Boram Kim

Nicolas Forquet
email: nicolas.forquet@irstea.fr

Pore-scale observation of deposit within the gravel matrix of a vertical flow constructed wetland

Keywords: deposit, constructed wetlands, thin section, pore distribution, clogging

Lifespan and well-operation of French vertical flow constructed wetlands (VFCW) depend on how the organic deposit forms and evolves within filter media. This study aimed to demonstrate the feasibility of thin section methods application to VFCW. Unfortunately, constructed wetland scientists are currently missing tools to observe how deposit physically occupies pore space. Thin sections allow a direct and undisturbed observation of filter media and deposit.

Undisturbed samples were taken from the surface of an experimental VFCW.

Water was exchanged with a solvent before resin impregnation to preserve the sample structure. Several thin sections were successfully produced. Results highlight that deposit significantly reduces pore space. It forms a structured media crossed by large channels which can participate to fast gravity-driven flow and media oxygenation. The deposit structure seemed also made of a large bundle of small pores less than 100 µm in radius. They can effectively store water by capillarity and provide a large surface for potential pollutant adsorption. Image analysis of thin sections provided hints at understanding the structuration of the porous media linked to organic matter deposition.

1.

Introduction

French first stage Vertical Flow Constructed Wetlands (VFCWs) differ from other wetland typologies on granular materials (gravel instead of sand). At the beginning of its operation, raw wastewater flows readily through coarse filter media. However, as time goes by, suspended solids (SS) settle and bacterial biofilm grows within interstitial pore spaces. On the surface, a deposit layer builds up and its structure evolves according to operating (feeding/resting alteration; batch volume, hydraulic and organic loads) and climate (season; rainfall) conditions. [START_REF] Molle | French vertical flow constructed wetlands: a need of a better understanding of the role of the deposit layer[END_REF] Finally plants, micro-and macro-fauna participate to the structuration of both deposit and porous media. [START_REF] Vergeles | Formation and properties of sediments in constructed wetlands for treatment of domestic wastewater[END_REF] All these phenomena significantly modify VFCW functioning as they age. Aging participates in improving VFCWs performance but may also be responsible of certain VFCW malfunctions. Furthermore, the accumulation of organic matter and its degradation products inevitably lead to clogging. [START_REF] Nivala | Clogging in subsurface-flow treatment wetlands: Measurement, modeling and management[END_REF] It makes necessary to regularly dredge out the surface deposit layer (every 10-15 years) and clogged porous media beneath on a long term. Therefore, understanding how aging acts on VFCWs may help improving their operation and our perception of their life cycle. Suspended solid filtration and deposit mineralization are controlled by mechanisms occurring at the pore scale. Only few scientists have tried to observe what is happening within a VFCW [START_REF] Molle | French vertical flow constructed wetlands: a need of a better understanding of the role of the deposit layer[END_REF][START_REF] Hua | The Role of Biofilm in Clogging Process in Vertical Flow Constructed Wetland[END_REF] and even less to look at the pore structure.

The thin section method, which is commonly used in soil science, [START_REF] Camuti | Preparation of polished thin sections from poorly consolidated regolith and sediment materials[END_REF] was chosen for the first direct observation of filter media structure. The objectives of the present work were to (1) evaluate thin section feasibility to VFCW, (2) observe deposit distribution within filter media and (3) quantify its impact on pore size distribution.

2.

Materials and methods

Sampling and preparation of thin sections

Sampling was performed in March 2015 from a pilot-scale VFCW located in Irstea's experimental hall at La Feyssine in the metropolitan area of Lyon (France, Figure 1). This experimental setup was meant to simulate granular characteristics of a real-scale first VFCW stage and put in operation since July 2012. From the beginning of its operation, it was fed with real domestic wastewater from a 400.000 p.e. treatment plant.

[6] Samples were taken from the surface of the VFCW using Kubiena tins (l×w×h: 55×75×40). They represented surface gravel layer with thin coat of organic deposit at the top.

In order to remove as much water as possible from the samples before hydrophobic resin impregnation, a solvent exchange in vapor phase was performed to ensure minimal disturbance to samples. [START_REF] Camuti | Preparation of polished thin sections from poorly consolidated regolith and sediment materials[END_REF] Sampled tins were equipped with a perforated lid at the bottom and placed on a raised platform above a bath of acetone. A container of anhydrous calcium chloride was also added to facilitate drying. A 0.79-0.80 range hydrometer was used for water removal monitoring. After several changes of acetone, dried samples (water content < 0.5%) were sent to the Stirling University where they were impregnated with polyester (crystic 7449) resin, cut vertically to obtain thin sections (around 30µm in thickness), polished and coverslipped for protection.

