Nitrogen removal in a coupled HRP and subsurface gravel bed constructed wetland.

Nitrogen transformation mechanisms that contribute to the highest nitrogen removal were investigated in a coupled HRP and SSCW units. The HRP unit was 4.6 m x 2.0 m x 0.6 m size while the SSCW was 7.5 m long, 1.5 m wide and 0.7 m deep. The SSCW was filled to above top water level with 19-mm diameter aggregates. To assess the performance of the system samples were collected and analysed for pH, dissolved oxygen, N03-N, N02-N, TKN and NH3-N. Samples were also tested for COD and total suspended solids. All analyses were conducted in accordance with the Standard Methods (1992). Two test phases were conducted during the study. In Phase I wastewater was retained for 5 days in the HRP and SSCW units. Phase II had an 8-day retention time in the HRP with 5-day retention time in the SSCW. Phase II produced a better performance with a 38.4% average total nitrogen removal compared to 33.3% for Phase I. The most efficient mechanisms were determined using a transformation model. Nitrification, sedimentation and denitrification processes were dominant in Phase I contributing 29%, 22% and 20%, respectively. Nitrification and denitrification dominated Phase II transformations with 32% and 22% contributions, respectively. The nitrification-denitrification route was considered the most significant mechanism for nitrogen removal in the coupled system with combined contributions of 49% and 54% in Phases I and II, respectively. The model predicted the overall nitrogen removals as 29% and 37% for Phases I and II, respectively. These compared well with removals obtained from field measurements where Phases I and II recorded 33.4% and 39%, respectively.