97 research outputs found
Oser les eaux usées?
Lâeau douce de bonne qualitĂ© pour les usages domestiques et industriels fait lâobjet dâune dure compĂ©tition. Dans ce contexte, lâutilisation des eaux usĂ©es pour lâagriculture permettrait de rĂ©server les eaux de meilleure qualitĂ© Ă ces autres secteurs. Les eaux usĂ©es contiennent ertainement des nutriments de valeur, mais elles sont aussi porteuses de risques sanitaires⊠Voyons cela de plus prĂšs.
Bien que gĂ©nĂ©ralement considĂ©rĂ©e comme inacceptable, lâirrigation Ă partir dâeaux usĂ©es est une pratique courante dans de nombreux pays ACP oĂč les infrastructures sanitaires et de traitement des eaux sont peu dĂ©veloppĂ©es et ne constituent pas une prioritĂ©. Un ingĂ©nieur hydraulicien en chef ghanĂ©en mâa dit un jour : ' Donnons dâabord de lâeau potable saine Ă tous, nous penserons aux eaux usĂ©es ensuite '.
Dans le monde, 20 millions dâhectares sont irriguĂ©s par des eaux usĂ©es, sans compter les usages de saison sĂšche et les eaux usĂ©es diluĂ©es dans les riviĂšres et les ruisseaux. Les eaux usĂ©es sont constituĂ©es de dĂ©chets liquides en suspension dans lâeau, mĂ©langĂ©s aux eaux de surface et Ă celles des nappes phrĂ©atiques.
Les eaux usĂ©es sont principalement des rĂ©sidus domestiques et industriels. En gĂ©nĂ©ral, 80 Ă 85% des eaux utilisĂ©es sont âperdusâ. Dans les villes et les citĂ©s ACP, les Ă©quipements sanitaires â quand ils existent â sont largement insuffisants pour la population. Je ne suis pas opposĂ© au traitement des eaux â les technologies existent : elles vont des filtres les plus complexes aux filtres plus simples comme ceux qui utilisent les lentilles dâeau et les bassins de stabilisation â mais il suffit dâun coup dâĆil sur la situation du Ghana et du Kenya pour comprendre que le traitement ne rĂ©soudra quâune toute petite partie du problĂšme. Câest un travail de longue haleine.
Agir ou disparaĂźtre
En 1995, Ă Accra, Ghana, les autoritĂ©s locales ont promulguĂ© un ArrĂȘtĂ© interdisant lâutilisation des eaux usĂ©es non traitĂ©es pour lâirrigation, mais aujourdâhui, sept ans plus tard, les agriculteurs les utilisent toujours. Câest une question de survie : je connais les risques liĂ©s Ă lâutilisation des eaux usĂ©es mais je ne peux pas les Ă©viter.
Utiliser les eaux usĂ©es prĂ©sente des avantages, liĂ©s notamment Ă leur forte teneur en nutriments, qui rĂ©duit les besoins et les coĂ»ts des engrais. Par consĂ©quent, de nombreux agriculteurs qui les utilisent sâen sortent mieux et peuvent mĂȘme crĂ©er des emplois. De plus, il sâagit souvent de petits paysans pauvres, vivant aux abords des villes, et si nous voulons lutter contre la pauvretĂ©, câest vraiment de ce groupe que nous devons nous occuper. Sâil nây a pas dâautres sources dâeau â ce qui est le cas le plus frĂ©quent â, nous devons trouver des stratĂ©gies pour rĂ©duire lâimpact nĂ©gatif de lâutilisation des eaux non traitĂ©es, car ces pratiques ne cesseront pas.
La principale contrainte est lâimage nĂ©gative de santĂ© publique liĂ©e aux Ă©lĂ©ments contaminateurs comme les mĂ©taux lourds et les agents microbiologiques pathogĂšnes. Mais dans de nombreux pays ACP qui ont un secteur industriel limitĂ©, les mĂ©taux lourds ne reprĂ©sentent pas un problĂšme majeur. Le gros problĂšme, câest plutĂŽt la contamination des cultures par les agents microbiologiques pathogĂšnes notamment par des bactĂ©ries et des helminthes Escherichia coli, Ă lâorigine de dysenteries et diarrhĂ©es. Peu dâagriculteurs portent des vĂȘtements de protection lorsquâils irriguent, car ils nâont pas Ă©tĂ© sensibilisĂ©s et ils nâont pas dâargent pour les acheter. Au Ghana, il est courant de voir un agriculteur manger en arrosant, ce qui peut provoquer des maladies et des infections de la peau. Pour Ă©viter ces problĂšmes, il faudrait former les agriculteurs.
