Wastewater use in urban and peri-urban agriculture

While more than half of Africa’s population already lives in an urban environment (especially small and medium cities), and the number of urban residents expected to rise up to 20.2% in 2050 (CSIS, 2018), agriculture still remains an important source of employment and could be a place of innovation for the environmental and demographical issues that most of these areas are facing. If the variety of climates, soils, cultures and economies cannot allow us to generalize our reflexions to the entire continent, we can however underline the importance of the water management in urban and peri-urban areas, as the line defining urban and rural is becoming blurrier, and urban agriculture is becoming increasingly important for the food autonomy and development of a lot of cities (H. De Bon et al., 2010). This tendency induces conflicts around water distribution, pollution, sanitation and between local agriculture and food importation. While the density of the cities imposes important quantities of water for the inhabitant’s consumption and sanitation, more and more farmers found themselves obliged to use wastewater in order to maintain the food autonomy of the cities nearby (J. Kihila et al., 2014).

Wastewater discharge next to a cropland, FAO (2017)

At first sight, urban and peri-urban agriculture might seem to be a solution to both environmental and economic issues, by reducing the food chain to a local scale and allowing inhabitants to maintain food security while still having access to the urban amenities. Indeed, wastewater contains a lot of nutrients (an irrigation rate of 1.5m per year could supply up to 225kg of Nitrogen and 45kg of Phosphorus per hectare per year, according to the study made by J. Kihila), and can be a cost-effective way to avoid freshwater over consumption and to reduce the cost of fertilizers. However, this use of wastewater also fosters the propagation of pathogens, can exposes farmers to toxic chemicals and endanger the consumers, especially when the products are consumed raw, because of the different pollutants such as faecal matter and heavy metals. For example, a study made by P. Antwi-Agyei et al. in 2015 has collected and analysed 159 samples of lettuce coming from wastewater irrigated fields in Accra, Ghana, and found out that 96% of them were tested positive for E. coli (a bacterium of which some strains can be pathogenic, while this one (R.V.C. Doigo et al., 2010) made in Niamey, Niger, underlines the presence of Salmonella (a protobacterium that can cause typhoid and paratyphoid fiver as well as salmonellosis) in addition to E. coli. Not treating wastewater in urban environment is also an environmental issue with the growing eutrophication of waterways (P.J. Oberholster, 2019), a situation where agriculture run-off and urban sewage’s nutrients accumulate and can lead to an oxygen-reduced water and an imbalance in the ecosystem, .  
Even though the dangerousness of this system is still difficult to assess, food-born diseases represents an important issue, with 1.8 million deaths in the world due to diarrhoeal diseases in 2005 largely attributed to food and drinking water (D.G. Newell, 2010), treating this water could be the best option to cut down risks of diseases.

However, most of the treating systems seems to be unattractive to farmers, as they are choosing wastewater because it is easy to have access to, and cost-effective. We can see here a discrepancy between the environmentalists’ studies and the practical reality of the smallholders farmers in theses areas of mutation, as most of the water treating systems are too complicated and expensive to buy and maintain throughout the years. As opposed to typical centralized infrastructures this study by N.A. Oladoja (2017) recommends the use of on-site, decentralized, gravity percolation of wastewater as one of the most effective way to ensure an appropriate management, a cost-effective, sustainable and low-energy infrastructure.

 Schematic representation of a typical centralized wastewater treatment system, Oladoja, N.A. Appropriatetechnology for domestic wastewater management in under-resourced regions of theworld (2017)
 

An example of gravity powered water treatment system can be found in the video linked below, which explains the construction of small-scale water treating systems in Yemen, where more than 500 000 suspected cases of cholera were detected in 2017 (WHO, 2017), mainly because of the use of contaminated wastewater in the food chain. This project was made in collaboration with Japan, the FAO and local communities in order to make sure that this infrastructure was adapted to the needs of the concerned population, and especially of the women who were the most affected by the cholera, and was developed along with a prevention campaign about safe water.

 
                                     FAO and Japan partner to fight cholera on Yemen's farms, 2019, YouTube

I found this topic especially interesting because it underlines how water and agriculture are linked to a wide range of other topics, especially in Africa, such as health, environment, gender and governance. I hope this article was clear and interesting, thank you for reading it!