[Figure 1] Sampling and thin section preparation (credits for pictures of sample impregnation and cutting: http://www.thin.stir.ac.uk/)

Image acquisition and post-processing

A stereomicroscope (LEICA MZ16; ×0.71) was used for magnified image acquisition of the prepared thin section. 25 images were taken per thin section. Illumination corrections were individually performed on these images before merging them (Figure 2). Gravels were manually identified using an edge-recognition algorithm (GIMP v2.8.16) and colored in black. A first binary image (Figure 3, a) corresponding to gravel was then extracted. Next, areas corresponding to deposit were distinguished thanks to the low value of their red component. A second binary image (Figure 3,b) was extracted associating gravel and identified deposit areas.

[Figure 2] Post processing.

The following steps were successively applied to those binary images (Figure 3).

(1) Morphological transformations (hole filling for gravels and majority transform for both images) were performed to remove minor roughness without affecting the main part of the structure. [START_REF] Rabbani | An automated simple algorithm for realistic pore network extraction from micro-tomography images[END_REF] (2) The porosity was estimated by dividing the number of pixels of the pore space by the total number of pixels. (3) Based on Rabbani et al., [START_REF] Rabbani | An automated simple algorithm for realistic pore network extraction from micro-tomography images[END_REF] we choose a watershed algorithm for image segmentation. [START_REF] Meyer | Topographic distance and watershed lines[END_REF] In image processing, the watershed algorithm is analogous to the notion of catchment basin in hydrology. The binary image of the pore space was obtained by taking the opposite of the binary image of gravel or gravel and deposit. Then the opposite of the distance separating each pixel from an edge (the limit between gravel / deposit and pore space) was computed. Minima represent watershed centers, which are analogous to pores. (4) Pores and throats (constriction between two pores) were identified and labeled.

(5) The surface of each pore was computed and the equivalent radius estimated.

Pores were then sorted according to their size and grouped into classes that were used to estimate both their cumulative frequency distribution and the cumulative pore space occupation. The pore space occupation of a class of pore is defined as the total surface of this class of pore divided by the porosity. Image processing was performed using MATLAB (R2013b).

[Figure 3] Binary images of gravel only (left) and gravel and deposit (right). Vertical cross-section of a sample taken from the surface of a VFCW.

Results and discussions

Three thin sections were successfully produced from samples. G. MacLeod from Stirling University reported that the only difficulty in working with the samples is the combination of both hard lithic material and soft organic material (deposit). This means that grinding to 30 microns has to be done very carefully to avoid loss of the deposit.

Hereafter we choose to only focus on results obtained from one thin section since they are similar for all three thin sections.

Visual observation of thin sections

Direct observations of the thin section (Figure 4) clearly highlight the migration of organic deposit in depth confirming its space occupation within filter media.

Nevertheless, deposit is not forming a coat on gravel surface. It forms an organic matrix within the pore space bounded to gravel at some location and structured by large channels (reaching 382 and 421 µm; Figure 4), providing macro-pores for preferential gravitational flows. The formation of these channels cannot be due to the sample drying process since the water / acetone exchange in vapor phase cannot cause shrinkage or cracking of deposit in such scale. [START_REF] Camuti | Preparation of polished thin sections from poorly consolidated regolith and sediment materials[END_REF] Creations of those channels may be therefore elucidated by the combination of deposit remobilization by flow, mineralization and cracking during rest periods. These large channels may also ease the oxygen penetration within the filtration layer despite the significant decrease of the porosity (from 0.51 to 0.33). For comparison, Morvannou et al. [START_REF] Morvannou | Which hydraulic model to use for vertical flow constructed wetlands?[END_REF] measured the porosity of clean filter material and filter material with deposit in a French VFCW. The values are 0.44 and 0.406, respectively.

[Figure 4] Magnified images of thin sections of VFCW filtering materials.

Figure 3 displays the binary images of gravel only and gravel and deposit. On image (a), gravels seem coarser on the upper part of the sample. We recall that samples were taken from surface. Therefore we can assume that some segregation took place at the surface and that finer grains were transported deeper.

