The effects of biofilm activities on nitrogen transformation in horizontal subsurface flow constructed wetlands

The effects of biofilm activities on nitrogen transformation in horizontal subsurface flow constructed wetlands were studied by using a developed mathematical model. The state variables modelled include organic, ammonia, and nitrate-nitrogen, which were sectored in water, plant and aggregate. Stella II software was used to simulate the nitrogen processes influencing the removal of nitrogen in the constructed wetland. The biofilm effects were evaluated basing on removal efficiencies of the above state variables. Two wetland units were planted with Phragmites mauritianus. Data collected from the first unit were used for model calibration while data from the second unit were used for model validation. The biofilm model parameters were obtained from literature. The major nitrogen transformation processes considered in this study were mineralization, nitrification, denitrification, plant uptake, plant decaying, and sedimentation. It was found that the major pathways leading to permanent removal of nitrogen in a horizontal subsurface flow constructed wetland system in ascending order are denitrification (29.9%), plant uptake (10.2%) and sedimentation (8.2%). A total nitrogen removal of 48.9% was achieved in this study. There was a nitrogen removal of 38.1% from plant-biofilm and suspended activities as compared to 25.1% from aggregate-biofilm and suspended activities. The averages in organic-nitrogen effluent concentrations were 0.39 gN/m2, 0.41 gN/m2 and 0.53 gN/m2 for suspended alone, aggregate-biofilm and suspended; and suspended and plant-biofilm, respectively.