Waste Management 29 (2009) 2765–2771

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Waste Management

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Assessment of total urban metabolism and metabolic inefficiencyin an Irish city-region

David Browne a,*, Bernadette O’Regan b,1, Richard Moles c,2

a Department of Transport, 44 Kildare Street, Dublin 2, Irelandb Centre for Environmental Research (CER), Foundation Building, University of Limerick, Castletroy, Irelandc Chemical and Environmental Sciences (CES) Department, University of Limerick, Castletroy, Ireland

a r t i c l e i n f o a b s t r a c t

Article history:Accepted 12 May 2009Available online 13 June 2009

0956-053X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.wasman.2009.05.008

* Corresponding author. Tel.: +353 16041585; fax:E-mail addresses: davidbrowne@transport.ie,

(D. Browne), Bernadette.ORegan@ul.ie (B. O’Regan), R1 Tel.: +353 61202552; fax: +353 61202568.2 Tel.: +353 61202817; fax: +353 61202568.

This paper aims to measure product and waste flows in an Irish city-region using the principles of metab-olism and mass balance. An empirical indicator to measure resource efficiency, using a ratio of waste dis-posal as a function of product consumption, was developed and it was found that total materialsmetabolic inefficiency fell by 31% from 0.13 in 1996 to 0.09 in 2002. The paper concludes by analyzingthe strengths and weaknesses of this indicator and its potential application in the field of sustainable con-sumption and resource efficiency as well as making suggestions to improve and strengthen the indicator.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The objective of this paper is to analyse product consumptionand solid waste generation in an Irish city-region in order to deter-mine the scale of consumption and waste output. This analysis isundertaken using a static model, which measures product andwaste flows in a particular year. A materials metabolic inefficiencyindicator is proposed, which relates the final disposal of materialand product wastes to consumption in order to highlight sectoralinefficiencies.

Section 1 discusses the role of the Laws of Thermodynamics inmeasuring material flows and metabolic exchanges and evaluatesempirical work on urban metabolism. Section 2 outlines the meth-odology and data sources used in this paper. Section 3 applies themetabolic inefficiency indicator to material and product consump-tion. Section 4 evaluates the results and offers final conclusions.The empirical metabolic inefficiency indicator is assessed to evalu-ate its strengths, applicability and weaknesses and suggestions aremade as to how it can be ameliorated as part of future work.

1.1. Thermodynamics and metabolism

Over evolutionary time, the natural system has developed thecapacity to utilise a unidirectional flow of energy to drive materialcycles. In contrast, modern industrial economies have evolved rap-idly on the basis of unidirectional material flows (Weston and Ruth,

ll rights reserved.

+353 16041180.davidbrowne2@gmail.com

ichard.Moles@ul.ie (R. Mole).

1997). Energy is degraded in the transformation of materials and, asa result, residual materials are released as waste products and linearflows of energy are dissipated in the form of waste heat (Ruth, 1995).

The Laws of Thermodynamics provide a basis for the physicalquantification of the interactions between natural systems andtheir surroundings (Weston and Ruth, 1997). The mass balanceprinciple, based on the First Law of Thermodynamics, states that‘‘mass inputs must equal mass outputs for every process step”and implies that residuals are inevitably produced as a result ofthe materials transformation system. Indeed, externalities associ-ated with production and consumption are pervasive and increasewith economic growth (Ayres, 1998).

Thus, the economy may be regarded as a materials processingsystem, in which useful low-entropy materials enter, undergo aseries of changes in their energy and entropy state and, after a timelag, the residual high-entropy materials or wastes are dissipated orreturned to the environment from various points in the economicprocess (Turner et al., 1994; Rebane, 1995).

