Attachment 5 - VIP Latrine Design Guides.pdf

TAG Technical Note No. 13

United Nations Development Programme Interregional Project INT/81/047 Executing Agency: World Bank

The Design of Ventilated Improved Pit Latrines by D. Duncan Mara, Technology Advisory Group (TAG) z ,- r '- 3'

UNITiD NATIONS 196. II90

- ' -1F , . ,. - 3 ^ =. A.

VI',dSa A joint United Nations Development Programme ,w, and World Bank Contribution to the International

UNIEDNAIOS 911 99 Drinking Water Supply and Sanitation Decade

Copyright © 1984 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

(i)

PREFACE

This Technical Note by D. Duncan Mara develops a general apRroach to the design of ventilated improved pit (VIP) latrines, based on TAG's_ recent experience in Botswana, Brazil, Ghana, Kenya, Lesotho, Tanzania and Zimbabwe. Further details of country-specific designs (currently for Botswana, Tanzania and Zimbabwe) are given in other Technical Notes in this series.

This paper is one of a series of informal Technical Notes prepared by TAG on various aspects of water supply and sanitation programs in developing countries. The initial emphasis of TAG was on the promotion of policy shifts from high-cost to low-cost on-site sanitation technologies. This emphasis is now being progressively directed to a focus on institutional development for on-site low-cost sanitation program delivery.

This note was originally prepared as an internal discussion document. Its wide distribution does not imply endorsement by the sector agencies, government, or donor agencies concerned with programs, nor by the World Bank or the United Nations Development Programme.

TAG is interested in receiving comments and suggestions on the paper, and, in particular, information on costs of technology, delivery and support systems, and generally, information on experience in program implementation.

All communication should be addressed to the Project Manager, UNDP Project INT/81/047, Water Supply and Urban Development Department, The World Bank, 1818 H. Street, NW. Washington, DC 20433.

Richard N. Middleton Project Manager

*/ TAG: Technology Advisory Group established under the United Nations Development Programme Global Project GL0/78/006 (renumbered on January 1, 1982; now UNDP Interregional Project INT/81/047: "Development and Implementation of Low-cost Sanitation Investment Projects"), executed by the World Bank.

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TABLE OF CONTENTS

Page

PREFACE ................................... **................. (i)

INTRODUCTION ..... o

VENTILATED IMPROVED PIT LATRINES .................................. I

General description

Odor control

Insect control

Component parts

SINGLE-PIT VIP LATRINES

Pit functions and design

Cover slab and foundation design ......... 0

Superstructure design

Vent pipe design

Fabrication of rural vent pipes

Relocation and emptying of pits

Double and multicompartment designs

Designs with soakaways

ALTERNATING TWIN-PIT VIP LATRINES ..... o

Pit function and design

Cover slab design

Superstructure and vent pipe design ....... o .............. o

Multicompartment units

Emptying of pits ......... ........... o

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APPLICABILITY AND CONSTRAINTS

Housing density

Water supply service level

Ground conditions ...... ............. *

Groundwater pollution

Sociocultural factors

DESIGN SELECTION CRITERIA

Favourable ground conditions

Adverse ground conditions ......... .*

Design examples

COSTS .......................................... 51

ANNEXES:

I. Soil stability criteria o . ......................... .. *. 55

II. Case studies ......... o

A. Zimbabwe ........... 0

B. Botswana

C. Tanzania

D. Ghana

E. Brazil .. *

III. Cost of Alternative Components for Rural Brick VIP

Latrines in Zimbabwe

INTRODUCTION

1. Excreta-related diseases are responsible for a large proportion of the morbidity and mortality in developing countries, especially amongst low-income communities in urban fringe and rural areas, where adequate water supplies and sanitation facilities are typically absent. Excreta control is thus of paramount importance if the incidence of these diseases is to be reduced. Recent research sponsored by the World Bdnk- has clearly shown that excreta-related diseases can be greatly reduced by:

(a) the provisions of sanitary toilets, of whatever type, which people of all ages will use and keep clean;

(b) the effective treatment of excreta or sewage prior to discharge or reuse;

(c) the provision of an adequate water supply so that water consumption is in the region of 30 to 50 liters per capita per day, which is normally the minimum requirement for the control of those excreta-related infections which have a water-washed mode of transmission; and

(d) an effective and sustained program of personal hygiene education by the responsible local authority.

2. Economic and financial constraints dictate that the water supply and sanitation technologies to be used for the control of excreta-related diseases in low-income communities must be affordable by these communities;

these technologies must therefore have low capital and operating costs. In rural areas and urban areas up to a population density of around 300 persons per hectare, the least-cost technically feasible sanitation technology will often be the ventilated improved pit (VIP) latrine (Figure 1). It is the purpose of this Technical Note to discuss general design criteria for VIP latrines and to review recent developments in VIP latrine design.

VENTILATED IMPROVED PIT LATRINES

General Description

3. Traditional (unventilated) pit latrines are a very common sanitation facility in many developing countries. They have however two serious disadvantages: generally they have a bad smell, as well as substantial numbers of flies and other disease-carrying insects breeding in them. Additionally, they are all too often poorly constructed, with the result that pit collapses are common. These disadvantages are substantially reduced in VIP latrines, which have been found to be socially very well accepted in those countries where they have been installed.

