In this paper an extended version of IWA's Activated Sludge Model No 3 (ASM3) was developed to simulate processes in waste stabilisation ponds (WSP). The model modifications included the integration of algae biomass and gas transfer processes for oxygen, carbon dioxide and ammonia depending on wind velocity and a simple ionic equilibrium. The model was applied to a pilot-scale WSP system operated in the city of Florianópolis (Brazil). The system was used to treat leachate from a municipal waste landfill. Mean influent concentrations to the facultative pond of 1,456 gCOD/m3 and 505 gNH4-N/m3 were measured. Experimental results indicated an ammonia nitrogen removal of 89.5% with negligible rates of nitrification but intensive ammonia stripping to the atmosphere. Measured data was used in the simulations to consider the impact of wind velocity on oxygen input of 11.1 to 14.4 gO2/(m2 d) and sun radiation on photosynthesis. Good results for pH and ammonia removal were achieved with mean stripping rates of 18.2 and 4.5 gN/(m2 d) for the facultative and maturation pond respectively. Based on measured chlorophyll a concentrations and depending on light intensity and TSS concentration it was possible to model algae concentrations.

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