Faucheux (1994) argues that ‘‘energy and mass conservation,together with entropic irreversibility, implies that unwanted by-products or waste energy are inevitable in the course of economicproduction and consumption”. The amount of residual waste mate-rials compared to the size of the active inventory of fixed capital orraw materials is a function of the efficiency of recycling and givesrise to the question of how big must the stock of waste materials beto allow a constant level of ‘useful’ materials in a stable system dri-ven by an unlimited exogenous energy flux (Ayres, 1999). The FirstLaw of Thermodynamics is useful for measuring material flows in asystem as material inputs and stock changes must equal materialoutputs. In this case study, the First Law of Thermodynamics isused to illustrate the principles of mass balance, as applied to anurban settlement.

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The Second Law of Thermodynamics or ‘Entropy Law’ statesthat the entropy of an isolated system increases and available en-ergy spontaneously dissipates, that is ‘‘the entropy of the physicaluniverse increases constantly because there is a continuous andirrevocable qualitative degradation of order into chaos” (Lozada,1995). However, all systems, whether isolated or not, are subjectto the same forces of entropic decay and any complex, differenti-ated system has a natural tendency to erode, dissipate, and unra-vel. The reason that open, self-organising systems do not rundown in this way is that they are able to import available energyand material from their host environments, which they use tomaintain their internal integrity. Such systems also export theresultant entropy in the form of waste and disorder to their hosts(Rees and Wackernagel, 1996).

The Second Law of Thermodynamics also suggests that allhighly-ordered systems grow and develop, that is increase theirinternal energy, during the process of entropy growth in larger sys-tems ‘‘at the expense of increasing disorder at higher levels in the sys-tems hierarchy” (Rebane, 1995). Thus, because such systemscontinuously degrade and dissipate available energy and matter,they are called ‘dissipative structures’ (Nicolis and Prigogine,1977). City-regions may be regarded as complex systems, whichcontinuously degrade and dissipate available energy and matterand appropriate ecosystem services from the local, regional andglobal hinterland (Rees and Wackernagel, 1996). The Second Lawof Thermodynamics is relevant to the case study in this paper, asthe particular city-region is an open, complex system, which pro-cesses material flows and inputs and discharges waste outputs.

It has been argued that complete recycling of waste products maywell be an energetic impossibility due to the physical and spatial dis-persion of waste products as low-entropy natural material resourcesand the energy expenditure required (Turner et al., 1994; Bianciardiet al., 1996; Ayres, 1997, 1999). Georgescu-Roegen (1976) arguesthat the principle of entropy applies to materials as well as energy,stating that matter is subject to an ‘irrevocable dissipation’ and thatirreversibility is also a feature of material transformations.

Indeed, a Fourth Law of Thermodynamics or Law of Matter En-tropy has been postulated, where material dissipation becomes thelimiting factor and ‘material entropy must ultimately reach a max-imum value’ (Mayumi, 1995; Ayres, 1998). However, Ayres andMiller (1980) argue that intrinsically scarce materials can be recov-ered, within an energy expenditure budget, and that is there is nolimit in principle to the degree of dematerialization that can beachieved in the long run (Cleveland and Ruth, 1997). The ‘FourthLaw of Thermodynamics’ is not empirically applied in this paper.

Resource efficiency may be defined as the amount of economicactivity generated from material extraction/throughput or energyconsumption and is the corollary of relative decoupling. Eco-effi-ciency may be defined as the ‘‘delivery of competitively-priced goodsand services that satisfy human needs and bring quality of life, whileprogressively reducing ecological impacts and resource intensitythroughout the lifecycle to a level at least in line with the Earth’s esti-mated carrying capacity” (WBCSD, 2000).

The material input per service (MIPS) indicator is defined as thematerial input of a product as a function of the number of servicesprovided and may be used to measure the environmental impactsof the production of services. It is an indicator of strong sustainabil-ity as it proposes absolute material reduction as well as an increasein the number of services provided by a single ‘service-delivery’machine or given level of input (Hinterberger et al., 1997).

Various policy targets for dematerialization have been sug-gested including the Factor 4 goal of increasing resource productiv-ity by halving global resource requirements and variants of theFactor X goal include Factor 2.5, which calls for an increase inproductivity of non-renewable raw materials, and Factor 10 (VonWeizsacker et al., 1997; Bringezu, 2003).

