TAG Technical Note No. 6
United Nations Development Programme
Interregional Project INT/81/047
Executing Agency: World Bank
Ventilated Improved Pit Latrines:
Vent Pipe Design Guidelines
by Beverley A. Ryan (Consultant), and
D. Duncan Mara,
Technology Advisory Group (TAG)
7*A), i q<f 3
aam
A oit United Nations Development Programme
C  %, j   and World Bank Contribution to the International
UNITED NATIONS 1981-1990 Drinking Water Supply and Sanitation Decade


Copyright © 1983
The International Bank for Reconstruction
and Development/THE WORLD BANK
1818 H Street, NW
Washington, DC 20433, USA
All rights reserved
Manufactured in the United States of America
This is a document published informally by the World Bank, as a joint
contribution with the United Nations Development Programme to the
International Drinking Water Supply and Sanitation Decade. The views and
interpretations in this document are those of the authors and should not be
attributed to the United Nations Development Programme or the World Bank, to
their affiliated organizations, or to any individual acting on their behalf.


PREFACE
This Technical Note by Beverley A. Ryan, and D. Duncan Mara sets
out preliminary guidelines for the design of vent pipes for ventilated
improved pit (VIP) latrines. These guidelines are based on extensive
fieldwork, done in Botswana and Zimbabwe in 1981/2, which showed that the
local wind speed and direction were the major factors responsible for
inducing updraught in the vent pipe; thermally induced updraught was found to
be of only minor importance.
The paper is one of a series of informal Technical Notes prepared
by TAG1/ on various aspects of water supply and sanitation programs in
developing countries. The papers were originally prepared as internal
discussion documents; their wider distribution does not imply endorsement by
the sector agencies, government, or donor agencies concerned with the
programs, not by the World Bank or the United Nations Development programme.
Comments and suggestions on the papers should be addressed to the Project
Manager, UNDP Project INT/81/047, Water Supply and Urban Department, the
World Bank, 1818 H Street, NW, Washington, DC, 20433.
Richard N. Middleton
Project Manager
Acknowledgements
The authors would like to express their gratitude to Dr. Peter R.
Morgan (Blair Research Laboratory, Harare) and Mr. James G. Wilson (Ministry
of Local Government and Lands, Gaborone) for their help to Ms. Ryan during
her field work in Zimbabwe and Botswana.
1/ TAG: Technology Advisory Group established under the United Nations
Development Programme Global Project GLO/78/006 (renumbered on January 1,
1982; now UNDP Interregional Project INT/81/047), executed by the World
Bank.


TABLE OF CONTENTS
Page
SUMMARY                  ******......                       ..........        (i)
INTRODUCTION                                                              ... ..0a*a 00aa 0a 000a0a  00  aaaaaaaaa 00aa 0aa 00  1
VENT PIPE DESIGN                                     *      * .*.*.*.*. *00*00 0 0aa00000a000000000 ***............  5
Ventilation rate .........oo*oo** a aa a aa a aa              .....     5
Pipe materials                                      ... ...........aaaaaaaaaaa00  0  0 0  0  0  5
External surface preparation                     .......000000000000009066 ..*009  6
Vent pipe length.          ...................aa   .........a.....      6
Vent pipe diameter                        **********a.......*.....a aaaaaa0  0aaa000  6
Design recommendations a****a*********a *a....                 ...o aa aaa   7
Location  .........                         ..,,.aaaaa.,,,aaaaa,, ,,a,aa,,,aaaaa,aaaaa   9
Fly screen specification                     ....................aa oooaaa*aaa  aa  10
SUPERSTRUCTURE DESIGN                   *******.****  a a aaa  a a..  a aa a  aa.......  10
ALTERNATING TWIN-PIT VIP LATRINES              **********..........a0,00a.......  11
MULTIPLE-PIT VIP LATRINES *******            *******aaaaaaa**********a......  11
FURTHER DATA ON FIELD EXPERIENCES .....00****      **00*****..........        11
ANNEXES
I.  Fabrication of rural vent pipes ...lo*06**************6..           12
II.   Construction details            *********************..........soooosoo  13


