Urban Metabolism

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Chapter 4  Methodology: A Model for Societal Metabolism

Development of a Sustainable Societal Metabolism Model

Modelling Approach

Whilst improvements in e ciency of resource use is often seen as a rst step towards sustainable resource management, holistic ‘whole-system’ design can facilitate more radical changes and achieve much more than a ‘softly softly’ approach (Hodson et al., 2012). The principles of UM and MFA, along with the sustainability basis of carrying capacity, show potential as a modelling tool for indicating the needs and production capacities of SIDS, and therefore whether and how they can become self-su cient. Uniting the two approaches would therefore generate a more insightful perspective by describing the metabolism not just by its gross inputs and outputs, but by tracking where material and energy ows go within the system after entering it, and where they arise from before exiting it (i.e. producing a ‘transparent’ metabolism, or opening up the ‘black box’).
A methodology borrowing from both styles of analysis would reveal a more detailed type of societal metabolism, in which the underlying vectors are more apparent { and therefore the relationships between infrastructure systems involved in resource management are readily identi able. Tracking the life cycle of each resource ow within the societal metabolism enables better understanding of the interactions between various systems of infrastructure required to support the population. These insights may be useful in determining whether the area in question has the potential to become fully sustainable and how it can be made so. An ‘urban metabolism 2.0’ proposed by Pincetl, Bunje, and Holmes (2012) seemed to suggest doing MFA and UM under one analysis, without uniting the approaches any other way, and lacked technical details on how the results could be interpreted together.
Though they maintain independent identities, the many philosophical similarities between MFA and UM have led to these distinct analyses becoming at times somewhat blurred in the literature. Their shared systemic mass balance approach has resulted in an increasing number of examples of UM studies with distinctly MFA characteristics in the way they subdivide ows and consider subsystems within the overall system of the societal metabolism, for example by Barles (2009) and Kennedy et al. (2010). In line with the stated research objectives of assessing the feasibility of a sustainable SIDS society, a methodology to characterise both the internal systemic relationships and external resource ows of the societal metabolism of a small island state was developed, as explained in the following sections, that combined the most useful aspects of both UM and MFA.
However, substantial di erences between the two approaches remain, and these had to be addressed in attempting to develop a single methodology applicable to SIDS. De ning the internal and external system boundaries were notably important in this context, in particular how to divide the boundary between human and natural systems, how to de ne distinct infrastructure systems, and how inputs and outputs should be grouped (see Section 4.1.4). Other issues and details dealt with include how to bring the concept of carrying capacity into UM and MFA (as discussed in Sections 1.3.3 and 4.1.3), how to characterise infrastructure in a way conducive to the intended methodology (in Section 4.2.2), how to generalise the model in software and build in dynamic capability (the development of which is detailed in Section 4.3), and how to present and interpret results (as shown in Chapters 5 and 6).

Conceptual Model Development

Because of the high levels of crossover between di erent infrastructure systems, the needs-based perspective of UM was taken as the basis for the conceptual model, to ensure basic needs would be met. This was based on the metabolic scheme centred on the human metabolism: ful lling needs and producing waste (as shown in Figure 4.1). The basic human needs and waste production were pared back to their simplest elements, then the infrastructures pertaining to each need/waste were grouped appropriately, as shown in Each type of mass goods were categorised as one of the UM ow types, being water, food, energy, materials, solid waste, wastewater, or emissions. These were then grouped as Inputs, Outputs, Imports, Exports, Re-exports, Local Production, and Losses (including waste), as follows:
Inputs were de ned as all available inputs into the system (Equation 4.1, equivalent to MFA Equation 3.1), as determined by the metabolic system boundaries (which will be discussed in Section 4.1.4);
Consumption was de ned as everything that is consumed locally (Equation 4.3) Outputs as all outputs from the system (Equation 4.2); and Production as everything produced locally from local resources.
The full set of MFA equations could not be used in this analysis, as data on hidden or unused ows were unavailable.