I/ R.G. Feachem, D.J. Bradley, H. Garelick and D.D. Mara (1983). Sanitation and Disease: Health Aspects of Excreta and Wastewater Management.

Chichester: John Wiley.

odors flies flyscreen thatch ~~~~~~roof

PVC brick vent spiral pipe g superstructure air -i currents lined

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Scemti diaramor entsIilatedimrvdptlrne

- 3 -

4. A VIP latrine differs from a traditional pit latrine in that it has a tall vertical vent pipe which has a flyscreen fitted at its top. The vent pipe is responsible for both odor and fly control, as explained in paragraphs 6 and 7 below.

5. There are two basic types of VIP latrine: the single-pit latrine and one with two pits, known as the alternating VIP latrine. The latter (described in paragraphs 45-52) is designed for the removal of the pit contents at regular intervals of two to three years; it is a permanent sanitation facility which is especially suitable for use in medium density urban areas.

Odor control

6. Fieldwork recently done in Botswana and Zimbabwe2/ has shown that the principal mechanism inducing ventilation in VIP latrines is the action of the wind blowing across the top of the vent pipe. The wind effectively sucks air out of the vent pipe and this air is replaced from the atmosphere via the latrine superstructure and squat-hole. Consequently there is a strong circulation of air from outside the latrine, through the superstructure and squat-hole, and up and out of the vent pipe. Thus, any odors emanating from the fecal material in the pit are exhausted via the vent pipe, and not via3 he squat-hole into the superstructure which, as a result, remains odor-free- . If the superstructure openings (doorways, etc.) face into the prevailing wind, the resulting increased air pressure within the superstructure increases the flow of air up the vent pipe and thus also helps to control odors in the latrine; the latrine should therefore be designed so that any openings face into the prevailing wind. Recommended vent pipe dimensions are discussed in paragraphs 27-38.

Insect control

7. Flies. The vent pipe controls flies in VIP latrines in two ways. Firstly, since flies are attracted to pit latrines by the fecal odors coming from them, almost all flies will try to enter the pit via the top of the vent pipe as that is the point from which the odors emerge; but they are prevented from entering by the flyscreen. Secondly, although a few flies may enter the pit via the superstructure and squat-hole and lay their eggs in the

2/ B.A. Ryan and D.D. Mara, Pit Latrine Ventilation: Field Investigation Methodology, TAG Technical Note No. 4; and Ventilated Improved Pit Latrines: Vent Pipe Design Guidelines, TAG Technical Note No. 6.

3/ An earlier explanation for the cause of ventilation was that the vent pipe absorbed heat from the sun and transferred some of this energy to the air inside the vent pipe, which consequently became less dense than the outside air immediately above it; it therefore rose out of the vent pipe and was replaced by air below, so establishing the air circulation pattern described above. The fieldwork done in Botswana and Zimbabwe showed that the shearing action of the wind and its direction relative to any openings (doorways, etc.) in the superstructure were much more important than the absorption of solar energy, except under very low wind conditions (see paragraphs 26 - 38).

-4-pit, the newly-emergent adult flies eventually resulting from these eggs fly instinctively in the direction of the brightest light; provided that the superstructure is reasonably well shaded, the brightest light seen by the flies is that at the top of the vent pipe: the flies thus fly up the vent pipe but their escape is prevented by the flyscreens. Fly control is very effective: in a 78-day monitoring period in Zimbabwe, only 146 flies were caught escaping from a VIP latrine, whereas 13, V 3 were caught from an unvented, but otherwise identical, pit latrine.-

8. Thus the screened vent pipe has three important roles in the successful performance of VIP latrines:

(a) it eliminates fecal odors in the superstructure;

(b) it prevents most flies from entering the pit; and

(c) it prevents those flies bred in the pit from escaping.

9. Mosquitoes. Culicine mosquitoes, which are the major nuisance

(biting) mosquitoes in the urban tropics and in many countries also the vector of Bancroftian filariasis, breed in wet pit latrines--that is, pits which extend below the groundwater table. Newly-emergent mosquitoes are not so attracted to light as are flies, and therefore not all of them will try to escape via the vent pipe: mang will leave via the squat-hole, even if the superstructure is well shaded.- Several substances which kill mosquito larvae can be added to the pit; for example, kerosene, used engine oil or chemical larvicides. An alternative is to place a mosquito trap (Figure 2) over the squat-hole. This 4s been found to be very effective in field trials in Dar es Salaam (Tanzania)-a; householders were keen to use them once they saw how many mosquitoes were being caught in the traps and they noticed as a result far fewer mosquitoes in their houses.

Component parts

10. Both single-pit and alternating twin-pit VIP latrines consist of the same basic component parts (Figure 1: see page 2):

4/ P.R. Morgan (1976). The pit latrine - revived. Central African Journal of Medicine, 23, 1-4.

5/ C.F. Curtis and P.M. Hawkins (1982). 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.

6/ C.F. Curtis (1981). Insect Traps for Pit Latrines. Mosquito News, 40

(4), 626-628.

