Abstract
The removal of organic material in free surface gravel bed filters was investigatedand modelled. Two pilot wetland units of 7.5m x 1.5m x 0.7m were constructedadjacent to the University of Dar es Salaam waste stabilization ponds to receivemaximum organic and hydraulic loading rates of less than 185 kg/ha/d and 110l/m 2 /d, respectively. These units received wastewater of domestic characteristicsfrom primary stabilization pond. The entire depth of 0.7 m of the wetland unitswere packed with gravel of size 8 ~ 25 mm and porosity of 35%. Samples werecollected from the influent and effluent of wetland units and were tested forchemical oxygen demand (COD), temperature, dissolved oxygen, pH and otherparameters useful for testing of the model. A semi-empirical model representingthe major processes taking place in the wetland units was developed byincorporating suspended biomass, biofilm biomass, and sedimentation andfiltration components. The model was optimized using computer program. Themodel was successfully developed and showed that biofilm of gravel mediacontributed significantly to COD removal in wetlands.
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