Sustainable resource use requires "clean cycles" and safe "final sinks".

In order to fulfill the objectives of environmental protection, today's focus on quantitative recycling rates must be amended by a more qualitative approach. Because modern products represent a mix of numerous and sometimes hazardous substances, ways must be explored to remove detrimental substances during recycling and to establish "clean cycles". On the one hand, such a "clean cycle" strategy will result in better recycling qualities of secondary products and less dissipation of hazardous substances during further product use. On the other hand, the elimination of hazardous substances during recycling requires sinks for the disposal of the eliminated materials. These topics are presented in general as well as by case studies. In particular, the sink issue is addressed, differentiating between sinks and final sinks and discussing the challenge to supply appropriate final sinks for all materials that cannot be recycled.

Introduction

On a global scale, there's a continuous increase of material extraction from the earth's crust (Weber et al., 2011), resulting in large accumulations of materials particularly in urban areas. While the anthropogenic material stocks are growing, the amount of waste flows and emission rates increase with a certain delay. To satisfy contemporary resource needs and to decrease environmental loadings, an obvious solution is to recycle as much as possible in order to substitute primary resources. Therefore, European legislation aims to increase recycling rates continuously. This quantitative approach does not take into account the presence of unwanted substances ending up in the second generation products. To avoid this, a so called “clean cycle” strategy is proposed.

“Clean cycle” strategy

Sustainable resource use is characterized by an ecologically acceptable impact on nature. Recycling strategies play a key role by extending materials' lifespan and reducing environmental impacts. Until now, the focus of environmental evaluation methodologies has been laid on emission rates, the composition of second life products has only rarely been considered in view of hazardous constituents, multiple recycling loops, and long term effects. There is no focus yet on the fact that today's recycling loops contain valuable as well as harmful substances. Even though cycling of hazardous substances is prohibited by waste management directives (Republik Österreich, 2008), the recycling quota regulation as well as economic constraints result in hazardous substances to remain in cycles. Hence, “tramp elements” accumulate in the anthroposphere with potential negative effects on a) product quality, and b) environment:

Some “tramp elements” accumulate finely dispersed in the product cycle and lower the quality of second generation goods made up from secondary resources. This becomes apparent for example in the plastic industry, where stabilizers like cadmium, lead and tin contaminate the recycling products (Fehringer et al., 1997), or in the steel industry, where copper contaminates the steel cycle (Gleich et al., 2004), or in packaging industry, where mineral oils contaminate paperboards used for food packaging (Kappen et al., 2012). In short, the goals of high recycling rates and high product qualities often contradict each other.

The accumulation of toxic “tramp elements” in the product cycle presents an increasing potential for the release of hazardous substances. The reason is that some fractions of a material are inevitable released throughout the entire life cycle (see Fig. 1). In other words, substances from the stock-in-use are converted into unrecoverable forms and dissipated into the environment (Ayres et al., 2002). For example, in 1983 about 10%–30% of refined copper left the product cycle and ended up in environment or landfills (Bureau of Mines, 1983, Nriagu and Pacyna, 1988). Exemplary, emission sources are roofs and brake wear. The products are exposed to abrasion, corrosion and weathering processes which effect dissipative material releases. The great majority of these dissipated materials accumulate in specific environmental compartments and may threaten to impact natural processes (Geiser, 2001). A case study on urban surfaces shows that about 1/3 of the diffusively emitted copper enters the waste management system. In contrast, roughly 2/3 is directly lost in an uncontrolled form to the environment (Rebernig, 2007). While the scientific community is increasingly aware of these kinds of material losses (e.g. Arx, 2006, Bergbäck et al., 2001, Burkhardt et al., 2007, Burkhardt et al., 2008, Obernosterer et al., 2003, Sörme et al., 2001), few guidelines were developed to keep dissipative losses from urban surfaces and abrasion on low levels (Hoffmann and Rudolphi, 2005, Zysset et al., 2002). In the future, dissipative material losses from all sources and their effects on the environment must be taken into account to determine sustainable resource use.

Fig. 1

Anthropogenic material stocks and flows.

In order to establish a sustainable resource use with no burden for future generations, hazardous substances have to be eliminated from material cycles. Otherwise recycling strategies run the risk to a) support a qualitative down-cycling of materials in a large scale format, and b) raise the potential for harmful material losses throughout the material life cycle.

There are two conclusions regarding a sustainable recycling policy which incorporates a “clean cycle” strategy:

For environmental protection and product quality reasons, the recycling quotes have to take qualitative characteristics regarding secondary materials into account. The dilution of unwanted substances in second generation products as well as the total release of harmful substances to the environment has to be taken under control.

