Quantification of greenhouse gas emissions from sludge treatment wetlands
Graphical abstract
Highlights
► The static chamber method can be adapted to determine gas emissions from STW. ► There is spatial and temporal variation in the CH4 and N2O emissions in the STW. ► Aerobic conditions before feeding are characterised by low CH4 and high N2O emissions. ► Fresh sludge feeding increases CH4 emissions and decreases N2O emissions. ► The Global warming potential of STW is lower than that of centrifugation and transport.
Introduction
Constructed wetlands (CW) constitute an alternative to conventional wastewater treatment systems to reduce pollutant discharge to water bodies. The application of this technique has increased during the last decades especially in the treatment of different effluents such as municipal and industrial wastewater, agricultural water, landfill leachate and runoff from managed peatlands (Vymazal, 2009). More recently, CW have been adapted to sewage sludge treatment, as a low cost and low energy demand process, to enhance sludge dewatering and stabilisation needed for agricultural uses.
Sludge treatment wetlands (STW) consist of shallow tanks (beds) filled with a gravel layer and planted with emergent rooted wetland plants (Fig. 1). In these systems, secondary sludge is usually pumped and spread on the wetland’s surface. Here, the sludge fed is rapidly distributed over the wetland and part of its water content is rapidly drained by gravity through the gravel layer; while another part is evapotranspirated by plants. In this way, a concentrated sludge residue remains on the surface of the bed where, after some days without feeding (resting time), thickened sludge is anew spread, starting the following feeding cycle. During feeding periods, the sludge layer height increases at a certain rate (around 10 cm/year). When the layer approaches the top of the tank, feeding is stopped during a final resting period (from 1 to 2 months to 1 year), aimed at improving the sludge dryness and mineralisation. The final product is subsequently withdrawn, starting the following operating cycle. Such systems have demonstrated a good applicability as an alternative sludge treatment for small (<2000 Population Equivalent, PE) and remote communities (Uggetti et al., 2011).
Removal of organic matter in wetlands is mediated by several microbial reactions such as aerobic respiration, denitrification, sulphate reduction, fermentation processes and methanogenesis (García et al., 2005). By means of these reactions inorganic gaseous compounds such as methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) can be released to the atmosphere (Kadlec and Wallace, 2009). These compounds are known as greenhouse gases (GHG) due to their contribution to the radiative forcing of the atmosphere and consequently to the Climate Change. Thus, their production by CW is a matter of concern, which needs to be clarified before the massive implementation of this technique.
It is well known that CH4 is produced in anoxic soils and sediments, while drained soils act as a sink for atmospheric CH4 due to methane oxidation through methanotrophs (Hanson and Hanson, 2006). Methane production is regulated by numerous factors, including oxygenation, water table, plant species and temperature (Grünfeld and Brix, 1999).
On the other hand, nitrogen is removed in agricultural soils, riparian buffer zones and natural or constructed wetlands by biological processes, mainly nitrification and denitrification (Brodrick et al., 1998, Bastviken et al., 2005, Maljanen et al., 2004, Groffman et al., 2000). Both nitrification and denitrification can lead to the emission of nitrogen oxides (Kampschreur et al., 2009). Formation of N2O depends on several environmental conditions such as availability of oxygen, carbon and nitrogen, and hydraulic loading rate (Knowles, 1982).
The gas dynamics are strongly affected by climatic factors, especially temperature and water content (Martikainen et al., 1993). Humidity conditions within the wetlands determine the location and extension of aerobic and anoxic processes. As a result, the gas fluxes in the systems have a strong seasonal and temporal variability (Liikanen et al., 2006).
Until now, greenhouse gas emissions have been measured from agricultural soils (Gregorich et al., 2005), rice fields (Ahmad et al., 2009), riparian buffer zones (Teiter and Mander, 2005), peatlands (Alm et al., 1999), municipal wastewater treatment plants (Sümer et al., 1995) and constructed wetlands (Søvik et al., 2006, Søvik and Kløve, 2007) (Table 1). To our knowledge, GHG emissions from sludge treatment wetlands have not been quantified, nor have been developed techniques to determine their gas emissions.
With the increasing application of STW, it is relevant to study their GHG emissions and their possible contribution to the radiative forcing of the atmosphere. Thus, the establishment of a simple and reliable technique to quantify gas emissions from STW is needed to determine and compare their potential environmental impact with alternative systems. The aim of this study is to establish a sampling and analysing method to determine GHG emissions from STW. The methodology is then applied to measure the spatial and temporal variation of methane and nitrous oxide emissions in a full-scale system. Finally, the atmospheric impact of STW is compared to that of conventional sludge management adopted in small communities.