Mon opinion â sans chercher Ă minimiser le phĂ©nomĂšne â est que le problĂšme sanitaire a Ă©tĂ© quelque peu exagĂ©rĂ©. Les nombreuses recherches entreprises sur la question ont Ă©tĂ© centrĂ©es sur les consĂ©quences nĂ©gatives pour la santĂ© plus que sur les consĂ©quences positives pour la production alimentaire. Ce dĂ©sĂ©quilibre a conduit les dĂ©cideurs politiques Ă promulguer des dĂ©crets contre lâutilisation des eaux usĂ©es. Cela a aussi faussĂ© la perception des opinions publiques. Les gens prĂ©tendent que lâutilisation des eaux usĂ©es dans lâirrigation est responsable de la malaria, du cholĂ©ra, de la lĂšpre et ainsi de suite. Mais ce nâest pas nĂ©cessairement le cas ; ces problĂšmes peuvent provenir dâautres activitĂ©s agricoles ou dâintrants comme le fumier. La contamination des cultures peut tout aussi facilement se produire sur les marchĂ©s ou chez les consommateurs eux-mĂȘmes. Nous devons approfondir la recherche pour identifier les effets rĂ©els de lâirrigation par les eaux usĂ©es et leur implication dans la santĂ© humaine comme dans la sĂ©curitĂ© alimentaire.
Cru ou cuit
Certaines pratiques de gestion permettent toutefois de rĂ©duire les effets nĂ©gatifs. PlutĂŽt que dâutiliser les mĂ©thodes dâirrigation par-dessus la plante, il vaut mieux utiliser lâarrosage direct des racines dans la mesure oĂč les sols et les plantes fonctionnent comme des filtres biologiques vivants. De plus, la plupart des agents pathogĂšnes vivent 15 Ă 30 jours et on peut arrĂȘter lâirrigation bien avant la rĂ©colte, ce qui rĂ©duit une partie des risques. Lâimpact sanitaire de lâirrigation par eaux usĂ©es est plus important sur des produits vĂ©gĂ©taux que lâon consomme crus, comme les salades ou les tomates, que sur les produits qui sont consommĂ©s cuits. La plupart des agents pathogĂšnes meurent Ă moins de 60°C.
En rĂ©sumĂ©, jâai toutes les raisons de penser que lâirrigation par eaux usĂ©es continuera Ă se dĂ©velopper dans un futur proche. Avec des niveaux de traitement plus rĂ©duits que pour lâeau de consommation, il devrait ĂȘtre possible dâutiliser les eaux usĂ©es sans risques pour lâagriculture. Mais pour cela, il faut sensibiliser les dĂ©cideurs, les donateurs et tous les acteurs pour quâils rĂ©alisent quâil sâagit dâun besoin rĂ©el et que nous nâavons pas de solutions de rechange.
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NĂ© au Kenya, Bernard Keraita est ingĂ©nieur hydraulicien. Il dirige le projet de recherche sur lâutilisation des eaux usĂ©es pour lâagriculture en Afrique de lâOuest, au sein du Bureau ghanĂ©en de lâInternational Water Management Institute (IWMI)
Les opinions exprimĂ©es dans ce Point de vue sont celles de lâauteur, et ne reflĂštent pas nĂ©cessairement les idĂ©es du CTA.Lâeau douce de bonne qualitĂ© pour les usages domestiques et industriels fait lâobjet dâune dure compĂ©tition. Dans ce contexte, lâutilisation des eaux usĂ©es pour lâagriculture permettrait de rĂ©server les eaux de meilleure qualit
On-farm treatment options for wastewater, greywater and fecal sludge with special reference to West Africa
Methodological challenges of measuring impacts of WASH on educational and health outcomes
Evidence on the impacts of improved Water, Sanitation and Hygiene (WASH) on health and educational outcomes continues to grow and is being used to advocate for an integrated approach to WASH programming. This paper shows that recent analytical work by the United Nations Childrenâs Fund (UNICEF) at the Eastern and Southern Africa Regional Office (ESARO) WASH section in Nairobi has revealed that there is a methodological challenge in building a robust evidence on the impacts of improved WASH on educational and health outcomes. The paper reveals that there remain large gaps in data collection and monitoring processes and that as a result inhibit efforts to make concrete inferences on the impacts of WASH on educational and health outcomes
On-farm practices for the safe use of wastewater in urban and peri-urban horticulture
The drafting of this handbook was initiated in October 2007,
in Ghana, by Ines Beernaerts, in the framework of the FAOKNUST
project on âEvaluation of non-treatment options for
maximizing public health benefits of WHO guidelines governing
the use of wastewater in urban vegetable production in Ghanaâ.
The Farmer Field School (FFS) approach was introduced in
the FAO-KNUST project and materialized in this handbook to
enhance the experimental learning of the (peri-) urban farmers.
