner posts inserted in the slab for attachment of the superstructure uprights. If the superstructure is to be built outside the slab area, the slab can be made square.


Some bricks are taken and placed on the ground to make a mould measuring about 1m X 1.2m. It is far better to lay a sheet of plastic on the ground first so that the concrete if formed on this plastic and does not touch the ground directly. This is particularly important when high strength concrete is formed.


Before the concrete is mixed 4 steel handles are made from steel bar and bent so they will form good handles and also anchor well into the concrete. Also four steel  “corner posts” are cut, ideally from angle iron (30mm X 30mm) or steel pipe (12mm or 15mm) with each piece being about 300mm long. A mould for the squat hole or pedestal hole is also required. The squat hole will measure about 150mm X 300mm being rounded off. The size of the hole for the pedestal will depend on the pedestal type used. Chicken wire or 3mm wire will also be required for reinforcing.


When all the bits and pieces are ready the concrete can be mixed. The mixture is one part cement to 3 parts sharp river sand. About 3 X 5litres of cement are required. Mix 1 X 5 litres cement  with 3 X 5 litres river sand at a time. The squat hole (pedestal hole) is positioned centrally and 30cm from the rear of the slab. Half the mix is added and then some 3mm wire or chicken wire is added for reinforcement. The mix is built up to a slightly greater thickness at the edges. The corner posts are set in the corners of the slab about 100mm from the edge with the concrete raised a little around them to increase the hold of the cement. The 4 handles are also set into the concrete about 75mm in from the side with two being placed on each long side of the slab about 300mm from each end. These will be used to lift the slab later with one person lifting each of the four handles. Once made, the slab is steel floated and left to start curing overnight. It is covered with cloth and watered regularly and can be lifted after 5 days curing. The diagram below shows the measurements for the rectangular slab.


Measurements of the rectangular “portable” latrine slab




1b. Making a square slab without corner posts.


This type is made if the superstructure is made outside the slab area. The slab is made 1 metre square (or 1m X 0.9m if 0.9m wide chicken wire is used) with a mix of 3 parts sharp river sand and one part cement. It can also be made with a weaker mix of 4 or 5 parts river sand to one of cement but the thickness should be increased to at least 50mm and this will make the slab a little heavier. Some experimentation will be required. A 0.9m X 1m slab can be made with 2 X 5l cement mixed with 3 times the volume of river sand. Half the mixture is added first, the chicken or reinforcing wire is then added and the remaining half of the concrete mix is then added and smoothed down. The handles are set in place and also the squat hole mould which is placed to the rear of the central position. If a pedestal is used, the hole for the pedestal can be made during casting, or if the pedestal is of the concrete type it can be caste into the slab itself. It is wise to allow the slab several days to cure - being kept wet at all times.



Measurements of the square “portable slab”


1c. Making a round slab


If there is a plan to upgrade the “arborloo” to a full Blair latrine using a round slab, it is best to make the slab with both a squat hole and vent pipe hole. Otherwise the slab is made in the same way as described above.







Measurements of round slab









Stage 2. Digging the pit


The pit is located in a suitable place, not only for its use as a latrine but its future location as a site for a tree. The pit is dug so that it measures about 0.8m square and between 1m and 1.5m deep. If a round slab has been made, a round pit can be dug - about 0.9m in diameter. The sides are cut down straight. In relatively firm soil there will be no need for a pit lining since the structure is light. If there is some doubt, make the hole smaller or add a “ring beam” of rocks or bricks cemented together around the head of the pit. The surroundings of the pit should be levelled off before the latrine is moved into position.


Stage 3. Moving the latrine slab into place


Once the pit is dug the latrine slab can be moved over it. If a light structure is attached to the slab, four people should be able to move the latrine unit complete, one taking each of the four handles. If there is a grass roof made, this may need to be separated to reduce the weight - but some testing will be required. The latrine slab or complete latrine, whichever has been made, is then mounted so that it sits squarely over the pit. Soil is then heaped up around the slab and tamped down hard.






Portable slab fitted with pedestal being moved on to a new pit.

Photo. Mutoroshanga at Eco-Ed Trust research site.







Rectangular slab with corner posts being fitted to a pit.

Photo. Mutoroshanga at Eco-Ed Trust research site.





Small men and young ladies can lift the portable latrine

In this case the latrine superstructure has already been made.

