Wastewater treatment by a natural wetland: the Nakivubo swamp, Uganda : processes and implications

F. Kansiime, M. Nalubega

Research output: Thesisinternal PhD, WU

Abstract

<p>An investigation to assess the capacity of the Nakivubo swamp, Kampala-Uganda (which has been receiving partially treated sewage from the city for more than 30 years now), to remove nutrients and pathogens was carried out. The aim of the study was to evaluate the potential of this swamp to remove nutrients and pathogens from wastewater in a sustainable way, with emphasis on describing and quantifying their pathways, transformations and budgets.<p>From field studies, water balance terms of channel discharges, rainfall, subsurface flows, evapotranspiration and seiches were measured or calculated from existing hydrometeorological data to form a water balance. Nutrients (N and P) and faecal coliforms (FC) transformations in the swamp were studied from four transects cut across the swamp. Vertical and longitudinal profiles of nutrients and pathogens were also constructed. Laboratory simulations were carried out to estimate nutrient fluxes into the plant and sediment compartments and to estimate the removal mechanisms of FC from the water column.<p>In this study differences in the morphological, hydraulic, physico-chemical, floristic and overall wastewater treatment performance between areas covered by the two major vegetation types <em>Cyperus papyrus</em> L. and <em>Miscanthidium violaceum</em> Robyns (about 80% and 20% of the study area, respectively) were elucidated. Papyrus is emergent at the swamp edges where the water level is more affected by the seasons (rainfall). It floats towards the centre and closer to the lake. The loose rhizomatous raft over which papyrus floats allows for fairly free fall-through of plant debris and decomposing matter onto the sediment via the water column resulting in high suspended solids content in the underlying water. This possibly slows, and sometimes restricts water flow in some areas. Due to the lower flows closer to the edges, a thick (up to 60 cm) layer of peaty material is also formed. The loose mat facilitates vertical mixing between the interstitial mat water and the water beneath the mat during the rise and fall of water/mat levels. This lead to a less steep gradient of nutrients over the vertical profile and facilitates nutrient uptake from the water column by papyrus vegetation.<p>In comparison, <em>Miscanthidium</em> vegetation is restricted to the middle of the swamp and is characterised by a thick (0.9 to 1.6 m) mat with highly interlaced roots, but low bulk density (60 - 300 kg/m <sup>3</sup> , surface to bottom). The thick mat helps the retention of falling plant debris on to its surface, where low rate decomposition and further mat accretion take place. The combination of material retention onto the mat surface and high water flows beneath results into a clearer water column and a very thin peat layer (maximum 10 cm) of poorly decomposed plant material. Further, the mat structure prevents free vertical and lateral mixing of the mat water with the water column beneath. This leads to reduced interactions of the plants with wastewater in these zones, and therefore less nutrient abstraction by plants from the wastewater in these zones.<p>The average waste water discharge in the swamp was estimated at 103,575 m <sup>3</sup> /d. Water flow is highly channelised and hydraulic retention times in the swamp during the rainy periods may be as low as 18 hours. Seepage is negligible. Water quality variations within the swamp showed that wastewater is not evenly transported to all parts of the swamp as it flows through.<p>The nutrient load into the swamp was 770 gN/m <sup>2</sup> /yr and 66 gP/m <sup>2</sup> /yr. Different nutrient uptake rates and plant tissue contents (N=1.3%, P=0.21% for papyrus and N=0.64% and P=0.15% for the <em>Miscanthidium</em> vegetated zones) plus the above structural differences in flows and retention times are partly responsible for the disparate purification efficiencies between the vegetation zones. In the papyrus vegetated zones, the average purification efficiencies were 67% N, and TP and 99.3% FC while in the <em>Miscanthidium</em> vegetated zones, it was lower at 55% N, 33% TP and 89.3% FC. The lower flows (about 20%) that went through the papyrus vegetated zones enabled higher retention times for these zones. The major mechanisms of nutrient removal in papyrus vegetated zones were identified to be plant uptake for the nutrients and attachment onto particulates followed by sedimentation, for FC and P. Predation and natural die-off of FC may be high especially in the root zones where micro-aerobic zones exist (mostly in papyrus zones).<p>The thick mat of <em>Miscanthidium</em> limits the number of live roots that can reach the water column to get nutrients from there. Since the bulk (80% near the lake) of the wastewater goes through this zone, then it means that the overall (swamp-wide) nutrient and pathogen removal efficiency from the wastewater is low (56% N, 40% TP and 91% FC).<p>Very low levels of oxygen were observed in the Nakivubo swamp (and very infrequently) due to the high oxygen demand exerted by decomposing organic matter in the swamp. Mostly, either hypoxic or anoxic conditions existed in most compartments of the swamp limiting nitrification although most physical and chemical variables were the range that would favour the survival of nitrifying bacteria. In the <em>Miscanthidium</em> mat, the low pH also possibly limited the viability and the activity of the nitrifiers in this zone.<p>The sharp decline in the concentration of pollutants from the swamp interface to the open waters of the Inner Murchison Bay can be explained by mixing and dilution in the lake. Combined effects of solar radiation, temperature, pH, biocides and the grazing protozoa may also be responsible for the lower FC numbers.<p>To protect the swamp and use it sustainably, efforts should not only concentrate on halting reclamation but also reducing the loads of effluents/pollutants being discharged into the swamp. Distribution of water over the large expanse of the upper and lower Nakivubo swamps in addition to creating a supplementary buffer system in the form of a forest wetland in the upper are suggested as the best sustainable management options. This should be supplemented with a proper wastewater collection and treatment to at least secondary level within the city.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Alaerts, G.J., Promotor, External person
  • Denny, P., Promotor, External person
  • Lijklema, L., Promotor
  • van Bruggen, J.J.A., Promotor, External person
Award date20 May 1999
Place of PublicationRotterdam etc.
Publisher
Print ISBNs9789054104209
Publication statusPublished - 1999

Keywords

  • uganda
  • waste water
  • waste water treatment
  • wetlands
  • wetland soils
  • sewage
  • sewage effluent disposal

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