Nitrogen removal buffer capacity of the Lubigi wetland in Uganda

Abstract

Lubigi wetland in Uganda receives sewage from Lubigi sewage treatment plant and polluted storm water from parts of Kampala city, which contain organic pollutants including nitrogen. This research was formulated to investigate and model the mechanisms and processes governing the transformation and removal of nitrogen in the Lubigi wetland. Wetland characteristics, flora and the transformation and removal of nitrogen were investigated in the Lubigi wetland in Uganda. Pertinent field investigations, surveys, data collection and laboratory tests and analyses were carried out. Nitrogen transformation and removal was modelled using STELLA II version 9.0®2006 software. The results revealed that the wetland main study area basin widths varied from 250 m to 450 m and deepest section of the wetland was 2.5 m. The study area basin total surface area and volume are 1,093,740 m2 and 1,073,060 m3, respectively. There are 9 dominant plants species, and the mean plants density, biomass and nitrogen content are 10.19 ± 4.69 plants/m2, 1.25 kgDWm−2 and 67.54 ± 37.9gNm−2, respectively. The mean influent and effluent discharges were 222,378 m3/d and 221,357 m3/d, respectively. The wetland main study area hydraulic residence times, vary between 6 h and 10 days depending on the season of the year. The major nitrogen transformation and removal mechanisms and processes are plants uptake (10.328 gNm−2day−1), sedimentation (2.467 gNm−2day−1) and denitrification (0.027 gNm−2day−1). It was concluded that wetland plants and sedimentation of organic nitrogen play a key role for nitrogen removal as they are responsible for removal of 67.54 ± 37.9 gNm−2 and 157.5 g gNm−2, respectively.