A framework for the evaluation of anthropogenic resources: the case study of phosphorus stocks in Austria

Journal of Cleaner Production xxx (2014) 1e14

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A framework for the evaluation of anthropogenic resources: the case study of phosphorus stocks in Austria Jakob Lederer*, David Laner 1, Johann Fellner 1 Christian Doppler Laboratory for Anthropogenic Resources, Institute for Water Quality, Resource and Waste Management, Vienna University of Technology, Karlsplatz 13/226.2, A-1040 Vienna, Austria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 October 2013 Received in revised form 30 April 2014 Accepted 7 May 2014 Available online xxx

Due to a predicted shortage of raw materials, increasing attention is drawn to the utilization of anthropogenic resources through recycling and urban mining. However, in order to assess the availability of anthropogenic resources as potential raw materials, a consistent concept for categorizing anthropogenic materials into reserves, resources and other occurrences is needed. This study presents a framework for the evaluation of anthropogenic resources, derived from the standard procedure for resource and reserve identification, evaluation, and classification of the U.S. Geological Survey for natural stock resources. The framework was applied to a case study on phosphorus (P) stocks in Austria. Results indicate that only 10% of the anthropogenic P stocks in Austria (one million tons in total) are extractable at subeconomic levels with production costs 5e10 times above the market price for P fertilizer. 70% of P stocks are not technically extractable and 20% of such a low grade that recovery is not practically feasible. Based on the assessment, it is found that the extractable amount of P could have been much higher if Prich materials were not mixed with low-grade materials during landfilling. Although the evaluation of anthropogenic P stocks in Austria was performed on a screening level, the application of the framework highlights that a consistent method for the evaluation of anthropogenic resources can provide a basis for enhanced utilization of anthropogenic resources. In future, further case studies are needed to demonstrate the application of the evaluation framework for various resources and in consideration of environmental, technological, and societal factors. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Anthropogenic resources Urban mining McKelvey diagram Resource classification Landfill mining Phosphorus

1. Introduction The increase in resource consumption recorded during the last two centuries led to policy initiatives, such as the European Commissions' resource strategy and the United Nations Environment Programme (UNEP) International Resource Panel (EC, 2011; UNEP, 2013). Moreover, numerous concepts have been designed to overcome the predicted resource shortage, such as the propagation of increased material efficiency (Allwood et al., 2011), sufficiency instead of material-consuming economic growth (Princen, 2003), and waste recycling, urban mining and similar waste-related concepts (Cossu, 2013; Tchobanoglous and Kreith, 2002). Material efficiency and sufficiency address a reduced input of natural resources

* Corresponding author. Tel.: þ43 1 58801 22653. E-mail addresses: [email protected], [email protected] (J. Lederer), [email protected] (D. Laner), [email protected] (J. Fellner). 1 Tel.: þ43 1 58801 22643.

into the anthropogenic system, while the waste-related concepts focus on output materials from anthropogenic utilization, called anthropogenic resources. In modern history, various meanings have been assigned to the term resource. Generally, resources are the means a subject uses to meet and feed its needs. Thereby, natural resources are transformed by the resources of capital and labor (Smith 1776). Before industrialization, in most of the then-agrarian societies, agricultural land and inputs were the most important natural resources. Industrialization shifted the focus to other raw materials, such as fossil fuels and minerals. Until the 1940s, scientific and political discussions as well as actual practices in Europe and the U.S., considered not only natural but also anthropogenic resources, such as wastes. The latter, in particular, was observed from the perspective of limited technologies and access to markets of natural resources (Klinglmair and Fellner, 2010; Strasser, 1999). As soon as technology and a new stage of economic globalization began in the 1950s, the discussion about anthropogenic resources temporarily declined, until it recurred in the 1970s (Strasser, 1999). Then, research not only

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Please cite this article in press as: Lederer, J., et al., A framework for the evaluation of anthropogenic resources: the case study of phosphorus stocks in Austria, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.05.078

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detected environmental pollution, a waste of materials, and a predicted shortage of raw materials (Boulding, 1966; GeorgescuRoegen, 1971; Meadows et al., 1972) but also found that for some materials, industrial production and consumption had built up significant anthropogenic material stocks which, for some materials (e.g., metals), may be on the same order of magnitude as natural stock reserves (see Fig. 1) (Kapur and Graedel, 2006; Klee and Graedel, 2004). Consequently, to date, approximately 30% of the copper consumed in Europe originates from secondary resources, and almost 70% of the US demand for iron and steel is met by scrap recycling (Rechberger and Graedel, 2002; US Geological Survey, 2009). These findings led to the consideration of the anthropogenic stock as an urban mine for secondary raw materials (Jacobs, 1969; Munro, 1984; Wittmer et al., 2003). Notwithstanding the fact that information about the magnitude of stocks is a minimum requirement to evaluate the resource potential of the anthropogenic stock, a consistent concept to assess the availability of these stocks as potential raw materials is needed. In view of the lack of concept for evaluating anthropogenic resources, Johansson et al. (2013) note that existing “mining concepts fail to help us navigate reliably in the complex technosphere, since they are disorganized, [ … ], and a clear categorization has not yet taken form”. Therefore they suggest to categorize anthropogenic stocks first based on their location in the anthroposphere (in their words technosphere), and second due to the mining concept that can be applied in order to extract secondary raw materials from the different stocks. Though this approach is useful in terms of organizing anthropogenic resource deposits, it does not answer the question on which part of the anthropogenic stock can be further considered for the development of an urban mining project. Consequently, an anthropogenic stock resource evaluation framework is required, as it is unlikely that the entire stock as shown in Fig. 1 is potentially available for extraction (Klinglmair and Fellner, 2010; Schneider et al., 2011). Contrary to anthropogenic stock resources, natural stock resources, like fossil fuels, metals, and minerals, are in the first step usually evaluated and classified based on their actual and potential exploitability, using evaluation frameworks like the resourcereserve classification developed by the USGS (1980). The evaluation procedure therein which is based on mineralogical (occurrence, grade, size) and economical (market prices, technology) factors, as well as the subsequent classification, are widely accepted, and it is beneficial to adapt and apply concepts like this to anthropogenic stock resources rather than design new ones. However, the question arises which of these procedures can be applied to anthropogenic resources and how? Hence, the overall-objective of this paper is to present a framework for the evaluation of anthropogenic stock resources based