-XXXXXXXXXXXX-

4t +. / Glass fiber fly screen

1mm sheet metal S t . 51 paint container base plate

Section through mosquito trap base plate of 1mm sheet metal cut to suit container and latrine

Mosquito trap in position over squat-hole rim of container lid fixed to base plate 10 10

- XrMosquito trap with container 10 EU ' I removed for emptying

Pattern for cutting fly-screen material

Figure 2. Squat-hole mosquito trap (6/).

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(a) the pit;

(b) the cover slab and its foundation;

(c) the superstructure; and

(d) the screened vent pipe.

There are minor design differences between the components for each type of VIP latrine, but the basic principles remain the same. The component parts and their design requirements are first described for single-pit VIP latrines;

specific differences for alternating twin-pit latrines are described in paragraphs 45-52.

SINGLE-PIT VIP LATRINES

Pit functions and design

11. Excreta are deposited directly into the pit, which has two essential functions:

(a) the liquid fraction of the excreta (mainly urine), together with the small amount of water that enters the pit from cleaning the cover slab, infiltrates into the surrounding soil; this may give rise to problems in soils which are not sufficiently permeable or which become unstable when saturated, and may also cause groundwater pollution (see paragraphs 15, 16 and 58); and

(b) the fecal solids in the excreta are digested anaerobically by bacterial activity - this results in the production of (i) gases such as methane, carbon dioxide and hydrogen sulphide which are exhausted from the pit via the vent pipe; and (ii) soluble compounds which are either further oxidized in the pit or are carried into the surrounding soil by the infiltrating liquid fraction.

12. The anaerobic digestion of the fecal solids, which proceeds rapidly at tropical temperatures, does not however remove all of the solids. Some compounds are biodegraded only very slowly; as a result there is a gradual accumulation of solids in the pit, although the rate of solids accumulation is much smaller than the rate of excreta addition. In dry pits (those that do not extend below the grqundwater table) the solids accumulation rate varies between 0.03 and 0.06 m per 7yerson per year, and in wet pits between 0.02 and

0.04 m per person per year.- Accumulation rates are lower in wet pits because biodegradation is faster under wet conditions than under the only just moist conditions in dry pits.

13. Volume. The required pit volume depends on the solids accumulation rate, the number of users and the desired life of the pit. In practice the

7/ Very little information exists on solids accumulation rates in pit latrines. The effects of climate and diet are largely unknown, although it is known that the addition of sullage to dry pits (e.g., by using the latrine for "bucket showers") reduces the rate of solids accumulation.

TAG is currently working on "pit biokinetics" and the results of this research will be published as a separate Technical Note in this series.

- 7 -pit must not be allowed to fill up completely (right to the underside of the cover slab), so a small free space at the top of the pit must be allowed for in the design; usually 0.5 m is sufficient for this. The effective pit volume m , which is the total volume less the free space volume, is calculated as the product:

solids accumulation number design rate, X of X life, mi3/person/year users years

The solids accumulation rate may for design purposes be taken as 0.04 and 0.06 m per person per year in wet and dry pits respectively; a lower value may be used if known to be locally more appropriate. These design values should be increased by 50% if bulky anal cleansing materials (for example, corn cobs, cement bags) are used, as these degrade only very slowly. The design life should be as long as possible; 10 years should be considered desirable. The longer the design life, the longer the interval between relocating or emptying the latrine, and so8ohe cost of the latrine (when calculated in annual terms) is generally lower.-

14.2 Dimensions. Usually the pit cross-sectional area is not more than 2 m in order to avoid cover slabs with large spans. In practice VIP latrines serving one household commonly have a diameter of 1-1.5 m or, in the case of square or rectangular pits, a width of 1-1.5 m; communal or institutional latrines can of course be much larger. The pit depth is then calculated from its required effective volume, and the tota4/ depth is this depth plus the desired free space which is normally 0.5 m.-

15. Soil perneability. The hydraulic loading on pit latrines is very low (less than 2 litres of excreta are added per person per day), so soils of lower permeability than would be considered for the disposal of septic tank effluent are still suitable for pit latrines. Soils with permeabilities as low as 2.5 mm per hour (such as clays and silty clays) are acceptable, provided expansive clays are not present.

16. Soil stability and pit lining. For the purposes of pit design soils can be considered as either stable or unstable. Stability is defined

8/ This assumes that the cost of extra depth in the pit is less than the present value of more frequent emptying or relocation. This may not always be true, especially in very deep pits in difficult soils.

9/ For example, for a circular pit with an effective depth of i. 5 m which serves six people with a solids accumulation rate of 0.06 m /person year, the pit life for various diameters can be calculated as follows:

Diameter Cross-secnional Effective volume Effective life

(m) area (m ) (m ) (years)

1.0 0.79 1.98 5.5

1.2 1.13 2.83 7.9

1.5 1.77 4.43 12.3

- 8 -as resistance to collapse, and should be assessed as described in Annex I.

Local ex87rience of pit latrines is useful: if pit collapses due to soil failure- have occurred, the soil should be considered unstable and lined as described in paragraph 18.