Pore-size distribution analyses

Segmentation aims at separating the pores. The result of segmentation for gravel only is

shown on Figure 5. The watershed algorithm is known for its tendency to over-segment.

We filtered out the smallest minima to limit the number of watershed and consequently the number of pore. Once a satisfying segmentation was reached, we estimated both the cumulative frequency distribution and the cumulative pore space occupation (Figure 6).

The presence of deposit transforms water path networks in two manners: i) general reduction of pore size and ii) creation of micro-scale pores.

[Figure 5] Segmentation results for gravel only. Vertical cross-section of a sample taken from the surface of a VFCW.

Flow in a gravel media is mainly driven by gravity and little water is kept by capillarity. The presence of deposit seems to change this behavior.

Limitations

The chosen method of image acquisition limits the resolution to 6.8 µm pixel -1 . Smaller pore structure cannot be observed. However, the observation at smaller scale might not have been worthful. The drying process may have caused cracks at similar scale that could bias interpretation.

Individual thin section provides a 2D representation of the pore structure that improves our understanding of deposit influence on water flow and pollutant transport.

A necessary further step will be the investigation on the spatial structure of deposit (3D observation). It could be achieved by using multiple parallel slices of the same sample or by X-ray tomography. [START_REF] Blunt | Pore-scale imaging and modelling[END_REF]

Conclusions

The presented work demonstrated the feasibility of the thin section method application to French VFCWs filter media observation. This method allows a direct and undisturbed observation of pore structure.

Results highlight that deposit significantly reduces pore space (porosity is reduced by 35%). Direct observation showed that deposit is not only attached to gravel but forms a granulated structure with large pores running through the assemblage of gravel and deposit. Numerical pore size distribution analyses pointed out that the deposit structure is made of a large bundle of small pores less than 100 µm in radius.

Image analysis of thin sections provides hints at understanding the structuration of the porous media linked to organic matter deposition. This is of critical importance when studying how water is stored and flows through the porous media. These promising results encourage us to continue in order to observe the three dimensional structuration.

Figure 6 a[Figure 6 ]

 66 ) shows a bimodal distribution for gravel with deposit. The lower mode includes pores ranging from 0 to 100 µm. It includes 61% of all pores but represents only 10% of the pore space. Water can be hold by capillarity in those pores where it moves slowly while larger pores remain available for fast gravity-driven flow. It corroborates assumptions made by Morvannou et al. [9] who used a dual-porosity model to accurately represent tracer movement in VFCW. Comparison of cumulative pore size frequency distributions (a) and cumulative pore space occupations (b) between gravel only and gravel with deposit.The network of micro-scale pores increases the interaction surface between water (containing pollutant, like ammonium, trace elements) and organic matter that provides potential sites of adsorption. It supports results showing high retention capacity for ammonium in VFCW fed with raw wastewater.[START_REF] Morvannou | Modeling nitrogen removal in a vertical flow constructed wetland treating directly domestic wastewater[END_REF]

Acknowledgements,

The authors wish to thank Georg MacLeod from University of Stirling for resin impregnation and cutting thin sections of samples. The authors would also like to thanks Claire Bertrand, Jérémie Aubert, Clément Crétollier and Adeline François for their technical support.

References:


Jean-Baptiste Bahers

Antoine Lacassagne

From the spatialization of LCA to its application in industrial ecology De l'ACV spatialisée à l'écologie industrielle

INTRODUCTION

The concepts of "territory" and "territorialization" lead to strong debates between geographers since the 1960-70s. The political, economic and social dimension of territory is discussed and analyzed according different points of view. To take examples for the most known, Michaël Storper studied the territorialized economic development, in order to understand economic activity depending on territorial resources [START_REF] Storper | The Regional World: Territorial Development in a Global Economy[END_REF], whereas Claude Raffestin talks about the `social space', using the idea of Henri Lefebvre, in order to designate a `production' that which makes a community from an ecosystem that is apportioned to it. Therefore, territory is the result of the production of actors [START_REF] Raffestin | Space, Territory, and Territoriality[END_REF]. This paper doesn't aim to discuss the legitimacy of territory's concept but we aim to use the term in order to make the link between industrial ecology and life cycle assessment.

