Quantification of greenhouse gas emissions from sludge treatment wetlands

Abstract

Constructed wetlands are nowadays successfully employed as an alternative technology for wastewater and sewage sludge treatment. In these systems organic matter and nutrients are transformed and removed by a variety of microbial reaction and gaseous compounds such as methane (CH₄) and nitrous oxide (N₂O) may be released to the atmosphere. The aim of this work is to introduce a method to determine greenhouse gas emissions from sludge treatment wetlands (STW) and use the method in a full-scale system. Sampling and analysing techniques used to determine greenhouse gas emissions from croplands and natural wetlands were successfully adapted to the quantification of CH₄ and N₂O emissions from an STW. Gas emissions were measured using the static chamber technique in 9 points of the STW during 13 days. The spatial variation in the emission along the wetland did not follow some specific pattern found for the temporal variation in the fluxes. Emissions ranged from 10 to 5400 mgCH₄/m² d and from 20 to 950 mgN₂O/m² d, depending on the feeding events. The comparison between the CH₄ and N₂O emissions of different sludge management options shows that STW have the lowest atmospheric impact in terms of CO₂ equivalent emissions (Global warming potential with time horizon of 100 years): 17 kgCO₂eq/PE y for STW, 36 kgCO₂eq/PE y for centrifuge and 162 kgCO₂eq/PE y for untreated sludge transport, PE means Population Equivalent.

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.