on existing natural stock resource evaluation procedures in order to classify anthropogenic stocks with respect to their potential exploitability based on mineralogical (occurrence, grade, size of deposit) and economic (market prices, technology) factors. By building on natural resource evaluation research it is also possible to directly compare the results of anthropogenic and natural stock resource evaluation. In order to test the applicability and further develop an existing framework, a case study can be used. To do so, a resource of particular relevance for modern humanity is selected, namely phosphorus (P) (Elser, 2012). P is a potentially critical resource because of its relevance as a non-substitutable macronutrient, its limited natural reserves, and its non-circular use in the economy (Cordell et al., 2009; Elser and Bennett, 2011; Gilbert, 2009; Ott and Rechberger, 2012). This particular role of P induced increasing efforts to recover P from anthropogenic resources such as waste water and solid wastes (Hermann, 2009; Kalmykova and Karlfeldt Fedje, 2013; Tan and Lagerkvist, 2011). Furthermore, a recent literature review by Chowdhury et al. (2014) on the material flows of P suggests the existence of a significant built-up of anthropogenic P stocks. However, an evaluation of the resource potential of these stocks in terms of prospection, exploration, and its resource potential from a mineral economic point of view has not been carried out yet. In order to test the applicability of the evaluation framework developed, it is applied to anthropogenic phosphorus stocks in Austria. 2. Definitions and concepts in natural and anthropogenic stock resource evaluation The interdisciplinary use of the term resource in cultural, social, and environmental sciences requires a clear definition of the terms used in this work, which are presented in Section 2.1. Afterwards, some current concepts used to evaluate natural and anthropogenic stock resources are briefly described in Section 2.2.

2.1. Terminology as used in this research 2.1.1. Natural and anthropogenic resources Natural resources are the physical material base located in the natural spheres (atmo-, bio-, hydro-, and lithosphere) intentionally transformed by human cultural resources (e.g., labor, technology, institutions, capital) to fulfill a specific function for human utilization (Ciriacy-Wantrup, 1944). This transformation leads to a translocation from the natural spheres to the man-made anthroposphere (Carol, 1956; Husar, 1994). As soon as a material enters

Fig. 1. Anthropogenic stock vs. reserve base including reference years (reference years for reserve base in brackets): Fe and Cd e 1985 (1996), Ag e 1991 (1996), Cu e 2000 (2000), Al e 2003 (2003) (Graedel, 2010; USGS, 1996, 2000, 2003).

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the anthroposphere, it can be called anthropogenic (Baccini and Brunner, 1991). The human utilization and transformation of natural resources has two main consequences. The first is the production of anthropogenic waste products, originating from consumer goods with short- (e.g., packaging material) or long-term (e.g., building materials) lifespans (Georgescu-Roegen, 1971). The second consequence is the buildup of material stocks in the anthroposphere (anthropogenic stocks), which are also the source of the aforementioned wastes and emissions from long-living consumer goods (Ayres, 1998). Because both anthropogenic wastes and stocks contain materials that are of potential use for humans to fulfill a specific function, they can be called anthropogenic resources. Although this term is also used by other disciplines (e.g., zoology applies it to human-induced food sources for wild animals, cp. Sterling et al. (2013)), it is widely used in the field of environmental science to describe these aforementioned human-generated resources (Ciacci et al., 2013; Reck et al., 2006).

2.1.2. Stock and flow resources An early classification of natural resources presented by Ciriacy
e Wantrup (1944) and later adopted by many authors (e.g., Guine and Heijungs, 1995; Korhonen, 2001; Ring, 1997) generally distinguishes between stock and flow resources. Natural stock resources are nonrenewable resources stocked mainly in the lithosphere without significant natural increases or decreases over short time periods. This group contains fossil fuels, metals, and minerals. In contrast, natural flow resources are renewable resources that are continuously available at different intervals, such as solar radiation, water, plants and animals. The main difference between them is the residence time at a particular location and the availability of a flow to replenish a stock. The link between stock and flow resources is that the prior is often built up by the latter. Analogously, anthropogenic resources can be classified as stock and flow resources. The first resource is mainly associated with buildings or landfills (Lichtensteiger, 2006) but also with environmental spheres, such as soils (Bergb€ ack et al., 1992). A further differentiation of these stock resources refers to the actual utilization of the anthropogenic stock, meaning whether it is currently in use €ck and Lohm, 1997; by humans or not (hibernating stock) (Bergba

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Kapur and Graedel, 2006). As with natural resources, the main difference between anthropogenic stock and flow resources is the residence time. The first has a relatively long residence time (e.g., buildings) compared to the latter, which are usually short-living consumer goods, such as food or packaging materials (Baccini and Brunner, 1991). From this differentiation, it becomes clear that anthropogenic flow resources can turn into stock resources, i.e., if directed towards landfills. Even though the paper at hand deals with anthropogenic stock rather than anthropogenic flow resources, this link between both is further explained in the following Subsection 2.1.3.

2.1.3. Interaction between natural and anthropogenic stock and flow resources The relationship between different anthropogenic and natural resources is illustrated in Fig. 2. The material-based link between natural and anthropogenic resources is established by primary raw materials used for production, which are extracted from natural resources. Transformation steps produce durable or non-durable consumer goods that are consumed and either stocked as products or disposed of as wastes after reaching the end of their product life. Stocks can be further distinguished into stocks that are in vs. out of use (e.g., connected vs. disconnected cables) (Kapur and Graedel, 2006). Depending on a number of factors, such as waste properties, available technology, and demand, waste may be recycled and is subsequently called a secondary raw material (Christensen, 2011). If no recycling takes place, the waste materials are stocked in deposits, e.g., in landfills. In each step of this chain of production, consumption, and waste disposal, diffuse emissions are released into the natural spheres. A conceptual material balance equation for the relationship between primary and secondary raw materials is given by Cossu (2013):

E ¼ DR þ DL þ

X

di þ I

where E is the primary raw materials, DR is the secondary raw materials immediately recycled, DL is secondary raw materials from landfill mining, di is diffuse emissions from various processes, and I is the immobilized materials in the stock.

Fig. 2. Relationship among natural and anthropogenic stock and flow resources in the geosphere.

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2.2. Evaluation concepts for natural and anthropogenic stock resources 2.2.1. Natural stock resources Extensive information for the evaluation of the exploitability of natural stock resources exists. The combination of this information with economic considerations resulted in the so-called McKelvey diagram developed by the U.S. Geological Survey (USGS) (McKelvey, 1972). Being the most widely-used approach in this field, it characterizes natural resource deposits with respect to the economic feasibility of extraction and geological certainty of their quantity and grade (see Fig. 3). The vertical axis of the diagram represents the economic viability of extracting natural resource deposits, classified as economic, marginally economic, and subeconomic deposits based on economic, metallurgic, legal, market, social and governmental factors. Depending on the confidence in the information on quantity and grade, natural resources are categorized as demonstrated, inferred and undiscovered resources, shown on the horizontal axis. Cross classification considering both axes helps to distinguish between reserves and resources. The latter is defined as “a concentration of naturally occurring solid, liquid, or gaseous materials in or on the Earth's crust in such form that economic extraction of a commodity is regarded as feasible, either currently or at some future time” (USGS, 1980). In contrast, a reserve is “that portion of an identified resource from which a usable mineral or energy commodity can be economically and legally extracted at the time of determination” (ibid.). Materials that cannot be considered as resource due to their low grade, their chemical constitution, or for other reasons, are termed as other occurrences. Various standards exist for the detailed evaluation of each natural resource deposit (e.g., CIMVAL Standards (2003); ECE (2010); SEC Guide (2005)), all of which consider evaluation criteria similar to those used by the USGS. The deposits known and investigated in this way are classified and summarized by the USGS according to the McKelvey diagram and are frequently published in the Mineral Commodity Summary (USGS, 2013).

