Nitrogen transformation in horizontal subsurface flow constructed wetlands I: Model development

The mathematical model took into account activities of biomass suspended in the water body and biofilm on aggregates and plant roots. The state variables modelled include organic, ammonia, and nitrate-nitrogen, which were sectored in water, plant and aggregates. The major nitrogen transformation processes considered in this study were mineralization, nitrification, denitrification, plant uptake, plant decaying, and sedimentation. The forcing functions, which were considered in the model, are temperature, pH and dissolved oxygen. Stella II software was used to simulate the nitrogen processes influencing the removal of nitrogen in the constructed wetland. One of the two-wetland units was used for model calibration and the second unit for model validation. The model results indicated that 0.872 gN/m2 d was settled at the bottom of the wetland and on gravel bed and roots of the plants. However, 0.752 gN/m2 d (86.2%) of the settled nitrogen was regenerated back to the water body, which means that only 13.8% of the settled nitrogen was permanently removed. Denitrification and nitrification were responsible for transformation of 0.436 gN/m2 d and 0.425 gN/m2 d, respectively. Uptake of nitrogen by plants was 0.297 gN/m2 d out of which 0.140 gN/m2 d was returned to the water body as plants decay. It was found that the major pathways leading to permanent removal of nitrogen in a horizontal subsurface flow constructed wetland system in descending order are denitrification (29.9%), plant uptake (10.2%) and net sedimentation (8.2%). A total nitrogen removal of 48.9% was achieved in this study.