TAG Discussion Paper Number 4 49957 United Nations Development Programme Interregional Project INT/81/047 Executing Agency: World Bank Ventilated Improved Pit Latrines: Guidelines for the Selection of Design Options D. Duncan Mara, Technology Advisory Group (TAG) ".'\ \=- v.~ ~7 ~_/il l~J A joint United Nations Development Programme "J;f '-r . .1!' and WorId Bank Contribution to the International UNITED NATIONS 1981-1990 Drinking Water Supply and Sanitation Decade LIST OF PUBLICATIONS BY THE 'IECHN01.iVl ADVISORY (,'R!XJP (TAG) UNDP L"lTERREGIONAL PIDJECT LW/81/04? WP /01 A ~el for t:re DeveloJl1elt of a Sel..f-relp Water Supply Program; by Colin Glennie. WP/02 Ventilated lDproved Pit Latrines: Recent Developnent: in~; by Peter Morgan and D. Duncan !Y',ara. 'IN/Ol MatOOds for Gatrering Socio-cultural Data for Water Supply am Sanitatiro Projects; by Mayling Simpson-Hebert (also in French). 'IN/02 Planning of Coommication Support (Infornation, lbtivatiro and Education) in Sanitation Projects and Programs; by HeE Perrett (also in Portuguese). 'IN/OJ 'lb:! Ventilated Inproved Douhle-Pit Latrine: A Construction Manual for Botswana; by JOM van Nostrand and JanES G. Wilson. 'IN/04 Pit Latrine Ventilation: Field I.nvestigatiro ~thodology; by Beverley Ryan and D. Duncan Mara. 'IN/OS Social Feasibility Analysis of IDrcost Sani.tatioo Projects; by Heli Perrett (also in Portuguese). 'IN/06 Ventilated Inproved Pit Latrines: Vent Pipe QUdelines; by Beverley Ryan and D. Duncan tls Sanitation in Lesotho; by Piers Cross. TN/08 Rural Ventilated Inproved Pit Latrines: A Field Manual for Botswana; by JaM van Nostrand and JanES G. Wilson. 'IN/09 Handbook for District Sanitation Coordinators; by Kebadire Basaako, Ronald D. Parker, Robert B. Waller and JanES G. Wilson. 'IN/10 Mama! 00 tiE Design, Constructioo and Maintpnaoce of Inw-rost Four-flush Waterseal Latrines in fulia; by A.K. Roy. TN/ll ~todng aOO Evaluation of Coommication Support Activities in L<:M-rost Sanitation Projects; by Heli E. Perrett. 'IN/ l 2 A Mmitodng aOO Evaluation McnJal for IDrcost Sanitatioo Programs in fu:lia; by Ronald Parlato. 'IN/l3 'lb:! Design of Ventilated Inproved Pit Latrines; by D. Duncan Mara. TN/l4 'lb:! Design of Smll Bore Sewers; by Richard Otis and D. Duncan Mara. 'IN/lS 1re Design of Pour-Flush Latrines; by D. Duncan Nara. DP/Ol Ventilated lDproved Pit Latrines: ZiJd:Jabwean Brick Designs; by Peter R. Morgan and D. Duncan Mara. DP/OJ Invol~ WooaJ. in Sanitatioo Projects; by Heli E. Perrett. TAG Discussion Paper Number 4 Ventilated Improved Pit Latrines: Guidelines for the Selection of Design Options D. Duncan Mara, Technology Advisory Group (TAG) A joint United Nations Development Programme and World Bank Contribution to the International Drinking Water Supply and Sanitation Decade Copyright © 1985 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 Discussion Paper, prepared by Duncan Mara, is one of the technical outputs of a meeting of TAG}j staff held in Nairobi, Kenya, during July 9-13, 1984. It seeks to clarify the current state of Ventilated Improved Pit latrine design, which often includes a bewildering set of design options such as single or twin pits, emptiable or non-emptiable, lined or unlined and raised above ground level or unraised. The ability to make a rational choice between these design options should lead to a less wasteful use of resources and enable an adequate and cost-effective sanitation technology to be readily provided to those in need. This document is being issued on a limited distribution basis to stimulate discussion and elicit ideas and contributions from readers. Comments and suggestions are invited tram readers who would like to contribute to the state of the art, particularly on specific issues raised in the paper. Unfortunately, TAG will not be able to respond to every individual reader but hopes to revise this paper in due course to reflect, as appropriate, the comments and contributions received from readers. The distribution of this Discussion Paper does not imply endorsement by the sector agencies, governments or donor agencies concerned with programs, nor by the World Bank or the United Nations Development Programme. Enquiries about the TAG program and the publications available and comments on this and other TAG papers should be addressed to the Project Manager. UNDP Project IN1'/81/047, Water Supply and Urban Development Department, The World Bank, 1818 H Street, N.W. Washington, lie 20433. Project Manager 11 TAG: Technology Advisory Group, established under the United Nations Development Programme, UNDP Interregional Project INT/81/047: Develop.ent and Iapleaentation of Low-cost Sanitation Investment Projects (formerly Global Project GLO/78/006), executed by the World Bank. ( ii) TABLE OF CONTENTS Preface .......................................................... i INTRODUCTION ..................................................... 