SUMMARY
These guidelines are based on extensive fieldwork, &,ie in Botswana
and Zimbabwe in 1981/82, which showed that the local wind speed and direction
were the major factors responsible for inducing updraught in the vent pipe;
thermally induced updraught was found to be of only minor importance.
These studies suggest that satisfactory odor control is achieved
with a ventilation rate of 10 m3/h; minimum vent pipe sizes to achieve this
are 100 mm diameter for AC or PVC pipes, 200 mm diameter for rural vent pipes
made from cement-rendered reeds, bamboos or similar materials, and 180 mm
square for brickwork. For permanent installations, especially in congested
urban areas where latrines may need to be placed very close to living
quarters, or in areas where mean wind speeds mal fall below 0.5 m/s, and
where minimizing cost is not an overriding cons'deration, adoption of a
ventilation rate of 20 m3/h will provide a greater factor of safety;
corresponding vent pipe sizes will be 150 mm for AC or PVC pipes, 200 mm for
rural vent pipes, and 230 mm square for brickwork. In all cases the vent
pipe should extend 500 mm above the highest part of the roof (or, in the case
of conically shaped thatched roofs, to the level of the roof apex); expanded
sections at the top of the vent pipe are unnecessary. Openings in the
latrine superstructure (e.g., doors) should face into the direction of the
prevailing wind in order to maximize ventilation rates. The fly-screen at
the top of the vent pipe should have apertures no greater than
1.2 mm x 1.5 mm in order to prevent the ingress and egress of insects.
Recommendations are given for methods of fixing (a) the vent pipe
to the superstructure and cover slab; and (b) the fly screen to the vent
pipe, and for squat-hole and pedestal seat covers (which do not impede the
air flow) and mosquito traps.


INTRODUCTION
1.        Research by the World Bankl/ has shown that ventilated improved
pit (VIP) latrines can provide most of the health and convenience benefits of
conventional waterborne sewerage but at a fraction of its costs. There are
various designs of VIP latrines (Figures 1-3) but they all differ from
traditional pit latrines in having a vertical vent pipe which has a fly
screen at its top and which leads directly from the pit beneath. The vent
pipe generates a strong updraft and so maintains a flow of air down through
the squatting plate. The effect of this air flow is to minimize odors in the
superstructure and to discourage breeding of insects (flies and mosquitoes)
in the pit. Most flies approaching a latrine will be attracted to the top of
the vent pipe by the fecal odors being discharged there, but the fly screen
prevents them entering the pit. Moreover, provided the superstructure is
kept reasonably dark, any flies that hatch in the pit are attracted to the
daylight at the top of the vent pipe but are prevented from leaving by the
fly screen; they eventually fall back into the pit and die.
2.        Systematic application of the principles of VIP latrines has so far
been limited to a few countries, notably Zimbabwe, Botswana, Ghana, Tanzania
and Lesotho. However, it is clear the VIP latrine (and its variant with
alternating twin pits) is one of the most appropriate sanitation technologies
for a wide variety of conditions in low-income rural and urban communities in
many developing countries.
Ventilation mechanisms
3.        Proper design of the vent pipe is essential for the correct
functioning of VIP latrines. Underdesign will normally cause problems in
odor and insect control, and overdesign will increase costs unnecessarily.
The purpose of this Technical Note is to give preliminary design guidelines
for VIP latrine vent pipes, based on the field experience of members of the
Technology Advisory Group (TAG) operating under the United Nations
Development Programme Interregional Project INT/81/047 executed by the World
Bank. This field experience has included an extensive program of monitoring
the ventilation performance of different types of VIP latrines in Zimbabwe
and Botswana.2/
1/        J.M. Kalbermatten, D.S. Julius and C.G. Gunnerson, "Appropriate
Sanitation Alternatives: A Technical and Economic Appraisal," World Bank
Studies in Water Supply and Sanitation, The Johns Hopkins University Press
(1982).
2/        B.A. Ryan, D.D. Mara and J.A. Fox, "Ventilation Mechanisms in
Ventilated Improved Pit Latrines," Research Report Series, Department of
Civil Engineering, University of Leeds, England (forthcoming). The work in
Botswana was funded by the United Kingdom Science and Engineering Research
Council, and that in Zimbabwe by TAG.


