Sanitation and Vector Control - Biological and Environmental Control of Disease Vectors - page 152

the superstructure and squat hole, which, if
designed well, should remain smell free. On top
of the ventilation pipe, a fl y screen should be
fi xed to prevent fl y and mosquito entry and exit.
Fly control in a VIP latrine works in two
ways. Flies attracted to the faecal smells coming
from the ventilation pipe will be unable to enter
because of the fl y screen, while fl ies emerging
from the drop hole and attracted by the bright
light coming from the ventilation pipe will
instinctively fl y towards the pipe, where they will
be trapped by the fl y screen and eventually die.
Unlike fl ies, emergent mosquitoes are not
attracted by light so that many will escape
through the squat hole; therefore additional
control methods are needed. These can include
the application of polystyrene beads, kerosene,
chemical larvicides and used engine oil to the
water within the pit, or the placement of a
mosquito trap or lid on the squat hole, and will
be discussed below.
The VIP has been shown to be ef ective in
controlling fl y populations; a study in Zimbabwe
retrieved only 146 fl ies escaping from a VIP
latrine over a 78 day period in contrast to almost
14,000 from an similar but unvented pit latrine
(Morgan, 1976). In a survey in Dar es Salaam,
Tanzania, 130 out of 192 latrines were found to
have a 'wet' pit, and 20% of these 'wet' pits were
positive for both fl ies and mosquitoes. However,
when a distinction was made between those
with a screened vent pipe and those without, the
VIP latrines with a screen were found to be free
of mosquito larvae (Curtis and Hawkins, 1982).
A well-designed VIP latrine should have its
door and openings facing the prevailing winds
and should maintain an air gap between the top
of the door and the doorframe roughly three
times the cross-sectional area of the vent pipe in
order to maintain ventilation within the pit
latrine (Mara, 1984). The ventilation pipe can be
made from a wide-ranging set of materials,
which include PVC, steel, bricks, anthill soil,
bamboo (with cell dividers removed), and
cement-rendered reeds or other natural
material. The length of the ventilation pipe is
critical and the pipe should be sui ciently long so
that the roof will not impede the fl ow of air over
the ventilation pipe. The ventilation pipe should
at least be 50 cm above the highest point of the
roof (Mara, 1984). Durability of the ventilation
pipe is key as it will be exposed to ultraviolet (UV)

light, which brittles PVC over time, and to
corrosive fumes (sulphides) from the pit's
decomposition processes. The diameter of the
ventilation pipe will depend on the air velocity
needed to achieve the suggested 20 m 3 day −1
ventilation rate (Mara, 1984). The internal
roughness and length of a ventilation pipe,
together with the fl y screen, will reduce the air
velocity and, as a result, a larger diameter pipe
might be needed. Mara (1984) in his VIP design
manual recommends a 150 mm diameter for
PVC pipes and a 230 mm diameter for bricks and
cement-rendered local material pipes. In more
exposed (rural) areas with wind velocities over 3
m s −1 , the diameter of PVC pipes can be reduced
to 100 mm and for brick and cement-rendered
local material pipes to 200 mm. The fl y screen
mesh aperture must not be larger than 1.2 mm
by 1.5 mm. A smaller size is not recommended
as it will obstruct the fl ow of air through the
ventilation pipe. Although it is tempting to use
bed net material for the mosquito screen on top
of the ventilation pipe, it is imperative that the
screen is made from a durable material that is
non-corrosive and can survive prolonged
exposure to UV light, rain, birds and lizards. PVC
glass fi bre screens and stainless steel are
recommended (Mara, 1984).
One of the drawbacks of a VIP latrine is
that it must have a close-fi tting door, but at the
same time it needs to allow space for ventilation,
as otherwise, bright light will come in through
the openings, of ering emergent fl ies and
mosquitoes an easy escape through the squat
hole. The requirement of a door almost always
means that a VIP latrine will require a brick or
wooden superstructure. This, together with the
cost of a roof, ventilation pipe and fl y screen,
often increases the costs of a well-designed and
functioning VIP latrine to beyond the means of
many of the urban and rural poor populations
(Cairncross and Valdmanis, 2006).

Pour fl ush latrines
The next step up on the sanitation ladder is the
pour fl ush latrine. In contrast to traditional and
VIP latrines, excreta in pour fl ush latrines are
not deposited straight down into a pit but fl ushed
away with water to a tank or pit that is in most
cases not placed under the squatting pan (Fig.
8.5). The squatting pan of a pour fl ush latrine is