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Nitrogen and phosphorus removal in substrate-free pilot constructed wetlands with horizontal surface flow in Uganda
KTH, School of Biotechnology (BIO).
KTH, School of Biotechnology (BIO).
2005 (English)In: Water, Air and Soil Pollution, ISSN 0049-6979, E-ISSN 1573-2932, Vol. 165, no 1-4, 37-59 p.Article in journal (Refereed) Published
Abstract [en]

In constructed wetlands (CWs) with horizontal sub-surface flow, nutrient removal, especially phosphorus, is limited because the root biomass fills the pore spaces of the substrate (usually gravel), directing wastewater flow to deeper wetland media; plants are not regularly harvested; the litter formed by decomposing vegetation remains on the surface of the substrate and thus does not interact with the wastewater; and the substrate media often used provide only limited adsorption. Effective nutrient removal including rootzone oxidation, adsorption and plant uptake therefore requires sufficient interaction of wastewater with the treatment media. We assessed the feasibility of biological nutrient removal from wastewater using substrate-free CWs with horizontal flow, planted with two tropical macrophytes namely, Cyperus papyrus and Miscanthidium violaceum. The objectives were to evaluate the system treatment efficiency under semi-natural conditions, and to assess microbial and plant biomass contributions to nutrient removal in the CWs. Results showed high removal efficiencies for biochemical oxygen demand, ammonium-nitrogen (NH4-N) and phosphorus (P) fractions in papyrus-based CWs (68.6-86.5%) compared to Miscanthidium (46.7-61.1%) and unplanted controls (31.6-54.3%). Ammonium oxidizing bacteria in CW root-mats (108-109 cells/gram dry weight) and residual nitrite and nitrate concentrations in the water phase indicated active system nitrification. Papyrus showed higher biomass production and nutrient uptake, contributing 28.5% and 11.2%, respectively, of the total N and P removed by the system compared to 15% N and 9.3% P removed by Miscanthidium plants. Compared to literature values, nitrification, plant uptake and the overall system treatment efficiency were high, indicating a high potential of this system for biological nutrient removal from wastewaters in the tropics.

Place, publisher, year, edition, pages
2005. Vol. 165, no 1-4, 37-59 p.
Keyword [en]
Constructed wetlands, Cyperus papyrus, Horizontal flow, Miscanthidium violaceum, Nitrification, Nutrient uptake, Plant biomass production, Substrate-free, Uganda
National Category
Other Environmental Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4950DOI: 10.1007/s11270-005-4643-6ISI: 000231156600004Scopus ID: 2-s2.0-23844438907OAI: oai:DiVA.org:kth-4950DiVA: diva2:7175
Note
QC 20100923. Uppdaterad från Accepted till Published (20100923).Available 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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