17. As a precautionary measure stable soils should be protected against possible failure (caused by, for example, gradual release of pore water pressure) by plastering the soil face with a 1 cm thick layer of cement mortar

(1 part cement, 5 parts sand). This is clearly only feasible when the groundwater table is below the pit base, and so suitable only for permanently dry or only seasonally wet pits. Linings for permanently wet pits are discussed in paragraph 19.

18. Pits in unstable soils must be fully lined, otherwise there is the risk--all too often realized--that the pit will collapse and the superstructure may fall into it. A wide variety of materials can be used to line the pit; for example, concrete blocks, bricks, cement-stabilized soil blocks, masonry, stone rubble, perforated oil drums, rot-resistant timber and wire-mesh-supported geofabrics [Figure 3 (a) through (f)]; local availability normally determines what material is used. Where blocks, bricks, masonry or stones are used, the lining joints should be fully mortared in the top half-metre of the pit; below this, the vertical joints should be left unmortared to allow the liquid fraction of the excreta to infiltrate into the soil. If the surrounding soil is very fine sand, for example, which would enter the pit through the open vertical joints, a thin (say, 100 mm) packing of fine gravel should be placed between the soil and the lining to prevent this.

19. It is difficult to excavate and line pits in areas with a permanently high groundwater table. If petrol or diesel driven portable pumps are available, the groundwater can be removed and short lengths of concrete pipe inserted as excavation proceeds; this "mini-caisson" approach is the most satisfactory, provided that the concrete pipes are made with sufficient holes for infiltration. Perforated oil drums coated with bitumfjlic paint are an alternative, but corrosion is a problem in the long term._

Cover slab and foundation

20. The cover slab and its foundation serve to isolate the pit from the atmosphere (to prevent the escape of flies and odors) and to support the superstructure and vent pipe. The foundation is generally a simple

10/ Pit collapse may, however, be due to poor engineering design: for example, the omission of a grassed slope or bund to carry away stormwater which is then able to erode the soil at the top of the pit; or by building too heavy a superstructure on inadequate foundations. It may also be due to poor "social design" of the latrine, where for example no allowance is made for local customs such as taking "bucket showers" in the latrine.

11/ Oil drums have been used in Zambia as aqua-privy tanks and have lasted approximately 10 years.

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- rsen

;t~~ ~ -.' , - " i , rTRWr y~~~~~~~~~~~~

N

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Figure 3 (a). Pit lining in open-joint brickwork (Zimbabwe)

$ {o .; ,- 4 o' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 2 a k -- 1,' 1 i . d~~~~j '"- -t

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£ .v sw.u 4.. q -

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Figure 3 (d). Pit lined with perforated oil drum (Kenya).

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Figure 3 (e. Ptlndwthmnrv oe AKny)

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Figure 3 (f). Pit lined with wire-supported geofabric

- (Botswana).

- 15 -ring beam of reinforced concrete or, more commonly and less expensively, bricks set in cement mortar; a single course of bricks laid on the ground surface with their inner edge flush with the pit wall is sufficient (concrete ring beams are of similar dimensions). Setting the base of the foundation on the ground surface enables a gentle grass slope or cement-stabilized soil to be made to carry away stormwater which might otherwise erode the upper part of the pit wall, so endangering the structural stability of the latrine.

21. The cover slab must be strong enough to support the weight of the superstructure, vent pipe and user. It should also feel safe and should not deflect detectably when the latrine is being used. The cover slab, which should be flush with the outer edge of the foundation, can be made from reinforced concrete (Figure 4) or from rot-resistant timber (Figure 5) which is covered with soil and then mortared. The cover slab has two holes in it:

the squat-hole and one for the vent pipe. The size of the squat-hole is important: it should not be large enough for a child to fall through; key-shaped or pear-shaped openings (Figure 6) with a maximum width of 200 mm are generally used. If the locally preferred defecation posture is sitting, rather than squatting, a simple pedestal seat can be provided (Figure 7).

22. Where the preferred posture is squatting, it is important that the surface of the cover slab should slope towards the squat-hole in order to provide drainage for urine and the water used to clean the cover slab. The recommended slope is 5%. Although the slab could be cast with this surface slope, it is usually simpler to plaster the slab to give the required slope, and also to provide a smooth finish, after the superstructure has been built. It is also worth considering whether foot-rests are to be provided;

although not strictly necessary, they may be a social requirement and they have the advantage that, provided they are in the correct position themselves (Figure 6), they help to locate the user directly over the squat-hole and so minimize fouling of the cover slab with excreta. A glass-fiber reinforced plastic cover, with integral squat-hole and foot rests, which is set in cement on the coverslab within the superstructure is shown in Figure 8.

23. It is important that the squat-hole is not kept covered when the latrine is not in use. Squat-hole (or pedestal seat) covers interfere with the essential circulation of air which is responsible for fly and odor control (paragraphs 6 and 7). Traditional (unventilated) pit latrines often rely on squat-hole covers to control fly breeding, but they are not only unnecessary in the case of VIP latrines but also positively detrimental to their proper operation 2 This is an important point to stress in user education programs. -

12/ If covers or seat lids are required for sociocultural or aesthetic reasons, then they must be raised clear of the slab or seat so that an air space of at least 25 mm is left when the cover or lid is in the "closed" position. Further details are given in the references referred to in footnote 2.

