Abstract
Due to poor urban sanitation farmers in and around most cities in developing countries face highly polluted surface water. While the sanitation challenge has obvious implications for environmental pollution and food safety it can also provide ‘free’ nutrients for irrigating farmers. To understand the related dimensions, a box-flow model was used to identify the most important water and nutrient flows for the Ghanaian city of Kumasi, a rapidly growing African city with significant irrigation in its direct vicinity. The analysis focused on nitrogen and phosphorus and was supplemented by a farm based nutrient balance assessment. Results show that the city constitutes a vast nutrient sink that releases considerable nutrients loads in its passing streams, contributing to the eutrophication of downstream waters. However, farmers have for various practical reasons little means and motivation in using this resource of nutrients. This might change under increasing fertilizer prices as the nutrient load will continue to increase by 40% till 2015 assuming a widening gap between population growth and investments in water supply on one side and investments in sanitation on the other. However, even a strong investment into flushing toilets would not reduce environmental pollution due to the dominance of on-site sanitation systems, but instead strongly increase water competition. Key options to reduce the nutrient load would be via optimized waste collection and investment in dry or low-flush toilets. The latter seems also appropriate for the city to meet the water and sanitation Millennium Development Goals (MDGs) without increasing water shortages in toilet connected households.
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Acknowledgements
This article and the MSc. thesis on which it is based were supported and financed by Eawag, IWMI, the Swiss Federal Institute of Technology Zurich (ETH) and the Swiss National Center of Competence in Research North-South (NCCR North-South). Special thanks go to Bernard Keraita for the farm interview data, and Daan Van Rooijen and Richard Kuffor from IWMI for their valuable discussions and inputs. We also thank the various partners in Kumasi that contributed to the system analysis and data collection, particularly the Department of Civil Engineering of KNUST, the Kumasi Metropolitan Assembly and the Ghana Water Company Limited.
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Erni, M., Drechsel, P., Bader, HP. et al. Bad for the environment, good for the farmer? Urban sanitation and nutrient flows. Irrig Drainage Syst 24, 113–125 (2010). https://doi.org/10.1007/s10795-009-9083-9
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DOI: https://doi.org/10.1007/s10795-009-9083-9