2.2.2. Anthropogenic stock resources Contrary to natural stocks, the potential utilization of the anthropogenic stocks lacks clear and widely accepted evaluation concepts. The most recent step in this direction has been presented by Johansson et al. (2013) who started with a classification of the anthropogenic stocks by the types of extraction of secondary raw materials that can be applied. Therein, they distinguish between inuse mining, hibernation mining, dissipation mining, landfill mining, slag mining, and tailing mining. The differentiation in mining type is based on factors, such as the current state of stock utilization (in-use vs. hibernation mining), location and process origin (slag vs. tailing mining), and degree of human control (landfill vs. dissipation mining). From these types of mining, the most referenced concept of anthropogenic stock resource evaluation is associated with socalled landfill mining, which is the recovery of wastes stored in landfills. A literature review by Krook et al. (2012) shows that most landfill mining studies simply determine the composition of the landfilled material. After the grade of valuable materials in the landfilled waste, excavation and treatment technology is prominently discussed, followed by studies on the benefits of landfill mining. However, the realization process itself and a detailed economic analysis are only included in a few publications. Another literature review on the same topic by Jones et al. (2013) distinguishes among the anthropogenic resources targeted for recovery, which are energy, materials or land. Against this background, they introduce the concept of enhanced landfill mining (ELFM), which aims to recover all of these resources. Evaluation in this concept refers to material composition (Quaghebeur et al., 2013), economic (Van Passel et al., 2013), technological (Bosmans et al., 2013), and environmental considerations (Jones et al., 2013). Contrary to that, €ndegård et al. (2013) apply life cycle assessment (LCA) to a hyFra pothetical landfill mining project, dealing with the adjacent uncertainties of data and results by Monte Carlo Simulation. Unlike landfill mining, tailing mining is not only frequently discussed in literature but also widely practiced, particularly for precious metals. Concentrations are usually higher in tailings than in other deposits, such as slags, which are often used as filling material in building construction. On the other end of the spectrum is mining dissipated materials (dissipative mining), which is typically

Fig. 3. McKelvey diagram illustrating resource and reserve terminology as used by the USGS (1980).

Please cite this article in press as: Lederer, J., et al., A framework for the evaluation of anthropogenic resources: the case study of phosphorus stocks in Austria, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.05.078

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neither practiced nor discussed because the material concentrations of resources are simply too low (Johansson et al., 2013). The so-called mining of the hibernating stock is a relatively new field in environmental systems analysis. Krook et al. (2011) quantified the total stock of copper in selected power grids, the share of hibernating copper stock out of use, and the costs of extracting this stock. This evaluation is two steps further than the evaluation performed by Pauliuk et al. (2014), which quantified metal stocks in urban water systems. 3. A framework for the evaluation of anthropogenic stock resources The methodological framework presented is derived from natural stock resource evaluation. Therefore, the most commonly applied approach on the global level developed by the USGS (1980) is combined with the first exploration steps of detailed mining projects and anthropogenic resource accounting by material flow analysis MFA (Brunner and Rechberger, 2004). After summarizing the most import steps for evaluating natural stock resources (Section 3.1), a brief description of the framework is presented in Section 3.2. 3.1. Natural stock evaluation procedures 3.1.1. The USGS resource-reserve approach The most renowned source for natural resource classification is the Mineral Commodity Summary from the USGS (2013), which collects data on the worldwide deposits of the most relevant materials based on the USGS (1980) approach as introduced by McKelvey (1972). Country experts collect and compile this data usually from authorities, mining companies, or scientists. Each datum refers to a deposit that has been investigated. The investigation criteria for each deposit first refer to the quantity and quality of the deposit (e.g., size, grade, type of mineral, continuity of enrichment, wanted and unwanted associated materials), which can be established by various direct and indirect prospection and exploration methods (e.g., geological maps, air photographs, sample drilling and excavation). Part of the exploration is then in a second step the valuation of the deposit and a project feasibility study. The latter also includes a deeper consideration of legal, environmental, and technological aspects (Hartman and Mutmansky, 2002). Using these data, the USGS classifies each deposit with the McKelvey diagram (see Fig. 3). The classification takes at least two indicators into account. The first indicator is the geological knowledge of the deposit (size, grade), expressed as the level of certainty, which also reflects the stage of prospection and exploration. For instance, the geological knowledge and thus the certainty of size and grade of a deposit already exploited is greater than that for an anticipated deposit for which only maps and satellite images exist. The terms used in the classification are therefore demonstrated, inferred and undiscovered resources. The second indicator is the economic viability of exploiting the deposit, which is determined by various technological, social, environmental, legal, and market factors (Hartman and Mutmansky, 2002; Sinclair and Blackwell, 2002). For classification, McKelvey (1972) suggested that profitable mineable deposits are economic reserves, while deposits that are exploitable for a production price below 1.5 times the actual price are marginal reserves. The differentiation between subeconomic resources and other occurrences is more complex according to him, as technologic advance can hardly be predicted in the long term. As an example, McKelvey mentions that the cutoff grade of copper has been reduced between 1900 and 1974 by a factor of ten. However, no further suggestion is made how to deal with this problem.

