A dynamic water quality model was applied in order to investigate self-purification processes in highly loaded canals in the centre of Bangkok, capital city of Thailand. Oxygen production by aquatic plants induces a significant diurnal variation of the dissolved oxygen concentration. The corresponding profiles of heterotrophic growth and BOD5 concentration demonstrate the limiting impact of oxygen shortage during night time. Both self-purification mechanisms - biological degradation and settling - are considered and water-sediment interactions are calculated. Simulation results and measurement data are summarized by mass balance schemes which offer a telling characterization of the complex system.

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