The eco-efficiency of urban material metabolism refers to theamount of social services per unit resource consumption or perunit pollution discharge during the process of urban materialmetabolism (Yan and Zhifeng, 2007). In this paper, an alternativeexpression of resource efficiency is presented, which expressesthe ratio between product disposal and consumption rather thanthat between economic or social benefit and environmentalpressures.

1.2. Urban metabolism

Alberti (1996) argues that a systematic analysis of urban sus-tainability should consider the direct transformation of the physi-cal structure and habitat, use of renewable and non-renewablenatural resources, release of emissions and wastes, and humanhealth and well-being. Cities and their immediate hinterland, i.e.city-regions, appropriate the ecological output and life supportfunctions of the global hinterland and, therefore, a prerequisite isthe sustainable use of these appropriated ecosystem services (Reesand Wackernagel, 1996).

This may imply that resource consumption and waste genera-tion are within the carrying capacity or assimilative capacity ofthe regional or global hinterland, as measured using material flowaccounting (MFA), metabolism accounting or ecological footprint(EF) analysis. Human carrying capacity, which is the maximumentropic load that can safely be imposed on the environment, iscorrelated with the carrying capacity of the environment or its ‘‘maximum persistently supportable load” (Rees and Wackernagel,1996). Alternatively, it could imply that the urban settlement issufficiently resilient and dynamic to adapt to material and energyfluxes and system bifurcations.

The concept of urban metabolism is used to measure socio-eco-nomic metabolism at the city-region level (Wolman, 1965; Boydenet al., 1981; Douglas, 1983; Girardet, 1992). It represents a holisticapproach to urban planning, exploring the interactions among re-source flows, urban transformation processes, waste streams andquality of life (Rotmans et al., 2000). Kennedy et al. (2007) defineurban metabolism as ‘‘the sum total of the technical and socio-eco-nomic processes that occur in cities, resulting in growth, productionof energy, and elimination of waste”. Newman (1999) postulates thatmeasuring urban material metabolism should include resource in-puts/production and waste outputs, as well as other criteria suchas livability, human amenity and health, employment, education,housing, accessibility and community.

Examples of urban metabolism studies that have been evalu-ated include Tokyo (Hanya and Ambe, 1976); Hong Kong (New-combe et al., 1978); Sydney (Newman et al., 1996); SwissLowlands (Baccini, 1997); Taipei (Huang et al., 1998); Vienna(Hendriks et al., 2000), Hong Kong (Warren-Rhodes and Koenig,2001), Greater London (Chartered Institution of Wastes Manage-ment, 2002), Cape Town (Gasson, 2002); and Shenzhen City in Chi-na (Yan and Zhifeng, 2007). This paper aims to add to the literatureon urban metabolism and identify trends and data gaps by measur-ing metabolic flows in an Irish city-region.

2. Methodology

The city-region selected was Limerick City and its environs,which is the primary urban centre in the Mid-West region in theRepublic of Ireland and whose population increased by 10% from79,137 in 1996 to 86,998 in 2002 (CSO, 2007). This case studywas selected as part of a research objective to apply a number ofmethods to an Irish settlement to (i) measure current sustainabil-ity and trends of sustainable urban development; (ii) evaluate dataavailability for developing an ‘urban sustainability toolkit’ for

D. Browne et al. / Waste Management 29 (2009) 2765–2771 2767

Ireland, and (iii) to assess commonalities and differences acrossdifferent sustainability metrics.

In order to complete a material and product flow analysis forthe study area and assess urban metabolic inefficiency, it was nec-essary to quantify (i) production, imports and exports of raw mate-rials; (ii) manufactured good production and trade; and (iii) wasteproduction or generation. It was also necessary to identify methodsof waste management and treatment. Consumption was calculatedby the sum of production and imports minus exports. Theapproach adopted is that recommended by the EUROSTAT method-ological guide for material flows.