-2-
Fly screen
Flat roof
Vent pipe
Spiral - shaped
superstructure
Reinforced
concrete stab
Brickwork
collar
I-*--- Pit
ig                                           I
I                                            1
Figure 1. Zimbabwean spiral VIP latrine.


ÿþFly screen
PVC vent pipe -
plug over                                                  Pedestal
unuse  pî   -Superstructure
unused pit                                                     unit
concrete blockwork
lining and foundation
Figure 2. Botswanan alternating twin-pit VIP latrine.


-4-
0
o                      ___           _IQIIo
PLAN                                          Fly screen
Superstructure                         --Vent pipe
Squat hole                             Removcible concrete stabs
Pit               Pit lining
SECTION                          ¯¯    ¯    ¯~     :
Figure 3.   Ghanaian alternating multiple-pit VIP latrine for communal or
institutional use.


- 5 -
4.        As a result of this VIP latrine ventilation monitoring program (the
methodology of which is described in a separate Note in this series3/), it
is now clear that the two most important factors governing the ventilation
rate in VIP latrines are the local wind speed and its direction; thermally
induced updraught caused by the absorption of solar radiation by the external
surface of the vent pipe was found to be of relatively minor importance. The
action of the wind as it passes over the top of the vent pipe is to create a
suction pressure within the vent pipe; wind speeds of 2 m/s and above, which
are not uncommom in developing countries, were found to induce air velocities
within the vent pipe ("venting velocities") of at least 0.7 m/s. Venting
velocities were found to increase substantially when the latrine super-
structure opening faced into the wind: the wind is channelled into the
latrine through the openings on the windward side, down through the squat
hole and up the vent pipe; venting velocities of 1.2 m/s or more were
commonly observed.
VENT PIPE DESIGN
Ventilation rate
5.        The vent pipe must be designed so that it can induce sufficient air
flow through the latrine to leave the superstructure odor-free. All of the
latrines studied in Botswana or Zimbabwe were completely free of fecal odors,
although a few of them had a slight odor of ammonia from urine splashed on to
the squatting plate. Odorless conditions were associated with vent pipe air
flow rates of 10 m3/h and above; this corresponded to approximately 6 changes
per hour of the superstructure air volume. This ventilation rate appears, on
the basis of the information currently available, to be a reasonable minimum
requirement in cases where costs must be kept as low as possible (such as in
most rural latrine programs). However, the field studies also show that a
flow rate of 20 m3/h, which would provide a good factor of safety for odor
control, can be relatively easily achieved, and this higher rate would be a
desirable design standard in, for example, urban programs providing permanent
(emptiable) latrines located close to living quarters.4/
Pipe materials
6.        A wide variety of different materials has been successfully used to
form vent pipes: for example, asbestos cement (AC), polyvinyl choloride
(PVC), unplasticized PVC (uPVC), bricks, blockwork, cement-rendered reeds,
cement-rendered hessian supported on steel mesh, and even anthill soil;
bamboo with the cell dividers removed can also be used. (Methods for the
construction of cement rendered reed and hessian pipes and other essentially
3/        Beverley A. Ryan and D. Duncan Mara, "Pit Latrine Ventilation:
Field Investigation Methodology," Technical Note TAG/TN/04.
4/        A ventilation rate of 20 m3/h is also recommended for WC rooms in
the United Kingdom ("Ventilation of Internal Bathrooms and WCs in Dwellings",
Technical Digest No. 170, Building Research Establishment, Watford, England,
1981).