Industrial ecology and life cycle assessment are both scientific fields which are supported by different applications and methods. Industrial ecology is supposed to understand how to design sustainable industrial systems, whereas life cycle assessment is a method useful to quantify the environmental impacts of a product or service. These issues are important to address according to the territory of implementation, because the sustainability of industrial system is very dependent on local, urban or regional context. That's why these methods probably need to be improved with a spatial analysis. How to implement the territorial dimension regarding industrial ecology and life cycle assessment? How spatialization of life cycle assessment can improve the industrial ecology application?

The first part will discuss the term of territory regarding industrial ecology and its political and economic application (towards circular economy). The second part will be more focus on life cycle assessment, and how it can be useful for industrial ecology implementation.

INDUSTRIAL ECOLOGY, CIRCULAR ECONOMY AND THE TERRITORY

From the territorial dimension in industrial ecology…

The industrial ecology is a scientific field which is structured with the analogy of natural ecosystems (Ehrenfeld, 2004) and supported by the "International Society of Industrial Ecology" (ISIE) and the Journal of Industrial Ecology created in the 1990s. It requires engaging profound changes in the industrial system towards "ecostructuration strategies" (Bourg, 2002). Suren Erkman points out the beginning of this concept during the 1960s, with the pioneers Robert U. Ayres and Preston Cloud in the USA, Jacques Vigneron in France and Peter Baccini in Swisstzerland who worked on the relationship between ecology and economy, and the expression of the oxymoron "industrial ecosystem" (Erkman, 2004).

The industrial ecology field is much known to promote the implementation of operational projects such as the "industrial symbiosis". It aims at developing closing loops of material and energy flows within an industrial area, such as the example of Kalundborg symbiosis which is very often reminded (Christensen, 2006;Jacobsen, 2006). However, industrial ecology can't be limited to the industrial symbiosis and provide research regarding several issues like: Eco-industrial Development, Socio-Economic Metabolism, Sustainable Urban Systems, Organizing Sustainable Consumption and Production, Environmental Extended Input Output, Life Cycle Sustainability Assessment. These topics are all present in the last collective work of the ISIE [START_REF] Clift | Taking Stock of Industrial Ecology[END_REF] and it shows the variety of social, territorial and economic systems which are studied. If we can consider that all of these issues can be studied regarding in their spatial and social dimensions, the concept of territory isn't really discussed. According to our review, the role of territory is seen by the authors, as:

-An area of singular characteristics with industrial boundaries, such as economic activity zones or harbor [START_REF] Chertow | Uncovering" Industrial Symbiosis[END_REF][START_REF] Mirata | Industrial symbiosis networks and the contribution to environmental innovation: The case of the Landskrona industrial symbiosis programme[END_REF][START_REF] Schiller | Analyzing networks in industrial ecology-a review of Social-Material Network Analyses[END_REF]. In this case, the territory is merely a given area where the park is situated, but it also provides synergy's opportunities because of the geographical proximity of industrial stakeholders. For some authors, the territory plays a role in the synergies as resources provider or project facilitator [START_REF] Beaurain | L'écologie industrielle comme processus de développement territorial: une lecture par la proximité[END_REF][START_REF] Boons | The dynamics of industrial symbiosis: a proposal for a conceptual framework based upon a comprehensive literature review[END_REF]. -An administrative area within it is possible to study society's metabolisms.

Therefore, the territory is correlated to spatial planning. According to the geographical level, it concerns urban, regional or national plans which aim at optimizing the circulation of material and energy flows [START_REF] Billen | L' Ecosystème Belgique: essai d'écologie industrielle[END_REF][START_REF] Fischer-Kowalski | Socioecological Transitions and Global Change: Trajectories of Social Metabolism and Land Use[END_REF][START_REF] Kennedy | Mainstreaming Urban Metabolism[END_REF][START_REF] Ferrao | Sustainable Urban Metabolism. 1 re éd[END_REF]. -The localization of environmental impacts. These works are focused on life cycle assessment and consumption-based accounting. Hence, the emissions and wastes arose on territory (Life Cycle Sustainability Assessment approach, see for example (Tom Wiedman in Clift et Druckman 2016) and it comes from an area of resource's consumption [START_REF] Druckman | The carbon footprint of UK households 1990-2004: A socio-economically disaggregated, quasi-multi-regional input-output model[END_REF].

