2.1.1 Calculations based on the USEtox models
The USEtox models describe the toxic impacts of chemical contaminants released to the environment on humans and freshwater aquatic organisms. The environmental fate submodel is a multimedia model consisting of a range of homogeneous compartments at three geographical scales (i.e. urban, continental and global) and each representing a specific part of the environment (e.g. air, water, soil). The fate submodel accounts for removal processes and intermedia transport processes and represents steady-state conditions. The exposure pathways (for human toxicity) covered in the USEtox models are related to the compartments air, drinking water, aboveground produce (in USEtox 2.0 terminology or “exposed produce” in USEtox 1.01 terminology), belowground produce (in USEtox 2.0 terminology or “unexposed produce” in USEtox 1.01 terminology), meat, dairy and fish. Some of the exposure pathways are not applicable in the context of this study, however. For instance, in Sweden, it is illegal to spread sewage sludge on grazing land. For a detailed description of the USEtox models, the reader is referred to the literature (Rosenbaum et al.
2008; Rosenbaum et al.
2011; Fantke et al.
2015). The USEtox calculations were performed based on the USEtox models downloaded from the USEtox homepage (
www.usetox.org).
Four sets of CFs were obtained based on the USEtox models. That is, each USEtox model version (i.e. USEtox 1.01 and USEtox 2.0, respectively) was run with two parameterisations. The first parameterisation corresponds with the default parameterisation; the second parameterisation consists of model parameters modified with intent to reflect the local conditions (i.e. Västra Götaland County) to the extent feasible. The parameter values used are provided in Table
S1 in Online Resource 1 (Electronic Supplementary Material).
The potential burden of disease related to the metals and organic contaminants contained in sewage sludge applied to agricultural land was calculated as shown in Eq.
1. Values shown in parentheses for the disease burden per case were taken from EC (
2010).
$$ {\mathrm{BoD}}_i={E}_i\times \left({CF}_i^c\times {\mathrm{DALY}}_i^c+{CF}_i^{nc}\times {\mathrm{DALY}}_i^{nc}\right) $$
(1)
BoD
i
burden of disease for contaminant i [DALY]
E
i
emission of contaminant i [kgemitted]
CF
i
c
characterisation factor (cancer) for contaminant i [cases kgemitted
−1]
CF
i
nc
characterisation factor (non-cancer) for contaminant i [cases kgemitted
−1]
DALY
i
c
disease burden per case (cancer) for contaminant i (11.5) [DALY case−1]
DALY
i
nc
disease burden per case (non-cancer) for contaminant i (2.7) [DALY case−1]
2.1.2 Calculations based on the SLAtox model
The SLAtox model addresses only the uptake of chemical contaminants through agricultural produce. The SLAtox model is based on the assumption of steady state in the soil compartment. That is, the amount of a given contaminant added to the soil (
E
i
) is assumed to be equal to the amount removed through loss processes such as volatilisation, leaching, degradation and plant uptake. The intake fraction of a given contaminant (iF
i
) through ingestion of agricultural produce was calculated as the fraction of plant uptake relative to all loss processes, furthermore allowing for food losses (i.e. edible produce lost between harvest and retail) and food waste (i.e. edible produce wasted in retail and by final consumers) (Eq.
2). The first-order rate constants for biodegradation were taken from the USEtox 2.0 database. Values shown in parentheses were chosen by the authors. The first-order rate constants for plant uptake, leaching and volatilisation were calculated based on ECB (
2003) and VKM (
2009) according to Equations
S1 to
S5 in Online Resource 1 (Electronic Supplementary Material).
$$ {\mathrm{iF}}_i={f}_{\mathrm{fl}}\times {f}_{fw}\times \frac{k_{\mathrm{plant}}}{k_{\mathrm{plant}}+{k}_{\mathrm{volat}}+{k}_{\mathrm{leach}}+{k}_{\mathrm{biodeg}}} $$
(2)
iF
i
intake fraction for contaminant i [kgintake kgemitted
−1]
f
flfood loss factor (0.9) [–]
f
fwfood waste factor (0.7) [–]
k
biodegfirst-order rate constant for biodegradation in topsoil (USEtox 2.0 database) [day−1]
k
plantfirst-order rate constant for plant uptake from topsoil (Equation
S1) [day
−1]
k
leachfirst-order rate constant for leaching from topsoil (Equation
S2) [day
−1]
k
volatfirst-order rate constant for volatilisation from topsoil (Equation
S3) [day
−1]
The iFs obtained through the SLAtox model were used in combination with human health EFs taken from the USEtox 2.0 database and applied to the emissions of chemicals resulting from the application of sewage sludge to agricultural land as shown in Eq.
3. Values shown in parentheses for the disease burden per case were taken from EC (
2010).
$$ {\mathrm{BoD}}_i={E}_i\times {\mathrm{iF}}_i\left({EF}_i^c\times {\mathrm{DALY}}_i^c+{EF}_i^{nc}\times {\mathrm{DALY}}_i^{nc}\right) $$
(3)
BoD
i
burden of disease for contaminant i [DALY case−1]
E
i
emission of contaminant i [kgemitted]
iF
i
intake fraction for contaminant
i (Equation
2) [kg
intake kg
emitted
−1]
EF
i
c
effect factor (cancer) for contaminant i (USEtox 2.0 database) [cases kgintake
−1]
EF
i
nc
effect factor (non-cancer) for contaminant i (USEtox 2.0 database) [cases kgintake
−1]
DALY
i
c
disease burden per case (cancer) for contaminant i (11.5) [DALY case−1]
DALY
i
nc
disease burden per case (non-cancer) for contaminant i (2.7) [DALY case−1]
The SLAtox model was implemented in Microsoft Excel 2011. For metals, the SLAtox model distinguished between two types of crops: root crops (belowground produce) and leafy crops (aboveground produce). To avoid double counting, it was assumed that half of the sludge is put on agricultural land where aboveground produce is grown and half of the sludge is put on agricultural land where belowground produce is grown. For organic contaminants, bioconcentration factors (BCFs) were available only for belowground produce. Therefore, it was assumed that all of the sludge is put on agricultural land where belowground produce is grown. For some metals (e.g. arsenic, chromium, antimony), the USEtox database provided EFs for two different oxidation states. As the monitoring data did not make such a distinction, we considered both oxidation states in the calculation of disease burden for individual metals. To avoid double counting, it was assumed that half of the respective metal was present in each of the two respective oxidation states.
2.1.3 Accounting for accidental ingestion of sewage sludge
In LCA, it is a common practice to base the comparison of the performance of different product or service systems, or different life cycle phases of a product or service system, on a typical situation when technical systems operate according to the design specifications. Because of a significant variability in the procedures and locations used for sewage sludge management, RA studies often include occupational, recreational, as well as residential exposure pathways. In the context of sewage sludge management, the frequency of the occurrence of operating conditions where the system does not work according to the design specifications may warrant consideration of non-routine operation scenarios in LCA. In this study, we estimated the amount of treated sewage sludge accidentally ingested that would lead to a disease burden equal to the disease burdens estimated by the USEtox and SLAtox models.