- 6 -
rural vent pipes are described in Annex I). Whatever material is us'i, its
durability (in particular corrosion resistance), availability, cost and ease
of fixing in place are important factors. Thus, vent pipes made, for
example, from thin galvanized steel sheet are not recommended as they are
prone to corrosion, especially in humid areas. PVC pipes become brittle when
exposed to high sunlight intensities, and thus it is better to use PVC pipe
made with a special stabilizer to prevent damage by ultra-violet radiation;
however, this grade of PVC may not be generally available. Cost is particu-
larly important in rural VIP latrines; for example, the use of a PVC pipe,
rather than a cement-rendered reed pi e, more than doubles the cost of a mud
and wattle latrine in rural Zimbabwe. /
External surface preparation
7.        In areas where the mean wiud speed is less than 0.5 m/s6/, the
external surface of the vent pipe should be painted black in order to
increase the absorption of solar radiation and thus the magnitude of the
thermally-induced venting velocity. In areas where the mean wind speed is
above 0.5 m/s the color of the vent pipe is not important.
Vent Pipe Length
8.        The vent pipe should be sufficiently long so that the roof does not
interfere with the action of the wind across the top of the vent pipe. With
flat roofs, the top of the vent pipe should be at least 500 mm higher than
the roof, and in the case of sloping roofs the vent pipe should be 500 mm
above the highest point of the roof. When the roof is made from thatch and
shaped conically, as is common in rural Zimbabwe and Botswana, the vent pipe
(which is generally the cement-rendered reed type) should be at least as high
as the apex of the roof.
9.        It is very important to ensure that the vent pipe is straight and
vertical in order to allow as much light as possible to shine down the pipe
into the pit and so attract any newly emergent flies up the vent pipe.
(Galvanized steel sheet vent pipes with a 900 bend were installed in a humid
coastal area in Africa; not only did this effectively prevent fly control,
but it also induced severe corrosion at the bend due to moisture accumulation
there.)
Vent Pipe Diameter
10.       The internal diameter of the vent pipe depends on the required
venting velocity necessary to achieve the desired ventilation rate, and this
in turn depends on such factors as the internal surface roughness of the pipe
and its length (which determine the friction losses), the head loss through
5/        P.R. Morgan and D.D. Mara, Ventilated Improved Pit Latrines: Recent
Developments in Zimbabwe, World Bank Technical Paper No. 3, Technology
Advisory Group, The World Bank, Washington, DC (1982).
6/        Wind speed data can be obtained from local meterological stations
(or often from the nearest airstrip).


- 7 -
the fly-screen and squat-hole mosquito trap7/, and the wind direction. Thus
cement-rendered reed vent pipes, for example, need to have a much larger
diameter than AC or PVC pipes since their internal roughness is considerably
considerably greater; brick vent pipes, which have a square cross-section,
also need to be larger not only because the roughness is greater but also
because a square cross-section is inherently less efficient than a circular
one in inducing updraft.8!
11.       In Zimbabwe vent pipes have been constructed with an expanded top
section, with the intention of compensating in part for the large head loss
across the fly screen. However, during the field studies it was found that
this feature did not significantly increase venting velocities, and it is
therefore not recommended.
12.       From the results of the latrine ventilation monitoring program in
Zimbabwe and Botswana (Table 1), it is clear that:
(a) the ventilation rate increases as the internal
diameter of the vent pipe increases;
(b) PVC and AC pipes of the same diameter and length
perform equally well; and
(c) the performance of cement-rendered reed and other
rural vent pipes is equal to that achieved by AC or
PVC pipes of approximately half the diameter.
Design Recommendations
13.       On the basis of these field studies, the following minimum vent
pipe sizes are recommended for various applications (see Table 2). These
sizes are probably slightly conservative (that is, result in slight over-
design) and should ensure satisfactory latrine operation under normal
conditions and with normal maintenance by the householder (keeping the
flyscreen clear of debris and ensuring that any cover used does not impede
the air flow down through the seat or squat hole).
7/        Squat-hole mosquito traps have been recommended to prevent the
escape of mosquitoes breeding in pits that extend below the water table
(C.F. Curtis, "Insect traps for pit latrines", Mosquito News, 40, 626-628,
1980). Annex II gives construction details for these traps.
8/        D.R. Wills, E.W.G. Dance and G.T. Blench, "The Design and
Performance of Natural Flue Terminations," Gas Council Research Communication
No. GC61, Institute of Gas Engineers, London (1959).


