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

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 beta = 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 beta 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.

Use of vetiver grass constructed wetland for treatment of leachate.

Performance of Constructed Wetland planted with vetiver grasses for the treatment of leachate was investigated in controlled experiments involving horizontal subsurface flow constructed wetland (HSSFCW). The HSSFCW experimental unit had two cells, one planted with vetiver grasses and another bare. Both units were packed with limestone gravel as substrate and were operated with equal hydraulic loading and hydraulic retention time. Collected samples of influents and effluents were analysed for COD, Cr, Pb, Fe and pH. The results showed that vetiver grasses tolerated leachate with high loading of COD up to 14,000 mg L(-1). The planted cell outperformed the unplanted cell in terms of COD, Cr, Pb and Fe removal. The systems showed optimum points for COD and Pb removal as a function of feed concentrations. The optimum COD removal values of 210 mgm(-2) day(-1) at feed COD concentration of 11,200 mg COD L(-1) and 89 mgm(-2) day(-1) at feed concentration of 7,200 mg COD L(-1) were obtained for planted and unplanted cells respectively. Similarly Pb removal values of 0.0132 mgm(-2) day(-1) at 1.0 mg Pb L(-1) and 0.0052 mgm(-2) day(-1) at 1.04 mgPb L(-1) were obtained for planted and unplanted units respectively. Removal of Fe as a function of feed Fe concentration showed a parabolic behaviour but Cr removal showed linear behaviour with feed Cr concentrations in both units. The system showed very good removal efficiencies with Cr and Fe but poor efficiencies were recorded for Pb.

The use of Toddalia asiatica (L) Lam. (Rutaceae) in traditional medicine practice in East Africa.

Toddalia asiatica (L) Lam. (Rutaceae) has been used by traditional health practitioners in East Africa for management of diseases, however, the extent of its usefulness has not been established to date. Fieldwork for this study was carried out in the Lake Victoria Basin between March and September 2006. The purpose was to collect ethnomedical information that will serve as a basis for further studies to establish current and potential medicinal uses. The ethnomedical information was obtained through interviews using semi-structured questionnaires. Consultative meetings were also conducted with traditional health practitioners and other members of the communities in Kenya, Uganda and Tanzania. Results of this study show that Toddalia asiatica is collected in the wild, prepared mostly as decoctions or concoctions and administered orally. It is used for the management of a number of disease conditions. The most frequently cited diseases were stomach problems (78%) followed by malaria (25%). Cough (22%), chest pain (13%), food poisoning (8%), sore throat (7%), were also mentioned among other disease conditions treated. Validation studies of therapeutic claims will be carried out at a later date.

Pumice soil: a potential wetland substrate for treatment of domestic wastewater.

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.