Both national and case-study material production and tradedata were collated for Standard International Trade Classification(SITC) Divisions 00-43 from a number of sources, including interalia the Food and Agriculture Organisation (FAO), the InternationalEnergy Agency (IEA), the Irish Central Statistics Office (CSO), theBritish Geological Survey, United Nations Industrial CommodityStatistics, and the Irish Environmental Protection Agency (EPA).In general, material input and production data were national andoutput waste data were settlement-specific. Where specific datawere not available for the settlement, proxy factors and ratios wereused to disaggregate national data.

The sectors that were analysed include (i) food and organicwaste; (ii) textiles and leather; (iii) paper and cardboard; (iv)chemicals, rubber and plastic products; (v) metallic products andother household durable manufactured goods; (vi) industrialmachinery and equipment; (vii) transport machinery and equip-ment; (viii) wood products and furniture; and (ix) constructionmaterials and non-metallic mineral products. These are classifiedunder the CSO Standard Classification of Industrial Activity, i.e.NACE3 and SITC classification.

Data for product manufacturing in Ireland were collated fromannual CSO PRODCOM reports. PRODCOM is a statistical classifica-tion used by EUROSTAT for the collection and dissemination ofstatistics on the production of manufactured goods.4 Where pro-duction data were only available in monetary terms or as generalsales data, export data were used to calculate the weight of productsales, which was estimated by dividing the value of product salesby the value of one exported tonne for that year in each industrialsector.

Potential weaknesses of PRODCOM data have been identified aspart of a Scottish case study but may also more generally apply toother jurisdictions, including Irish data, i.e. (Chambers et al., 2004):

(1) PRODCOM data may vary significantly between years due todifferences in estimation.

(2) They do not cover products of coal and lignite mining, peatextraction, extraction of crude petroleum and natural gas,the manufacture of coke and refined petroleum products,processing of nuclear fuel and recycling.

(3) There are cases where data are available in volumetric termsand appropriate conversion factors are not available, or theproduct description is vague and a conversion factor for asimilar product has to be applied.

(4) For certain industries data are only available in units of mon-etary value.

(5) There may be errors, which are inherent in any statisticaldata set.

(6) It may be difficult to validate data using other sources, dueto either poor data availability or incompatibility of differentsources.

3 http://www.cso.ie/surveysandmethodologies/classification_indus_act.htm, lastaccessed April 2009.

4 http://epp.eurostat.ec.europa.eu/portal/page?_pageid=2594,58778937&_dad=portal&_schema=PORTAL, last accessed April 2009.

(7) There may be gaps in data availability due to confidentialityor commercial sensitivity.

Product flow or national production data were only available ata national level and, in order to account for material and productflows in the Limerick city-region, population and average house-hold expenditure proxies had to be used. This was necessary dueto gaps in available primary data for the Limerick city-region andthe difficulty in measuring bottom-up flows for an urban conurba-tion of that magnitude over a time-series. National per capita sec-toral and aggregate consumption of manufactured goods andproducts was estimated by the sum of national domestic produc-tion and imports less exports. Average weekly household Limerickand national expenditure on food and manufactured householdgoods, including clothing/textiles, household durable and non-durable goods, and miscellaneous paper and other items, wastaken from the 1999–2000 National Household Budget Survey(NHBS) micro-data.

This was used to estimate expenditure ratios or proxies, whichwere then used to estimate per capita consumption of goods byLimerick residents, as can be seen in Table 1. The expenditureproxy ratio for household durable goods was used for metallicproducts and other household durable manufactured goods andwood products and furniture. Population proxies were used forindustrial machinery and equipment; transport machinery andequipment and construction materials and non-metallic mineralproducts. A population approach was taken in order to disaggre-gate national data and due to lack of data on use of industrial ortransport machinery at local level.

Transport machinery does not relate to vehicle sales or manu-facturing but rather to fixed infrastructure. An alternative ap-proach could be to adjust industrial machinery according to thelevel of industrial activity in the area and to include annual localvehicle sales as a proxy for transport activity and material flows.This could be undertaken as part of future work. A ratio of residen-tial and non-residential construction activity was used to estimateconsumption of construction materials in the Limerick area.