The best practices described in this handbook were designed
and field-tested in a project funded by the CGIAR Challenge
Program on Water and Food (CPWF) under the coordination
of the Kwame Nkrumah University of Science and Technology
(KNUST) and the International Water Management Institute
(IWMI). This initiative was taken to enable the project to go
beyond research and ensure the sustainability of the results
No water to waste, not even wastewater
With competition stiff for high quality fresh water for domestic and industrial users, the use of wastewater for agriculture can release better quality water for these other sectors. Wastewater unquestionably contains valuable nutrients, but poses health risks too. A look at the nuances.
Wastewater irrigation is a reality. Though generally unacceptable, irrigation using untreated wastewater exists and is practised in many ACP countries, where sanitation and treatment infrastructures are poorly developed and, besides, are not a priority. A senior government water engineer here in Ghana once told me, 'The first priority is clean drinking water for all, then we shall think of wastewater.'
Worldwide, 20 million hectares are reportedly under wastewater irrigation, but it is much more if we consider seasonal users who use it during the dry seasons, and those who use it in diluted forms from streams or rivers. Wastewater is basically the liquid waste suspended in water, mixed with groundwater or surface water.
The main sources of wastewater are domestic and industrial. As a general rule, 80 85% of water used is wasted. Most populations in ACP towns and cities have outpaced the sanitation facilities, if these facilities are present in the first place. I am not opposed to treatment the technologies are there, ranging from the more
complex trickling filters to simple ones, such as using duck weed and stabilisation ponds but if I just take the cases of Ghana and Kenya at a glance, then treatment will solve only a very small part of this wastewater problem. It is going to be a long-term exercise.
A do or die situation
In Accra, Ghana, the local authorities enacted a bye-law in 1995 prohibiting the use of untreated wastewater in irrigation, but today, 7 years on, farmers are still using it. It is a 'do or die' situation. I know the risks of wastewater use, but stopping it is very impractical.
A major advantage of using wastewater is that it contains high levels of nutrients, reducing the need for and cost of fertilisers. Consequently, many farmers using wastewater are better able to support themselves and their families and often create extra employment. In addition, many of these people are poor, small-scale farmers living on the fringes of towns and cities and I say, if we are aiming at poverty alleviation, this is the group that we have to focus on. If alternative sources of water are not available, which is usually the case, we have to do more research to come up with better strategies to reduce the negative impacts while they continue using untreated water.
The main constraint that often overshadows the benefits of wastewater irrigation is its negative public health image because of its contaminant constituents, such as heavy metals and microbiological pathogens. But in most ACP countries with limited manufacturing industry, heavy metals are not a major problem. The great worry in these countries is crop contamination from microbiological pathogens, including Escherichia coli bacteria and helminths that cause dysentery and diarrhoea. Few farmers wear protective clothing while irrigating, due partly to lack of awareness and partly to lack of money to buy the gear. Here in Ghana it is fairly common to see a farmer eating while irrigating. This exposure can cause diseases and skin-related infections. Farmers should be trained to avoid these problems.
Without playing it down, I personally believe that the health issue is being exaggerated somewhat. Most research on this subject has focused on negative impacts on health rather than on positive impacts
on food provision. This has led to an unbalanced impression among policy-makers who have gone ahead to make bye-laws that are against wastewater use. In turn, this has created distorted perceptions in public opinion. People claim that wastewater use in irrigation causes malaria, cholera, leprosy and so on. But this is not necessarily the case; these problems could stem from other farm activities or inputs such as manure. Besides, crop contamination can occur just as easily at market and consumer levels. We really have to do more research to clearly point out the actual effects of wastewater irrigation and their implications for both health and food security.
Raw or cooked
Nonetheless, several management practices can be implemented to reduce the negative effects. Instead of using overhead irrigation methods such as watering cans and sprinklers, applying water directly to plant roots is more appropriate, since soils and crops act as living bio filters. Furthermore, most pathogens die in 15 30 days, so if irrigation can be stopped well before the crops are harvested, then part of the risk can be reduced. The impacts of wastewater irrigation on crops that are eaten raw, such as lettuce or tomatoes, are likely to pose greater health risks than crops that are cooked before eaten. If crops are cooked well, most pathogens die at less than 60°C.
In short, I have every reason to believe that wastewater irrigation will expand in the near future. With lower levels of treatment required compared to fresh water for consumption, it must be possible to use wastewater safely in agriculture. But we need to make policy-makers, donors and all stakeholders aware that this is a real need and that we are short of alternatives.
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Born in Kenya, Bernard Keraita is an irrigation and water engineer. He currently works with the International Water Management Institute (IWMI) in its Ghana office, where he coordinates the Wastewater Use in Agriculture Research project that covers the West African region.
The opinions expressed in Viewpoint are those of the authors, and do not necessarily reflect the views of CTA.With competition stiff for high quality fresh water for domestic and industrial users, the use of wastewater for agriculture can release better quality water for these other sectors. Wastewater unquestionably contains valuable nutrients, but poses..
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