1 VIP LATRIHE DESIGN OPTIONS 3 Relative cost and affordability ••••••••••••••••• 3 Pj.t e.lllPtying •••••••••••••••••••••••••••••••••••• 5 Alternating pits •••••••••••••••••••••••••••••••• 7 Lining the pits ••••••••••••••••••••••••••••••••• 9 Raising •••••••••••••••••••••••••••••••••••••••• 11 In-house latrines •••••••••••••••••••••••••••••• 13 Selection algorithm •••••••••••••••••••••••••••• 13 ANNEXES: I. Latrine Emptying Costs •••••••••••••••••••••••••••••••• 17 II. Soil Stability Criteria ••••••••••••••••••••••••••••••• 18 (iii) FIGURES Figure 1. Non-emptiable, unlined and unraised single-pit VIP latrine ••••••••••••••••••• ~........ 2 Figure 2. Vacuum tanker specially designed for emptying VIP latrine •••••••••••••••••••••••••••••• 4 Figure 3. An alternating twin~pit VIP latrine • • • • • • • • 41 • • • & ...... . 6 Figure 4. VIP latrine pits lined with rough coral and concrete blocks ••••••••••••••••••••••••••••••• 8 Figure 6. An in-house, emptiable and lined single-pit VIP latrine ••••••••••••••••••••••••••••••••••••••• 12 INTRODUCTION 1. The ventilated improved pit (VIP) latrine is now widely recognized as one of the most important, affordable and appropriate on-site sanita~ion technologies for use in both urban and rural areas of developing countr es. The Technology Advisory Group (TAG) has been one of the major agencies responsible for its promotion in several countries (principally in Africa and South America) and has published s~veral Technical Notes describing the design of various types of VIP latrines.lJ 2. This Discussion Paper provides guidelines that will enable a design engineer to make a rational selection of the VIP latrine design option most appropriate to the community for which he is responsible. The designer is aiming at a technical choice that will be affordable to the householder and will give the best long-term service at the least coste It is assumed for the purpose of this paper that a VIP latrine of some sort is the most appropriate sanitation facility for the community under consideration, and also that the reader is aware of how VIP latrines function and how they are able to control odors and insec~s (these details are covered in the appropriate TAG publications).1J What remains for the designer is to select the type of VIP latrine that is compatible with available resources as well as with the prevailing physical, sociocultural, institutional, economic and financial conditions. In doing so, special emphasis is placed on technology costs and affordability. 3. For convenience, VIP latrine options can be considered as: (a) Design options, which cover the basic types of VIP latrines. (b) Construction options, which cover the various ways in which the basic VIP latrine types can be built. In general, the selection of the most appropriate design option is more critical than the choice of the most suitable construction option. TAG Technical Note No. 13 covers the various construction options (and also design calculations for effective pit volume); therefore, this Discussion Paper is restricted to a consideration only of the various design options. l! A complete list of TAG publications is given inside the front cover of this document. - 2 - flyscreen roof vent pipe superstructure 'pit ~Iined 'with 'cement mortar Figure 1. Non-emptiable, unlined and unraised single-pit VIP latrine. This is usually the least expensive design option. - 3 - VIP LATRINE DESIGN OPTIONS 4. A VIP latrine m..ay be, d,e,signed in several ways. For example, it may be a single-pit unit or an alternating twin-pit system. The latter is always designed as a permanent, emptiable fac.ility, whereas the former mayor may not be designed to be so. Emptying mat/be done manually or mechanically by using specially designed vacuum tankers •....:.. Additionally, pits mayor may not need to be lined (to prevent structural collapse), and they may be partially raised above ground level (to minimize problems in rocky or high groundwater table areas). In high-density areas in Zimbabwe, single,-pit VIP latrines with soakaways have been used to prolong the effective life of the latrine. These latrines are largely experimental at this stage; moreover, it is debatable whether they should be classified as