In France, a new scientific field emerges towards social and spatial analysis of industrial ecology, named "territorial ecology". This concept is defined by Sabine Barles as: "it is an industrial ecology that is considered in a spatial context and that takes into account the stakeholders and, more generally, the agents involved in material flows, questions their management methods and considers the economic and social consequences of these flows" [START_REF] Barles | Society, energy and materials: the contribution of urban metabolism studies to sustainable urban development issues[END_REF]. This definition leads to the organization of an interdisciplinary field of research (N. [START_REF] Buclet | Ecologie industrielle et territoriale : Stratégies locales pour un développement durable[END_REF][START_REF] Junqua | Ecologie industrielle et territoriale : COLEIT 2012[END_REF], within social sciences and land planning have a strong role. According to us, the territorial ecology corresponds to (Bahers 2014): -A territorialized approach of flow circulations and stakeholder's system, -An implementation of this "resource-waste" ecology in the regional and urban plans. This new field is very close to the social ecology of the Institute of Social Ecology in Vienna [START_REF] Haberl | Social Ecology[END_REF]). However, it differs in that social ecology is mainly focused on national territory (Austria, Germany, India, China) instead of regional or urban areas as suggested in the territorial ecology.

… to the territorialization of circular economy

The new program of circular economy, which has a great success these days, is supposed to fulfill a renewed vision of resources and waste management. According to the French environmental agency (ADEME), industrial symbiosis, products ecodesign, sustainable consumption and the 4R-V (Reduction at source, Reuse of a product, Recovery, Recycling and Valorization of residual material) are the pillars of this political and economic program.

The territorial dimension in the French implementation of circular economy is just mentioned because it requires facilitating "the reinforcement of cooperation between economic actors at the relevant scale taking into account the principle of proximity" 1 , as laid down in the first regulatory definition in 2014 within the law of "Energetic Transition" i . We can see towards this definition, that there is no indication on which spatial scale, the circular economy strategies are supposed to be implemented. According to the European Union, the territorial approach disappears completely, since the first target is to promote the efficiency of resource's use in order to maximize the economic growth (EC, 2014;2015) 23 .

Therefore, few authors use the territorial approach to interrogate the political program of circular economy. Gregson et al. (2015) use the variable of spatial distance to criticize these initiatives which are unable to reuse all the waste at a local scale. The emerging circular economy, according to them, "entails challenges borne of a conjuncture of politically created markets, material properties and morally defined material circuits" (Gregson, 2015). The political action of circular economy depends on the issue's appropriation of local actors. This perspective is also studied by researchers, who talks about "generative spaces" for the development of circular economy from citizen engagement (Hobson, 2015). Ghisellini et al. [START_REF] Ghisellini | A Review on Circular Economy: The Expected Transition to a Balanced Interplay of Environmental and Economic Systems[END_REF] considers that the perspectives of circular economy should be different according to the socio-technical levels (providing information to policy makers at the macro level, development of industrial symbiosis at the meso level, and selling of a service (instead of a product) or its refurbishment and remanufacturing at the micro level). Nevertheless, the spatial issue is only mentioned at the meso level, regarding the Eco-industrial parks. The industrial symbiosis seems thus to reveal the territorial dimension of circular economy. It requires a comprehensive knowledge of local context in order to be implemented, especially regarding local actors interests.

Industrial symbiosis as spatialized systems

We have seen industrial symbiosis as the most famous example of industrial ecology approach, forming a whole system by setting up synergies between economic agents. The most known type of synergies is the recycling of a waste or a byproduct of one enterprise into a material for an another one. However, literature shows other types of synergies: they can be organizational, relational or strategical (Massard, 2011) (2015). Closing the loop -An EU action plan for the Circular Economy, Disponible sur < https://ec.europa.eu/priorities/jobs-growth-andinvestment/towards-circular-economy_en> Another aspect of those synergies is the agreement to realize the material exchange and to assure them from the economical fluctuation of raw materials exchange rates. Moreover than relationship based on byproduct recycling, an industrial symbiosis can be defined through the agreement between enterprises which would be otherwise all alone (Jensen, 2012). Setting up of agreements can be facilitated by spatial proximity of the agents allowing a mutual trust (Jensen, 2012;Chertow, 2000). This spatial proximity leads enterprises to share their technical knowledge, best practices and investigations through mutualism, vital for the right working of the symbiosis (Christiansen, 1994in Ehrenfeld, 1997). Nevertheless, an industrial symbiosis is a spatialized system, in terms of materials and immaterials exchanges, leading agents to a social proximity, depending of their motivations, aims but also to face up environmental and territorial constraints (Chertow, 2000). However, according to several authors [START_REF] Van Berkel | Comparability of industrial symbioses[END_REF][START_REF] Sokka | Analyzing the Environmental Benefits of Industrial Symbiosis[END_REF], industrial symbioses legitimacy is based on the hypothesis that their synergistic exchanges are greener, demonstrated qualitatively but not quantitatively. We can ask ourselves about the quantification of the environmental benefits of industrial symbioses, to valid this hypothesis, often seen on industrial symbioses studies. How environmental evaluation tools can quantify the environmental benefits of industrial symbiosis, which is a spatialized system as we saw? We focus on this study on Life Cycle Assessment (LCA), a sturdy tool to assess environmental assessment of complex systems such as industrial symbiosis.

