23 – Latrine pit design Developing knowledge and capacity GGUIDEUIDE 2233 About WEDC in water and sanitation The Water, Engineering and What makes us stand out from the Development Centre is one of the crowd is our outreach to practitioners. world’s leading education and research We use our knowledge base and our institutes for developing knowledge applied research work to develop and capacity in water and sanitation the capacity of individuals and Latrine pit design for sustainable development and organizations throughout the world, emergency relief. promoting the integration of social, technical, economic, institutional and We are committed to the provision This guide examines some of the factors that need to be taken into account environmental activities as foundations of effective, evidence-based and when planning and designing a latrine pit (or twin pits), including the for sustainable development. appropriate solutions for the location of a latrine, its shape, volume, liquid capacity and life. improvement of basic infrastructure Visit our website to find out more about and essential services for people living our postgraduate and professional in low- and middle-income countries. development programmes (MSc, Contents With over 45 years of experience, we Diplomas and postgraduate certificates Pit location......................................................1 offer expert advice and quality learning available at the University or by Key design features ......................................1 opportunities for sector professionals. distance learning); our research; our Distance from a water source ......................2 advisory services; our international Founded in 1971, WEDC is based in the conferences; and our extensive range Depth to water source ...................................2 School of Civil and Building Engineering of information resources which are Lateral separation .........................................2 at Loughborough University, one of the free to download from our knowledge top UK universities. Being a part of a Shape ..............................................................2 base. leading university gives us a recognised Volume ............................................................8 platform of independence and quality. http://wedc.lboro.ac.uk Liquid capacity ...............................................9 Pit life ..............................................................9 Associated reading: Latrine pit excavation and linings Water, Engineering and Development Centre School of Civil and Building Engineering Refer to other guides in this series for further information about the range of latrine Loughborough University types suitable for low-income communities. Leicestershire LE11 3TU UK T: + 44 (0) 1509 222885 Linkedin: WEDC UK E: [email protected] Twitter: wedcuk W: wedc.lboro.ac.uk YouTube: wedclboro wedc.lboro.ac.uk/knowledge 2 9 © WEDC, Loughborough University, 2014 Liquid capacity In urban areas, where pit emptying, is carried out it makes more sense to limit Author: Bob Reed Quality assurance: Rebecca Scott Pour-flush latrines and those that are also used for bathing or laundry can store pit depth to about 1.5 metres. People Edited and illustrated by Rod Shaw Technical illustrations: Ken Chatterton a significant amount of liquid. In dry pits usually prefer to have a small amount of the contents emptied more frequently, Designed and produced by WEDC Publications table this liquid will tend to accumulate on the surface of the pit sludge and than a large amount occasionally, as it is gradually rise until the surface of the pit a more affordable option. This guide is one of a series of published learning resources which are available for wall allows the liquid to infiltrate into the purchase in print or available to download free of charge from the WEDC Knowledge surrounding soil (Figure 7). Base. Any part of this publication, including the illustrations (except items taken from In impermeable soils, the use of pour- Why don’t we use the top half other publications where WEDC does not hold copyright) may be copied, reproduced flush latrines or allowing wastewater metre of a pit latrine? or adapted to meet local needs, without permission from the author/s or publisher, from bathing and laundry to enter the A pit is usually considered full when provided the parts reproduced are distributed free, or at cost and not for commercial pit quickly could fill the pit with liquid. the contents reach 0.5m from the pit ends and the source is fully acknowledged. Please send copies of any materials in This problem is seen most often in peri- slab. There are a number of reasons which text or illustrations have been used to WEDC at the address given below. urban areas where water consumption is for this. higher. The solution is to either: If the pit is to be replaced when it is Published by WEDC, Loughborough University • empty the solids out of the pit more full, then 0.5m of soil on