Carrying Capacity Through UM and MFA

Proponents of UM and MFA point out that the analyses could have an important role in sustainable development, particularly in infrastructure planning (Kennedy et al., 2010); however, these approaches seem to focus more on resources themselves than the human need for and consumption of these resources, even though this consumption is key to the sustainability of a city and its supporting hinterland. If a methodology based in UM and MFA is to be useful in sustainability assessment and infrastructure planning, they must be examined through the lens of carrying capacity.
If a societal metabolism needs to be circular to be sustainable (as suggested by Meijer et al., 2011, and shown in Figure 3.3), then the concept of carrying capacity needs to be introduced into the planning and development of that metabolism, no matter the size of the hinterland (such as in Sundkvist, Jansson, Enefalk, & Larsson, 1999). For SIDS, oceanic boundaries historically kept the islands’ populations in balance with the available natural resources; i.e. there was no spare land from which to draw additional resources.
Technology allows an arti cial separation from the carrying capacity of any given area, along with a usually higher material standard of living, however even a carrying capacity enhanced by technology has its limits. At present, SIDS import a large amount of their food, energy, and materials, relying on shipping to support a more and more western lifestyle which would otherwise be nearly impossible given the islands’ limited natural and human resources; it is probably safe to say that their natural carrying capacity is being exceeded, and arguably so is that of the entire planet (Rockstrom et al., 2009).
Carrying capacity as a concept can be di cult to nail down because of the in uence of fossil fuel energy and continually advancing technology. Does it refer to the maximum population possible, consuming the absolute minimum physical requirements for survival, or does everyone get to consume as much as they want, thereby drastically reducing the total population that can sustainably live in a given area?
Determining the number of people that an island is capable of supporting rst requires standardisation of how much they consume. The present level of input and consumption of mass goods by UM and MFA enables analysis of carrying capacity without adjusting expected living standards.
Using the Consumption calculated in Equation 4.1 as Total Resource Supply, Resource Availability (as mass or energy per capita) may be determined according to Equation 4.4:
If Input (from Equation 4.1) is used instead of Consumption, Resource Availability will reveal the theoretical maximum in a perfectly e cient system, i.e. if no waste were created.
Because of the elastic relationship between supply and demand in any free economy, the Resource Availability term in Equation 4.4 can also be equated to Resource Demand, if existing living standards are to be maintained. Equation 4.4 can thus be rearranged as follows, to re ect the maximum population an area can currently support:
Or, looking at the population that an area can sustainably support (in other words, its Carrying Capacity), the Total Resource Supply term can be modi ed to only account for locally-produced goods (using Local Production, from Equation 4.1).
Equation 4.5 re ects the dynamic nature of supply and demand in that an increase in resource availability (or, amount available or required per capita) will cause a decrease in carrying capacity, unless supply is proportionally increased. Through Total Resource Supply (Equation 4.6), it relates infrastructure to carrying capacity.
These same equations can be rearranged to show the total resource supply required or available, as follows:

