Determination of the solid–water distribution coefficient (Kd) for pharmaceuticals, estrogens and musk fragrances in digested sludge
Introduction
The issue of pharmaceutical and personal care products (PPCPs) and their metabolites in the environment has raised increasing concern in recent years. PPCPs have been found in a wide range of environmental samples including wastewater, surface, ground- and drinking water (Kolpin et al., 2002; Boyd et al., 2003; Kanda et al., 2003; Glassmeyer et al., 2005; Loraine and Pettigrove, 2006) as well as in sludge and sediments (Ternes et al., 2002; Andersen et al., 2003; Joss et al., 2004).
Sorption can be one of the key factors controlling the removal of organic pollutants in sewage treatment plants (STPs). It may also influence the rate of other processes such as biodegradation and photolysis (Bekbolet et al., 1999). Consequently, the sorption properties of PPCPs are crucial for understanding their fate and impact in the environment.
Sorption depends on the physico-chemical properties of the solids and the chemicals involved (Bowman et al., 2002; Holthaus et al., 2002), as well as on ambient conditions such as pH, ion strength, temperature or the presence of complexing agents (Spark and Swift, 2002).
Several approaches have been used to determine the affinity of a given substance to solids, which are based on the octanol–water partitioning coefficient (Kow) and on the organic carbon-based coefficient (Koc) of the respective compound. Both are reasonable assumptions for non-specific lipophilic interactions. However, concerning PPCPs, recent studies revealed significant deviations from available Kow and Koc coefficients, which could be explained by the fact that pharmaceuticals often contain polar functional groups (e.g. carboxylic moieties, aldehydes and amines), which might interact only with special parts of organic matter (OM) or with minerals, and hence the Kow and Koc values are inappropriate to simulate their sorption properties in sludge samples (Golet et al., 2003; Ternes et al., 2004). Therefore, for modelling the sorption constants of pharmaceuticals, a new approach is needed considering the properties and the polarity of the functional groups of their molecules (Goss and Schwarzenbach, 2001).
Sorption of highly hydrophobic compounds such as PAHs, PCBs and insecticides has been deeply studied over the last decades (Poerschman and Kopinke, 2001; During et al., 2002; Schwarzenbach et al., 2003; Stevens et al., 2003). However, the database about the sorption of moderately polar substances such as pharmaceuticals is much weaker. Recent developments of the analytical methodology for measuring PPCPs in the sludge allowed for the determination of the sorbed quantities, and Kd and Koc values of PPCPs in primary and secondary sludge have been reported (Simonich et al., 2002; Artola-Garicano et al., 2003; Golet et al., 2003; Clara et al., 2004; Holbrook et al., 2004; Ternes et al., 2004; Andersen et al., 2005; Urase and Kikuta, 2005).
In contrast, no data related to digested sludge could be found. During wastewater treatment in STPs, an important fraction of PPCPs is recycled together with the solids to the sludge treatment line, not only those compounds with high sorption properties, but also those substances with low Kd values, because these streams have high concentrations of solids, but also a significant amount of water (>90%). However, this part of the treatment is not often considered when studying the fate of PPCPs in STPs due to the lack of data about either Kd values or sorbed amounts in digested sludge. As it has been found that an important fraction of these compounds is eliminated during sludge anaerobic digestion (Carballa et al., 2006, Carballa et al., 2007a, Carballa et al., 2007b), to have accurate Kd values in digested sludge would avoid the expensive and time-consuming analysis in the sludge phase. These Kd values in digested sludge might not be similar to those obtained for primary and secondary sludge since the digested sludge characteristics in terms of composition (mainly OM), structure and morphology are different, and moreover they could be modified during anaerobic digestion (temperature and pH changes) as well as by the pre-treatment methods (alkaline, thermal and oxidative) applied during advanced treatment of sewage sludge.
The objective of the current paper is to determine the solid–water distribution coefficient (Kd) and the organic carbon normalized distribution coefficient (Koc) for selected PPCPs in digested sludge in order to supply a feasible tool to evaluate PPCPs’ fate during sludge treatment. These values will be compared with the modelled ones using the simple Kow approaches and with those reported in literature for primary and secondary sludge.
Section snippets
Anaerobic digestion pilot plant
The anaerobic digestion pilot plant consists of two lab-scale (10 L) continuously stirred anaerobic digesters. One of them was operated in the mesophilic range (37 °C) and the other in the thermophilic one (55 °C). The digesters were inoculated with methanogenic biomass coming from an upflow anaerobic sludge bed (UASB) reactor operated under mesophilic conditions with sacharose as substrate. The feeding, common for both reactors, was a mixture of primary and secondary sludge (70:30, v/v) collected
Organic carbon distribution coefficient (Koc)
The organic carbon distribution coefficient (Koc) was calculated from the solid–water distribution coefficient (Kd) and the organic carbon fraction (foc) of the digested sludge as shown in the following equations (Schwarzenbach et al., 2003):where X is the PPCP concentration in the sludge (μg kg TSS−1) and S is the PPCP concentration in the aqueous phase (μg l−1).
Modelling of Kd and Koc values
Koc values were modelled as described in Eq. (3) using the pH-dependent octanol–water distribution (Dow), which
Results
Table 3, Table 4 show the results obtained (average values with the standard deviations) for the mesophilic and thermophilic digested sludge at different operational conditions, respectively.
Considering the standard deviations of the results, the Kd values obtained were not significantly influenced by operational conditions (SRT, sludge pre-treatment, temperature) of the anaerobic digestion as long as the pH remained almost constant (between 5.4 and 6.9) during the whole operation (Table 1).
Conclusions
In this study, the solid–water distribution coefficient (Kd) and the organic carbon distribution coefficient (Koc) of 12 substances, including musk fragrances, pharmaceuticals and estrogens, in mesophilic and thermophilic digested sludge have been determined from measurements in the liquid and sludge phases.
These coefficients can be used to evaluate PPCPs’ fate during sludge treatment, thus avoiding the expensive and time-consuming analysis of sludge samples. Furthermore, they provide a
Acknowledgments
This work formed part of the EU Poseidon Project (EVK1-CT-2000-00047) and it was also financially supported by grants from the Xunta de Galicia (2004) and the University of Santiago de Compostela (2005). Special thanks go to Dirk Löffler from BfG and to Alfredo Alder and Elvira Keller from EAWAG for their support during PPCP analysis.
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