- 8 -
Table 1. Measured ventilation rates in VIP latrines in Botswana and Zimbabwe
Vent pipe details           Super-       Venti-     Superstructure
structure     lation       air volume
Material    Internal diameter     volume        rate         changes
(mm)             (m3)        (m3/h)        per hour
(a) Botswanan alternating twin-pit VIP latrines
PVC             110              2.5           18             7
PVC             160              2.5           36            14
PVC             200              2.5           49            20
PVC             100              2.5           18             7
(b) Zimbabwean single-pit VIP latrines
PVC             100              1.8           11             6
PVC             150              1.8        18 - 47        10 - 26 a/
AC              150              1.8           18             10 b/
Reed/cement          280              1.8           32             18
Pole/soil            280              1.8           32            18
Hessian/wire
mesh/cement          250              1.8           43            24
Brick                230 square       1.8           36            20
a/ The differences are due to differing wind directions.
b/ The test was carried out under adverse wind conditions.


-9-
Table 2. Vent pipe material and (m) recommended minimum internal size for
various pipe materials
AC or PVC          Brick      Cement-rendered
reed or hessian
Permanent installations,
mean wind speed below
3 m/s (design venting
velocity 20 m3/h)            150 mm dia    230 mm square     250 mm dia
Permanent installations,
mean wind speed above
3 m/s (design venting
velocity 20 m3/h)            100 mm dia    180 mm square     200 mm dia
Rural installations,
minimum-cost urban
installations                100 mm dia    190 mm square     200 mm dia
Location
14.       The latrines should be located away from overhanging branches and
anything else that might impede the action of the wind across the top of the
vent pipe. The vent pipe itself should be located on the windward side9/ of
the superstructure, as also should any openings (doorways, windows, gaps
between roof and the walls). If, however, it is impossible to have both the
vent pipe and any openings on the windward side, at least one of them must be
(and this should preferably be the openings). Minimum opening requirements
are discussed in paragraph 20 below.
15.       In general the vent pipe should be located on the outside of the
superstructure, since it is difficult and expensive to ensure a rain-proof
and wind-tight seal if the vent pipe passes through the roof. Moreover, in
very sheltered areas, thermally-induced ventilation may be more important
than that due to the wind and thus the vent pipe must be placed outside the
superstructure. In urban areas especially, external vent pipes could be
subject to damage by vandals, although no reports of this occurring have yet
been made.
16.       The vent pipe must be rigidly fixed to the superstructure and the
cover slab; design details are discussed in Annex II.
9/        The direction of prevailing wind may be ascertained from the local
wind rose which is normally available from meterological stations.


- 10 -
Fly screen specification
17.       The purpose of the flyscreen is to prevent the passage of flies and
mosquitoes; in order to achieve this, the mesh aperture must not be larger
than 1.2 mm x 1.5 mm (smaller apertures will result in greater reduction in
air flow, due to frictional losses, and so a mesh with apertures as close to
1.2 mm x 1.5 mm as possible should be used). The screen must be made of
material that is corrosion-resistant, since it must withstand strong sun-
light, high temperatur,.s, intense rainfall, and the corrosive environment of
the vented gases. Experience from Zimbabwe suggests that PVC-coated glass
fiber screens are effective for at least five years; the material is
relatively cheap (approximately US$2 per m2), and is suitable where a latrine
will be abandoned within five years or where there are good assurances that
the screens will be renewed as necessary. For more permanent installations,
where cost is not of such great importance, it would normally be better to
instal stainless steel screens, although this may cost five times as much as
the cheaper PVC-coated glass fiber alternative. Other materials, in particu-
lar synthetic fibers such as nylon and polyester, appear to offer attractive
alternatives to PVC-coated glass fiber, but there are no field records on
their use and so they cannot, at this stage, be recommended with confidence.
18.       It is important to ensure that the fly screen is tightly fixed to
the top of the vent pipe in order to prevent access by insects. With AC and
PVC pipe the fly screen can be simply glued with epoxy resin to the pipe end
(which should be filed to remove any sharp edges which might damage the fly
screen); alternative fixing details and those for rural vent pipes are given
in Annex II.
19.       When the fly screen is in place there should be no obstruction to
the wind flow across the top of the vent pipe. In order to obtain maximum
wind shear and hence updraught, it is important that the fly screen presents
a clear, flat face to the wind. Fixing cowls, which reduce wind shear by
inducing turbulence, should not be used.
SUPERSTRUCTURE DESIGN
20.       Openings (doorways, etc.) in the superstructure should be located
on only one side of it, preferably the windward side. It is extremely impor-
tant to avoid openings on opposite sides, as this would significantly reduce
the pressure difference causing updraught in the vent pipe, In latrines
designed with doors the minimum size of ventilation opening(s) should be at
least three times the cross-sectional area of the vent pipe (to allow for
head losses in the superstructure).
21.       Squat-hole cover plates are unnecessary if the superstructure is
kept reasonably dark inside, as it is the case in the spiral latrines used in
Zimbabwe. If, however, the superstructure is not dark it becomes necessary
to shade the squat-hole so the emergent flies are attracted up the vent pipe
rather than to the squat-hole. A suitable cover can be readily made from
plywood and so shaped as to fit between the footrests; strips of wood of
25 mm square cross-section must be screwed to the underside of the
longitudinal edges of the cover in order to permit the free passage of air.