J: -

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V~~~~I t'4 ISO~~~~~~~~~~~~~~~~~'1 r~ ~ ~ :

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Figure 4. Reinforced concrete cover slab (Tanzania).

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I~~~~~~~~~~~~~~~~~~~- -n* vo 4.1~~~~~~~~~

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Figure 5. Timber cover slab (Zimbabwe).

- 18 -

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-- E20 20~~~~~~~0

Figure . Squathole gemetries

Figure 6. Squat-hole geometries.

- 19 -

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Figure 7. Glagsfiberreinforced plastic covet fo covr sabforVI larins Kena)

- 20 -

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Figure 8. Glass-f iber-reinforced plastic pedestal seat for VIP latrine (Brazil).

- 21 -

Superstructure design

24. The function of the superstructure of any type of latrine is to provide the user with privacy, comfort and protection from the elements. There are two additional functions in the case of VIP latrines: (a) to provide sufficient shade over the squat-hole so that newly emergent flies are not attracted to leave the pit via the squat-hole; and (b) to channel air through the squat-hole and up the vent pipe, in order to control both flies and fecal odors.

25. Provided the superstructure is able to perform these functions, its design details are relatively unimportant from a strictly technical point of view. The superstructure can be built in a wide variety of forms and from a wide variety of materials [Figures 9 (a) through 9 (d)]. In urban areas materials such as brick, blockwork or ferrocement are often used; the roof can be tiled or made from a thin concrete slab, corrugated steel or asbestos cement sheet. In rural areas, it is generally more appropriate to use local materials such as mud and wattle, thatch or sun-dried earth blocks; the roof is often made from thatch. The design adopted in any one locality depends largely on social preference and the availability of materials; in general, the superstructure form should be architecturally similar to the local houses, and this principle normally determines what materials are used. In this way not only are local sensibilities taken into account and so not offended, but (especially in rural areas) traditional housebuilding skills can be used and the householder knows how to repair the superstructure, for example, after damage during the rainy season.

26. Latrine entrance. Traditionally the latrine is entered through a doorway, with the door providing the user with privacy. It is very important that the door remains closed while the latrine is not in use; if it is left open, any newly-emergent flies in the pit will be presented with an alternative source of bright light and they may not therefore try to escape via the vent pipe, but leave the latrine via the squat-hole and superstructure. Fly control, which is one of the principal advantages of VIP latrines, therefore becomes ineffective. Self-closing doors can be used (a counterweight attached to the top of the door via a rope and pulley is sufficient for this purpose, as shown in the early example from the USA in Figure 10). Alternatively, the latrine should be locked on the outside;

this is often done by the users in order to prevent casual use of the latrine by unauthorized people such as passers-by or neighbors without latrines. If doors are used then there should be a sufficient air gap (equivalent to 1 least three times the cross-sectional area of t vent pipe) at the top.- of the door to maintain ventilation. In Zimbabwei- it was found that doors were undesirable not only because they were frequently left open with

13/ The air space could be provided (at least partially) at the bottom of the door, but this may be social;j unacceptable if the latrine user's feet can be seen from the outside.

14/ P.R. Morgan and D.D. Mara (1982). Ventilated Improved Pit Latrines:

Recent Developments in Zimbabwe. TAG Working Paper No. 2. The World Bank.

- 22 -

.v ~ ~ ~ ~ -- ,

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Fgr 9 (a) Ferceet spra VI arn

-~~~~uertutr (Zimbbwe)

- 23 -t * 4

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- 24 -fl N ~ ~ ., bnt aO S w1 .

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Figure 9 (c). Mud and wattle VIP latrine superstructure with thatched roof (Zimbabwe).

- 25 -

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Fiur 9.. (d). Prcs fercmn VI latin *~~~~uertutr (Brazil).

- 26 -

Vent with fly screen

Fly screenil

Pit ventilator

Tight fifting lflue at least 4 self closing -selfdColoorst M it _ Tsiglht Ififting

. .4 r | seat cover Metal seat support

Earth backfill

Original F _ ground elee

\>' > \evel Xgs = Pit \,\Earth>\- \ \. Rough lumber

\or open joint \ \\masonry c

< ~ At least 2'

\\\\ \\\\\ Ground water table level

Figure 10. Early pit latrine design from the United States showing self-closing door. [Reproduced by courtesy of John Wiley Inc. from F.B. Wright, "Rural Water Supply and Sanitation", 1939.]

- 27 -resultant poor fly-control, but also because wood is expensive, hinges rust and occasionally the doors were removed and chopped up for firewood. The superstructure was then redesigned with a spiral form to avoid the need for doors [Figure 9 (see page 22); see also Annex II].

Vent pipe design

27. Vent pipes of a wide variety of different materials have been used successfully: for example, asbestos cement (AC), polyvinyl chloride (PVC), unplasticized PVC (uPVC), bricks, blockwork, cement-rendered reeds, cement-rendered hessian supported on steel mesh, and even anthill soil;

large diameter bamboo with the cell dividers removed could also be used.

