Modelling phosphorous removal in constructed wetland

This research work is carried out so as to assess the performance of constructed wetland piloted within the Department of Water Affairs, Gaborone, Botswana. The treated effluent is reused for irrigation and people are concerned about the quality of effluent used. The wetland has four cells and each cell has flow rate of 4m3/day and hydraulic retention time of approximately 5 days. The first cell (Cl) is set as a control with sand only, the second one (C2) planted with Typha Latifolia, the third one (C3) with Papyrus Cyperus and the last one (C4) with Phragmites Mauritianus. The samples obtained in this pilot system for phosphorus analysis were filtered through a what man 0.45tim filter paper and analyzed using the IC machine. The data from C 1, C2, C3, and C4 showed average effluent phosphorus (PO4-3) concentration of 0.63mg/1, 0.26mg/1, 0.19mg/1 and 0.16mg/1 of which when translated to percentage removal cells have 93.6%, 97.4%, 98.1% and 98.4% respectively. From the effluents it can be noted that C4 is the best macrophyte to be used in treating wastewater. DWA constructed wetland is in compliance with the Botswana Bureau of Standards (BOBS) BOS 93:2004 which allows 0.5mg/l of phosphorus concentration in the effluent that is to be discharged into the environment. A model was developed using the Stella 6.0.1 software and it shows that sand adsorption is the main process involved in phosphorus removal, then plants uptake. The model proved to model the processes in the wetland and this was indicated by a good correlation factor of R2 = 0.55, 0.74, 0.68 and 0.72 for C1, C2, C3 and C4 respectively.