Nitrogen removal buffer capacity of the Lubigi wetland in Uganda

Highlights

• Field investigations, data collection and analyses of water, sediments and plants were carried out in the Lubigi wetland.

• The mechanisms governing nitrogen dynamics in the wetland were modelled using STELLA II software.

• The dominant pathways of the influx nitrogen were through plants' uptake (40.8%) and sedimentation (42.8%).

• The studied section of the wetland removed 20.1% of total nitrogen, which is equivalent to 987 tons/year.

• Wetland plants and sediments played a key role for nitrogen removal in the wetland.

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.

Introduction

Natural wetlands provide a wide variety of bio-physical and socio-economic functions such as promotion of biodiversity, preservation of water quality, storage of floods and control of erosion (Novitzki et al., 1997; Fisher and Acreman, 2004; Muraza et al., 2013). However, they are facing increasingly serious threats from watershed anthropogenic environmental degradation and human and infrastructural encroachments (Co'zar, 2007; Kayima, 2020). With increasing population, urbanization and industrialization, wetlands are increasingly becoming recipients of raw or partially treated wastewater discharges in many countries of the world (UNEP, 2013; Kayima et al., 2018a, Kayima et al., 2018b). For example, wastewater treatment plants of the urban areas of major Ugandan cities discharge their effluents into the nearby natural wetlands (Ministry of Water and Environment, Uganda, 2015), although they have capacities to remove some pollutants from water and wastewater, and thus to some extent filtering and purifying it (Kansiime, 2004; Kayima et al., 2018a, Kayima et al., 2018b; Kayima, 2020). This demand for particular urgency in their efficient management and sustainable utilisation (Ministry of Water and Environment, Uganda, 2015). However, in accordance with Gumm (2011), wetlands in Kampala are inadequately protected because of lack of accountability, conflict of interest and lack of resources, which eventually lead to misuse of the wetlands.

One of the wetlands severely strained from anthropogenic activities is the Lubigi wetland, which is one of the largest Lake Kyoga drainage basin wetlands. The anthropogenic activities affecting this wetland include illegitimate disposal of solid wastes and municipal wastewaters (African Development Fund, 2008) and construction of major projects in the wetland such as high-tension electric power line and a sewage treatment plant (Watebawa, 2012; Kayima, 2020). Lubigi wetland also receives storm water discharge from Nsooba-Lubigi drainage channel with average capacity of about 220,000 m³/day (Kayima and Mayo, 2018). All these human activities in and around the wetland has resulted into the degradation of the wetland. In accordance with Habonimana (2014), GIS mapping showed that 40% of the wetland was degraded, which is well above the national average of 30%, which may reduce its buffering capacity.

The Lubigi wetland buffers Lake Kyoga against pollution from some parts of Kampala city (Kayima, 2020). One of the important functions of the wetland is reduction of nitrogen from raw or partially treated wastewater (Metcalf and Eddy Inc.; 2003; Vymazal, 2007; Kansiime et al., 2007; Mayo et al., 2018). Excessive concentrations of nitrogen can stimulate excessive growth of aquatic flora, which can lead to blockages of receiving fresh water bodies and interfere with navigation (UNEP, 2013). Eutrophication of natural water bodies can lead to dissolved oxygen depletion and toxicity to aquatic fauna, which can cause the death of fish and render water septic and odorous and therefore unfit for human and animal use (Vymazal, 2007). High nitrate levels can lead to methamoglobinaemia in infants and increased cancer risks in human beings (Senzia, 2003).

As nitrogen exists in wastewater in various forms, transformation of nitrogen in the wetland from one form of nitrogen to another may involve volatilization of NH₃, nitrification, denitrification, uptake by plants and microorganisms, sedimentation, regeneration from benthic layer, anaerobic ammonia oxidation, mineralisation, sorption and desorption (Vymazal, 2007; Mayo et al., 2018). Some of these transformation routes leads to permanent nitrogen removal. The reduction of nitrogen in wetland ecosystem is influenced by multiple factors including uptake by plant biomass and microorganisms (Lee et al., 1975; Bigambo and Mayo, 2005), sedimentation (Johnston et al., 1984; Kayima, 2020) and denitrification (Lowrance et al., 1984; Jordan et al., 1993; Mayo et al., 2018). Therefore, the performance of the wetland will depend on the wetland characteristics that enhance reduction of the pollutant (Fisher and Acreman, 2004; Vymazal, 2007). The main objective of this research study, is to investigate the wetland ecological factors, and the processes governing the fate of nitrogen in the Lubigi wetland.