Methods for the construction of cement-rendered reed and hessian pipes and other essentially rural vent pipes are described in paragraphs 30-32. Whatever material is used, its durability (including corrosion resistance), availability, cost and ease of construction are important factors. Thus, vent pipes made, for example, from thin galvanized steel sheets 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.i2- Cost is particularly important in rural VIP latrines; for example, the use of a PVC pipe, rather than a cement-rendered reed ' e, more than doubles the cost of a mud and wattle latrine in rural Zimbabwe.RI

28. Length. 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 also be 500 mm above the highest point of the roof. When the roof is made from thatch and shaped conically, the vent pipe should be at least as high as the apex of the roof.

29. Diameter. The internal diameter of the vent pipe depends on the required venting velocity necessary to achieve the recommended ventilation

15/ In Zimbabwe it has been found that PVC pipe manufactured to Central African Standard K21, "Unplasticized polyvinyl chloride pressure pipe", (Standards Association of Central Africa, Harare, 1971) performs more satisfactorily than that manufactured to the less stringent requirements of British Standard BS 3505 or ISO Standard 3127. See also R.W. Doughty, UPVC pipes and fittings for hot climates, in: Institution of Public Health Engineers 1982 Handbook, London.

16/ P.R. Morgan and D.D. Mara (1982). Ventilated Improved Pit Latrines:

Recent Developments in Zimbabwe. TAG Working Paper No. 2. The World Bank.

- 28 -rate of 20 m3/hr_Z7/, 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 the flyscreen and squat-hole mosquito trap (if any), 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 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-sgution is inherently less efficient than a circular one in inducing updraft.- Current recommendations for the minimum internal size of vent pipes are as follows:

AC or PVC 150 mm diameter Brick 230 mm square Cement-rendered reed or hessian 230 mm diameter (and other rural types)

In exposed locations where wind speeds are greater than 3 m/s, the minimum diameter of AC and PVC pipes may be reduced to 100 mm, and to 200 mm in the case of "rural" vent pipes.

Fabrication of rural vent pipes

30. 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 30 cm external diameter. The flyscreen is then fixed to one end. 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 has hardened, the vent pipe is fixed in position with the mortared half next to the latrine superstructure 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 1 - 2 cm wide strips may also be used.

31. Cement-rendered hessian and wire-mesh vent pipes. Spot-welded mild steel mesh (4 mm bars at 100 mm centers), 2.5 m long and 0.8 m wide, is rolled into a tube to give an internal diameter of approximately 25 cm. Hessian or jute fabric is then tightly stitched around the outside of the tube, and the flyscreen 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 1 cm. The vent pipe is then fixed in place.

17/ B.A. Ryan and D.D. Mara, Pit Latrine Ventilation: Field Investigation Methodology, TAG Technical Note No. 4; and Ventilated Improved Pit

Latrines: Vent Pipe Design Guidelines, TAG Technical Note No. 6.

18/ D.R. Wills, E.W.G. Dance and G.T. Blench (1959). The Design and

Performance of Natural Flue Terminations. Gas Council Research Communication No. GC61. London: Institute of Gas Engineers.

- 29 -

32. Anthill soil vent pipes. Well-kneaded anthill soil is rolled into "sausages", approximately 10 cm in diameter and 90 cm long, which are made into circles of approximately 28 cm internal diameter. The vent pipe is constructed in situ from these circles; vertical reinforcement with short lengths of reed of 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 flyscreen is attached to the upper end and then a thin coat of cement mortar (1 part cement, 6 parts sand) applied.

33. External surface preparation. In areas where the mean wind speed is less than 0.5 m/s, 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.

34. Locati?n. The latrines should be located at least 2 m 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 side of the superstructure, as also should any openings (doorways, windows, gaps between the roof and walls). If, however, it is impossible to have both vent pipe and any openings on the windward side, at least one of them must be (and this should preferably be the openings). It is extremely important to avoid openings on opposite sides, as this would significantly reduce the pressure differences causing updraft in the vent pipes. 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).

35. In general, the vent pipe should be located on the outside of the superstructure, since it is more difficult and expensive to ensure a rainproof and wind-tight seal between the roof and a vent pipe going through it.

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 on its sunny side and painted black. However, in urban areas especially, external vent pipes could be subject to damage by vandals, although, as yet, there have been no reports of this happening.

36. The vent pipe must be rigidly fixed to the superstructure and the cover slab; design recommendations are given in Figure 11.

37. Flyscreen specification. The purpose of the flyscreen is to prevent the passage of flies and mosquitoes; therefore the mesh aperture must not be larger than 1.2 mm x 1.5 mm (smaller apertures are not recommended as they will result in decreased ventilaL on rates, due to increased frictional losses). The flyscreen must be made of corrosion-resistant material that is able to withstand intense rainfall, high temperatures and strong sunlight.

PVC-coated glass-fiber screens have been extensively used in Zimbabwe as they are inexpensive (around US$ 0.50 per latrine), but after five years or so they become very brittle and susceptible to damgfe by birds and lizards. It is preferable to use stainless steel screens- which last indefinitely;

19/ For example, 16 mesh, 28 s.w.g. screening.

