Conventional and source-separating urban sanitation systems are compared with regard to their ecological sustainability using the methodology of Life Cycle Assessment. A substance flow model of all relevant processes in a settlement with 5,000 inhabitants is set up and evaluated with environmental indicators for resource demand and emissions to air, water, and soil. The comparison shows that source separation does not necessarily result in a system with less environmental impacts. If the conventional system is energetically optimized and equipped with extended nutrient removal, its impact is comparable to the source-separating systems. However, source separation has the potential to offer ecological benefits depending on the system configuration. Especially the input of toxic heavy metals to agriculture with sewage sludge can be substantially lowered if separately collected urine and faeces are used as organic fertilizer.

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