Modelling Oxygen Transport in a Reedbed-constructed Wetland Purification System for Dilute Effluents

This paper describes a simple model of oxygen supply to aerobic microorganisms in a horizontal reedbed-constructed wetland. The model simulates oxygen transport through the water by diffusion and convection, and via the macrophyte plants to the microorganisms that reside on their roots. The model also describes nitrogen (N) transformation processes for organic material with high biological oxygen demand (BOD), ammonium and nitrate. Some of these transformation processes are aerobic and therefore constrained if the oxygen supply is inadequate. Parameter values have been selected for equations describing oxygen transport and N transformation rates to fit data from a functioning reedbed processing sewage from a rural community. This reedbed achieves a large reduction in ammonium concentration in the effluent. Since it is unlikely that so much ammonium could have been lost by volatilisation alone, this implies rapid transport of oxygen to the microorganisms to convert the ammonium to nitrate. This rate is substantially higher than that which can be explained by diffusion and convection through the water alone, suggesting an important role for oxygen transport via the plants to the roots.