System Boundaries

In this methodology, it was necessary for the whole country to be included geographically, including its exclusive economic zone. This decision was made because background of the study came from SIDS sustainable development, but it also has broader theoretical relevance to the development of the UM eld. In the context of the metabolism concept being proposed as a potential aid in infrastructure planning, the infrastructure supporting a city which does not lie within city boundaries and also supports rural communities, such as municipal water supply and national electricity grid, cannot be ignored. From this perspective, there is no reason not to consider the rural hinterland as part of the extra-urban metabolism, and shift from an urban metabolism to a regional or country-wide metabolism, or even a global one, more true to the idea of a full societal metabolism. Because of its focus on physical needs and quantitative modelling, social aspects were outside the scope of the study, although cultural characteristics are used where relevant to interpret elements of the results and analysis.
The di culty of drawing meaningful system boundaries is a prevalent issue for UM and MFA. No common de nition exists for ‘urban area’ and ‘city’; this hinders straightforward comparison between UM methodologies and results (Weisz & Steinberger, 2010). If it is to be applied to SIDS in the context of sustainable development, UM presents speci c challenges. For example, re-exports (goods that are shipped to one location with the intention of shipping them on to another location) can contribute signi cantly to SIDS’ GDP but are not part of the indigenous metabolism. Similarly, tourism is an essential component of most SIDS’ economies, but due to their remoteness tourists travel a long distance (usually by air) to reach their destination. SIDS reap the bene ts of this long-distance travel but it is not within their means to make this transport more e cient, so do the emissions and resources used belong to the SIDS or the tourists’ home countries? The chosen system boundaries could clearly have a signi cant e ect on the results.
This di culty is echoed in previous MFA studies, such as that of the waste management strategies for Oahu, Hawai’i, whose authors note the complication of the blurring of material ow categories (Eckelman & Chertow, 2009); in this case, 70,000 t of paper were consumed within the economic system, but 360,000 t of waste paper were generated. The disparity was believed to have occurred by waste paper originating as packaging or as other products, such as catalogues, however the exact portion of basic unprocessed paper that initially entered into the economy was not ascertainable due to the complexity of the system. This demonstrates the di culty of extricating individual ow information in complex systems such as a societal metabolism, and the importance of meaningful categorisation of ows.
Although there are many components within infrastructure, they are not all equal with regards to the supply of basic needs that underpins the UM approach of characterising ows as food, water, energy, and materials. While buildings, roads, and communications are vital aspects of infrastructure in every society, they are not directly responsible for ful lling basic needs. In the case of roading, which is responsible for delivering { but not producing { items necessary for sustenance, transportation infrastructure is indirectly measurable in Materials supply, and energy use for transport purposes is contained in the distinct category of Transport Systems. The latter combines road transport with sea-borne transport, examples of important parts of the wider infrastructure network that are necessary to the metabolism but not directly productive. Thus buildings and communications infrastructure are not speci cally separated despite their usual status as items of ‘built infrastructure’. However, the materials they consume and wastes they produce are quanti ed via materials consumption and Construction & Quarrying Systems.
It was unnecessary to form a complete inventory of the area’s natural systems, because only the anthropogenic (societal) metabolism was investigated. Though these may be useful in future modelling, for this study a full account of the resources present in natural ecosystems was outside the scope. For example, sh taken for consumption or export were included, but sh left in the sea were not. Likewise, oxygen ows were not included except via fossil fuel combustion; though it is present in natural processes such as respiration, oxidation, and photosynthesis, the large atmospheric reservoir allows the assumption that oxygen is in in nite supply for sustainability purposes. Water and air ows are not typically included in MFA studies (Perez Manrique et al., 2013), however since in this methodology (in accordance with normal UM analysis) greenhouse gas emission outputs are accounted, it is logical that the in ow corresponding to these out ows should be counted. Mineral resources may be considered part of an island’s material stock (and may also to contribute to their exports) but they may not yet be exploited, in which case they are not yet a material ow and may never become one if the source is economically unviable. Thus, like the ows of natural resources which may never cross paths with the societal metabolism, mineral resources were not quanti ed except where currently extracted for use in the human ecosystem (also known as the anthroposphere).
The conceptual model developed takes into account agricultural systems, which do not typically gure in UM due to their negligible occurrence in urban areas, though they are important to the societal metabolism for food supply and provision of various materials. The broader category of agriculture was subdivided into animal and plant production systems, and food processing was likewise split into household and industrial activities, after the MFA by Neset et al. (2006). Quanti cation accuracy in the agricultural sector can be problematic due to the natural variability between areas, the lack of measured inventory for many natural resources and processes, the crossover in the paddock between plant systems and animal systems (such as cows grazing under coconut trees), and di cultly in separating out data on subsistence where it is common for subsistence farming to overlap with commercial farming, for example selling surplus crops at market.
The activities of non-livestock animals were not considered due to the lack of data, so ecologically signi cant ows such as movement of pollen and scavenging were not quanti ed. Those included from agriculture were those which had direct human interference, such as creation of organic waste, seed collecting, and deforestation emissions. Major natural ows, such as plant uptake of rainfall and insolation, were quanti ed for comparison and completeness, but were not counted in the totals for the societal metabolism, as these sources are only indirectly used by humans. Heat emissions and non-greenhouse gases are sometimes included in UM (for example, Decker et al. (2000)); however this was not feasible for Samoa as there were no data available.
The socio-technical infrastructure systems pertaining to the anthropogenic needs and waste products (as discussed in Section 4.1.2) were identi ed, along with each system’s inputs and outputs and the uxes between systems. The anthroposphere is underpinned by the natural (eco)systems and cycles which provide vital resources such as sunlight for energy and photosynthesis, rainfall which feeds crops and animals as well as supplying household needs, pollination services, and abiotic resources such as rock and gravel.
Though the anthroposphere depends on these resource ows, measurement of the various and uctuating components of the natural ecosystem’s metabolism in its entirety was not possible within the limits of the study. Furthermore, air and natural water ows are not typically considered in MFA (Perez Manrique et al., 2013). An overview of the ows of major resources and their context within the system boundary is shown in Figure 4.3.
The system boundaries as determined by this methodology resulted in the development of an ‘infrastructure network’ diagram showing the role of each infrastructure system from Obernosterer et al., 1998). The system boundary is indicated by the dotted line;
NB: This is not a defini ve account, but intended to show an overview of the flows of major resources and stocks (ellipses denoting individual reservoir types) lying on the dotted line are partially their context within the system boundary.
included in the study, to the extent that data is available and their inclusion is relevant to the anthropospheric ows.
within the societal metabolism. An expanded (but still very simpli ed) schematic showing the interactions between infrastructure systems in Samoa and ow vectors entering and exiting Samoa’s societal metabolism is shown in Figure 4.4. Where multiple items overlap within the metabolism, these have been simpli ed into a single arrow for legibility. Flows are delineated as constituting one of the four main categories of UM before the human component of the metabolism (denoted by dotted arrows) wherein they are consumed and converted into waste products. Inputs, or ‘sources’ are the vectors from which material ows enter the anthroposphere, for example from the natural ecosystem, the abiotic sphere, the ocean, the atmosphere, or imports (shipping or air freight). Outputs, or ‘sinks’ are the reverse, and include endpoints such as land lls (or the local environment in the case of dumped waste), the ocean, the atmosphere, and exports.