Photo. Woodhall Road at Aquamor research site.


Stage 4. Making the superstructure


There are many ways of making the superstructure. If the structure is to be made directly on the slab, the four steel “corner posts” are used to start the construction with 4 posts being erected on them. These posts can be made stout reeds, bamboo or other suitable straight poles cut about 1.8m tall and wired on to the steel posts. Further supporting timbers in reed, bamboo or other materials are then made attached to the 4 posts to add strength. These will be four at the top to join the uppermost parts of the posts.


In its simplest form, reeds or grass can be added between the corner posts supported by wires which join the posts. The aim is to give the interior of the latrine privacy using a light material. Reed mats can also be used. Woven fertilizer bags can also be used, but their appearance is not so good, but they can be used whilst more suitable material is being found. A roof can be added in reeds or grass thatch, although if built in the traditional way the roof will be heavy, and will need to be made so that it can be separated from the main structure before the latrine is moved.


Alternatively the superstructure can be built separate to the slab in poles and grass. This may be the preferred method of construction, although when the time comes to move the site of the latrine, both slab and structure will need to be moved independently. A roof may be added if desired, but this will add weight to the structure and take extra time to build. A considerable amount of innovation can be used to design and make the structure so it is light, strong and attractive in appearance.




The superstructure of the portable latrine can be made in many ways

This one is made of reeds and fertilizer bags.

Photo. Woodhall Road at Aquamor research site.








Tree being planted next to “arborloo.” The pole and grass superstructure shown on the right, which is built outside the slab area offers more room inside the latrine and may be preferred to the version built on the slab. It will require relocation once the portable slab has been moved to another site however. The choice of which type to built is left the owner who has the responsibility to manage and maintain the unit.

Photo. Mutoroshanga at Eco-Ed Trust research site.









This “ArborLoo” slab is fitted with a pedestal

Photo. Mutoroshanga at Eco-Ed Trust research site.



Dealing with rainwater and flooding


The pit used with the “arborloo” is shallow and never more than one metre deep and usually about half a metre deep. This has the advantage that ground water is less likely to penetrate the pit, but the disadvantage that rainwater which accumulates on the slab may drain into the pit and could even fill it up - at least temporarily (the slab can act as a water harvester!). When the pit becomes flooded, either by rain or as a result of a raised water table, the pit walls may collapse if the soil is not firm. The very much moistened pit contents are more subject to fly breeding. However there are a number of precautions that can be taken to overcome or reduce some of these problems.




If the “arborloo” is built with a roofless structure, the rain will fall directly on the slab, and because of its slope - towards the squat hole - rainwater will run into the shallow pit. After a heavy storm, it may even fill it. This should be avoided. The best way is to make a roof over the structure. This will divert rainwater away from the area of the slab, although it may not reduce the problem altogether. 


Another way is to make the latrine slab so that the central squatting area is very slightly raised above the outer slab level, and thus water will tend to drain away from the hole. This also poses some problems however, since it is best from a cleaning and maintenance point of view that slab washing water drains into the pit - also that water flooding off the slab does not undermine the support of the slab around the pit. Some local debate and experimentation will be required.


If a pedestal is used, the part of the slab which supports the pedestal itself can be slightly raised above slab level and thus rainwater will tend to drain to the outside of the slab. The pedestal itself should be covered with a lid or plastic sheet to reduce rainwater entering the pit through the seat area. It should be covered in any case to reduce odours and fly breeding - but this should already be under control as a result of soil and ash additions to the pit contents.


If water is led from the central slab to the rim of the slab, and drains away outside the slab, it is essential that some sort of raised ring beam, cement mortared in position, be built around the pit and the surrounding soil raised to the height of the slab. Grass should be planted in this soil. Nevertheless maintenance will be required to avoid excessive erosion around the latrine which could lead to latrine collapse. Of course when the latrine is moved on - the ring beam should be removed - as the ground is prepared for tree planting - but the cement mortar can be made weak - 10:1 with sand - and can easily be broken up and the bricks or stones reused on the next pit. 




If the area where the latrine is built is liable to flooding, then it is very wise to mount the slab on a “ring beam” i.e. a series of bricks or stones cement mortared together around the rim of the pit on which the slab is placed. These can be one or two bricks high. Soil is heaped up around the bricks - thus elevating the height of the slab and latrine above ground level. Unless the soil is very loose, it will probably be unnecessary to line the pit itself. Obviously in slightly looser soils, the pit can be dug smaller in area, thus allowing for a greater overlap of the slab around the open pit. Local experimentation will always be required - and this should always consider local conditions including flooding and rainfall. Local knowledge of the area should always be used.




