Modelling of Nitrogen Transformation and Removal in a Constructed Mangrove Wetland
A mathematical model was developed to simulate nitrogen transformation and removal in a horizontal surface flow constructed mangrove wetland receiving domestic sewage from septic tank. The wetland used the already existing mangrove plant species Avicennia marina, that had average height of 4 m. STELLA® 8.1.1 software was used to simulate nitrogen transformation processes and removal. From model simulation, it was found that, the volatilization process was the major removal route of nitrogen from the water phase, accounting for 64% (12 mg/m2.d) of the inflow nitrogen load (18.74 mg/m2.d); followed by sedimentation process that accounted for 25.1% (4.7 mg/m2.d); then de-nitrification, microorganism uptake and plant uptake that together accounted for 6% (1.12 mg/m2.d). The residual nitrogen in the water phase accounted for 5% (0.92 mg/m2.d). For intermediate processes, 6.1 mg/m2.d on N-Soil was transformed to NH3-N through regeneration process, 5 mg/m2.d of Org-N was transformed to NH3-N through mineralization process and 0.13 mg/m2.d of NH3-N was transformed to NO3-N through nitrification process.
The developed model predicted reasonably well the behaviour of constructed mangrove wetland in nitrogen transformation and removal as evidenced by the validation results of simulated and observed data that have good linear regression coefficients (R2) of 0.68 for ammonia-nitrogen, 0.48 for organic nitrogen and 0.61 for nitrate nitrogen. From the findings, the mode can be useful for design of constructed mangrove wetlands in coastal zones in tropical and subtropical regions.