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RÉUTILISATION DES FÈCES ET DES URINES PROVENANT DE L'ASSAINISSEMENT ÉCOLOGIQUE
Le concept d' assainissement écologique (ecosan) a été expliqué dans la fiche technique Assainissement écologique : un concept . Cette note décrit plus en détail les méthodes utilisées pour rendre les matières fécales et l'urine sans danger pour un usage agricole et fournit des conseils sur les méthodes d'application.
Les excréments d'une personne peuvent fournir presque suffisamment de nutriments pour produire environ 250 kg de céréales ( grains) par an (Strauss, 2000). C’est l’une des raisons pour lesquelles la ressource a été utilisée tout au long de l’histoire à la fois dans l’aquaculture et dans l’agriculture (le sous-produit d’ecosan est généralement utilisé dans cette dernière). Historiquement, les excréments n'ont pas toujours été traités avant utilisation. Ecosan, correctement utilisé, permet de réutiliser les excréments tout en minimisant les risques pour la santé. Cette ressource précieuse doit être utilisée en toute sécurité ; le « diagramme F » montre les nombreuses voies par lesquelles les maladies provenant des excréments peuvent être transmises et les obstacles à ces infections. Lorsqu’on envisage la réutilisation de l’urine et des matières fécales d’ecosan, la barrière la plus intéressante est le blocage de la transmission des matières fécales depuis les champs. La figure 1 identifie les toilettescomme moyen de prévenir la transmission. La réutilisation des excréments d'ecosan conserve la barrière des toilettes, mais, au lieu de contenir des matières fécales, les toilettes ecosan les traitent avant leur application. De plus, ecosan peut réduireles mouches et prévenir la pollution des eaux souterraines et d'autres sources d'eau douce .
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Cette note technique explique brièvement quand les risques sont associés à la réutilisation et qui est à risque à chaque étape. Il existe quelques explications sur les méthodes de traitement pour les trois types de produits, à savoir les matières fécales, l'urine et l'urine et les matières fécales combinées. Enfin, quelques directives pratiques sont données sur la manière d'épandre les excréments et de le faire en toute sécurité.
Étapes d'exposition
Il y a essentiellement six phases dans la réutilisation de l'urine et des matières fécales. Le tableau 1 montre ces phases et qui est à risque au cours de chaque phase. Il suggère également quelques procédures d'atténuation possibles à différentes étapes, dont certaines seront discutées plus en détail ultérieurement.
Tableau 1- Principaux points d'exposition lors de la réutilisation des excréments [Adapté de : OMS, 2006]
| Activité à risque | Voie d'exposition | Groupes à risque | Atténuation |
|---|---|---|---|
| Vider le récipient/la chambre de collecte | Contact | Entrepreneurs, Résidents, Communauté Locale | Vêtements de protection, Formation, Optimiser le traitement sur site, Eviter les déversements |
| Transport | Contact, propagation à travers l'équipement | Entrepreneurs, Communauté locale | Nettoyer l'équipement avant de l'utiliser sur d'autres matériaux, éviter le déversement |
| Traitement secondaire (hors site) | Contact, Vecteurs | Travailleurs, Communauté à proximité | Traitement efficace, Vêtements de protection, Restreindre l'accès, Réduire la propagation du vecteur |
| Application | Contact, Inhalation | Entrepreneurs, agriculteurs, communauté locale | Travailler les matériaux dans le sol, Restreindre l'accès au champ si la qualité n'est pas garantie, Vêtements de protection pour les travailleurs |
| Cultures, récolte, transformation et vente | Consommation, Manutention | Consommateurs, travailleurs, vendeurs | Prévoir un mois entre l'application et la récolte. Les aliments consommés crus présentent le risque le plus élevé. Vêtements de protection. Prévoir de l'eau pour le nettoyage. |
| Consommation | Consommation | Consommateurs | Pratiquer une bonne hygiène, bien cuisiner |
De nombreuses étapes pratiques visant à garantir que les excréments sont utilisés en toute sécurité sont abordées ci-dessous. Elles se concentrent principalement sur l'application, la récolte et la consommation des cultures . L’une des méthodes les plus simples pour atténuer tous les risques consiste à assurer un traitement primaire approfondi (sur site). Plus le traitement est effectué tôt dans la séquence des événements, mieux c'est, car les risques seront minimisés plus tard. Les méthodes utilisées pour tuer les pathogènes seront maintenant discutées afin de clarifier les facteurs affectant la survie des pathogènes dans les excréments.