Since the basic “arborloo” is not fitted with a vent pipe and a roof, it is essential that both wood ash and soil are added to the pit contents very regularly and ideally after every use. Unless this is done, fly breeding will begin and odours will be the same as a normal pit latrine. It is very desirable to also fit and use the cover made for the squat hole. The cover should be in place at all times, other than when the latrine is being used. A bucket containing soil and ash mixed together (about 2 parts soil to one of ash) should be placed within the latrine so that a few handfuls can be added after each use of the latrine. It is also important to add leaves and even kitchen scraps to the pit to help the composting process which turns the pit contents into humus suitable for planting trees. Trees will grow well in a loose, well-composted soil.


When the pit is about 2/3rd full, a new pit should be dug at a site nearby and the latrine slab and structure lifted up and moved on to the new site. If the slab and structure are separate then they must be moved independently. Four people should be able to move a portable latrine in a few minutes. If the latrine structure is made with poles and grass this will need dismantling and rebuilding.


The old pit contents are then mixed with some well composted soil  and the hole is then “topped up” with a mixture of compost and soil and well watered. A suitable young tree is planted in the centre of the hole and well watered. The most suitable trees for this are paw paw, guava, mango, mulberry  and avocado with the “passion fruit” - granadilla also growing well in such conditions. Citrus trees can also be tried. The cycle is repeated again once the second pit has filled up.


In more densely settled areas pits should be dug deeper so that the latrine/tree sites need to be changed less frequently. Pits can be dug up to about 1.5m deep. However some caution must be taken if the pits are not lined. The firmness of the soil must be established. Also trees have a habit of growing large in time. This must be taken into consideration when spacing the sites used for the successive latrines. The paw paw is a suitable tree for higher density areas and the granadilla will grow well on walls and fences near the latrine site. Avocado and mango do grow into large trees and more space will be required for them. One thing is certain - once trees start to bear fruit - they become very popular and the craving for more trees and more variety is inevitable.


Upgrading the “Arborloo”


The concrete slab which forms part of the “arborloo” can be used as the basis for building more permanent latrines. It is possible to both the square slab and also the round slab as a basis for the construction of a fully functional Blair latrine.


It is possible to add a screened vent pipe to the simple “arborloo,” thus converting it into a very simple VIP latrine. In the case of the round slab, it is advised that this is made with a vent pipe hole already cast into it. A plug is placed in this hole at first, but this can be replaced with a vent pipe later. If the squat hole or pedestal is covered with a lid, the screened pipe will trap flies and also draw out odours when the lid is removed during use. In the case of the square slab a second slab fitted with vent hole should be made and fitted over the pit.


A series of manuals has been produced for the upgrading of the “Arborloo” to a fully functional Blair Latrine, both for the square slab and the round slab. It is possible to start simply and cheaply and over the years upgrade to a permanent Blair latrine, even one of the composting type.





Arborloo” with reed stems and reed mats as a superstructure. The door is a rolled reed mat.

Photo. Woodhall Road at Aquamor research site.







Light weight Blair type latrines are light to lift when made of strong thin walled ferrocement.

These are not fitted with vent pipes but could be upgraded.

Photo. Office site. Mvuramanzi Trust.







An “arborloo” in a plantation of paw paw

Photo. Guruve at research site of Mvuramanzi Trust.









An “arborloo” in a plantation of banana.

Photo. Guruve at research site of Mvuramanzi Trust.











This “arborloo” superstructure is made from a steel frame surrounded by cemented hessian.

Note paw paw tree planted on left on an earlier site of the latrine.

Photo. Porta Farm at research site of Mvuramanzi Trust.







The latrine structure being moved.

Photo. Porta Farm at research site of Mvuramanzi Trust.













The latrine structure is laid down whilst the base slab and pedestal are moved to a new site.

Photo. Porta Farm at research site of Mvuramanzi Trust.






The slab and pedestal have been moved on to a new site. The old pit is exposed.

It will be topped up with soil and grass etc and a tree planted.

Photo. Porta Farm at research site of Mvuramanzi Trust.