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The effect of mass transfer on the removal rate constants of BOD5, NH3, NO3 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 BOD5, 1.88 for NH3, 1.20 for NO3 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. [ABSTRACT FROM AUTHOR]

Laboratory and fieldwork studies were carried out to evaluate the potential of pumice soil for use as a wetland substrate in wastewater treatment. The composition of pumice soil was analysed by x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. Adsorption kinetic studies were carried out in a semi-batch recycle system. Fieldwork tests were carried out on Subsurface Flow Constructed Wetland (SSFCW) cells planted with Phragmites mauritianus and Vetiveria zizanioides. The results have shown that pumice soil composition contains among other elements Al, Ca, Fe and Mg, which are positive indicators for phosphorus adsorption. The main minerals observed by XRD were augite, hematite, and sodium titanium silicate. Phosphorus adsorption kinetics have shown that phosphorus is adsorbed on pumice soil following first order kinetics and the adsorption was highly influenced by mass transfer. Approximately 3% of the phosphorus was removed by plant uptake. [ABSTRACT FROM AUTHOR]