Dynamics of a waste stabilization pond in Chile

A 5-modular units facultative waste stabilisation pond was experimentally s tudied for performance and efficiency, in relation to its original design. The system was located at La Esmeralda, Melipilla a small town of the centr al region in Chile. It was found that treated water did not comply with tre atment requirements due to overloading, flow dynamics, wrong assumptions re garding hydraulic mixing behaviour, and an overestimation of systems kineti cs. The overloading in terms of flow led to a retention time which was 82% of original design. Furthermore, bad mixing further decreased the designed residence time to only 18%. The effluent did not meet the WHO faecal confor ms guidelines for unrestricted irrigation. The modelling work on the batch faecal coliforms and coliphage removal concluded that the effective decay r ates were 1.359x1.087((T-20 degreesC)) day(-1) for coliforms, and 0.439x1.0 44((T-20 degreesC)) day(-1) for coliphages. The data was also well fitted b y a model that incorporated solar radiation. However, model discrimination indicated that the dependence on temperature gave a better and more sensiti ve fit with the observed data. As the original design considered a lower fa ecal coliforms decay ratio, namely 0.7 day(-1), the failure of the system h ad to relate more to mixing than overloading. Operational seasonal data (fl ow and retention time) and physical, chemical and microbiological parameter s, and tracer studies are included. Some solutions for improving the La Esm eralda WSP performance are also discussed.