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3.1.2. Natural stock resource deposit exploration One of the shortcomings of resource classification procedures like the USGS resource-reserve approach is the lack of objective criteria for classification and the dependency on the experience of the user, in this case the country experts performing the classification of resource deposits (Sinclair and Blackwell, 2002). This leads to a hardly reproducible methodology when applying the USGS approach to anthropogenic resource deposits. In order to present a consistent methodological approach for an anthropogenic stock resource classification, the procedure for establishing the data collection is derived from the evaluation of a detailed mining project as shown by Hartman and Mutmansky (2002). Thereafter, the first steps towards evaluating a mine are first prospection and second exploration. Prospection is the commodity or site-specific search for an ore. It involves the gathering of information on the size and grade of a deposit, usually by compiling data through literature, geological reports and maps, aerial and satellite images. The result is some rough information on a geological prospect. Exploration is the evaluation of the geological prospect due to its size, shape, grade, and profit potential. Therefore, on-site sampling methods like excavation and drilling are used to extract samples which can be analyzed and tested due to their quality (e.g. grade, appearance, compound, workability). By applying mathematical reserve estimates, the size can be calculated based on the collected samples. With all this data and considering the potential mining technology available, a feasibility analysis is carried out mainly focusing on valuating the deposit at hand. Based on these steps, the decision is made whether a prospect is intended to be developed for exploitation or not. 3.2. Anthropogenic stock evaluation procedure The developed procedure for anthropogenic stock evaluation consists of four steps and is schematically illustrated in Fig. 4. The individual steps of prospection, exploration, economic evaluation, and classification are described in detail in the Subsections 3.2.1 to 3.2.4 below. 3.2.1. Prospection of anthropogenic stocks Contrary to natural stocks, anthropogenic ones have been created by man. Therefore, some minimum information about the stocks built-up should be available. A method that aims to collect and illustrate this information is material flow analysis (MFA) as presented by Brunner and Rechberger (2004). Therefore, the first step in the evaluation procedure is to carry out a macro MFA study for the material under investigation. The result is a material flow diagram showing the most relevant material stocks in the anthroposphere, typically on a regional, national, or global level. The material stocks are further grouped according to common properties, as for instance shown in Graedel (2010) for metals, distinguishing between in-use stocks, stocks in unmined ores, stocks in tailings, stocks in process facilities, government stocks, stocks in manufacturing facilities, stocks in recycling facilities, and landfill stockpiles. 3.2.2. Exploration of anthropogenic stocks Having identified the most relevant anthropogenic stocks within the defined system, a more detailed exploration of the different stocks can take place. Therefore, data for each deposit in different stocks are collected and further processed. For some stocks (e.g. tailings, landfills), the same sampling techniques as for natural stock resources can be applied, i.e. excavation and drilling. However, if data quality and quantity is sufficient, published data can be used. This is the case for instance for

Please cite this article in press as: Lederer, J., et al., A framework for the evaluation of anthropogenic resources: the case study of phosphorus stocks in Austria, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.05.078

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Fig. 4. Procedure for the evaluation of anthropogenic stock resources.

landfill deposits, where the composition and quantity of materials landfilled has been recorded. The result of this step is an MFA with a higher resolution for stock-estimates than the bulk MFA mentioned in Subsection 3.2.1. Because all of the estimates are still associated with substantial uncertainties due to the paucity of data or limited system understanding, uncertainty ranges are assigned to each parameter used to quantify the material stocks. In this research, the specified uncertainty ranges correspond to ± two times the standard deviation. Assuming normally distributed quantities, this translates into a 95.4% confidence that the true value for the material stock estimate lies within the range given by mean ± twice the standard deviation. In MFA, the uncertainty of the substance stock depends on the information about the size of the stock (e.g., a solid waste landfill) and the substance concentration in the good (e.g., phosphorus in the solid waste which is landfilled) (Brunner and Rechberger, 2004; Cencic, 2012). ur,G is the relative standard uncertainty (RSU) that is approximately the size of the good stock G, and ur,c is the RSU that is approximately the true concentration of the substance S in the respective good. The RSU is specified as two times the relative standard deviation and is given in the percent (%) of the mean value. Under the assumption that both variables are independent of each other, the total RSU for the substance (ur,S) is calculated as follows:

ur;S ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2r;G þ u2r;c

(1)

The result is then expressed as

mS ±ur;S

(2)

where mS is the mean value of the substance stock size in mass units, and ±ur,S is the RSU given in percent of the mean value that specifies the distance to the upper and lower bound estimates. 3.2.3. Economic evaluation of anthropogenic stocks To extract the desired material and produce a marketable good, some technology is required. By selecting the technology applied, it is possible to give a rough estimate on the associated costs which forms the base for the subsequent classification of different types of deposits. Then, the actual costs C0 in V per mass unit (ton) of the desired material is calculated. Because a mine is usually not exploited within one year, the time span t for operating the mine must be assumed. During this time span, a cost increase i for all cost items must be predicted, usually by using data from national statistics departments. Then the average extraction costs C over the

period t are calculated by dividing the uniform series compound by t, as shown in Equation (3) (Wiley, 2002).

 . . C ¼ C0  ð1 þ iÞt  1 i t

(3)

To assess the economic viability, not only the costs, but also the average price that can be realized for the secondary raw materials or the products produced over the extraction period must be calculated. Therefore, the current market price of F0 in V per ton of material extracted is extrapolated with an average annual price increase of i taken from the same source as the price index for the cost increase. To calculate the average price F achievable over the extraction period, Equation (3) is adjusted by replacing the costs C by the price F (see Equation (4)).

 . . F ¼ F0  ð1 þ iÞt  1 i t

(4)

This formula is applied not only to the main target raw material and the corresponding product, but also to co-products produced (Torries, 1998). In the subsequent evaluation, extraction and production costs are compared to the price that can be achieved for the product. Based on the results of the evaluation, the anthropogenic resource stocks can be classified as shown in the following Subsection. 3.2.4. Classification of anthropogenic stocks after McKelvey (1972) and USGS (1980) The classification of each stock is based on McKelvey (1972) and USGS (1980) (see Subsection 3.1.1). It considers the economic viability of extracting a secondary raw material from a resource and producing a tradable good on one hand, and the knowledge of the existence of the resource on the other hand. For the economic classification, McKelvey suggests the following terms. Resources are economic or, as he put it, recoverable if they can be extracted with a profit. Therefore, the production costs must be below the product price achievable (C < F). Resources for which the production costs are higher than the price, but not by more than a factor of 1.5, are marginally economic (F < C < 1:5  F). Resources above this value are termed as submarginal or subeconomic. Unfortunately, no threshold is given to distinct between subeconomic resources and the large group of other occurences. This large group is not considered as a resource and consists of nonconventional and low-grade materials which are either not considered as even potentially economic, or materials which cannot be extracted at all due to legal issues or their chemical constitution. In order to distinguish between subeconomic and

Please cite this article in press as: Lederer, J., et al., A framework for the evaluation of anthropogenic resources: the case study of phosphorus stocks in Austria, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.05.078

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other occurrences, a threshold factor of 10 is suggested for this study. Deposits below this threshold are considered submarginal or subeconomic (1:5  F < C < 10  F), while those above the threshold are counted as other occurrences. The classification according to the knowledge, thus the certainty of the existence of the stock is structured as identified e demonstrated, identified e inferred, and potentially undiscovered. To perform this classification, the uncertainties determined for each stock are used. Identified e demonstrated resources are of proven existence (e.g., by documents from public authorities who have recorded the amount of waste landfilled) and knowledge is highly certain. Therefore, the part of the estimated stock corresponding to the lower RSU bound (mS e ur,S*mS/100) is placed in this category (the confidence that the actual stock of the material is at least this size is 95.4%). Identified e inferred resources are defined here as the amount of stocks between the lower uncertainty bound (mS e ur,S*mS/100) and the mean value of the stock (mS). The same amount of the material (due to symmetric uncertainty ranges) is designated as potentially undiscovered resources, which may exist but are highly uncertain (i.e., the confidence that the total stock is larger than the calculated mean is less than 50%). Finally, a cross-classification is performed considering both, economic viability and knowledge. Therein, reserves are resources that are both identified e demonstrated and economically extractable. The reserve base further includes the part that is identified e demonstrated and not profitably extractable with current technology and market conditions (vertically classified as marginally economic).