- 11 -
If a seat cover is used with pedestal seats, a small block of wood, 25 mm
thick, should be screwed to its underside at the front for the same reason.
In high groundwater table areas where mosquito breeding in pit latrines
occurs, the use of squat-hole mosquito trap has been proposed by some
researchers (details are given in Annex II); however, there are few field
data yet on the willingness of householders to use these traps, nor their
effect on reducing the ventilation rate.
ALTERNATING TWIN-PIT VIP LATRINES
22.       Twin-pit VIP latrines should be designed with a vent pipe for each
pit and there should be no interconnection between the two pits. The cover
over the squat hole of the pit not in use must provide an effective seal
which prevents air entering the pit. If these precautions are not followed,
the ventilation performance will be severely curtailed due to cross-flows
occurring between the two pits.
MULTIPLE-PIT VIP LATRINES
23.       With multiple-pit VIP latrines, which are used as communal or
institutional sanitation facilities, it is very important that the precau-
tions given in paragraph 22 are followed. If the multiple-pit latrine is
designed as a series of alternating twin-pit latrines, with each pit (other
than the two end pits) serving two cubicles (Figure 3), the vent pipe must be
large enough to provide adequate ventilation of both cubicles; experience in
Ghana suggests that a 200 mm internal diameter pipe is necessary.
FURTHER DATA ON FIELD EXPERIENCES
24.       The feature which distinguishes the VIP from older unsatisfactory
designs of pit latrines is the large diameter vent pipe fitted with an insect
screen. This addition makes the latrine hygienic and readily acceptable, but
does add significantly to the cost or effort involved in construction,
especially in the case of rural latrines. It is therefore important to
refine the design recommendations in this Technical Note in the light of
further field experience. A companion Technical Note (TAG/TN/04) sets out
procedures for formal monitoring of ventilation performance, but TAG would
also be most interested in receiving formal or informal reports on VIP
programs. Data either on actual ventilation performance (expressed in air
flow or changes of air per hour) or on satisfactory insect and odor control
(as determined by the users) should be related to factors such as the
location of the latrines relative to other buildings, trees, etc.; their
orientation and design; the arrangement, diameter and construction material
of the vent pipe; daily or seasonal variations in wind patterns; and user
behavior (such as keeping the flyscreen clear of leaves and cobwebs or
replacing mosquito traps). This Technical Note and others in the series will
be updated periodically to ensure that the most recent field data is
incorporated.