For residential construction activity, the level of house comple-tions in the jurisdiction was expressed as a ratio of the level of na-tional house completions. Non-residential construction wasdisaggregated using a population proxy. Per capita consumptionwas scaled up using the population of Limerick City and its environsin 1996 and 2002 to estimate total consumption in the city-region.Table 2 shows the material and product consumption in weightterms as well as the percentage change in both 1996 and 2002.

Data were also collated for the main waste streams, includinghousehold, commercial and industrial, as well as a number ofwaste streams, such as construction and demolition (C&D) waste,healthcare waste, waste oils, packaging waste, waste batteries,waste sludges, waste from electrical and electronic equipment(WEEE), polychlorinated biphenyls (PCB), scrap metals, end of lifevehicles and scrap tyres. The material composition of Limerickhousehold waste and national commercial waste was taken fromthe EPA 1998 National Waste Database Report and the 2002 Na-tional Waste Database: Interim Report (Crowe et al., 2000; Collinset al., 2004).

Household waste disposal was estimated from urban districtrecycling rates for Limerick and commercial and industrial wastedisposal was estimated from total waste generation and nationallandfill, disposal and recovery data, as shown in Table 3. Limer-ick-specific data on commercial recycling or industrial recoverywere not available so assumptions were made on national recy-cling and recovery rates. Metabolic inefficiency for household,commercial and industrial consumption was calculated for 1996and 2002 by estimating the ratio of final waste for disposal to prod-uct consumption for a number of sectors, as can be seen in Table 4.

2768 D. Browne et al. / Waste Management 29 (2009) 2765–2771

3. Results

Total consumption of durable and non-durable products andconstruction materials increased by 64% from 827,011 tonnes in1996 to 1,352,626 tonnes in 2002, as can be seen in Table 2. Table2 indicates also that the highest percentage changes in materialand product consumption between 1996 and 2002 are in woodproducts and furniture (+127%) and construction materials andnon-metallic mineral products (+79%).

Construction materials accounted for 76% of material and prod-uct consumption in 1996 and 83% in 2002, which implies that con-struction is heavily dominant in total consumption. If construction

Table 3Total household, commercial and industrial waste generation (Tonnes) and percentage ch

FoodTextiles and leatherPaper and cardboardChemicals, rubber and plasticsMetallic products and other household durable manufactured goodsIndustrial machineryTransport machineryWood products and furnitureConstruction materials and non-metallic mineral productsTotal

Table 4Materials and waste metabolic inefficiency ratios and percentage change, 1996 and 2002.

FoodTextiles and leatherPaper and cardboardChemicals, rubber and plasticsMetallic products and other household durable manufactured goodsIndustrial machineryTransport machineryWood products and furnitureConstruction materials and non-metallic mineral productsTotal

Table 2Material and product consumption (Tonnes) and percentage change, 1996 and 2002.

FoodTextiles and leatherPaper and cardboardChemicals, rubber and plasticsMetallic Products and Other Household Durable Manufactured GoodsIndustrial MachineryTransport MachineryWood Products and FurnitureConstruction Materials and Non-Metallic Mineral ProductsTotal

Table 1Average weekly household expenditure (CSO, 2002).

Sector Limeric

Food 80.78Clothing and textiles 38.07Paper and printed material 8.42Non-durable household goods and miscellaneous plastic products 26.93Household durable goods 20.79Total manufactured goods 94.21

is not included, it is estimated that total consumption of durableand non-durable products increased by less than 16% between1996 and 2002, which again shows the impact that the construc-tion sector has.

Total waste disposed of by various means in Limerick increasedby 18% from 106,432 tonnes in 1996 to 125,733 tonnes in 2002, ascan be seen in Table 3. Table 3 indicates also that the highest per-centage changes in total waste generation between 1996 and 2002are in metallic products and other household durable manufac-tured goods (+164%) and industrial machinery (+104%). Disposalof wood products and furniture shows a fall of 74% against anincrease in consumption of 127% in the same period, which

ange, 1996 and 2002.

