Effects of diffusional mass transfer on the performance of horizontal sub surface flow constructed wetlands in tropical climate

The effect of mass transfer on the removal rate constants of BOD5, COD and TSS was investigated in an experimental scale Horizontal Subsurface Flow Constructed wetland (HSSFCW) planted with Phragmites mauritianus installed at the University of Dar es salaam main campus. 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 pollutants removal rates in CW cells of different widths. For the tested velocity range of 3 – 46 m/day, the removal rate constants were observed to range between 0.31/day – 4.08/day, 1.16/day – 7.10/day and 0.84/day – 12.11/day for BOD5, COD and TSS respectively. Most of the removal rate constants for BOD5, COD and TSS were higher than the typically used temperature dependent removal rate constants. The relationship between flow rates and cell widths was established to guide the location of the baffle wall for enhancement of mass transfer and optimization of treatment performance. The correlation of the dimensionless groups of Reynolds Number (Re), Sherwood Number (Sh) and Schmidt Number (Sc) yielded mass transfer equations with values of (β) of 0.87, 1.05 and 0.92 for BOD5, COD and TSS respectively. These results indicate that the design values of pollutant removal rate constants used to size HSSFCWs are highly influenced by, among other factors, flow velocity. This study suggests the incorporation of mass transfer into CW design procedures to improve the performance of CW systems and reduce land requirements especially in tropical climate regions.