Modeling of Nitrogen transformation and removal in horizontal subsurface flow constructed wetlands during treatment of Domestic wastewater

Field investigations were made on pilot scale horizontal subsurface flow constructed wetlands units located at the University of Dar es Salaam main campus. Four units covering surface area of 40.7 m2 each were constructed downstream of the primary facultative pond and two units each of surface area 15.9 m2 were constructed downstream of the maturation pond. The performance of systems planted with local available macrophytes, Phragmites mauritianus or Typha domingensis were studied and mathematical model was developed to permit dynamic simulation of nitrogen interaction in horizontal subsurface flow constructed wetland treating domestic wastewater. Nitrification, denitrification, plant uptake, plant decaying and accretion of organic nitrogen are the processes considered in the model. Stella II software (STELLA ® 6.0.1) was used to simulate these processes. the system located downstream of the primary facultative pond, which were planted with Phragmites mauritianus, the denitrification process accounted for 0.219 gN/m2.d, which was 15.0°/o of the incoming nitrogen (1.458 gN/m2.d). Plant uptake accounted for 0.195 gN/m2.d (13.4°/o) from the system. f above ground biomass is harvested, 10°/o of incoming nitrogen will be removed from the system. About 6.2°/o of influent nitrogen load was unaccounted for. Similar trends of nitrogen dynamics were found in model for Typha domingensis with slight differences in internal cycling rates. The model developed for the system downstream of the maturation pond showed that, denitrification was major removal mechanism accounting for 20.5010 . The model presented in this work is suitable for design of horizontal subsurface flow constructed wetland receiving domestic wastewater for secondary treatment in tropical climates.