1996 2002 % Change

16,699 20,969 +262278 1881 �1722,656 29,055 +2835,072 17,194 �5115,455 40,833 +16423 47 +1042287 3096 +351252 332 �7410,710 12,326 +15106,432 125,733 +18

1996 2002 % Change

0.41 0.43 +50.5 0.61 +220.77 0.71 �80.52 0.33 �370.46 1.09 +1370.006 0.008 +330.38 0.32 �160.08 0.01 �880.02 0.01 �500.13 0.09 �31

1996 2002 % Change

40,271 48,885 +214511 3094 �3129,304 40,802 +3967,900 52,886 �2233,967 37,413 +104030 5973 +486071 9574 +5815,115 34,234 +127625,841 1,119,766 +79827,011 1,352,626 +64

k average expenditure (£) National expenditure Ratio

93.6 0.8635.11 1.088.33 1.0133.49 0.80426.86 0.774103.79 0.91

D. Browne et al. / Waste Management 29 (2009) 2765–2771 2769

indicates an improvement in resource efficiency over the period.There is no clear evidence as to why this has occurred but couldbe possibly due to longer product utilisation; a particular increasein the stock as a consequence of residential construction and pop-ulation growth or under-reporting of disposal.

Table 4 shows the materials and waste sectoral and overall met-abolic inefficiency ratios for 1996 and 2002, which are derivedfrom waste disposal in a particular sector as a function of productconsumption in that sector. This indicates that the greatest in-crease in metabolic inefficiency between 1996 and 2002 was inmetallic products and other household durable manufacturedgoods (+137%) with the highest ratios in 2002 being metallic prod-ucts and other household durable manufactured goods, followedby paper and cardboard and textiles and leather, as can be seenin Fig. 1.

It is possible that disposal rates and, as a result, the inefficiencyratio for metallic products, increased as a result of greater report-ing of disposal or due to the ‘flow’ nature of such consumption incontrast to the ‘stock’ nature of more durable goods such as furni-ture. Wood products and furniture show a sharp fall in inefficiencyof 88% from 0.08 in 1996 to 0.01 in 2002. Construction materialsand chemicals and plastics also show sharp decreases of 50%and 37%, respectively. The total metabolic inefficiency ratio de-creased by 31% from 0.13 in 1996 to 0.09 in 2002, as can be seenin Table 4.

4. Discussion and conclusions

Product flow analysis was used to measure metabolic ineffi-ciency by relating final disposal of wastes to consumption andwas developed in order to assess sectoral or component flowsrather than raw material flows and to provide a comparative basisfor other methods. The metabolic inefficiency indicator applied inthis paper was developed from mass balance and metabolism the-ory as an empirical method for assessing sustainability and dema-terialization and can be theoretically applied in the fields ofindustrial ecology, household metabolism and socio-economicmetabolism. This approach can be used to complement embodiedenergy analysis or carbon footprinting of production and consump-tion, e.g. as applied to the same case-study (Browne et al., 2008a,2008b, 2009).

0

0.2

0.4

0.6

0.8

1

1.2

Food Textiles Paper Chemicals Other Durables

Rat

io o

f Was

te D

ispo

sed

to F

inal

Con

su

Fig. 1. Comparison of sectoral material and wa

The objectives and aims of the indicator are to:

(1) Highlight the most inefficient sectors in terms of waste pro-duction and disposal relative to consumption.

(2) Integrate the consumption and output links of the lifecycleof product consumption with environmental impact by com-paring final waste disposal with consumption.

(3) Corroborate other resource or waste management indica-tors, for example it may be used to assess if inefficiency indi-cators correlate with absolute material or waste flows orsector recycling rates.

Although total material consumption and waste production in-creased between 1996 and 2002, metabolic inefficiency decreasedby 31% from 0.13 in 1996 to 0.09 in 2002. This can be expressedalso by comparing the increase in material and product consump-tion of 64% with the increase in total household; commercial andindustrial waste generation of 18%, which suggests relative decou-pling of waste generation from product consumption and, there-fore, a more efficient processing system. The overall reduction inmetabolic inefficiency in this period was largely due to the in-creased consumption of construction materials in Limerick in2002 and the fact that construction and demolition (C&D) wasteproduced showed little change.

