Modeling of nitrogen transformation and removal in horizontal surface flow constructed mangroves wetland

The potential use of Constructed Mangrove Wetlands (CMWs) as a cheaper, effective and appropriate method for nitrogen removal from domestic wastewater of coastal zone in peri urbarn cities, was investigated from August 2007 to September, 2008. Field investigations were made on horizontal surface flow constructed mangrove wetlands located at Jangwani beach in Dar es Salaam. Two wetland cells of 40m x 7m each were constructed to receive domestic sewage from septic tank of Belinda Beach Hotel. One cell was operated in a batch mode while the other operated in an intermittent continuous flow mode. Both wetland cells used the already existing mangrove plant specie Avicennia Marina. The performance of the wetland cells and comparison between the wetland cells in the treatment of domestic sewage were determined and a mathematical model was developed to permit simulation of nitrogen transformation and removal in the wetland cell operated in an intermittent continuous flow mode. In carrying out of modeling process, Stella II software (STELLA®8.1.1) was used. In the intermittent continuous flow wetland cell; the observed removal rates of NH3-N, NO3-N, TKN, PO4-P, COD and Faecal Coliforms were 85%, 76%, 61%, 35%, 49% and 98%, respectively. In the batch wetland cell; the observed removal rates of NH3-N, NO3-N, TKN, COD and Faecal Coliforms ware 28%, 20%, 14%, 9% and 45%, respectively. It was found that, the wetland cell that was operated in an intermittent continuous flow mode performed better than the one operated in batch mode. From model simulation, it was found that, the volatilization process was the major removal route of nitrogen, accounting for 35% (30 mg/m2.d) of the inflow nitrogen load. The model presented in this work is suitable for design of horizontal surface flow constructed mangrove wetlands receiving domestic wastewater for secondary treatment in tropical coastal zones.