This study was conducted to determine performance of free water surface (FWS) constructed wetlands located in the tropics in the removal of organic matter and to assess the importance of biofilm bacteria in the overall kinetics of organic matter (or chemical oxygen demand, COD) removal. Because constructed wetlands normally contain both biofilm and suspended bacteria, a kinetic model incorporating the activities of these two bacteria groups, dispersion number and hydraulic retention time, was employed. The model parameters essential for the calculation of COD removal in FWS constructed wetlands were determined from the experiments and some from the literature. The model was found satisfactory in predicting COD removal efficiencies in a pilot-scale FWS constructed wetland unit treating a domestic wastewater.

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