Les facteurs contribuant à la disparition du pathogène
Les facteurs énumérés dans le tableau 2 provoquent la mort des agents pathogènes présents dans les excréments. Un ou plusieurs de ces éléments rendront les selles sûres, notez que l'urine ne contient pas un niveau élevé d'agents pathogènes et qu'un certain niveau de stockage la rendra totalement sûre (voir ci-dessous).
Tableau 2 : Facteurs affectant les micro-organismes dans l'environnement [Adapté de Schonning et Strenstrom, 2004]
| Température | pH | Ammoniac | Humidité | Lumière solaire/UV | Autres organismes | Nutriments | Autres |
|---|---|---|---|---|---|---|---|
| La plupart des micro-organismes survivent bien à des températures basses (<5 ºC), tandis que la mort se produit à des températures élevées (40 à 50 ºC). Malheureusement, de nombreux procédés de compostage ecosan n'atteignent pas ces températures sur tout le volume du compost. | Les micro-organismes survivent mieux à pH W neutre (7), si un mélange devient très acide ou alcalin , les agents pathogènes mourront. Par conséquent, de la chaux est ajoutée aux toilettes ecosan pour augmenter le pH (un pH >9 est requis), plus le pH est élevé, plus le taux de mortalité est élevé (si pH >11-12). | Des niveaux élevés d' ammoniac tuent les agents pathogènes, des produits chimiques peuvent être ajoutés | Une faible teneur en humidité tuera les agents pathogènes, c’est le raisonnement derrière la déshydratation des latrines ecosan. Un objectif de teneur en humidité satisfaisant serait <25 %. | La lumière UV réduit le nombre d'agents pathogènes, ce qui entraîne une augmentation de la mortalité des agents pathogènes après application dans les champs. | Les micro-organismes vivent plus longtemps dans le matériel stérilisé, donc lorsque de la terre (ainsi que d'autres matériaux) est ajoutée, les bactéries concurrentes encourager mourir | En raison de la nature des agents pathogènes dangereux, ils sont incapables de rivaliser avec d'autres agents pathogènes pour les nutriments rares, ce qui entraîne la mort des agents pathogènes à partir des excréments. | La disponibilité de l’oxygène peut également affecter la survie des agents pathogènes, tout comme la présence de divers composés organiques et inorganiques. |
Ce tableau montre les facteurs qui tuent les agents pathogènes. Les agents pathogènes particuliers situés dans les selles et l'urine ne seront pas répertoriés en détail. Les sections suivantes examinent la meilleure façon de traiter chaque composant, à la fois séparément et lorsqu'ils sont mélangés, ainsi que des lignes directrices sur les pratiques d'application.
Fèces
Lorsqu’elles sont séparées de l’urine, les selles contiennent un niveau minimal de nutriments et un niveau élevé d’agents pathogènes. Malgré cela, il contient la majeure partie de la matière organique et constitue un très bon amendement du sol.
Pathogènes
Les agents pathogènes trouvés dans les selles peuvent être divisés en quatre catégories : bactéries W , virus W , protozoaires parasites W et helminthes W (vers parasites). Cette note ne décrira pas les agents pathogènes de manière très détaillée ; Les maladies qui peuvent être transmises comprennent le choléra W , la typhoïde W et l'hépatite A W .
Traitement des selles
There are numerous means of reducing the pathogen content in faeces, some can be advised as primary treatment (on-site; within the sanitation systems itself), whereas others should be recommended as a secondary (off-site) treatment. The main reason for employing primary treatment is to reduce volume and weight of faecal sludge this facilitates simpler storage, transport and secondary treatment, and in some cases reduces pathogen content to make further handling safer. Secondary treatment makes the faeces safe enough to return to the soil. The following are the main treatment methods.
Storage – Through storage of faeces a number of the processes within table 2 will take place. Storage is often recommended as a primary treatment method, the success displayed in practice is variable. The pH of the material, the moisture content, ambient temperature and biological competition will all affect die off, since these factors will vary continuously the level of pathogen die off will also vary. Different studies have shown the required storage time to be different however the WHOW guidelines (WHO, 2006) suggests that if the ambient temperature is between 2 – 20 ºC a storage period of 1.5 – 2 years will be sufficient, and for an ambient temperature of 20 – 35 ºC a storage duration of 1 year or more is needed. Storage is best applied alongside other measures and is limited by the die off of AscarisW (parasitic intestinal roundworm) eggs.
