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Constructed Wetlands and Deconstructed Discourses: Greenhouse gas fluxes and discourses on purifying capacities
Linköping University, The Tema Institute, Department of Water and Environmental Studies. Linköping University, Faculty of Arts and Sciences.
2002 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is a study within the sciences ofconstructed wetlands. It has two sections. Section one is a natural scienrific study ofgas fluxes from construcred wetlands and section two is an analysis ofhow natural sciences describe constructed wetlands.

The aim of section one was to determine methane and nitrous oxide fluxes between the atmosphere and constructed wetlands. The result from these gas flux studies showed that methane fluxes ranged between -375 and 1739 mg m-2 day1, whereas the nitrous oxide fluxes ranged from consumption at -350 to emissions at 1791 Jlg m-2 h-1. The annual inregrated methane fluxes were 87 mg m-2 day1, whereas the inregrated nitrous oxide fluxes resulred in an overall average of 126 µg N2O m-2 h-1. There were large temporal and spatial variations in gas fluxes but no differences between night and day. The methane rates were lower than the average rate, 225 mg m-2 day1, found at a constructed pilot scale wetland treating dairy wastewater in New Zealand (Tanner, 1997), but the cited measuremenrs were only taken during one monrh in mid-summer. The average and range of methane flux values in my study also showed that the methane emission rates were of the same magnitude as those observed from natural wetlands with similar vegetational composition. The nitrous oxide emissions observed in the Nykvarn study were ca. three to ten times higher than in previous studies of constructed wetlands treating wastewater, whereas the nitrous oxide consumption rate was approximately 80 times higher. The nitrous oxide fluxes were 3 to 180 times higher compared to other wetland systems. Emission factors were calculated according to the methods presenred by the Inrergovernmenral Panel of Climate Change (IPCC). The calculated nitrous oxide emission factors from Nykvarn were always lower (0.02 to 0.27%) compared to the default factor provided by the IPCC (0.75%).

The aim of section two was to analyse descriptions on purifying capacities in constructed wetlands in selected scienrific articles. This was made with help of a discourse analysis, which can be explained as a study of communication. The results from this analysis showed that the wetlands were described in terms of self-purifying, biochemical cycles. The cycle concept is used in all three discourses (case I to 3) but it is comprehended differently. In case I the cycling processes are treated as inrrinsic and unproblematic in relation to purification. In case 2, the purifying cycles are treated as dynamic, complex and problematic, but humans could influence them and develop their inrrinsic ability. In case 3 th~ cycling processes became a tool, allowing us to grasp, handle, act upon and influence the wetlands. In the thesis these differences are discussed with respecr to scienrific ideals including language in general and metaphors and concepts in parricular as weil as to the ambiguous relationship between humans and wetlands. Finally, the thesis conrain a reflection on the studies discussed in section I and II. The reflection aims at developing new paths for of gas flux investigations (with biosemiotic perspectives) in research fields related to constructed wetlands.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2002. , 106 p.
Series
Linköping Studies in Arts and Science, ISSN 0282-9800 ; 253
Keyword [sv]
växthuseffekter, vattenrening, miljöaspekter
National Category
Social Sciences Interdisciplinary
Identifiers
URN: urn:nbn:se:liu:diva-29577Local ID: 14953ISBN: 91-7373-321-0 (print)OAI: oai:DiVA.org:liu-29577DiVA: diva2:250394
Public defence
2002-04-26, Elysion, Hus T, Campus VAlla, Linköpings universitet, Linköping, 10:15 (English)
Supervisors
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2014-08-29Bibliographically approved
List of papers
1. Nitrous oxide exchanges with the atmosphere of a constructed wetland treating wastewater: Parameters and implications for emission factors
Open this publication in new window or tab >>Nitrous oxide exchanges with the atmosphere of a constructed wetland treating wastewater: Parameters and implications for emission factors
2003 (English)In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 55, no 3, 737-750 p.Article in journal (Refereed) Published
Abstract [en]

Static chamber measurements of N2O fluxes were taken during the 1998 and 1999 growth seasons in a Swedish constructed wetland receiving wastewater. The dominating plant species in different parts of the wetland were Lemna minor L., Typha latifolia L., Spirogyra sp. and Glyceria maxima (Hartm.) and Phalaris arundinacea (L.), respectively. There were large temporal and spatial variations in N2O fluxes, which ranged from consumption at -350 to emissions at 1791 µg N2O m-2 h-1. The largest positive flux occured in October 1999 and the lowest in the middle of July 1999. The average N2O flux for the two years was 130 µg N2O m-2 h-1 (SD = 220). No significant differences in N2O fluxes were found between the years, even though the two growing seasons differed considerably with respect to both air temperature and precipitation, 15% of the fluxes were negative, showing a consumption of N2O. Consumption occured on a few occasions at most measurement sites and ranged from 1-350 µg N2O m-2 h-1, 13-43% of the variation in N2O fluxes was explained by multiple linear regression analysis including principal components. Emission factors were calculated according to IPCC methods from the N2O fluxes in the constructed wetland. The calculated emission factors were always lower (0.02-0.27%) compared to the default factor provided by the IPCC (0.75%). Thus, direct application of the IPCC default factor may lead to overestimation of N2O fluxes from constructed wastewater-treating wetlands.

National Category
Social Sciences
Identifiers
urn:nbn:se:liu:diva-46586 (URN)10.1034/j.1600-0889.2003.00034.x (DOI)
Note

The originial title of this publication was: Nitrous oxide exchanges with the atmosphere of a constructed wetland treating wastewater : Important parameters and implications for emission factors.

Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2014-09-12Bibliographically approved
2. Methane emissions from a constructed wetland treating wastewater: Seasonal and spatial distribution and dependence on edaphic factors
Open this publication in new window or tab >>Methane emissions from a constructed wetland treating wastewater: Seasonal and spatial distribution and dependence on edaphic factors
2004 (English)In: Water Research, ISSN 0043-1354, E-ISSN 1879-2448, Vol. 38, no 18, 3960-3970 p.Article in journal (Refereed) Published
Abstract [en]

Constructed wetlands for wastewater treatment have many advantages. They can be used for several purposes, for example, to reduce levels of organic matter and nutrients, and to retain toxic metals. However, most wetlands are inherently net sources of gaseous compounds like methane and nitrous oxide, which are of environmental concern due to their rapid current accumulation in the atmosphere and their potent global warming capacity. In order to determine the flux of methane from a constructed wetland a study was conducted over two growth seasons on a pilot scale wetland constructed to reduce nutrient levels in secondary treated wastewater. The emissions for the spring to autumn period averaged 141 mg CH4 m-2 d-1 (S.D.=187), ranging from consumption of 375 mg CH4 m-2 d-1 to emissions of 1739 mg CH4 m-2 d-1. The spatial and temporal variations were large, but could be accounted for by measured environmental factors. Among these factors, sediment and water temperatures were significant in all cases and independent of the scale of analysis (r2 up to 0.88).

Keyword
CH4 fluxes, Greenhouse gas, Seasonal and spatial distribution, Wastewater
National Category
Social Sciences
Identifiers
urn:nbn:se:liu:diva-45628 (URN)10.1016/j.watres.2004.07.008 (DOI)
Note

The original title of this article was: Seasonal and Spatial Distribution of Methane Emissions in a Constructed Wetland Treating Wastewater : dependence on edaphic factors.

Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2014-09-12Bibliographically approved
3. Diurnal variation of methane and nitrous oxide fluxes from a constructed wetland with emergent macrophytes
Open this publication in new window or tab >>Diurnal variation of methane and nitrous oxide fluxes from a constructed wetland with emergent macrophytes
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In order to obtain valid integrated estimates of annual fluxes of gases like methane and nitrous oxide using daytime measurements, it is essential to know their diurnal variations. Therefore, this study was done to supplement measurements of the daytime emission of these two gases from a constructed wetland in southern Sweden. The wetlandconcerned is a pilot scale system of ponds designed to reduce nitrogen levels in the wastewater from a neighbouring sewage treatment plant before it is released into a nearby river. Measurements were made during three occasions spread over the I998 and I999 growth seasons in order to cover young, mature and early senescent phases of the plants present (Typha latifolia L. and Lemna minor L.). Care was taken to capture the morning peaks in gaseous emissions (if present) that have been reported to occur sometimes. The rates of methane emission showed minor diurnal variations, which correlated with fluctuations in carbon dioxide assimilation rates together with wind speed (R2 = 0.55), supporting the hypothesis that pressurised ventilation of plants is involved in the fluxes. Only once was any indication of a morning peak in the methane emissions observed. Nitrous oxide emissions were also correlated with the wind speed (R2 =o.8o), so nitrous oxide was likely transported in the same manner. A conceptual model is presented, accounting for the effects of several importantvariables, such as production, consumption and different gaseous transport mechanisms,that explains the small diurnal variations observed in the methane emissions. The emission rates of the two gases rarely followed the same pattern, and sometimes even had reversed trends. The relatively small diurnal variations could be largely ignored, because of the much larger variations in daytime rates observed over the seasons due to a range of other factors.

National Category
Social Sciences Interdisciplinary
Identifiers
urn:nbn:se:liu:diva-79119 (URN)
Available from: 2012-06-29 Created: 2012-06-29 Last updated: 2012-06-29Bibliographically approved
4. Constructed wetlands - nature's kidneys or cleaning machines?: An analysis of papers on the self-purifying capacity of constructed wetlands
Open this publication in new window or tab >>Constructed wetlands - nature's kidneys or cleaning machines?: An analysis of papers on the self-purifying capacity of constructed wetlands
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This paper focuses on the purifying capacity of wetlands constructed to treat wastewater. Fifty-nine scientific papers about such wetlands, published during the period 1992-1997, were assessed. The aims of the study were, first, to examine how constructed wetlands were described and, second, to analyse the concepts and metaphors related to wetland purifying capacity. In the papers studied, the wetlands were described in terms of self-purifying, biochemical cycles. Particular metaphors, such as "agent", make it possible to convey a specific concept of a wetland's role in wastewater purification. However, the characteristics of self-purification were depicted in three very different ways in different papers. In some cases they were described as simple, stable and effective. In others they were described as complex, dynamic and very variable. Finally, they were sometimes paradoxically described as a combination of the two, for example, as simple and complex, stable and dynamic or effective and with variable efficiency. Thus, there was a conflict in the papers between the idealised view of the intrinsic purifying power of the wetlands and the actual results of purification. The paradox of wetlands being both simple and complex can be explained by the different explanations of the process of purification, as well as by the ambiguous relationship between humans and wetlands.

Keyword
constructed wetland, purifying capacity, description, concept, metaphor, cycles
National Category
Social Sciences Interdisciplinary
Identifiers
urn:nbn:se:liu:diva-79122 (URN)
Available from: 2012-06-29 Created: 2012-06-29 Last updated: 2012-06-29Bibliographically approved

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