Effect of diffusional mass transfer on the performance of horizontal subsurface flow constructed wetlands in tropical climate conditions

摘要

The effect of mass transfer on the removal rate constants of BOD5,NH3,NO3 and TKN has beenninvestigated in a Horizontal Subsurface Flow Constructed Wetland (HSSFCW) planted withn. The plug flow model was assumed and the inlet and outlet concentrationsnwere used to determine the observed removal rate constants. Mass transfer effects were studied bynassessing the influence of interstitial velocity on pollutant removal rates in CW cells of differentnwidths. The flow velocities varied between 3–46 m/d. Results indicate that the observed removal ratenconstants are highly influenced by the flow velocity. Correlation of dimensionless groups namelynReynolds Number (Re), Sherwood Number (Sh) and Schmidt Number (Sc) were applied and log–lognplots of rate constants against velocity yielded straight lines with values β ¼ 0.87 for BOD5, 1.88 fornNH3, 1.20 for NO3 and 0.94 for TKN. The correlation matched the expected for packed beds althoughnthe constant β was higher than expected for low Reynolds numbers. These results indicate that thendesign values of rate constants used to size wetlands are influenced by flow velocity. This papernsuggests the incorporation of mass transfer into CW design procedures in order to improve thenperformance of CW systems and reduce land requirements.