Life Cycle Assessment as a tool to quantify the environmental benefits of an industrial symbiosis

LCA is a normalized method to quantify the potential environmental issues and environmental impacts through the whole life cycle of a product (ISO14044, 2006). Life cycle assessment is an iterative method composed by four phases: the goal and scope definition, the life cycle inventory, the impact quantification and the interpretation of the results of each phase (ISO14040, 2006). Several research work were done to improve the LCA methodology last two decades but very few were applied to industrial symbiosis, showing methodological issues specific to industrial symbiosis [START_REF] Mattila | Methodological Aspects of Applying Life Cycle Assessment to Industrial Symbioses[END_REF]. The first one is the quantification of the environmental benefits, by the comparison with a reference system, a reference scenario built to satisfy the same functional unit than the symbiotic scenario (ISO14044, 2006;[START_REF] Martin | Quantifying the environmental performance of an industrial symbiosis network of biofuel producers[END_REF]. The functional unit is an essential point to realize the LCA of industrial symbioses when the aim of the study is to quantify their environmental benefits. Nevertheless, the specific recycling of material flows occurring in industrial symbioses makes the realization of this reference scenario sensitive to methodological assumptions of the author.

The most used reference scenario in the literature is the one based on the studied industrial symbiosis without synergies between the enterprises of the system [START_REF] Van Berkel | Quantifying Sustainability Benefits of Industrial Symbioses[END_REF][START_REF] Martin | Quantifying the environmental performance of an industrial symbiosis network of biofuel producers[END_REF]. To build a scenario without synergies, the author of the LCA should ask which raw material consider instead of the non-recycled byproduct and how this non-recycled byproduct should be managed for every single synergy of the system. This can be realized with system expansion, considering more raw materials production and more byproducts to manage than the symbiotic scenario. However, this method has weakness from the necessity of using several assumptions [START_REF] Mattila | Methodological Aspects of Applying Life Cycle Assessment to Industrial Symbioses[END_REF]. The selection of the suitable reference scenario reduces the overestimation of environmental benefits of the industrial symbiosis [START_REF] Mattila | Methodological Aspects of Applying Life Cycle Assessment to Industrial Symbioses[END_REF].

To consider this sensitivity, different assumptions can be tested by producing several reference scenarios, allowing the realization of a sensitivity analysis [START_REF] Martin | Quantifying the environmental performance of an industrial symbiosis network of biofuel producers[END_REF]. The sensitivity on reference scenario produces a variability of reference scenarios usable and so a variability of the environmental impacts of those reference scenarios. When making the comparison with the symbiotic scenario, the variability of the environmental impacts of the reference scenarios causes a variability of the environmental benefits of the industrial symbiosis studied. There is a need to quantify this variability to know if the industrial symbiosis leads to environmental benefits and to quantify them.

A territorial approach to consider industrial symbioses specificities and quantify its environmental benefits

To face up the reference scenario's variability without realize too much reference scenarios, the realization of a reasonably worst case reference scenario and a reasonably best case reference scenario can be done to quantify the widest variability of those reference scenarios (ILCD, 2010), and so on, to mark out the environmental impact of the industrial symbiosis between those two scenarios. We can assume that all the reference scenarios realizable have an environmental impact included between those two extrema.

Moreover, the environmental impact of the industrial symbiosis can be compared to those of the best case and the worst case.

-If the industrial symbiosis has less environmental impact than the reasonably best case reference scenario, there is a strict environmental benefit of the industrial symbiosis, whatever the reference scenario chosen.

-If the industrial symbiosis has more environmental impact than the reasonably worst case reference scenario, there is a strict environmental detriment of the industrial symbiosis, whatever the reference scenario chosen.

