Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.1.1 Pit latrine
A pit latrine collects excreta in a pit dug in the ground
beneath the toilet structure. If the soil is loose the pit needs to be lined
with, for example, loose bricks to prevent the wall from collapsing. During
storage in the pit decomposition of the organic substances takes place under
anaerobic conditions. As described in Section 2 (2.3) the anaerobic
decomposition releases gases (carbon dioxide and methane) and reduces the volume
Seepage of water into the surrounding soil takes place through
the sides and bottom of the pit. During seepage further decomposition of organic
matter by soil bacteria takes place reducing the BOD of the water. There will
also be die-off of bacteria and viruses during storage and as the water
percolates through the soil. Nutrients are generally not removed by bacteria
under these conditions, so pollution of groundwater will occur.
Control of odour and insects are important with a pit latrine.
This is achieved by having a vented pit (Figure 2.15). The vent acts to draw
odour and insects into the pit and up the vent. Gases (methane and carbon
dioxide) produced by the decomposition of the excreta also leave through the
vent. Natural convection can be relied upon by ensuring that the vent protrudes
well above the roof of the housing. Facing the vent towards the sun (southward
in the Northern hemisphere and northward in the Southern hemisphere) and
painting the vent black to maximise absorption of heat from the sun will help
venting by heat convection. The heated air in the vent rises and draws air from
the toilet. Ventilated improved pit (VIP) toilets are widely used in Africa (see
Regional Overview for Africa).
Pit latrines pose problems when groundwater is shallow and the
pit is in groundwater or close to it. There is no soil barrier to protect the
water quality of the groundwater, and mosquitoes may breed inside the pit. A pit
is also difficult to dig when the ground is rocky. Pit latrines should not be
used in these cases.
The pit will eventually fill with faecal sludge and needs to be
emptied. The period between emptying depends on the size of the pit and its
usage. It is desirable to design the pit to store at least one year of sludge
production. Emptying requires mechanical suction of the sludge. The sludge
requires treatment prior to re-use or disposal (see Section 2 (5)). Two
adjoining pits can be used alternately. Further decomposition of sludge in a
full pit takes place while the adjacent pit is in use. Its content after further
decomposition can be manually removed.
An alternative way of dealing with a full pit is to dig another
pit and relocate the sanitary platform and toilet housing to the new pit. The
full old pit can then be covered with soil, preferably of greater than 15 cm
depth to prevent disease vectors (rodents and insects) from burrowing into it.