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Focus: urban metabolism, a balance of materials and energy flows

In the context of the implementation of the Resources and Waste strand of the Employment-Environment Alliance, Brussels Environment has conducted a study designed to quantify the flow of the materials, water and energy into the Brussels Region, which are then consumed, transformed or stored, and which then leave. The metabolic balance carried out in this context showed in particular that, due to its urban character and the predominance of the tertiary sector, the Brussels Region is characterised by a highly linear economy which is dependent on the exterior. The study also highlighted the quantitative size of certain flows, including in particular those related to the construction, agriculture and food sectors, as well as the fuel and petroleum product sectors.

The transition of cities to a more circular economy

Cities are significant consumers of energy and materials, which are either stored for more or less extensive periods, or which leave in the form of exported products or, more often, in the form of waste, emissions into the air and in water, and heat.

An analogy is commonly made between the way cities or regions function, and the way ecosystems function. However, if there are analogies with natural ecosystems, this modus operandi fundamentally differs nonetheless, in terms of the flow characteristics:

  • the predominance of anthropogenic flows (fuels and electricity, tap water, food products, manufactured goods, waste and emissions, pollutants, etc.) in relation to natural flows (solar energy, cycles of water, nitrogen, phosphorus, carbon and oxygen with, among others, photosynthesis, etc.);
  • the generally linear circulation of flows (very little re-circling of flows by reuse or recycling in the city or through synergies with businesses);
  • very strong dependence of cities in relation to flows coming from outside the system, due in particular to this highly linear circulation of flows, and the high density of populations and economic activity.

This modus operandi of cities has a significant environmental impact both "upstream" of the city, following the bulk importation of resources extracted outside of urban territories, and within the city and "downstream" of it, following various emissions which cause air, water and soil pollution.

For these territories, the transition from an economy which is very resource-hungry and primarily linear ("extract-manufacture-consume-dispose") towards a more restrained and circular economy  ("reduce-reuse-recycle") represents a major challenge, not only from an environmental perspective, but also from an economic perspective (reduced use of resources, decrease in the dependence on external territories, technological innovation and improvement of competitiveness) and a social perspective (creation of local jobs which cannot be outsourced easily, decrease in environmental pressures).

At the Brussels level, this transition from a linear economy towards a circular economy which creates jobs represents an objective which is clearly part of the regional policy, and is included in particular in the majority agreement 2014-2019 and the 2025 strategy for Brussels.

Balance of the materials and energy flows in the Brussels-Capital Region

In general, a territorial metabolism balance (or urban metabolism if this exercise is applied at the city level) consists of quantifying the flows of materials and energy which enter the territory, which are then consumed, transformed and stored there, and which then leave. The flows studied, the level of perspective, the possible analyses carried out (e.g. of the consumers and producers of different flows) or the methods used for quantifying and their accuracy, all vary considerably from one balance to another depending on the objectives pursued and the resources and data available.

The study into the metabolism of Brussels is one of the founding components of the Regional Programme for a Circular Economy (see focus on the Employment-Environment Alliance) to support the development of an industrial ecology programme.  Industrial ecology is intended to develop, at the level of the industrial system, an organisational method characterised by the optimal use of resources and a high level of recycling of materials and energy, in particular by developing synergies between businesses (the local reuse of production residues, the pooling of certain services and equipment). This approach involves analysing the flows of materials, water and energy in order to identify the emissions and waste of businesses - or, by extension, of the economy - which could be re-injected into the local economy.

It is in this context that Brussels Environment carried out the study into the urban metabolism of the Brussels Region in 2014, using data pertaining essentially to 2011.  Due to its highly transversal character, the project involved numerous partners both within Brussels Environment (the involvement of numerous departments) and within external service providers (a consortium of several universities and consultancies).

The overall balance of the main flows of energy, water and materials is illustrated in the chart shown below. We can distinguish between incoming, internal and outgoing flows, as well as the stock of materials present in the Brussels Region.