-Between those two extrema, there is a relative environmental benefits, depending of the reference scenario chosen.

The results of our literature review show two different cases with two different approaches to realize the worst case reference scenario:

-If there is currently an industrial symbiosis on the territory, the worst case reference scenario is the studied industrial symbiosis is realized considering the enterprises of the industrial symbiosis without recycling linkages [START_REF] Sokka | Analyzing the Environmental Benefits of Industrial Symbiosis[END_REF]; titre chapitre [START_REF] Eckelman | Life Cycle Energy and Environmental Benefits of a US Industrial Symbiosis[END_REF][START_REF] Dong | Achieving carbon emission reduction through industrial & urban symbiosis: A case of Kawasaki[END_REF][START_REF] Martin | Quantifying the environmental performance of an industrial symbiosis network of biofuel producers[END_REF][START_REF] Daddi | Using Life Cycle Assessment (LCA) to measure the environmental benefits of industrial symbiosis in an industrial cluster of SMEs[END_REF].

-If the industrial symbiosis studied is prospective, the worst case reference scenario is built with average technologies satisfying the same functional unit than the symbiosis [START_REF] Liu | Life cycle assessment of an industrial symbiosis based on energy recovery from dried sludge and used oil[END_REF][START_REF] Blengini | Eco-efficient waste glass recycling: Integrated waste management and green product development through LCA[END_REF][START_REF] Ammenberg | Improving the CO2 performance of cement, part III: the relevance of industrial symbiosis and how to measure its impact[END_REF][START_REF] Mohammed | Identification of an environmentally friendly symbiotic process for the reuse of industrial byproductan LCA perspective[END_REF]) . This second option can be done realizing an overview of the technical solutions used in the territory, to consider local technologies satisfying the functional unit chosen.

The realization of the reasonably best case scenario needs to show an improvement from the reasonably worst case. The consideration of processes using marginal technologies (e.g. renewable energy instead of fossil energy), and best available technologies to improve the current ones can lead to a best case to substitute worst cases processes, according to our research. Indeed, the implementation of new industrial processes, spatialized like industrial symbiosis, is realized considering economical, social and political motivations (Chertow, 2000). Technical improvements assessed should consider local political priorities, regulation (e.g. waste regulation), and be limited by the availability of raw materials, land availability for the facilities, local competencies to develop and to operate those new technologies but also the social acceptation of the neighborhood.

The interviews of local agents regarding issues and constraints about the different best case reference scenarios assessed lead to the definition of the reasonable best case reference scenarios. Therefore, a strong lever is the satisfaction of most of the local agents considering their own strategical aims and technical improvement needs. This resulting reference scenario has a reasonable probability to be implemented on the territory in the hypothesis that the industrial symbiosis is not developed. The key agents for this evaluation are the local politicians, local experts knowing the environmental issues of the territory, institutions in charge of the waste management and local inhabitants.

CONCLUSION

In conclusion, it appears that a territorial approach is required to assess the environmental benefits of industrial symbiosis. There are a lot of perspectives to improve the LCA methodology with spatial, social and political dimensions, according to the territoriality of industrial symbiosis.

Therefore, urban and territorial metabolism is a powerful concept which could be interesting to mobilize. It provides keys to understand the socio-ecological dynamics [START_REF] Bahers | Métabolisme territorial et filières de récupération-recyclage : le cas des déchets d'équipements électriques et électroniques (DEEE) en Midi-Pyrénées[END_REF][START_REF] Buclet | Sous la direction de Nicolas[END_REF];) towards a better knowledge of the space of resources and actor's system. Territorial metabolism gives also opportunities to examine the metabolic links between territories, their environments and their supply and disposal territories as a mean to transition towards a circular economy. The analysis of the inter-territorial relationships from a metabolic functioning point of view constitutes a privileged entrance to reach this perspective.

  THE LIFE CYCLE ASSESSMENT OF INDUSTRIAL SYMBIOSIS: HOW SPATIALIZATION OF LCA CAN IMPROVE THE QUANTIFICATION OF THE ENVIRONMENTAL BENEFITS OF AN INDUSTRIAL ECOLOGY APPLICATION

  .

	1	Traduced by ourselves

2

European

Commission (2014)

. The Circular Economy: connecting, creating and conserving value, Disponible sur < http://ec.europa.eu/environment/circular-economy/> 3 European Commission