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Hydraulic loading, stability and water quality of Nakivubo wetland, Uganda
KTH, Superseded Departments, Biotechnology.
Makerere University.
KTH, Superseded Departments, Biotechnology.
KTH, Superseded Departments, Biotechnology.
2004 (English)In: African Journal of Aquatic Science, ISSN 1608-5914, E-ISSN 1727-9364, Vol. 29, no 2, 213-220 p.Article in journal (Refereed) Published
Abstract [en]

Nakivubo wetland, which has performed tertiary water treatment for Kampala city for the past 40 years, is ecologically stressed by agricultural and infrastructural developments. Field studies were carried out to assess the hydraulic loading, pollution profile, stability and water quality of this wetland. The upper and lower Nakivubo wetland receive 4.13-7.66 x 104 and 3.50-10.32 X 104m3/day of water respectively, of which 48.3-57.9% of total hydraulic loading to the upper wetland was carried by sampling station S1. The influent water to the upper wetland had a total BOD5 and NH4-N loading ranging from 2.6-4.4 x 103kg BOD/day and 0.79-1.68 x 103kg NH4-N/day respectively. The National Water and Sewerage Corporation's effluent constituted a large proportion of BOD and NH4-N loading into Nakivubo wetland. Zinc, copper and chromium were detected in trace amounts at most sampling stations. However, lead was occasionally detected at Kibira channel (station S5) at a concentration of 0.4mg/l, which is higher than the permitted Ugandan discharge limit of 0.1mg/l (NEMA 1999). The wetland showed a very high removal efficiency for BOD, ranging from 77.4%-86.3%, compared to ammonium-N which ranged from -66.1% to 33.1% indicating limitations with the nitrification process. A low self-purification for zinc, copper and chromium was also observed in the upper Nakivubo wetland, possibly due to poor plant-wastewater interaction resulting from wetland drainage. In the lower Nakivubo wetland conductivity and dissolved oxygen were generally higher in papyrus- than in Miscanthidium-vegetated zones. However, the BOD and ammonium-N loadings did not vary significantly (P = 0.217 and P = 0.359 respectively) between the two vegetated zones.

Place, publisher, year, edition, pages
2004. Vol. 29, no 2, 213-220 p.
Keyword [en]
heavy metals, hydraulic loading, Nakivubo wetland, stability, water quality
National Category
Other Environmental Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4946DOI: 10.2989/16085910409503812Scopus ID: 2-s2.0-14844288339OAI: oai:DiVA.org:kth-4946DiVA: diva2:7171
Note
Uppdaterad från accepted till published: 20101028. QC 20101028Available from: 2005-02-20 Created: 2005-02-20 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Optimizing processes for biological nitrogen removal in Nakivubo wetland, Uganda
Open this publication in new window or tab >>Optimizing processes for biological nitrogen removal in Nakivubo wetland, Uganda
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The ability of Nakivubo wetland (which has performed tertiary water treatment for Kampala city for the past 40 years) to respond to pollution and to protect the water quality of Inner Murchison Bay of Lake Victoria was investigated. The aim of this study was to assess the capacity of Nakivubo wetland to remove nitrogen from the wastewater after its recent encroachment and modification, in order to optimize biological nitrogen removal processes using constructed wetland technology.

Field studies were performed to assess the hydraulic loading, stability and water quality of this wetland. The distribution and activity of ammonium-oxidizing bacteria (AOB) in Nakivubo channel and wetland were also investigated, and the significance of the different matrices in biological nitrogen transformations within the two systems elucidated. Studies to optimize nutrient removal processes were carried out at pilot scale level both in container experiments and in the field using substrate-free constructed wetlands (CWs) planted with Cyperus papyrus and Miscanthidium violaceum which were adapted to the local ecological conditions.

Results showed that Nakivubo wetland performs tertiary treatment for a large volume of wastewater from Kampala city, which is characterised by large quantities of nutrients, organic matter and to a lesser extent metals. Mass pollutant loads showed that wastewater effluent from a sewage treatment plant constituted a larger proportion of nitrogen and phosphorus and biochemical oxygen demand (BOD) discharged into the wetland. The upper section of Nakivubo wetland exhibited high removal efficiencies for BOD, whereas little or no ammonium-nitrogen and metals except Lead were removed by wetland. Studies further showed that nitrifying bacteria existed in the wetland but their activity was limited by oxygen depletion due to the high BOD in the wastewater and heterotrophic bacteria from the sewage treatment plant. Distributional studies indicated the presence of more AOB in surface sediments than the water column of the lower section of Nakivubo channel, an indication that nitrifiers settled with particulate matter prior to discharge into the wetland, and thus did not represent seeding of the wetland. The significant reductions in concentrations of BOD compared to ammonium and total nitrogen in the channel and wetland wastewater confirmed this finding. Whereas suspended nitrifiers upstream of Nakivubo channel equally influenced total nitrogen balance as those in surface sediments, epiphytic nitrification was more important than that of sediment/peat compartments in the wetland, and thus highlighted the detrimental impacts of wetland modification on the water quality Inner Murchison Bay and Lake Victoria as a whole.

Performance assessment of pilot-scale container experiments and field-based CWs indicated highly promising treatment efficiencies, notably in papyrus-based treatments. Plant biomass productivity, nutrient storage, and overall system treatment performance were higher in papyrusbased constructed wetlands, and resulted in effluent that met national discharge limits. Thus, papyrus-based CWs were found to be operationally efficient in removing pollutants from domestic wastewater.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. viii, 62 p.
Keyword
Environmental technology, Ammonium-oxidizing bacteria, Biological nitrogen removal, Coliform retention, Constructed wetlands, Cyperus papyrus, Miljöteknik
National Category
Other Environmental Engineering
Identifiers
urn:nbn:se:kth:diva-134 (URN)91-7283-962-7 (ISBN)
Public defence
2005-02-25, Kollegiesalen, Valhallavägen 79, Stokcholm, 14:00
Opponent
Supervisors
Note
QC 20101028Available from: 2005-02-20 Created: 2005-02-20 Last updated: 2010-10-28Bibliographically approved

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