If “clean cycles” are established, the safe disposal of specific substances removed from the cycles is mandatory. These contaminated material flows have to enter safe “final sinks”. Consequently, a safe “final sink” concept with corresponding policy instruments has to be developed.

Fig. 2 displays the resulting material flows based on a “clean cycle” strategy. An optimum mix of primary and clean secondary material keeps the material cycle alive. Impurities and dissipative material losses are directed to safe final sinks.

Fig. 2

Material flows based on a “clean cycle” strategy (Stumm and Davis, 1974, modified).

The need for final sinks

In 1965 Albert Wolman was the first who analyzed cities as metabolic systems (Wolman, 1965). He highlighted the fact, that material use is directly linked with emissions and waste. The discussion about the need for sinks to take up substance flows became public awareness in 1987, where the Brundtland report states the importance of “ultimate sinks for the by-products of human activities” (United Nations, 1987). In 1996, Joel Tarr – an environmental historian – has drawn the attention to “the search of the ultimate sink” from a historical perspective (Tarr, 1996). At the same time Marina Alberti highlighted the fact that “cities cannot sustain themselves without drawing on the carrying capacity of their hinterland or region at the back end of their metabolism” (Alberti, 1996). Now it's time to develop a systematic “sink concept” that links urban output flows with manmade or natural sinks. The definition of sink indicators can also support the evaluation of ecological sustainability (Döberl and Brunner, 2004), and thus contribute to the reduction of the risk of overloading sinks, such as today's excessive flow of greenhouse gases to the sink “atmosphere” (Brunner, 2010).

From a materials management point of view, the anthroposphere can be seen as flow through reactor with a distinctive storage function. Material flows leave the anthropogenic material cycle (see Fig. 2), due to a) the second law of thermodynamics (dissipative losses occur) and, b) the diminishing returns (secondary resources). The core question is: Where should these off-flows end up? The answer on a general level is: They have to end up in final sinks. A more concrete answer remains to be developed because at present, the sink concept is still a vague framework. Based on previous definitions, the authors put the following working hypothesis forward for discussion:

A “sink” is defined as a process that receives anthropogenic material flows that have no positive value for present societies.

A “final sink” is a sink that either destroys a substance completely, or that holds a substance for a very long time period.

In order to exemplify the term “sink” and “final sink”, two case studies are discussed. They focus on: a) the retention in the environment of anthropogenic materials that have been previously lost by dissipation, and b) the elimination of copper from the steel cycle and the deposit of recycling residues.

Glaciers act as receptor for a broad range of dissipative, airborne losses like heavy metals or persistent organic pollutants (POPs). Thus glacier ice is a “sink” that can become a secondary source of pollutants (Salomons, 1998). Recently, a Swiss study (Bogdal et al., 2009) identified an accelerated release of pollutants from melting Alpine glaciers. This might result in an accumulation of hazardous substances in the food chain up to fishes or plants growing in glacier water. The case study exemplifies the term sink as used here for the temporary storage process “glacier ice”, and reveals that so far many anthropogenic flows of hazardous substances have not yet reached an environmentally safe “final sink”.

Due to technological constraints, it is difficult and costly to remove copper during steel recycling. So, traces of copper remain and accumulate in the product cycle. This results in down cycling and lower product quality. In order to achieve appropriate product qualities, steel companies select scrap fractions on the resource market depending on the copper concentration. A study about sustainable metal management (Gleich et al., 2004) indicates, that in the future, copper concentrations will become so high that specific scrap fractions cannot be used as secondary resource in the steel industry anymore. Scenarios were identified as reasonable that remove part of the scrap from the recycling stream in order to dispose them off in manmade sinks like landfills.

The example shows that a) steel is a temporary sink for copper and the steel cycle eventually stresses long term sinks like landfills, and b) the selection of scrap depends on quality criteria. So, steel recycling quotes are quality driven and can't be achieved by prescribed recycling rates. The exclusive definition of recycling rates on a quantitative base blinds out the qualitative constraints of recycling. In order to guarantee a clean steel cycle, recycling policies have to consider product qualities too.

Concluding, both the whole range of dissipative material losses1 and impurities within products have to be directed to safe final sinks. If a safe final sink can't be identified for ecological, technical or economic reasons, material design strategies have to be developed that consider final sink limitations, such as in the case of fossil fuels where a safe mediate term sink for carbon is missing.

Conclusion

In order to achieve sustainable resource management, a strategy directed towards clean cycles and safe final sinks needs to be developed. A clean cycle strategy delivers an optimum instead of a maximum of secondary resources through elimination of hazardous substances from material cycles. This ensures the generation of quality proven recyclables for multiple life cycles of products without the risk to shift problems into the future. Removing hazardous substances from material cycles requires final sinks where these substances can be stored safely for geological time periods.