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List of Figures 
List of Tables 
1 Introduction
1.1 Background
1.2 Small Island Developing States
1.3 Sustainability Frameworks
1.4 Case Study: Samoa
1.5 Summary
2 Objectives & Scope 
2.1 Objectives
2.2 Scope
2.3 Research Contribution
3 Literature Review 
3.1 Methodologies Reviewed
3.2 Urban Metabolism
3.3 Material Flow Analysis
3.4 Comparison of Urban Metabolism & Material Flow Analysis
3.5 Alternative Approaches
4 Methodology: A Model for Societal Metabolism 
4.1 Development of a Sustainable Societal Metabolism Model
4.2 Quantitative Model Development .
4.3 Software Modelling
4.4 Modelling Attempts
4.5 Summary
5 Results & Discussion: Characterisation of Samoa’s Societal Metabolism 
5.1 Overview
5.2 Inputs
5.3 Outputs
5.4 Overall Mass Balance
5.5 Infrastructure Systems Interdependency
5.6 Summary
6 Analysis: Sustainability Assessment & Carrying Capacity 
6.1 Samoa’s 2009 Sustainability Assessment
6.2 Samoa’s 2009 Carrying Capacity
6.3 Samoa’s Future Sustainability Assessment
6.4 Samoa’s Future Carrying Capacity
6.5 Implications for Infrastructure Planning
6.6 Methodology Assessment
6.7 Summary
7 Conclusions 
7.1 Key Findings
7.2 Future Work
A Methodology to Support Infrastructure Development for Sustainability

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