Section snippets
Site description and sludge characterisation
La Guixa is a small wastewater treatment plant (1000 Population Equivalent, PE) located in the province of Barcelona (Spain), which treats 100 m3/d of urban wastewater in an activated sludge with extended aeration system. In this facility, 5 wetlands with a total surface of 210 m2 were established in 2007 to treat waste activated sludge (Fig. 2). The wetlands were planted with Phragmites australis (common reed) with a density of 4 plants/m2. The STW are fed with thickened sludge from a pipe
Adaptation of the static chamber method to STW
The static chamber method was mostly successfully adapted to the determination of gas emissions from STW. An example of the linear regression found in this study is shown in Fig. 4. In some cases, mainly for CH4, the increase in gas concentration in the chamber was not linear within time. The probable reason for the high methane concentration at the beginning of the measurement was the release of bubbles during lid placement. The bubbles have high methane concentration but low nitrous oxide
Conclusions
This study focused on the establishment of a simple and reliable method for the determination of methane and nitrous oxide emissions from STW. The methodology was then applied to measure greenhouse gas emissions and to determine GWP from STW. The following conclusions can be drawn from this study.
- 1.
The static chamber method can be successfully adapted to the determination of gas emissions from STW.
- 2.
In spite of the spatial and temporal variation in the CH4 and N2O emissions in STW, there is no
Acknowledgements
This work was carried out with financial support of the Spanish Ministry of Environment (MMARM, Project 087/PC08). The contribution of Depuradores d’Osona S.L., Ana Ferrer, Anna Pedescoll, Lina Tyroller and Marja Maljanen is appreciated. E. Uggetti is grateful to the Technical University of Catalonia for her PhD scholarship and to the CUR of DUIE of Generalitat de Catalunya for her mobility scholarship (BE-DGR2009).
References (41)
- et al.
Greenhouse gas emission from direct seeding paddy field under different rice tillage systems in central China
Soil and Tillage Research
(2009) - et al.
Potential denitrification in wetland sediments with different plant species detritus
Ecological Engineering
(2005) - et al.
Effect of key design parameters on the efficiency of horizontal subsurface flow constructed wetlands: long-term performance pilot study
Ecological Engineering
(2005) - et al.
Greenhouse gas contributions of agricultural soils and potential mitigation practices in Eastern Canada
Soil and Tillage Research
(2005) - et al.
Nitrous oxide production in riparian zones and its importance to national emission inventories
Chemosphere Global Change Science
(2000) - et al.
Methanogenesis and methane emissions: effects of water table, substrate type and presence of Phragmites australis
Aquatic Botany
(1999) - et al.
Nitrous oxide emission during wastewater treatment
Water Research
(2009) - et al.
Temporal and seasonal changes in greenhouse gas emissions from a constructed wetland purifying peat mining runoff waters
Ecological Engineering
(2006) - et al.
Carbon dioxide, nitrous oxide and methane dynamics in boreal organic agricultural soils with different soil characteristics
Soil Biology and Biogeochemistry
(2004) - et al.
Carbon in tropical wetlands
Geoderma
(1997)
Greenhouse gas emissions from a constructed wetland plants as important sources of carbon
Ecological Engineering
Emission of N2O and CH4 from a constructed wetland in southeastern Norway
Science of the Total Environment
Accumulation of organic solids in gravel-bed constructed wetlands
Water Science and Technology
Emission of N2O, CH4 and CO2 from constructed wetlands for wastewater treatment and form riparian buffer zones
Ecological Engineering
Sludge dewatering and stabilization in drying reed beds: characterization of three full-scale systems in Catalonia, Spain
Bioresource Technology
Sludge treatment wetlands: a review on the state of the art
Bioresource Technology
Technical, economic and environmental assessment of sludge treatment wetlands
Water Research
The use constructed wetlands with horizontal sub-surface flow for various types of wastewater
Ecological Engineering
Winter CO2, CH4 and N2O fluxes on some natural and drained boreal peatlands
Biogeochemistry
Method for determining emissions factors for the use of peat and peatlands-flux measurements and modeling
Boreal Environmental Research
Cited by (51)
Promotion of full-scale constructed wetlands in the wine sector: Comparison of greenhouse gas emissions with activated sludge systems
2021, Science of the Total EnvironmentCarbon footprint of constructed wetlands for winery wastewater treatment
2020, Ecological EngineeringCitation Excerpt :Direct GHG (i.e. CO2, CH4 and N2O) emissions generated in the septic tank (scenario W1), the constructed wetlands (scenario W2) and the activated sludge system (scenario W3) were measured by using a Gasmet DX4015 Fourier transform infrared (FTIR) gas analyser. The measurements of CO2, CH4 and N2O fluxes were done using the static chamber method for the constructed wetlands (scenario W2) (Chen et al., 1997; De la Varga et al., 2015; Uggetti et al., 2012) and the floating chamber method for the activated sludge treatment plant (scenario W3) (Czepiel et al., 1995; Hwang et al., 2016; Ribera-Guardia et al., 2019). Two campaigns were carried out during the vintage season (August/September 2018) and the rest of the year (February/March 2018).