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2.6 billion people in the world lack access to satisfying sanitation. In addition to the indignity and uncleanliness in their situation, untreated excrements pollute ground and surface waters, with both health and environmental hazards as consequences. The extreme urbanization rate in many developing countries have worsened the situation and complicated the implementation of sustainable solutions. At the same time, the soils in sub-Saharan Africa are generally poor and the use of fertilizers much lower than on all other continents. The recent trend of heavily increasing food prices has made the urgent matter of securing food-supply even more pressing. Hence, sanitation systems that secure health and environment condition, and that also enable reuse of the nutrients in human excrements as fertilizers have the potential of being beneficial for many reasons.

Three scenarios for systems handling liquid household waste (urine, faeces and greywater) in Kumasi were defined: the Urine diversion, the Biogas, and the Waterborne scenario. A model based on material flow analysis (MFA) and life cycle assessment (LCA) was constructed to evaluate the environmental performance of the scenarios. The main model variables were nitrogen, phosphorus and organic carbon, and the evaluation focused on eutrophication and potential reuse of nutrients in urban and peri-urban agriculture.

The results showed that a local nutrient reuse approach did not appear applicable to dense, urban areas, since the production of fertilizers was much larger than the need. It seemed however, to be a feasible option in more spatial areas, where farms and back yard cultivation are more common. The future city development was concluded to be an important factor in the choice of sanitation system. Continued practice of urban and peri-urban agriculture give reason for at least partly local-reuse-oriented systems, while decrease of agriculture within the city area may speak in favor of more centralized solutions. The existing use of cheap poultry manure from farms in the peri-urban area may weaken the arguments for reuse-oriented local sanitation systems in Kumasi.

The waterborne and the biogas scenarios made reuse of treated wastewater possible (e.g. for irrigation). Eutrophying effect depended highly on the amount of water that was assumed to be reused: Full reuse made the waterborne scenario the least eutrophying and the urine diversion the most, whereas no reuse resulted in the lowest eutrophying effect for the urine diversion scenario and the highest for the biogas scenario.

The waterborne scenario was associated with much higher water consumption than the other two scenarios, a problem in a city with already deficient water providing systems.  It required less truck transports than the other scenarios, but its construction and operation are likely much more energy demanding.