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4.2. Prospection of anthropogenic P stocks considered The basis for the selection of the anthropogenic P stocks is a material flow analysis (MFA) carried out by Egle et al. (2014), who determined relevant P stocks in Austria (see Table 1). While stock number one is not considered in this work, the sizes of stocks number two, three, four, and five are taken from this table. The remaining, but most relevant stock for this work, is waste management (number six), which has to be calculated in the subsequent exploration Section 4.3. Based on information from Egle et al. (2014), it can be split into seven stock types, namely municipal solid waste (MSW) landfill I, MSW landfill II, municipal sewage sludge (MSS) landfill I, MSS landfill II, MSW bottom ash landfill, mixed ash landfill, and building materials. One stock not considered by Egle and colleagues is the P in steel production slags. Due to its historical use as P fertilizer and the presumed size of the stock (Matsubae-Yokoyama et al., 2009), it is considered in this work. Contrary to that, P from industrial waste landfills is not considered, as neither the work of Egle et al. (2014), nor other literature sources suggest that it will yield relevant quantities of P (Quaghebeur et al., 2013). A further description of the calculation of the missing stock figures is given in the following Section 4.3 on the detailed exploration of the prospected stocks, using the data from the Austrian federal waste management plans (BMLFUW, 2001, 2006, 2011). Additional sources are described in the appropriate Subsections 4.3.1 to 4.3.8. The time range of stock formation considered is between 1960 and 2009. 4.3. Exploration of anthropogenic P stocks considered

4. Case study phosphorus stocks in Austria In order to test the developed methodology, the procedure is applied to anthropogenic phosphorus (P) stocks in Austria. Before going into the details of the case study, a brief overview on natural P resource deposits is presented in Section 4.1.

4.1. Natural phosphorus stock resources Natural P resources from currently mined phosphate rocks have an average P concentration of 2e17% wt (Van Kauwenbergh, 2010). In 2002, the USGS recorded worldwide 1635 phosphate resource deposits of different origin and sizes, starting with 4350 t of P. The smallest deposit actually mined contains 6520 t of P (Chernoff and Orris, 2002). At present, mining of low-grade P reserves (6e12% wt of P) starts when phosphate prices are at 600e740 V/t, and mining of ultra-low grade P reserves (1e6% wt P) occurs if prices are at 1300e1500 V/t (Sverdrup and Ragnarsdottir, 2011; Van Kauwenbergh, 2010). The bulk of natural P resource deposits is located in Morocco, the US, South Africa, and Russia. Austria has no natural P resources.

4.3.1. MSW landfills I This stock considers only MSW landfills closed after 1990. The data used to calculate the amount of stock present is taken from Lunzer et al. (1998) and Laner et al. (2008), resulting in 78 million m3 of wet MSW. The RSU of the good (ur,G) is set to 10%, which reflects the uncertainty of the assumed bulk density of 1,000 kg/m3 of landfilled MSW (Tchobanoglous and Kreith, 2002). The P concentration in MSW landfills varies between 0.1 and 0.3% wt € (Kaartinen et al., 2013; Ostman et al., 2006). A mean value of 0.2% wt is used in this study, with the concentration RSU (ur,c) of ±50%. Applying Equation (1) yields a total RSU (ur,S) of ±51%. Between 3 and 5% of the generated municipal sewage sludge (MSS) is co-processed with MSW in mechanical-biological treatment plants and later deposited at MSW landfills (Domenig, 1998; Oliva et al., 2009; Scharf et al., 1997). This amount is added to the value determined for MSW landfills I by using P concentrations explained in the Subsections 4.3.3 and 4.3.4 on MSS landfills. 4.3.2. MSW landfills II For MSW landfills closed before 1990, the quantity of landfilled MSW has not been recorded. In the early 1990s, 250 kg MSW/

Table 1 Anthropogenic P stocks in Austria in 2008 from Egle et al. (2014) and the corresponding mining concept after Johansson et al. (2013). Stock size is given in t, and the stock change (D stock) is given in t/a. No

Stock name

Stock description

1 2 3 4

Animal husbandry Agricultural soil Other soils and forest Household & infrastructure

5 6

Water bodies Waste management

P stocked in living animals P stocked in soils; input through fertilizing exceeds the output P stocked in soils and trees; output exceeds the input P stocked in garden soils through the use of fertilizers and wood in buildings P in particulate matter is stocked in river and lake sediments Most P is stocked in landfills (65%), although some is also in building materials, such as cement, due to co-incineration (35%)

D Stock

Mining concept

7,000 12,000,000 6,300,000 125,000

33 þ5,500 500 þ2,500

None Dissipation mining Dissipation mining Dissipation mining

6,000 Not determined

þ1,300 þ8,700

Dissipation mining In-use (building materials) & landfill mining (landfills)

