Biochemical oxygen demand (BOD) reaction kinetic and mass transfer in horizontal subsurface flow constructed wetland

The BOD5 reaction kinetics and mass transfer from three constructed wetlands of different sizes and flow rates planted with Phragmites mauritiamts were studied. The plug flow model was assumed and the concentrations of BOD5 were used to determine the overall removal rate constant of dissolved organic matter. The rate constants were determined at inlet flow rates of2m3/d, 4m3/d, 7.7m3/d, and 44m3/d. Their Corresponding rate constants were found to be 0.06/d,0.1/d, 0.15/d and 0.7/d, respectively. The removal rate constants were observed to increase with increasing flow velocity. This implies that, they obtained rate constants were heavily influenced by mass transfer in the system. Correlation of dimensionless groups namely Reynolds Number, Sherwood Number and Schmidt Number was applied to the obtained rate constants. Plot of the log K against log u yielded a straight line with x = 0.86.The correlation matched the literature values of x for packed beds although the constant x obtained was higher than the literature values for laminar situation in the wetland, i.e. x = 0.67.It was concluded that, the removal rate and thus the efficiency of constructed wetlands for the removal of BOD is highly influenced by the hydrodynamics situation in the wetland. Because of the strong dependence on mass transfer, all measures that increase mass transfer will increase the performance of constructed wetlands.