Heat Treatment/Composting – High temperature is one of the most reliable methods of reducing pathogen content, thermophilic composting is one method of ensuring high temperatures. Although many toilets are described as composting they do not actually maintain these conditions which require good management to hold a temperature of > 50 ºC for > 1 week (WHO, 2006) (despite this the storage conditions can still render faeces safe). Faeces can also be solar heated whilst composting to increase the temperature. Composting is best applied as a secondary large scale treatment process where it can be adequately managed. Small scale composting needs further evaluation.
Alkaline Treatment – This can take place using either ash and lime or urea. Ash and lime can be added at a household facility level to provide a primary level of treatment, raising the pH above 9 will assist die off in combination with storage. Further benefits include reduced smell, reduction of flies (as material covers faeces) and assistance in moisture content reduction. At least 1 – 2 cups (200-500 ml) of ash and/or lime should be added after each defecation (or enough to cover the faeces) (Schönning and Stenström, 2004). Urea is an additive used for elevating the pH level of faeces, it can also add to the fertilizer value. It is generally accepted as a method for larger scale secondary treatment when professionals can handle the chemicals.
Incineration – This process is accepted to be for larger scale secondary treatment when it needs to be assured that no pathogens remain, the resulting ash will have a lower nutritional value as the nitrogen is lost. Moving the waste to incineration will pose a health risk to workers. Further evaluation of incineration in practice still needs to take place.
Practical guidelines for application of faeces
The following practical recommendations should be followed when applying faeces to land, treatment processes may allow survival of some pathogens and precautions should be taken (Slob, 2005):
. • personal protective equipment such as gloves should be worn and washing of equipment and hands should be practiced;
. • a period of one month should be observed between fertilizing and harvesting, allowing further die off of pathogens whilst the material is on the crops;
. • large levels of phosphorous in faeces make this a good indicator for application rates, more detailed information can be found from EcoSanRes (n.d:b);
. • equipment for sanitised material should not be used for un-sanitized material;
. • faeces should be worked into the soil to minimise exposure to humans or animals;
. • crops that are consumed raw (except fruit trees) should not be fertilized with faecal material;
. • faeces should be buried deep, but not below the rooting depth of the crop; and
. • faeces should be added to soil before planting of crops.
Following these guidelines will minimise the risk of disease transmission. Four separate methods to apply faeces are suggested by Slob (2005):
. • ploughing with tractor or animal drawn;
. • burying faeces under a layer of plain soil forming a bed;
. • placing faeces into channels and covering with unmixed soil; and
. • placing into holes close to where crops will be planted and covering with soil.
Storage
Stored material should be kept dry before use, this could be achieved by elevating and covering the material. Animals and children should not be allowed access to the stored material, simply fencing off the area can achieve this.
Alternative use for faeces
If use of faeces in agriculture is not culturally acceptable the material could be mixed with animal manure and added to a bio-gas digester, although this technology is yet to prove very reliable. If responsibly handled, faeces can also be buried in shallow pits and a tree planted on top, in a similar vein as an 'aborloo'. If use is not possible at all the faeces should be disposed of safely.
Urine
Urine carries the majority of nutrients and few pathogens. This is one of the reasons for employing urine diversion methods as the nutrients are preserved. This section will run through the main methods of urine treatment, the methods for application and other practical recommendations.
Pathogens in urine
Pathogen content in urine is very low, the majority being present due to cross contamination with faeces. Therefore it is of primary concern not to allow faeces into diverted urine.
Treatment
Due to the lower pathogen content the treatment methods of urine are much simpler than for faeces. Storage remains the main methodology. In all cases it is preferable that urine is not diluted, as undiluted urine increases die-off of pathogens and prevents mosquitoes breeding. Treatment options are outlined below.
Storage – This has long been the accepted method of treatment, the level of pathogen dieoff will vary with additional factors, most notably temperature. At a household level it is generally acceptable for the urine to be applied to land without storage as long as the crop is for the households own consumption and a month passes between fertilising and harvesting. This is advised because person-to-person transmission of disease within the household is of greater probability than transmission of disease through contaminated crops. Table 3 shows the recommended storage times at different ambient temperatures.