Stock size

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capita/year were landfilled (Lunzer et al., 1998). Using this amount for the whole period between 1960 and 2009 and subtracting the MSW from MSW landfills I, the amount is 15.7 million t. Due to the low data quality, a RSU of ±50% is assumed for the size of the goods. The P concentration and the associated RSU are assumed to be the same as for MSW landfills I, yielding a total RSU of 71%. 4.3.3. MSS landfills I Data on the landfilling of MSS has been collected and published frequently since 1991. A decrease in the share of landfilled MSS from 35% (1991) to 8% (2009) has been observed, while at the same time, the amount of sludge generated annually increased from 170,000 to 256,400 t of dry matter (dm) (Domenig, 1998; Oliva et al., 2009; Scharf et al., 1997). Uncertainties exist due to variable dm contents. With an average dm content of 30% wt ranging from 25 to 35% wt, the RSU for the MSS is ±17% (Fytili and Zabaniotou, 2008). The P concentration of sludge increased from 1.5% wt dm to 2.7% wt dm between 1991 and 2009, with a mean value of 2.1% wt dm (Oliva et al., 2009). The resulting RSU for the P concentration is ±30% (Egle et al., 2014), leading to a total RSU of 43%. 4.3.4. MSS landfills II Because there is no data available on MSS landfills before 1991, a steady increase from zero MSS landfilled in 1960 to 170,000 t of dm per year in 1991 is assumed, leading to 2,720,000 t of dm in total. Of this 30% is assumed to be landfilled with a P concentration of 1.5% wt of dm (Oliva et al., 2009). The other 70% of MSS generated have either been incinerated or utilized in agriculture. Landfilling is assumed to occur in separate compartments. Due to a lack of data, the RSU is assumed to be quite high, namely ±50% for the size of the good and ±50% for the concentration. The total RSU of this stock estimate results then to 71%. 4.3.5. MSW bottom ash landfills Since 1991, 3.2 millions of MSW bottom ash with a concentration of 0.8% wt (range of 0.6e1.0) of P have been landfilled in Austria (Dahl et al., 2009). In one of the incinerators, approximately € hmer 161,690 t of MSS have been co-incinerated with the waste (Bo et al., 2006; Stubenvoll et al., 2002). This value is simply added to the bottom ash landfills. The corresponding RSUs are ±33% for the concentration and ±10% for the good, resulting in a total RSU of 34%. 4.3.6. Mixed ash landfill In Vienna, MSS is incinerated in a mono-incineration plant and then landfilled together with the bottom ash from MSW incineration. The amount of landfilled fly ash from the MSS monoincineration, available since 1981, is 583,500 t of dm, with an average P concentration of 7% (Domenig, 1998; Oliva et al., 2009; Scharf et al., 1997). The MSS incineration ash was mixed with 3,500,000 t of MSW bottom ash (P concentration of 0.8% wt). Therefore, the average concentration of this mixed ash landfill is 1.7% wt. Based on the relatively detailed figures for both, the RSUs are ±10% for the goods and ±20% for the P concentration (Mattenberger et al., 2010), resulting in a total RSU of 22%. 4.3.7. Building materials P-rich wastes, such as MSS and carcass meal, are co-incinerated in cement kilns and power plants. In both cases, the P ends up in the building material. Since 1991, 110,000 t of dm of MSS containing 7,000 t of P (cP ¼ 2.3%wt dm) and 474,000 t of bone meal/carcasses containing approximately 26,500 t of P (cP ¼ 5.6% wt dm) have been co-incinerated (Domenig, 1998; Egle et al., 2014; Oliva et al., 2009; Scharf et al., 1997). An RSU of ±20% is assumed for the size of the goods in both stocks, while the concentration RSU is assumed to be ±50%, leading to a total RSU of 54%.

4.3.8. Steel slags Even though the slags from steel production are not used as fertilizer anymore and are instead applied in building construction (cement, road bed material), they still contain significant amounts of P. A raw estimate based on slag generation during steel production (150 kg of slag per t of steel), the average P concentration in steel production slags (3%), and a steel stock of 62,479,000 t yields a total of 281,000 t of P (Gara and Schrimpf, 1998; Graedel, 2010). Given the RSU of the material (±50%) and substance concentration (±50%), a total RSU of 71% is used. 4.4. Economic evaluation of anthropogenic P stocks Extraction and production cost estimates vary greatly depending on the ore material and the available technology, which is presented in this section for the landfill stocks. For all of the other stocks, no estimates are performed, as recovery to produce a secondary mineral P fertilizer is considered to be unfeasible for various reasons, such as the chemical appearance (e.g. P fixed in a concrete or asphalt matrix), the low grade (P in water bodies and soils), the low size, or a combination of these factors (P in private households). The technology presented is assumed to produce a product comparable to mineral fertilizer. The actual costs C0 for extraction and production of mineral fertilizer are described in detail in the following Subsections (4.4.1 to 4.4.4). The actual market price F0 for natural P based mineral fertilizer is 1,590 V per t of P in triple superphosphate (TSP) (World Bank, 2013). To calculate C and F, a time span of 30 years of deposit exploitation is assumed, starting from 2013. Based on national statistics data for the period between 1986 and 2012, the price index i for the infrastructure-building and transport sector and thus the extraction and production costs for the P fertilizer are assumed with 2% p.a., while during the same period the average annual price increase for mineral fertilizer has been 4% p.a. (Statistik Austria, 2013). The same amount is applied to co-products produced. For calculating both, costs and prices, Equation (3) and Equation (4) are used. 4.4.1. MSW landfills I The MSW landfills are excavated and re-cultivated at a cost of 10 V/t MSW (LUANW, 2004). The costs for transport include a flat rate of 5 V/t and a distance-based fare of 0.05 V/km/t MSW (ALPHA-HUF, 2011; Bernhard et al., 2011). The MSW is then incinerated at net-cost of 70 V/t raw waste in a grate furnace incinerator that transfers the bulk of the P into the bottom ash (Stubenvoll et al., 2002). Energy recovery from incineration is already included in this cost item. The bottom ash produced is subsequently treated (metal separation, grinding, thermo-chemical treatment) at a cost of 150 V/t bottom ash (38 V/t raw waste) to produce a fertilizer product with similar fertilizing effects as Thomas € mer, 2013). phosphate (Hermann and Adam, 2006; Nanzer, 2012; Ro A detailed description of the thermo-chemical process can be found in Egle et al. (2013) and Mattenberger et al. (2010). Additional revenues can be obtained from the recovery of other secondary raw materials as co-products. Due to the technology selected, only iron scrap in MSW landfills can be considered. Based on data from the Austrian Federal Environmental Agency (Bernhard et al., 2011), an average ferrous metal content of 4% in MSW landfills and a secondary raw material price of 100 V/t for iron can be assumed in order to calculate the revenues per ton of phosphorus fertilizer produced. This calculation further assumes that 100% of the iron found in the MSW can be recycled. Contrary to ferrous scrap from waste incineration, the metals separated during thermo-chemical treatment cannot be