If C&D is not included, then consumption increased by 16% from201,170 tonnes in 1996 to 232,860 tonnes in 2002. On the otherhand, waste increased by 18.5% from 95,722 tonnes in 1996 to113,407 tonnes in 2002. Therefore, metabolic inefficiency in-creased by 2% from 0.476 in 1996 to 0.487 in 2002. Thus, we cansee that, if construction is not included, the metabolic inefficiencyratios for both 1996 and 2002 are higher but there is only a slightincrease.

Table 4 shows that, in 2002, the highest materials metabolicinefficiency ratios were for metallic products and other householdmanufactured products (1.09); paper and cardboard (0.71); andtextiles (0.61). The sectors with the lowest inefficiency ratios in-clude industrial machinery (0.008); construction materials andnon-metallic mineral products (0.01); and wood products and fur-niture (0.01).

Sectors that showed an increase in sectoral metabolic ineffi-ciency between 1996 and 2002 include metallic products and otherhousehold manufactured products (137% increase); industrial

IndustrialMachinery

TransportMachinery

WoodProducts

ConstructionMaterials

Total

19962002

ste metabolic inefficiency, 1996 and 2002.

2770 D. Browne et al. / Waste Management 29 (2009) 2765–2771

machinery (33% increase) and textiles and leather (22% increase),as can be seen also in Fig. 1. It is likely that these increases are aresult of higher disposable income and discretionary spending onhousehold goods and greater disposability of consumer durableproducts.

Sectors that showed a decrease include wood products and fur-niture (88% decrease); construction materials and non-metallicmineral products (50% decrease) and chemicals, rubber and plastic(37% reduction). This indicates that these sectors have becomemore efficient in terms of recovery and recycling, compared withmaterial and product consumption.

It is worth noting that the sectors with the lowest inefficiencyratios are associated with production of durable goods. On theother hand, metallic products and other household durable manu-factured products, which includes non-durable goods, has thehighest inefficiency ratio in 2002 and shows a 137% increase be-tween 1996 and 2002. This may be a result of faster replacementand less durability in this particular market, for example the highreplacement rates of items such as mobile phones or personal com-puters, compared with household furniture or built infrastructure.

Table 4 indicates also that waste generation in the metallicproducts and other household durable manufactured goods sectorin 2002 is higher than product consumption, giving an inefficiencyindicator of 1.09. This suggests that waste products may only begenerated or come ‘on-stream’ after a time lag and total annualwaste generation can be higher than material or product consump-tion in that year. Thus, it may be argued that accounting for timelags in a more dynamic model could strengthen the indicator.

In comparison, the highest projected industrial waste quantitiesin 2001 were in food products, beverages and tobacco (48% of to-tal), basic metals and fabricated metal products (22%), and pulpand paper products (9%) (Meaney et al., 2003). In terms of indus-trial sector recovery rates, the highest in 2001 were in wood andwood products (95%), pulp, paper and paper products (93%), andfood products, beverages and tobacco (82%) (Meaney et al.,2003). This indicates that metabolic inefficiency ratios do not defacto correlate with either waste generation or recycling ratesand, thus, may be used to highlight other elements of the life cycle.This illustrates that waste generation is a function of throughputand that final waste generation may be managed by reducing atsource or along the lifecycle. Thus, product lifecycle efficienciesmay be increased by designing for long-life durability rather thanshort-life obsolescence.

Strengths of the indicator include:

(1) It provides a useful indicator of resource efficiency in that itrelates both end-points of the product use life cycle, that isconsumption and waste disposal.

(2) It can be adapted to calculate industrial sector productionefficiency by estimating the ratio of sectoral waste genera-tion to material inputs and production and can, therefore,be used in the field of industrial ecology or as a corporatesustainability metric.