Table 3: Recommended storage time for urine based on ambient temperature [Adapted from: Schönning and Stenström, 2004]
| Storage Temperature (degree celcius) | Storage Time | Possible pahtogens present after storage | Recommended crops (a) |
|---|---|---|---|
| 4 | > 1 month | Viruses, protozoa | Food and fodder crops to be processed |
| 4 | > 6 months | Viruses | Food crops to be processed; fodder crops (b) |
| 20 | > 1 month | Viruses | Food crops to be processed; fodder crops (b) |
| 20 | > 6 months | Probably none | All crops(c) |
a– The recommended crop is for larger systems where crops will be consumed byindividuals other than members of the household where the urine was collected.
b- Not grasslands for production of fodder.
c – For food crops consumed raw leave one month between fertilising and harvesting andwork the urine into the ground where the edible part of the plant is above the soil surface.
Other Treatments – Storage is the only treatment method that has been widely practiced to date. Evaporation of urine to concentrate nutrients has been attempted but is not yet efficient enough to practice. Increasing the temperature and/or pH of the urine will speed up inactivation further. Increasing temperatures above 20ºC have not yet been practiced (Schönning and Stenström, 2004).
Application of Urine
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Urine can be dealt with in a number of ways. As mentioned above, direct use should only be practiced on a household level, on a larger scale one should practice storage and re-use. In some ecosan toilets the urine will automatically go to a selected soil bed for soaking or to fertilise a small group of plants
(e.g. the Kerala double vault dehydration toilet). The various options are shown in figure 2.
The following are practical recommendations when utilising urine from ecosan facilities (WHO, 2006; Schönning and Stenström, 2004; EcoSanRes, n.d:a; EcoSanRes, n.d:b; Slob, 2005):
. • urine should ideally be worked into the soil, this could be achieved via mechanical means or it could be subsequently watered into the soil i.e. with irrigation water (as mentioned urine should ideally not be stored diluted);
. • the urine should be applied close to the ground to minimise aerosol formation, this could be done manually or, on a larger scale, with agricultural equipment;
. • if Schistosoma haematobiumW is endemic in a region the urine must not be used near freshwater sources;
. • a general rule of thumb is that the urine from one person in 24 hours can be applied to 1m of land per growing season (guidelines based on local fertiliser requirements can be found from EcoSanRes (n.d:b));
. • use of urine should stop between approximately 2/3 and 3/4 of the time between sowing and harvest, after this time the plants reach their reproductive stage and take up less nutrients; and
. • if the urine is likely to have suffered cross contamination further precautions such as protective clothing should be worn. Hand washing should always be practiced.
Alternative use for urine
If the use of urine is not culturally acceptable in food production, the urine can also be added to compost piles to assist the decomposition process. As mentioned the urine can also be soaked away, evaporated or discharge to a plant box connected directly to the toilet.
Mixed faeces and urine
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In some forms of ecosan urine diversion does not take place and the faeces are mixed with the urine. In this case the waste is treated in the same way as faeces. The most established ecosan systems using this method are the Fossa Alterna and the Aborloo. If removed from the pit the resulting compost is similar to that from just faeces, carrying no offensive smell and resembling soil (figure 3).
Pathogens
The pathogens in the material will be the same as faeces, the difference will be that many useful nutrients contained within the urine may be lost in the process. Assuming enough educational should ideally be used, as this utilises the most useful aspects of both components.
Figure 3: Inspecting mixed compost. support and cultural acceptance urine diversion
[Source: Morgan, 2007]
Treatment
Treatment processes are relatively similar to those for faeces, but the inclusion of urine makes the systems less dry and therefore less susceptible to treatment from reduced moisture content. The process mostly involves competition with other organisms (within the soil or added material) and increased temperature. Secondary treatment typically takes place by leaving the waste in the ground and using an alternate pit. Some methodologies will then remove the waste from the pit after a set period of time (long enough to allow pathogens to die off -approximately 1-2 years in tropical conditions), whilst others simply use the nutrients by planting a tree directly onto the pit once it becomes full.
Application and storage advice
In application and storage mixed urine and faeces should be treated the same as separated faeces.
General guidance on application
Alongside the points above there are some more general recommendations (Schönning and Stenström, 2004):
- urine diversion is generally recommended;
- twin pit collection within in a sealed vault is preferable;
- every effort should be taken to reduce faecal contamination of diverted urine;
- solar heating can increase die-off of pathogens;
- toilet paper should be collected separately unless composting or incineration are the intended treatment processes;
- anal cleansing material should not be collected with urine;
- stones used for anal cleansing should be collected separately, vegetable material used for this purpose can be added to the faeces compartment;
- contents of nappies/diapers should be emptied into the faecal compartment;
- other material such as sanitary napkins should only go into the faecal compartment if they are degradable, otherwise they should be treated as solid waste; and
- if diarrhoea is prevalent extra absorbent material may have to be added.