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recovered as they are mainly present as metal chlorides (Mattenberger et al., 2010). 4.4.2. MSW landfills II The same technology and costs including revenues from iron scrap as for MSW landfills I are applied, except that the excavation and re-cultivation costs are higher (20 V/t raw waste). This is because most of these landfills are below a size of 10,000 m3, leading to higher unit excavation prices (Laner et al., 2008; LUANW, 2004). 4.4.3. MSS landfills I and II The sludge is excavated and transported at the same costs as given for MSW landfills I. Pre-treatment costs of the sludge (dewatering from 30 to 40% wt dm content) are 10 V/t raw sludge, and incineration net-costs amount to 70 V/t raw waste (Stubenvoll et al., 2002). The residual fly ash is thermo-chemically treated at a cost of 120 V/t ash (14 V/t raw sludge with 30%wt dm content) (Hermann and Adam, 2006). The costs are assumed to be lower than for bottom ash because no grinding is required. Contrary to MSW landfills, no valuable co-products are considered in the MSS landfills. 4.4.4. MSW bottom ash landfills (pure or mixed with MSS mono incineration fly ash) For bottom ash landfills, the costs for excavation are highest due to the efforts involved in breaking the rock-like material (30 V/t bottom ash) (LUANW, 2004). The costs for ash treatment (metal separation, grinding, thermochemical treatment) are 150 V/t bottom ash. Ferrous scrap recovery from bottom ash landfills is not considered as it is assumed that it has already taken place directly after incineration. 4.5. Classification of anthropogenic P stocks in Austria The total size of the identified anthropogenic P stocks in Austria is 1,020,000 t of P. Based on the current estimates of average TSP prices over the next 30 years (3,000 V/t), none of the identified resources can be economically extracted and thus can't be classified as a reserve. Furthermore, the reserve base containing marginally economic and identified e demonstrated resources is zero. A total of 102,000 t of P (10% wt of the total stock) is classified as subeconomic, with production costs approximately 5e10 times above the average TSP prices (MSS ash mixed landfills, MSS landfills I þ II, MSW bottom ash landfills). The residual bulk of P is either low-grade materials in MSW landfills (194,000 t of P or 18% of the total stock) where extraction costs are approximately 30 times higher than the TSP price, or not extractable materials (784,000 t of P or 72%). Regarding certainty, 49% of the identified resources are demonstrated, and 51% are inferred stocks. However, if only extractable stocks (subeconomic and low-grade materials) are considered, only 15% and 14% of the total stocks are demonstrated and inferred, respectively. Table 2 gives an overview of the size and classification of the anthropogenic stocks of P in Austria. The results for each individual stock are presented in the following Subsections 4.5.1 to 4.5.7, followed by an illustration in Figs. 5 and 6. The given P stock figures are the mean value for each stock. 4.5.1. MSS mixed ash landfills A total of 62,000 t of P was stored in the mixed MSS incineration fly ash and MSW incineration bottom ash landfill. The size of the deposit is within the range of natural deposits, and the average concentration of P is 1.7% wt; thus, is comparable to ultra-low grade phosphate rock (Chernoff and Orris, 2002; Van Kauwenbergh, 2010). The MSS incineration fly ash counts for 61% wt of the P but only 16% wt of the total mass, meaning that the admixing of MSW

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Table 2 McKelvey diagram for anthropogenic phosphorus stocks in Austria formed between 1960 and 2009 (the uncertainty ranges of the estimates form the basis for the distinction between demonstrated, inferred, and potentially undiscovered resources). Identified resources

Economic Marginally economic Subeconomic Other occurrences: low-grade Other occurrences: not extractable Total

Demonstrated

Inferred

0 0 70,000 89,000 720,000 1,020,000

0 0 32,000 105,000

Potentially undiscovered resources 0 0 32,000 105,000 374,000 511,000

incineration bottom ash reduces not only the average P concentration but also the extractability because the resulting ash mix has stronger binding properties and is more difficult to excavate and process than pure fly ash. Both are responsible for the higher average production costs of approximately 14,000 V/t P compared to phosphate rock. Because these costs are not too far from the threshold of F ¼ 3,000 V/t P, they can be classified as subeconomic. 4.5.2. MSW bottom ash landfills There are two landfills with pure MSW bottom ash compartments in Austria of the same size, in terms of total bottom ash landfilled. The first contains approximately 8,000 t of P because MSS has been co-incinerated with the MSW, resulting in a P concentration of approximately 1.5% wt. If no MSS has been coincinerated, as in the second landfill, the concentration is only 0.8% wt, which corresponds to only 4,000 t of P. The average production cost for both landfills is 22,000 V/t P, with a lower value for landfill one (17,000 V/t P) and a higher one for landfill two (32,000 V/t P). Therefore, both are classified in the subeconomic group. 4.5.3. MSS landfills I Approximately 800,000 t of dm of sludge has been landfilled in six sewage sludge landfills since 1990, containing approximately 16,000 t of P. The size of the landfills in terms of P is between 1,600 and 4,500 t of P and is relatively large compared to other anthropogenic stocks but in the lower range if compared to natural deposits. While the average P concentration of 0.6% wt is relatively low, even compared to ultra-low grade natural phosphate, the excavation is considered quite simple because the sludge is relatively soft. However, transport and processing might be challenging due to emissions from anaerobic degradation. The average production costs are approximately 26,000 V/t P, which is approximately nine times the threshold value. Thus, these stocks are subeconomic. 4.5.4. MSS landfills II MSS that has been landfilled before 1990 contains approximately 12,000 t of P. However, these landfills, which are scattered all around Austria, are relatively small, resulting in higher production costs compared to MSS landfills I, particularly for excavation. Additionally, the P concentration is lower because P removal from waste water was not widely established then. These landfills are also considered subeconomic. 4.5.5. MSW landfills I Approximately 163,000 t of P are stored in 135 MSW landfills, with sizes of between 4 and 21,000 t of P. The average P concentration in most landfills is 0.2% wt, much lower than in mineable natural deposits. However, in six landfills where mechanical-biological

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Fig. 5. Costs of producing phosphorus fertilizer from the main anthropogenic stocks of P in Austria, which were formed between 1960 and 2009. The dotted lines show the thresholds for economic feasibility (lower line: subeconomic-marginally economic; upper line: other occurrences-subeconomic).

Fig. 6. Size and classification of the main anthropogenic phosphorus stocks (including uncertainty ranges of estimates) in Austria.

treatment residues containing sewage sludge have been landfilled, P concentrations of about 1% wt are to be expected. The average production costs are approximately 82,400 V/t P, so it is hardly imaginable that recovery will ever be economically feasible. Subsequently, these landfills, even though they represent the largest theoretically minable anthropogenic stock, are not subeconomic but rather a low-grade material.

4.5.6. MSW landfills II Approximately 31,400 t of P are stored in these landfills, with average production costs of 93,000 V/t P. In addition to the high costs and the low P concentration of 0.2% wt, the fact, that their size is small and their location in some cases is unknown, decreases their potential to a level below the prior stock type (MSW landfills I), meaning that they are in the low-grade material group.

