A 3-modular units facultative waste stabilisation pond was experimentally studied for performance and efficiency, in relation to its original design. The system was located at La Esmeralda, Melipilla a small town of the central region in Chile. It was found that treated water did not comply with treatment requirements due to overloading, flow dynamics, wrong assumptions regarding hydraulic mixing behaviour, and an overestimation of systems kinetics. The overloading in terms of flow led to a retention time which was 62% of original design. Furthermore, bad mixing further decreased the designed residence time to only 18%. The effluent did not meet the WHO faecal coliforms guidelines for unrestricted irrigation. The modelling work on the batch faecal coliforms and coliphage removal concluded that the effective decay rates were 1.359×1.087(T-20°C) day-1 for coliforms, and 0.439×1.044(T-20°C) day-1 for coliphages. The data was also well fitted by a model that incorporated solar radiation. However, model discrimination indicated that the dependence on temperature gave a better and more sensitive fit with the observed data. As the original design considered a lower faecal coliforms decay ratio, namely 0.7 day-1, the failure of the system had to relate more to mixing than overloading. Operational seasonal data (flow and retention time) and physical, chemical and microbiological parameters, and tracer studies are included. Some solutions for improving the La Esmeralda SWP performance are also discussed.

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