Conclusion
There are clearly many important considerations for householders when using ecological sanitation. This technical brief has described the main treatment methods and application techniques, wherever possible the following control measures should all be practiced (WHO, 2006):
. • treatment of excreta;
. • crop restriction (e.g. only applying material to crops that are either non-food crops or require cooking before eating).
. • practice recommended excreta handling and application techniques;
. • allow recommended time between fertilising and harvesting/consumption;
. • practice appropriate food preparation (e.g. peeling, washing and cooking); and
. • restrict exposure to excreta and implement hygiene education.
The WHO guidance notes on this topic have recently been revised and provide excellent guidance on the topic (WHO, 2006), as do publications by Schönning and Stenström (2004) and Jönsson et al (2004).
The benefits of ecological sanitation are potentially high, following these guidelines will assist in ensuring that these benefits are achieved safely and hygienically. It is worth bearing in mind the barriers illustrated in figure 1 and ensuring that communities practice all possible initiatives to reduce the risk of faecal-oral disease. Simple hand washing devices are detailed in Morgan (2007) that could be installed near to ecosan facilities and other toilets, they should also be adopted for use by those applying material from ecosan facilities to fields.
See also
References and further reading
Toilets that Make Compost: Low cost, Sanitary Toilets that Produce Valuable Compost for Crops in an African Context. Morgan, Peter (2007), EcoSanRes Programmer, Stockholm Environment Institute, Sweden.
http://www.ecosanres.org/pdf_files/ToiletsThatMakeCompost_lowres_greyscale.pdf
Human Waste (Excreta and Wastewater) Reuse. Strauss, Martin (2000) SANDEC, Switzerland.
http://www.eawag.ch/organisation/abteilungen/sandec/publikationen/publications_wra/downl oads_wra/human_waste_use_ETC_SIDA_UA.pdf
Logistics Aspects of Ecological Sanitation in Urban Areas: Case Study in Low-income Community in Dehli, India. Slob, Marieke (2005)WASTE, The Netherlands.
http://web.archive.org/web/20120215150046/http://www.ecosan.nl/page/813
Guidelines for the Safe Use of Wastewater, Excreta and Greywater: Volume 4, Excreta and Greywater Use in Agriculture. WHO (2006), World Health Organisation (WHO), Geneva, Switzerland.
Guidelines for the Safe Use of Urine and Faeces in Ecological Sanitation Systems. Schönning, Caroline and Stenström, Thor Axel (2004), EcoSanRed Programme, Stockholm Environment Institute, Sweden.
http://www.ecosanres.org/pdf_files/ESR_Publications_2004/ESR1web.pdf
Ecological Sanitation. Esrey, S. A., Gough, J., Rapaport, D., Sawyer, R., Simpson-Hebert, M., Vargas, J. and Winblad, U. (1998)SIDA, Stockholm, Sweden.
http://www.ecosanres.org/pdf_files/Ecological_Sanitation.pdf
Guidelines for the Safe Use of Urine and Faeces in Ecological Sanitation Systems. EcoSanRes (n.d:a), EcoSanRes Fact Sheet 5. EcoSanRes, Sweden.
http://www.ecosanres.org/pdf_files/Fact_sheets/ESR5lowres.pdf
Guidelines on the Use of Urine and Faeces in Crop Production. EcoSanRes (n.d:b). EcoSanRes Fact Sheet 6. EcoSanRes, Sweden.
http://www.ecosanres.org/pdf_files/Fact_sheets/ESR6lowres.pdf
Should Ecological Sanitation Carry a Health Warning? Assessing the Health Risks of Ecological Latrines. Scott, Rebecca (2006), WELL Briefing note 27. WELL, Loughborough University.
http://www.lboro.ac.uk/well/resources/Publications/Briefing%20Notes/BN27%20Ecological% 20sanitation.htm
Guidelines on the Use of Urine and Faeces in Crop Production. Jönsson, H., Stintzing, A. R., Vinneras, B. and Salomon, E. (2004), EcoSanRed Programme, Stockholm Environment Institute, Sweden.
http://www.ecosanres.org/pdf_files/ESR_Publications_2004/ESR2web.pdf
Useful websites
WASTE (a Dutch NGO) is doing a large amount of work on ecological sanitation. Information can be found at www.ecosan.nl
A Swedish funded ecological sanitation research group provide a wide range of useful information at www.ecosanres.org
The German international cooperation enterprise for sustainable development, GTZ, provide a wealth of technical information at www.gtz.de/ecosan