VIP latrines or aqua without chutes. For these reasons they are not included in the list of options .. Relative costs and affordability 5. The four basic sets of design options considered in this discussion paper are as follows: (a) non-emptiable/emptiable; (b) non-alternating (single pit)/alternating (double pits); (c) unline'd/lined; and (d) not raised/raised~ The negative option is intentionally first in each case because it is less expensive than the correspond tive option.. As a r:esul t, the least-cost VIP latrine design option is , unlined, single-pit latrine that is not raised ahove Figure I).. This is the option bes t sui ted for the lowes t income group in. a communi tY·D At the other extreme is the raised, fully lined, emptiable VIP latrine, which can cost more than twice as much. This is beel-lUSe about 40,-50% of the cap! tal costs arise from pit lining, and and maintenance costs are always higher for emptiable options. An implication of that whenever pit lining becomes imperative, the lowest detention period should be chosen so that pit volumes and. hence costs can be minimized to enhance affordabillty.. An even on affordability can be achieved when there is access to credit~ 2/ Boesch A., and R. Schertenleib (1985): Emptying On'-Stte Ex.creta. Disposal Syst.ems: Field Tests wi'til Mechanized Equipment is Gaborone (Botswana), No. 03/85 .t> l1&ure 2. Vacuum tanker specially designed for emptying VIP latrine. - 5 - Pit emptying 6. The key points to note about pit emptying are: (a) Emptying is essential if (i) there is insufficient space available for the relocation of a single-pit VIP latrine when it becomes full; or (ii) an in-house latrine facility (see paragraph 13) is required and affordable. (b) Emptying is desirable if a shallow pit is required (for example, to minimize pollution of the local groundwater if this is used as a source of supply). (c) The main advantage of emptying is that it permits the facility to be a permanent one, and one that can be located in-house; a second advantage (which may be important in some societies) is that it makes reuse of the excreta product possible .. (d) Pit emptying has several disadvantages: (i) the purchase, operation and maintenance of specialized pit-emptying equipment may be required, depending upon the community type and the nature of the sludge (Figure 2); (ii) higher operational costs; and (iii) pit lining is required if the pits are to be emptied mechanically; it is highly recommended if they are to be emptied manually. (e) Mechanical emptying is required to remove the contents of single-pit latrines, which will contain fresh excreta and therefore may also contain viable pathogens. - 6 - /: ," /~10 years? BI. surface? (See note A). (See note J~ YES NO C). NO YES r " Is a fully lined (or raised and .. ..... NO fully lined), non-emptiable, single-pit VIP latrine the YES cheapest option? (See note E). , SELECT Is manual emptying socially ALTERNATING feasible and cheaper than YES .. mechan~~empt~ng? ~~~~~~~~~~~~~~~~T~~~~~~~~~ TWIN-PIT VIP LATRINES. (See note Fl. (See note G). NO " Is mechanical emptying NO institutionally feasible? ~~-------------~~ ... (See note H). YES ., Are mechanically emptied alternating twin-pit VIP latrines cheaper than YES mechanically emptied single-pit VIP latrines, or do they convey institutional benefits? (See note I). NO , SELECT Are mechanically emptied single-pit MECHANICALLY VIP latrines without soakaways less I-Y...;.E_S~_ _ _~_~~_~_ _ _ _ _ _ _~__-"o.M EMPTIED expensive than those with r SINGLE-PIT soakaways? (See note J). VIP LATRINES. (See note G). NO SELECT ... - SINGLE-PIT VIP LATRINES WITH SOAKAWAYS. (See note K). - 15 - Notes to Selection A1gorito. A. Space for relocation of non-empt1able, single-pit VIP latrines is usually available in rural areas and low-density urban areas. The designer should consider the "alternating single-pit" option discussed in paragraph 9. B. As defined in Annex II. c. Unpickable rock within 1 m or groundwater within 0.3 m either permanently or seasonally. D. The pit will require lining if the answer to question 2 is "no," and both lining and raising if the answer to question 3 is "yes." E. The designer should compare the alternative of smaller, emptiable pits. F. Mechanical emptying is a broad term and covers, for example, manually operated diaphragm pumps that could be mounted on an animal-drawn cart, which also carries a small sludge tank as well as more sophisticated vacuum truck systems. G. The pits will generally require lining and also ra1Sl.ng if the groundwater table 1s high (as defined in note C above). Use manual emptying if feasible for alternating twin-pits but mechanical emptying always for single-pits. H. Is there an institutional capability for organizing regular pit-emptying and vehicle maintenance programs? Are spare parts, fuel and other expendable items available? I. The designer, in seeking to answer the above question, should consider various emptying frequencies--for example, alternating twin-pits with two-, three-, and four-year cycles and sing1e-p.its with three-, flve- and ten-year cycles. Institutional benefits include flexible emptying time, no treatment costs and instant reuse of the excreta product. - 17 - TAG/DP/04 ANNEX 1 LATRINE EMPTYING COSTS 1. The use of modern, powerful vacuum tankers for emptying VIP latrines is often thought to be very expensive. While it is true that few reliable field data are available to calculate emptying costs with any precision (this is essentially because work has only recently begun on the design and fabrication of suitable tankers), it is nonetheless possible to make "order of magnitude" estimates of the costs involved. 2. Let it b~ supposed for a communiti of 6,000 households that a suitable vacuum tanker (such as the BREVAC!.!') has a c.i.f.cost of US$75,OOO, and that it can service 10 VIP latrines a day for lOa days per year. One such tanker can thus service (10 x 200), or l,OOO latrines annually. 3. If the tanker is assumed to have operating and maintenance costs of U8$3U,000 per year, and if the tanker is written off after three years (which is unlikely in practice but serves to increase the costs in this illustra- tion), then the total annualized cost for emptying 2,000 pits each year out of 6,000 pits assumed to be designed for emptying on a three-year cycle is as tallows: Vacuum tanker (~ US$75,000 amortized at 12% over 3 years) US~31,L.00 o & M cost US$30,000 TUTAL ANNUAL CuSTS US~61,200 This is equivalent to an annual household (latrine) cost of (US$bl,200/bOOO), that is, U8$10.2, or a monthly payment of US~0.85. This is not likely to be unaffordable. Since the tanker can reasonably be expected to last more than three years, and since its operating and maintenance costs are likely to be much less than U8$30,000 per year, the actual cost to the householder will probably be even less than U8$0.85 per month. 2 / These order-of-magnitude calculations therefore indicate that mechanical emptying of VIP latrines is likely to be economically feasible in urban areas, provided the off-shore costs (in foreign currency) are affordable. Jj "BREVAC: a Mechanized Method of Emptying Sanitation Chambers," BRE Information Paper No. 84, Building Research Establishment, Watford, England, lY84. 11 Estimated costs of pit-emptying in Brazil are reported to be U8$0.40-0.50 (1985 dollars) per household per month. - 18 - TAG/DP/04 ANNEX II SOIL STABILITY CRITERIA 1. This Annex describes three alternative, simple field tests for soil stability. On the basis of these tests, the designer of VIP latrines can decide whether a pit needs to be fully lined, as described in paragraph IS above. Test A 2. This is the simplest test. Soil samples are taken by hand auguring; one sample should be taken every 50 cm to a depth of 3 m. Each sample is then hand-rolled to form a rough cylinder of approximately 2 cm diameter and 5 cm long. After sun-drying for two days or, preferably, oven-drying for two hours at 100 C, the sample is gently crushed between one's thumb and fingers. Unstable (cohesionless) soils crush easily, whereas stable (cohesive) soils do not. This test requires some experience, and it is therefore a good idea to practise the test on soils of known particle size distribution and undrained shear strength. Test B 3. This is the standard soil mechanics measurement of particle size distribution~ A soil can be considered stable if it contains more than 30% clay « 0.002 mm). It is simpler to measure the combined sand and silt fraction (> 0.002 mm), which should not therefore exceed 70%. Test C 4. This test is the measurement of the undrained shear strength of soil samples and is thus applicable only to cohesive soils. It is done in the field by the standard soil mechanics vane test procedure...!! Soils with an undrained shear strength of less than 15 kN/m2 are likely to be able to support normal superstructure and covers lab loads (which may exceed 20 kN). As a reasonable precaution, ~its excavated in soils with an undrained shear strength of less than 20kN/m should be fully lined. l! This is described in, for example, British Standard BS 2004: 1972.