TAG/TN/06                           - 12 -                  ANNEX I
Page 1
FABRICATION OF RURAL VENT PIPES
(a)   Cement-rendered reed vent pipes
Local reeds, approximately 1 cm diameter, are tied together with
wire or string to form a mat measuring 2.5 m by 1 m. The mat is then rolled
around four or five rings of green saplings to form a tube of some 300 mm
external diameter. The fly screen is then fixed to one end (Annex II).
Cement mortar (1 part cement, 3 parts sand) is applied to the tube along its
entire length but only around half its circumference; when this had hardened,
the vent pipe is fixed in position (Annex II) and the other (outer) side then
plastered with cement mortar. Thin poles or bamboo sticks may be used
instead of reeds. Larger bamboo sticks, split longitudinally into 10-20 mm
wide strips, may also be used.
(b)   Cement-rendered hessian wire mesh vent pipes
Spot-welded mild steel mesh (4 mm bars at 100 mm centres), 2.5 m
long and 0.8 m wide, is rolled into a tube to give an internal diameter of
approximately 250 mm. Hessian or jute fabric is then tightly stitched around
the outside of the tube, and the fly screen fixed to one end by stitching
with string or thin galvanized wire. Cement mortar (1 part cement, 2 parts
sand) is then applied by brush to the hessian surface in thin layers, to a
final thickness of at least 10 mm. The vent pipe is then fixed in place
(Annex II).
(c)   Anthill soil vent pipes
Well-kneaded anthill soil is rolled into "sausages," approximately
100 mm in diameter and 900 mm long, which are made into circles of
approximately 280 mm internal diameter. The vent pipe is constructed in situ
from these circles; vertical reinforcement with short lengths of reed or thin
bamboo (or other suitable material) can be driven in between adjacent circles
as construction proceeds. When the vent pipe has been built to a height of
2.5 m, its external surface is smoothed off by adding more soil; the fly
screen is attached to the upper end (Annex II) and then a thin coat of cement
mortar (1 part cement, 6 parts sand) applied.


TAG/TN/06                     - 13 -                       ANNEX II
Page 1
CONSTRUCTION DETAILS
This Annex gives construction details for:
(a) fixing the vent pipe to the superstructure and cover slab
(Drawing No. VP/01);
(b) fixing the fly screen to the top of the vent pipe
(Drawing No. VP/02); and
(c) squat-hole mosquito traps (Drawing No. VP/03).1/
1/ The mosquito trap design is based on that given by C.F. Curtis and P.M.
Hawkins, "Entomological studies of on-site sanitation systems in Botswana
and Tanzania", Transactions of the Royal Society of Tropical Medicine and
Hygiene, 76 (1), 99-108, 1982.


500 minL
Either: standard
pipe  fitting
or as Detail A
Detail B
General arrangement
Sockep bend cud
built into wall
Detail A
Method    of fixing vent pipe to superstructure
S   Socket end cut                       Cement mortar
off and cast into                    filler
concrete slab
________UNDPtt                                                  Interregional P5roject INT/ 81/047  (m
PVC pipe roughened with
solvent cement and sand                                                               VIP VENT     PIPE   DESIGN
Detail B
AMternative     methods of fixing vent pipe to ssub Vent pipe fixing details
nDimensions in mm                        Org. No. VP 01


Glass fiber screen with                                                        Glass fiber screen with
1.5 x 1.5 mm openings                                                          1.5 x1.5 mm openings
Pipe end sanded to
X:XXXXXXXXXXXX X                                    XXXXXXXXXXXXXXXXX
X                                                    X                X
X   X                                   X                X
X       X                                            X                X
X       X                                            X                XN
75     x                         PVC collar to fit   100
I        over pipe           10w
..       X                                            X                X
X                x
x                x       Galvanized binding
PVC or AC vent pipe                                  wire (min.1mm dia.)
or Jubilee clip
or Nylon tie
Alternative methods of fixing fly screen to AC and PVC vent pipes
Glass fiber screen with
1.5 x1.5mm openings
X XXXXXXXXXXXXXXXXXXXXXXX<X
Fly screen fixed with
X                          X
SX       galvanized wire
x                           x
150                                X
X                          X
x                          x        Cement mortar
X                          X
X                          X
X                          X
X                          X
X X.
Reed or split bamboo
framework                      UNDP Interregional Project INTI811047
VIP VENT PIPE DESIGN
Method of fixing fly screen to a'rural' vent pipe                           Fly screen fixing details
Dimensions in mm IDrg. No. VPI 02


TAG/TN/06                               -16-                     ANNEX II
Page 4
x x xx x xx xx x
lo
Glass fiber fly screen
lmm sheet meta(                    -     - 5paint container
base plute
d
Section through mosquito trap
base plate of lmm
sheet metal cut
to suit container
and latrine
Mosquito trap in position
over squat-hole                                       rim of container tid
fixed to base plate
10             10
Mosquito trap with container
10                       .removed for emptying
i/
UNDP Interregional Project INTI81/047
Pattern for cutting
fly-screen    material                        Squat--hole mosquito trap
SÃœnsensinnmm                                          Drg.N.VP/O03