Environmental assessment of anaerobically digested sludge reuse in agriculture: Potential impacts of emerging micropollutants
Introduction
Huge amounts (average 30 kg dry matter/inhabitant·year) of sewage sludge (SS) are generated all over the world from sewage treatment plants (STPs) and its management in an economically and environmentally acceptable way has become a matter of increasing importance during the last few years (Cartmell et al., 2006, Murray et al., 2008, Peters and Rowley, 2009). Among the available technologies, anaerobic digestion (AD) is commonly used at medium and large STPs and final reuse in agriculture has risen significantly since the European Union banned the disposal of SS at sea in 1998. In Spain, 65% of SS was disposed to land in 2006 and it should increase up to 85% by 2015 (PNIR, 2008).
The problem is that biosolids constitute a complex mixture that, in addition to nitrogen and phosphorus, often contains inorganic (metals and trace elements) and organic pollutants, such as endocrine disrupting compounds (EDCs) and Pharmaceutical and Personal Care Products (PPCPs) (Buyuksonmez and Sekeroglu, 2005, Jones-Lepp and Stevens, 2007). Many studies have investigated the phytotoxic effect of heavy metals in soils amended with biosolids (Lopez-Mosquera et al., 2000, McDonald et al., 1994), but surprisingly, there is little information about EDCs and PPCPs fate in soils, particularly after land application of sewage sludge. Recent studies have shown that PPCPs move rapidly (within minutes) from agricultural fields to adjacent surface waters via tile drainage systems (Lapen et al., 2008) and the octanol–water partition coefficient (Kow) is commonly considered the key factor of runoff transport potential of PPCPs (Sabourin et al., 2009). However, the injection of biosolids slurry below the soil surface could effectively eliminate surface runoff of PPCPs (Topp et al., 2008). In addition, PPCPs are poorly biodegraded by the bacterial community present in SS-amended soils (Redshaw et al., 2008). Therefore, undesirable effects, such as intake by plants, leaching into the groundwater and negative impact on the terrestrial organisms are not excluded.
Life Cycle Assessment (LCA) is a holistic tool increasingly used to evaluate the environmental burdens associated with a product, process or activity (Baumann and Tillman, 2004, ISO 14040, 2006). LCA has been widely used for examining both sludge treatment and end-use options (Peters and Lundie, 2002, Lundin et al., 2004, Okuno et al., 2004, Hospido et al., 2005, Houillon and Jolliet, 2005, Svanstrom et al., 2005, Cartmell et al., 2006, Murray et al., 2008, Peters and Rowley, 2009). In contrast, the application of LCA to evaluate the environmental impact associated with the presence of micropollutants in wastewaters or sludge is up to date limited. Muñoz et al. (2008) have quantified the potential environmental impacts on ecotoxicity and human toxicity of wastewaters containing up to 98 compounds, including priority and emerging pollutants and, in another study, the same authors have used LCA to compare several wastewaters reuse scenarios involving different tertiary treatments and including 84 pollutants (Muñoz et al., 2009). Two similar studies (Høibye et al., 2008, Wenzel et al., 2008) have applied LCA to evaluate different advanced wastewater treatment technologies (sand filtration, ozonation and membrane bioreactors) and their effect on several micropollutants (organic substances, heavy metals, estrogens and pathogens). In all these works, either sludge treatment was disregarded or incineration was assumed as the disposal pathway. For that reason, it is stressed the importance of evaluating the environmental consequences of other sludge routes, such as land disposal.
This paper makes use of LCA to evaluate the reuse of anaerobically digested sludge in agricultural land, focusing on the possible impacts caused by emerging micropollutants. Moreover, the influence of different operational conditions applied during the anaerobic digestion process on the digested sludge quality has been analyzed. To our knowledge, this is the first study addressing those issues and it will provide the missing information about the contribution of the sludge treatment to the potential impacts of emerging micropollutants on ecosystems and human health.
Section snippets
Material and methods
According to the ISO 14040 standard (2006), an LCA study comprises four stages: goal and scope definition, inventory analysis, impact assessment, and interpretation.
Comparative results
Fig. 1 and Table 2 display the comparative results from the impact assessment stage among the different scenarios studied. As each impact category is expressed in its corresponding unit of reference (i.e. kg PO43−-eq for EP, kg CO2-eq for GWP and kg PDB3-eq for HTP and TTP), the results in Fig. 1 are presented as relative values (%) in order to be visually comparable on the same axis.
Eutrophication: analysis in detail
Eutrophication covers all potential impacts of excessively high
Conclusions
This paper has quantified the environmental impact associated to the application of digested sludge on agricultural soil in terms on eutrophication, global warming as well as human and terrestrial toxicity. The novelty of the analysis is the inclusion of emerging micropollutants, in particular 13 PPCPs, associated to the sludge (dissolved and sorbed).
From an environmental point of view, the disposal of undigested sludge is the less suitable alternative. Only the results on GWP contradict this
Acknowledgments
This research was funded by postdoctoral contracts for Dr. Almudena Hospido and Dr. Marta Carballa from the Xunta de Galicia (IPP-06-57 and IPP-08-37, respectively). The authors also thank the Spanish Ministry of Education and Science for its financial assistance (Projects CSD2007-00055 and CTQ2007-66265/PPQ) and the Xunta de Galicia (Project 09MDS010262PR). We would like to express our gratitude to Dr. Ivan Muñoz (University of Almeria, Spain) for his kind provision of the characterisation
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Both authors equally contributed to the work.