The transformation and removal of nitrogen was studied in a pilot high rate pond with a surface area of 10.2 m 2 and water depth of 60 cm. The pilot unit received wastewater from an existing field scale primary facultative pond at the University of Dar es Salaam. Wastewater samples were collected from the influent and effluent of high rate pond and were analyzed for physical-chemical parameters in the laboratory and in situ. An appropriate model complexity was selected, from which a conceptual

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... odel was then developed to model various processes in the system using STELLA 6.0.1 software. The study demonstrated that dominant nitrogen transformation processes in HRP were nitrification and denitrification, which transformed 0.95 and 0.87 gN/m 2 •d, respectively. These were followed by mineralization (0.37 gN/m 2 •d), ammonia uptake by microorganisms (0.34 gN/m 2 •d), volatilization (0.30 gN/m 2 •d), sedimentation (0.24 gN/m 2 •d), and regeneration (0.15 gN/m 2 •d). Uptake of nitrate was not observed because of microorganisms preference for ammonia, which was abundant in the pond. The major nitrogen transformation mechanisms in high rate pond were denitrification, net sedimentation and volatilization, which accounted for 69.1%, 7.1% and 23.8% of the total permanent removal mechanisms of nitrogen in High Rate Pond.