top of the frequently so that there is more room wastes will make the area safe to ISBN 978 1 84380 173 3 for liquid storage; walk over so anyone treading over This is one of a series of WEDC guides on sanitation. For a comprehensive list of all • dig an overflow pit to take the excess the site will not sink in. published guides, please visit: http://wedc.lu/1mlHcnD liquid; It will also prevent domestic animals • find alternative disposal methods for Also available in French, translated by SOLIDARITÉS INTERNATIONAL from burrowing into the pit, looking the bathing or laundry liquids; or for food. • use another sanitation system The soil layer will prevent any fly designed to accommodate the larvae that may have been breeding additional liquid (such as a septic in the waste from escaping. tank with infiltration). Furthermore, the increased Pit life Water, Engineering and Development Centre incidence of problems with odour, The life of the pit is the time taken for it School of Civil and Building Engineering flies and splashing as the content to fill. Loughborough University reaches close to the top. The pit Leicestershire LE11 3TU UK In rural areas, single pits should usually contents will also be visible which be dug as deep as possible so that it may offend users and dissuade T: + 44 (0) 1509 222885 Linkedin: WEDC UK takes a long time for them to fill. A them from using the latrine. E: [email protected] Twitter: wedcuk minimum design life of ten years is W: wedc.lboro.ac.uk YouTube: wedclboro recommended. 8 1 Volume Pit location Wet pits or dry pits? The volume of a pit affects how long it A latrine is no use if users can’t reach it. This is particularly important if the latrine is a Pits that are partly below the water will take to fill and hence its life before long way from the house and/or if some of the users have difficulty walking. So access table and therefore permanently emptying or replacement. Estimating to the latrine is a major consideration in the location of the latrine which will in turn contain water are known as ‘wet how long that will be is not easy. The key impact on the design of the latrine pit. pits’. factors affecting the rate at which a pit fills are: Generally, micro-biological activity Key design features is greater in wet conditions than dry • the number of users; When planning and designing a latrine, ones. Also the solids can be more • the volume of fresh human waste make sure: efficiently compacted under water deposited into the pit; • to plan to build the latrine as close as (there will be fewer voids). • the volume of other wastes; and possible to the living area; It is also thought that the flow of • the access path is at least 120cm water through the pit carries more • the sludge accumulation rate. wide and built of materials that give a of the wastes into the surrounding In the absence of local knowledge smooth but non-slip finish; soil than happens in a dry pit. the figures given in Table 3 could be • slopes and steps are kept to a This is why the sludge accumulation used. These figures are long-term minimum and if essential slopes are rate for wet pits is much lower than accumulation rates. In shallow pits (such designed at less than 1 in 12; for dry ones. as are used for twin pit latrines) these rates will be too low. In this case, you • a hand rail next to the steps or a should increase the figures by 50%. slope is included; and • the path is designed to be easy to see at night. Assuming there is no possibility or concern as a result of polluting groundwater, the location of a pit for excreta disposal depends on the type of latrine used. Liquid Liquid percolation zone Fresh unconsolidated waste Consolidated waste Remember a latrine will last a family many years. All the users may be fit and active when the latrine is Figure 1. Consideration should be given built but this may not be the case in to all users, particularly those with years to come. Figure 6. Liquid percolation zone in a pit latrine mobility difficulties 2 7 1m 1m 1m Simple pit latrines should be built away Depth to water source 2m from the home to prevent nuisance from Table 1 suggests the minimum distance odour and flies. A distance of about 10 between the bottom of the pit and the 0.5m metres is usual but some users feel highest level of the groundwater table to 0.5m that the convenience of a nearby latrine ensure total removal of organic wastes. outweighs the inconvenience caused by If the pit meets this criteria then it can be 2.5m the odour and flies, so choose to locate located as little as 10m from the water 1m them closer to the home. source. Pour-flush latrines can be built very close to, or as part of a house, provided Lateral separation the house has good foundations to If the depth to the water table is less prevent subsidence and the pit lining is than that suggested in Table 1 then strong enough to withstand forces placed the groundwater will be contaminated and require further time to purify itself.