The
fossa alterna
“alternating pit”
A method of recycling human waste
Volume II
Peter Morgan
Introduction
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