SOO minL Either: standard pipe fitting or as Detail A

Detail B

General arrangement sivrnized steel a built into wall

Detail A Method of fixing vent pipe to superstructure

0a a t Socket end cut Cement mortar off and cast into filler concrete slab tW P o * e -* ~ ~ ~~~.:

PVC pipe roughened with solvent cement and sand

Detail B Alternative methods of fixing vent pipe to slab

Figure 11. Recommendations for fixing vent pipe to cover slab and superstructure.

- 31 -even though they are more expensive (US$ 25 per m2 ex works), the cost is small (less than 5%) in comparison with the total cost of the latrine.

38. It is important to ensure that the flyscreen is tightly fixed to the top of the vent pipe in order to prevent access by insects. Design details are shown in Figure 12(a) and (b). When the flyscreen is in place there should be no obstruction to the wind flow across the top of the vent pipe.

Relocation and emptying of pits

39. When single-pit VIP latrines become full, there are two options available to their owners: the construction of a new latrine on an adjacent site, or emptying the existing (full) latrine. In rural areas, construction of a new latrine, reusing as much as possible from the old latrine (for example, the cover slab and vent pipe), is generally the preferred solution, as space for the new latrine is usually available. Manual emptying poses health risks due to the excreted pathogens that may be present in the fresh fecal material at the top of the pit, and in any case this is often not a socially acceptable task; and mechanical emptying is not normally feasible in rural areas. In urban areas manual emptying has the same disadvantages, but mechanical emptying might be feasible, especially if the pit were wet, as the procedure is essentially similar to desludging septic tanks (although the solids at the bottom of deep pits may be highly compacted and therefore difficult to remove by the standard vacuum equipment used to desludge septic tanks). Dry pits are considerably more difficult to desludge mechanically than wet pits (see paragraph 52). A better solution in urban areas is to use single-pit VIP latrines with soakaways (paragraph 42) or alternating twin-pit VIP latrines (paragraph 45).

Double and multicompartment designs 20/

40. A recent development in southern Zimbabwe, where there is a strong sociocultural preference for separate latrines for male and female members of the household, is the use of double compartment VIP latrines of the type shown in Figure 13. In stable soils each compartment may have its own pit, but in unstable soils a common pit with a fully mortared brick dividing wall is used. In rural areas especially, this type of latrine is generally too expensive for one household but, in order to reduce costs to an acceptable level, neighboring households have been willing to share a single latrine of this type, with the men from both households using one of the compartments and the women the other.

41. Multicompartment VIP latrines have been developed for schools and other institutions. These are essentially the same as the double compartment unit but of course with more compartments. The number of compartments depends on the number of users, with a design guideline of 10 persons per unit (20 persons per unit in non-residential institutions, such as schools). The number of users per unit can be increased to 30 if soakaways are provided as discussed on the next page.

20/ P.R. Morgan and D.D. Mara, VIP Latrines: Zimbabwean Brick Designs, TAG Discussion Paper TAG/DP/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 /remxxxx xx,. remove sharp edges /xxxxxxxxxxxxxx x X~~~~~~~~~~~~~~ r X xxxxxxxa-I xX xX xxxxx x 75S xx roPv PVC collar to fit 100p xi x x~~~~~~~~~ N 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 l 1.5 mm openings \

NN-xxxxxxxxxx xxxx XxNX Xx x X Fly screen fixed with X X l l/ galvanized wire

X X

X x Cement mortar

Reed or split bamboo framework

Method of fixing fly screen to a'rural' vent pipe

Figure 12 (a). Recommendations for fixing flyscreen to vent pipe.

Belled end section of pipe.

Cut.

| Flyscreen

PVC pipe. Belled end. PVC vent pipe.

Figure 12 (b). Details for fixing flyscreen to Ventpipe (Brazil), (The belled end of 100 mm PVC pipe is cut off and used as a collar to hold the flyscreen in place.)

* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-4 k

U~~~~~~~~~

Figure13. V in Z.

-- L -w -ti-

-|:||t'ks A- _ it;S} t** +:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~1 i.-- A o -v - #S { + t ~ ~ ia s s-- w---n-4ies s t~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~A ss-^t s s R ~-s

Figure 13. Double-compartment VIP latrine (Zimbabwe).

- 35 -

Designs with soakaways 21/

42. Another recent development in Zimbabwe has been VIP latrines with adjacent soakaways (Figure 14). These have been designed to increase the pit life in peri-urban areas where the number of people using a single latrine can be as high as 20, or even, exceptionally, 30. The latrine pit, including its base, is completely sealed with cement mortar in stable soils or with fully mortared brickwork in unstable soils. At a point 2.25 m above the pit base a 75 mm diameter PVC pipe with a sanitary tee is installed which leads to an adjacent soakaway which is at least 1 m away from the pit. The soakaway has a diameter of 1.5 m and a depth of 2 m; it is lined with unmortared bricks to a depth of 1.4 m. At this depth a reinforced concrete cover slab is placed on the bricks and the remaining space above it backfilled.

43. This type of pit latrine (which essentially resembles an aquaprivy with a vent pipe in place of the drop-pipe) has been used only in Zimbabwe for the last seven years; thus, it has not been possible so far to estimate its total useful life. Early indications are that it is performing very well in periurban areas. With regular desludging of the pit (say, every five years), it may be expected to last for at least 30 years.

