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Natural wastewater treatment systems (WWTSs) for urban areas in developing countries are subjected to large fluctuations in their inflow. This situation can result in a decreased treatment performance. The main aims of this paper are to introduce resilience as a performance indicator for natural WWTSs and to propose a methodology for the identification and generation of realistic disturbances of WWTSs. Firstly, a definition of resilience is formulated for natural WWTSs together with a short discussion of its most relevant properties. An important aspect during the evaluation process of resilience is the selection of appropriate disturbances. Disturbances of the WWTS are caused by fluctuations in water quantity and quality characteristics of the inflow. An approach to defining appropriate disturbances is presented by means of water quantity and quality data collected for the urban wastewater system of Coronel Oviedo (Paraguay). The main problem under consideration is the potential negative impact of stormwater inflow and infiltration in the sanitary sewer system on the treatment performance of anaerobic waste stabilisation ponds. [ABSTRACT FROM AUTHOR]