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Modeling of Dissolved Oxygen Transfer Capacity of Constructed Wetlands Treating Domestic Wastewater

R. J. Kimwaga

Abstract


Dissolved oxygen (DO) concentration is one of the principal control parameters in the performance of horizontal subsurface flow constructed wetland (HSSFCW). DO is important parameter since most important degradation processes (carbon degradation, nitrification) require aerobic conditions and thus sufficient oxygen supply is of high importance. Effective supply of air significantly improves the treatment performance of HSSFCW especially Nitrogen removal. Various aeration strategies can be tested by means of modeling and computer simulation. In this study, a model of dissolved oxygen transfer was developed to predict the rates of oxygen transfer capacity in the HSSFCW over a wide range of inlet conditions (flow rates, BOD5, COD, NH3 and NO3). The predicted oxygen transfer rates are based on performance of BOD5, COD, NH3 and NH3, in which the latter is the most difficult parameter to remove in HSSFCW. The study was carried out in the HSSFCW located at the outlet/downstream of Maturation Waste Stabilizations Ponds (MWSP) located at the University of Dar es Salaam main campus, Tanzania. The HSSFCW measuring 3m (width), 9m (length) and 0.6m (depth) was planted with Phragmite Mauritiunus and packed with 8mm – 32mm diameter limestone gravel. The simulation results were compared with data originating from the field. The model that best reflected the temporal distributions of the DO concentrations of the HSSFCW was the one-dimensional diffusion-convection equation with a variable overall oxygen transfer coefficient in the source term for dissolved oxygen. The model results from this study shows that the oxygen concentration from diffusion is 27 gO2m-2day-1 while convection is about 5.7 gO2m-2day-1. The measured and predicted DO concentrations are closely correlated as they gave R2 values of 0.87 suggesting that a model can be used for predicting dissolved oxygen transfer capacity in HSSFCW.

Keywords


Dissolved Oxygen, Rate, Oxygen Transfer, Convection, Diffusion, HSSFCW

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