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

Abstract

In this paper a mathematical model for prediction of nitrogen transformation in horizontal subsurface flow constructed wetlands was developed. Two horizontal subsurface flow constructed wetlands were designed to receive organic loading rate below 50 kg/ha/d and hydraulic loading rate of 480 m 3/ha/d from a primary facultative pond. Two rectangular shaped units each 11.0 m long, 3.7 m wide and 1.0 m deep and bottom slope of 1% were constructed and filled with 6–25 mm diameter gravel pack to a depth of 0.75 m. Each unit was planted with Phragmites mauritianus with an initial plant density of 29,000 plants/ha. The plants were allowed to grow for about four months before sampling for water quality parameters commenced. Samples were collected daily for about three months. Dissolved oxygen, pH and temperature were measured in situ and ammonia, total Kjeldahl nitrogen, nitrates, nitrite and Chemical Oxygen Demand were measured in the laboratory in accordance with Standard Methods. 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/m 2 d was settled at the bottom of the wetland and on gravel bed and roots of the plants. However, 0.752 gN/m 2 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/m 2 d and 0.425 gN/m 2 d, respectively. Uptake of nitrogen by plants was 0.297 gN/m 2 d out of which 0.140 gN/m 2 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.