Effect of diffusional mass transfer on the performance of horizontal subsurface flow constructed wetlands in tropical climate conditions
Water Science and Technology
Waste Stabilization Ponds and Constructed Wetland Research and Development Group, University of Dar es Salaam, P. O. Box 35131, Dar es Salaam, Tanzania
The effect of mass transfer on the removal rate constants of BOD 5, NH 3, NO 3 and TKN has been investigated in a Horizontal Subsurface Flow Constructed Wetland (HSSFCW) planted with Phragmites mauritianus. The plug flow model was assumed and the inlet and outlet concentrations were used to determine the observed removal rate constants. Mass transfer effects were studied by assessing the influence of interstitial velocity on pollutant removal rates in CW cells of different widths. The flow velocities varied between 3-46 m/d. Results indicate that the observed removal rate constants are highly influenced by the flow velocity. Correlation of dimensionless groups namely Reynolds Number (Re), Sherwood Number (Sh) and Schmidt Number (Sc) were applied and log-log plots of rate constants against velocity yielded straight lines with values β = 0.87 for BOD 5, 1.88 for NH 3, 1.20 for NO 3 and 0.94 for TKN. The correlation matched the expected for packed beds although the constant β was higher than expected for low Reynolds numbers. These results indicate that the design values of rate constants used to size wetlands are influenced by flow velocity. This paper suggests the incorporation of mass transfer into CW design procedures in order to improve the performance of CW systems and reduce land requirements. © IWA Publishing 2011.
Constructed wetlands; Design procedure; Design value; Diffusional mass transfer; Dimensionless groups; Horizontal subsurface flow constructed wetlands; Log-log plots; Low Reynolds number; Mass transfer effects; Outlet concentration; Phragmites; Plug flow models; Pollutant removal; Removal rate constant; Schmidt numbers; Sherwood numbers; Straight lines; Tropical climates; Flow velocity; Inlet flow; Mass transfer; Packed beds; Reynolds number; Velocity; Wetlands; Rate constants; ammonia; nitrate; nitrogen; total kjeldahl nitrogen; unclassified drug; ammonia; ammonium; biochemical oxygen demand; constructed wetland; correlation; diffusion; flow velocity; fluid flow; grass; mass transfer; nitrate; organic nitrogen; perennial plant; performance assessment; pollutant removal; subsurface flow; tropical environment; article; biochemical oxygen demand; concentration (parameters); diffusion; flow rate; land use; Phragmites mauritianus; plant; pollutant; tropic climate; wetland; Biological Oxygen Demand Analysis; Facility Design and Construction; Hydrodynamics; Models, Theoretical; Tanzania; Tropical Climate; Water Movements; Water Pollutants, Chemical; Water Purification; Wetlands; Phragmites mauritianus