44. An even more recent development has been to discharge the effluent from a line of VIP latrines into a short small bore sewer (75 mm diameter, laid at a gradient of I in 200) which leads to a communal soakaway. This is a very useful option in areas where there is insufficient space on each plot for an individual soakaway. Moreover, it is possible to upgrade this type of latrine to a low-volume cistern-flush toilet for added user convenience.

Alternating twin-pit latrines

45. Alternating twin-pit VIP latrines (Figure 15) have two separate pits, each with their own vent pipe, but only one superstructure. The cover slab within the superstructure has two squat-holes, one over each pit. Only one squat-hole and pit are used at a time. When this pit is full, generally after one to three years, its squat-hole is covered up and the second pit put into service; after a further period of one to two years, when this pit is full, the contents of the first pit are removed to enable it to be used again. This alternating cycle is repeated indefinitely. This type of VIP latrine is thus a permanent sanitation facility suitable for use in urban areas where there is insufficient space on each housing plot for two or more single-pit VIP latrines. Alternating twin-pit latrines have even been "retrofitted" to replace existing in-house bucket latrines and so provide an indoor sanitation facility (Annex II). Many of the design details for alternating twin-pit VIP latrines are the same as for the single-pit type; specific differences are described on page 37.

21/ P.R. Morgan and D.D. Mara, VIP Latrines: Zimbabwean Brick Designs, TAG Discussion Paper, TAG/DP/01, World Bank. (Publications in the TAG Discussion Paper series are not routinely distributed to the recipients of TAG Working Papers and Technical Notes, but are available from the Project Manager on request.)

- 36 -

Cocee slab 75 mm scum

Cement Mortar

4 $ ~~~brickwork _

Sludge

Figure 14. Schematic diagram of a VIP latrine with soakaway.

W o~' j.. _.iA!kJt._ A _

Figure 15. Alternating twin-pit VIP latrine (Tanzania).

-38 -

Pit function and design

46. The function of the pit is to store the excreta until they can be safely removed. With the sole exception of a few ova of the human roundworm, Ascaris lumbricoides, alj21 xcreted pathogens die within 12 months at temperatures above 200C.- In most developing country climates, one year is therefore the minimum storage requirement for each pit. Usually, to provide some degree of flexibility in the design of latrine emptying schedules (especially to allow for breakdown of any mechanical equipment and for seasonal problems of access), a minimum period of two years is specified.

Calculation of the necessary pit volume (paragraph 13) shows that in most cases the pits are quite small: for example, 3for a family of 10 each pit should have an effective volume of only 1.2 m , assuming a solids accumulation rate of 0.06 m3 per person per year only and a two-year storage time.

Consequently the pits can be much shallower (often less than 1 m total depth) than in the case of single-pit VIP latrines, and this may be a significant advantage if groundwater pollution must be avoided (paragraph 58), or if soil conditions are difficult (for example, if there is shallow, unpickable rock).

47. The pit shape is normally rectangular and the pits may extend either to each side of the superstructure or to its rear (Figure 16). The pits are lined as necessary (paragraph 16), with any suitable locally available material (such as brick, concrete or cement-stabilized soil blocks) being used to build up the partition wall between the two pits. This partition wall must have a good foundation and be fully mortared to prevent any cross-flows of air between the pits which would interfere with the ventilation and might cause odors to enter the superstructure. For the same reason, the cover slab must be firmly bedded with mortar on the partition wall, as well as on the brick or blockwork collar.

Cover slab design

48. The cover slab is usually made in reinforced concrete in three or more sections (Figure 17): a central section with two squat-holes and holes for the two vent pipes, and at least two removable covers (one for each pit) to allow access for emptying. The edge details of the cover slab sections shown in Figure 17 are important as there must not be any gaps between the central and outer sections to allow the escape of either flies or odors. A lime mortar, or a weak cement mortar if lime is not available, should be used to bed the removable slab sections to the central section and to the collar.

As with single-pit VIP latrines, the cover slab should be sloped towards each squat-hole (paragraph 22).

Superstructure and vent pipe design

49. The superstructure and vent pipe design details are essentially similar to those for single-pit VIP latrines (paragraphs 24-38). Alternating twin-pit VIP latrines have been installed inside existing houses, with the

22/ R.G. Feachem, D.J. Bradley, H. Garelick and D.D. Mara (1983). Sanitation and Disease: Health Aspects of Excreta and Wastewater Management.

Chichester: John Wiley.

- 39 -

0 0 0 0 o 0

0 0

Figure 16. Alternative pit geometries for alternating twin-pit VIP latrines.

- 40 -

* s e to _ P

-J ~ A-

40 4

4 0~ ~~~~~

L |100 VrieslOj. 1001 Va ries 100o4

Figure 17. Coverslab details for alternating twin-pit VIP latrines.

- 41 -pits accessible from outside (Annex II); in some cultures such an arrangement may be socially preferable to external superstructures.

Multicompartment units

50. Multicompartment alternating twin-pit VIP latrines have been developed in Ghana for use in rural institutions such…