(3) It can be used in analysis of household consumption metab-olism by disaggregating between durable product consump-tion or Net Additions to Stock (NAS) and non-durableconsumption or annual material or product flows.

(4) It may be used as a complementary indicator with recyclingrates and waste generation to indicate waste generation as afunction of material throughput in urban, household or soci-etal metabolism.

(5) It allows for policy formulation in the field of industrial ecol-ogy and sustainable consumption.

The research shows the difficulty in undertaking a regional orurban material flow analysis, particularly in an Irish context. Mate-

rial and product consumption data had, in most cases, to be disag-gregated from national data using proxy factors, althoughsettlement-specific municipal waste data were used. In addition,there was a lack of settlement-specific industrial data, which wasa significant weakness, and national data had to be disaggregatedusing employee number ratios.

The results also show the disparity in data availability betweendifferent countries; with some statistical organisations collatingdisaggregated regional material extraction and consumption data,through either bottom-up or input–output (IO) analysis. This com-plicates comparative analysis and suggests the need for standard-ized data collection. The lack of data availability limited theinferences that can be drawn from the results with regards to localsustainability in the particular case study and part of future workmay involve a bottom-up assessment to test the methodologyand facilitate a time-series analysis.

In addition, one of the current flaws of the methodology, as ap-plied in this paper, is that it relates consumption of durable prod-ucts and building materials for construction and infrastructure,which have a lifetime longer than 1 year and may be regarded asNet Additions to Stock (NAS), to annual waste generation and dis-posal. One of the problems with trying to correct this is the ques-tion of how to assign a useful residence time for materials orproduct consumption and this was not attempted as part of this re-search. Ideally, the sectors would be analysed using a dynamicmodel, which imputes realistic residence times for material orproduct flows to reflect the variability in residence time of materi-als in economic stocks and this may be attempted as part of futureresearch. In addition, the indicator could be used for annual flowssuch as non-durable product consumption or industrial processingand inventory analysis.

It could be argued that the methodology may be improved bydifferentiating between societal, household and industrial metabo-lism and this could form part of future work on ameliorating themethodology. This would facilitate the imputation of residencetimes in industrial metabolism based on common inventory stocksand turnover. Similarly, non-durable goods in the householdmetabolism may be assumed to have a residence time less than ayear, thus allowing for calculation of household metabolicinefficiency.

A key research question that could potentially be answered iswhether wastes arising in a given year are higher relative to mate-rial inputs entering the system because societal metabolism isbecoming more inefficient or because materials that have beensequestered or captured in the economic stock for several yearshave finally entered the waste stream. The Laws of Thermodynam-ics guarantee that all raw material inputs will ultimately becomewastes and that the only uncertainties are the time incidence, loca-tion or dispersion of waste materials and whether they will emergein homogenous or heterogeneous form.

Notwithstanding this, the objective for sustainable industrialproduction, and indeed a sustainable economic system, should beto ensure that materials and energy are embodied in long-lifeproducts, which facilitate effective and efficient recovery and recy-cling, rather than factoring in calculated obsolescence in produc-tion. This can ensure that recovering or recycling materials arepotential strategies for reducing the consumption of ‘virgin mate-rials’ and so can improve resource productivity. Indeed the ineffi-ciency ratio indicator may be used to show that a policy, whichpromotes consumption of durable rather than non-durable or dis-posable goods, may be more sustainable.

This research shows that, even if metabolic inefficiencies are re-duced and relative decoupling is achieved, if resource throughputand material consumption continue to increase, then increases inrecycling and recovery rates will only partially mitigate the prob-lem. Therefore, it is concluded that material inputs need to be

D. Browne et al. / Waste Management 29 (2009) 2765–2771 2771

reduced at source in accordance with the waste hierarchy andincentives should be in place to ensure more sustainable personaland household consumption.

Acknowledgements

The authors wish to gratefully acknowledge the funding assis-tance, awarded by the Irish Research Council for Science, Engineer-ing and Technology (IRCSET) under the Embark Initiative of theIrish National Development Plan (NDP) 2000–2006.

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