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4.5.7. Other stocks Even if all other anthropogenic stocks are not considered as potentially economic for the production of secondary mineral fertilizer, three of them should be briefly discussed herein. Attention should be first given to the stocks in building materials (34,000 t of P) and steel and iron slag (280,000 t of P) simply because the Pcontaining input materials for both stocks have historically been used as fertilizers (sewage sludge, bone meal and carcass, and steel slag). Although there were some good reasons for abandoning this practice, current technological progress could make it possible to overcome a number of challenges attached to the recovery of these flows, particularly in terms of hygienic aspects (in the case of bone meal and sewage sludge) or hazardous substance emissions (in the case of sewage sludge and steel slag). In addition, it should be noted that the huge stocks in soils of 275,000 t of P are not entirely lost to the system. Although they may not serve as a secondary raw material for mineral P fertilizer production (and hence, are not designated a resource in the context of this evaluation), a change in cultivation practices like crop rotation or fallow would mobilize some of the P stocked and potentially make it available for plant production (Childers et al., 2011). A summary of the size of all the anthropogenic stocks investigated is shown in Fig. 6. 5. Discussion and conclusions The absence of a framework for anthropogenic resource evaluation led us to the adaptation of standard natural resource evaluation procedures in this study. This framework was used to investigate anthropogenic phosphorus stocks in Austria and their resource value. The evaluation, based on an MFA of phosphorus flows in Austria and an economic assessment, indicates that approximately 70% of the huge anthropogenic P stock, which amounts to 1 million t of P, is not extractable in order to produce a product similar to mineral

11

Table 3 McKelvey diagram for global natural phosphorus stocks after Wagner (2003) (bold) and national Austrian phosphorus stocks from this study (italic, in brackets), given in kg P/capita. Identified resources natural (anthropogenic) Economic Marginally economic Other occurrences

Undiscovered resources natural (anthropogenic)

700 (0) 2,200 (0) 56,000,000 (120)

60,000 (0)

P-fertilizer from natural P deposits. The reasons are, first, the nature of the P-stock (e.g., soils, households & infrastructure); second, the dilution of relatively P-rich material, such as waste water and MSW (e.g., water bodies, building materials); and third, the transformation and direction of P-rich materials, such as MSS, MSW, and slags, to hardly accessible matrices (e.g., steel and iron slags used in building industry; building materials). Of the 30% P stock that is, in theory, extractable, two thirds (20% of the total) is stored in MSW landfills that have such a low P grade that future extraction is considered practically infeasible. Hence, only the remaining one third (10% of total) of the P stocks has a concentration and constitution worthy of further studying the mineable potential beyond the screening evaluation presented in this study. Based on the results of our evaluation, the enthusiasm for anthropogenic stock compared to natural stock seems disproportionate e at least for anthropogenic P stocks in Austria. The subeconomic but theoretically extractable component of the anthropogenic P stock corresponds to approximately 120 kg P per capita. This is equal to only 3% of the 4,000 kg of P per capita of the worldwide natural stock resources (USGS, 2013). Table 3 shows the role of anthropogenic P stocks from Austria compared to the global resource estimate based on the McKelvey diagram (1972) carried out by Wagner (2003). Both are shown on a per-capita basis,

Fig. 7. Costs of producing phosphorus fertilizer from the main anthropogenic stocks of P in Austria, which were formed between 1960 and 2009, including copper recovery and avoided landfill aftercare costs. The dotted lines show the thresholds for economic feasibility (lower line: subeconomic-marginally economic; upper line: other occurrencessubeconomic).

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namely the Austrian anthropogenic P stock divided by the Austrian population of 8.6 million, and the global natural P stock divided by the world population of 7.2 billion. Although these figure might be different for some of the materials mentioned by Gordon et al. (2006), the dissipative and dilutive nature of anthropogenic processes noted by Singh (2013) suggest that a similar statement may be valid for many of them. In order to determine the significance of anthropogenic stocks for other materials, the presented framework should be applied to these materials. Even though the screening-type application of the framework for natural mineral stock resources used by the USGS for anthropogenic stocks proved to be a useful initial step towards the evaluation of the latter, many questions remain for subsequent research. Three such potential areas of future study are discussed below. First, the application of the evaluation framework should be extended to include anthropogenic flow resources, as they create future stocks. Consequently, the future extractability of P from these stocks depends on the management of current material flows. For instance, if the MSS fly ash had not been diluted with MSW bottom ash in mixed ash landfills, the MSS ashes would have formed a resource deposit of 38,000 t of P with a concentration of 7% P, extractable at approximately 3,000 V/t of P, and thus slightly above the economic/marginal economic threshold limit. These values are almost in the range of medium-grade natural phosphate deposits that are already being mined (Wagner et al., 1999), and recovery could make Austria independent from mineral P fertilizer net-imports of currently 16,000 t per year for approximately 2 ½ years (Egle et al., 2014). In Switzerland, the management of specific anthropogenic P-flows adheres to this principle, as MSS is incinerated in mono-incineration plants, and the ashes must be landfilled in separate compartments to excavate them in the future when more elaborate fertilizer-production technology is available (Hermann, 2009). Second, the presented framework was adequate for a screeninglevel evaluation, but for a more concrete assessment the technological, legal, environmental, and societal aspects of mining anthropogenic stock resources need to be considered in more detail. In this study, it was assumed that current technology can produce a fertilizer from the anthropogenic resources that would be considered similar to those of natural phosphorus resources. In fact, this is not yet the case, but promising attempts have been made by various groups (Egle et al., 2013). However, a detailed evaluation should consider these findings. Furthermore, the anthropogenic P stocks in soils have been classified as not extractable for the production of a P mineral fertilizer. Though this is true, it does not mean that this P is lost, like for example the one stored in building materials. As aforementioned in Subsection 4.5.7, P in soils can be mobilized by different cultivation practices. How to include this point within an anthropogenic resource evaluation procedure like the one presented, should be subject of further elaboration. Third, the material composition of anthropogenic stocks should also be given further consideration. Like natural resource deposits, anthropogenic deposits can contain more than one valuable material. Natural copper deposits, for instance, contain a significant tin content, and in a feasibility study for mining such a deposit, both materials are considered, even though only one is in the focus of the study (Torries, 1998). An example of an anthropogenic stock containing more than one potentially mineable material is MSW landfills, which are already involved in current landfill mining projects for various materials that are typically recovered using specific excavation and processing technologies (Jones et al., 2013; Van Passel et al., 2013). Though a brief examination that hypothetically considers first the recovery of 100% of the copper in the MSW landfills based on default values for the copper concentration

of 0.14% and a current price for copper scrap of 4,500 V/t (Bernhard et al., 2011), and second the avoided costs for the aftercare of MSW and MSS landfills of 25 V/t (after Buchert et al. (2013)) and 20 V/t for slag landfills (after BMLFUW (2013)), does not change the figures significantly. As shown in Fig. 7, the cost/price ratio of the subeconomic resources (mixed ash landfill, MSW bottom ash landfills, MSS landfills I þ II) slightly decrease to a factor of four to eight. The reduction for MSW landfills I and II is much higher, from 30 to 20, but still way above the defined threshold limit of ten. From the three examples given, it is apparent that extensive research is needed regarding the extraction, mining, and recovery of anthropogenic resources. Similar to natural resources, the evaluation of anthropogenic resources needs to account for the resource quantity and quality as well as technological and economic factors.

Acknowledgements The presented work is part of a large-scale research initiative on anthropogenic resources (Christian Doppler Laboratory for Anthropogenic Resources). The financial support of this research initiative by the Federal Ministry of Economy, Family and Youth and the National Foundation for Research, Technology and Development is gratefully acknowledged.

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