The incoming flows to the Brussels Region included in the balance are:

  • the imported international and inter-regional flows (including a part of transit traffic which cannot be distinguished from imports): energy (natural gas, electricity, petroleum products, biofuels and wood, coal), materials (minerals, fuels, agriculture and food, metallurgy, other categories including clothing, household appliances, furnishings, newspapers and books, etc.) and tap water;
  • natural incoming water flows (rainfall, waterways and canals);
  • anthropogenic flows which enter the territory but which are not economically recovered (waste water produced in the Flemish Region which is treated by water treatment plants in the Brussels Region).

It is important to point out that, due to a lack of data, imports of materials have been estimated based on various assumptions and approximations and consequently have a considerable margin of inaccuracy. However, we can also observe that the balance shown below includes a double counting, since it records both incoming flows of energy consumed in the Brussels Region in the form of GWh (gas, electricity, but also liquid or solid fuels) as well as the incoming flows of materials, including fuels which are partly intended for Brussels consumption, in the form of kilotonnes.

The outgoing flows of the Brussels Region take into account:

  • exported international and inter-regional flows (including a part of transit traffic which cannot be distinguished from exports):  materials (same as above)
  • outgoing flows generated by human activities, namely waste flows (waste not treated by the incinerator and bottom ashes and fly ashes originating from the incinerator), the flows of waste water (effluence from the water treatment plants which exit the Brussels Region via the Senne) as well as flows of greenhouse gas;
  • outgoing flows of water (evaporation and evapotranspiration, waterways and canals) as well as gaseous flows.

We can observe that the flows corresponding to emissions of atmospheric pollutants other than greenhouse gases, as well as emissions of pollutants into the water, have been considered in the context of this project, but have not been integrated into the overall project (the quantifying of these flows, calculated by Brussels Environment on the basis of simulations, is however available, cfr. various indicators of emissions of atmospheric pollutants and focus on the emissions of pollutants in surface water).  However, the natural gaseous flows (mainly linked to photosynthesis) as well as the consumption of oxygen due to the combustion process, have not been taken into account for this balance.

The internal flows representing the flows which are produced and treated in the Brussels Region, namely:

  • the primary production of energy (essentially through the production of electricity by the electric power station, linked to the incinerator) or 1150 GWh (or 1150 billion Wh);
  • water catchment (VIVAQUA, among others) or slightly more than 2 million m3 of water;
  • the quantities of waste water treated at the 2 regional water treatment plants, or 130 million m3 of water;
  • the waste flows processed by the incinerator, or 448 kt (or 448,000 tonnes);
  • the flows of materials (or 500 kt) added to the stock of materials already present in the Brussels Region.

The production of biomass for consumption purposes (production of wood and market gardening) is also considered as an internal flow.  However it is not included in the overall and material balances.

The material stock (184,921 kt) recorded in this balance represents a somewhat cursory evaluation of the mass of the following elements:

  • construction material making up residential buildings, offices and shops (certain types of buildings such as schools and industrial buildings have not been taken into account);
  • vehicles (private vehicle stock registered in the BCR and the stock of the STIB);
  • the water en sewage network, the electricity network, the rails of the tram/metro/train network, the road network;
  • small and large electrical appliances owned by Brussels households.

Urban metabolism of the Brussels-Capital Region, 2011 (2012 for certain data on non-municipal waste)
Source: ICEDD-ECORES-BATIR on behalf of and with the contribution of Brussels Environment 2014

The creation of this balance is based on multiple data sources as well as, for certain flows, various assumptions and estimates. As a result, the accuracy of the data presented in this balance is highly variable depending on the flow in question, and some quantifications should be considered above all as orders of magnitude.

The main sources of data used are:

  • for energy: the regional energy balance;
  • for the greenhouse gas emissions: the inventory of emissions (Brussels Environment);
  • for incoming and outgoing flows of materials: extra-national imports and exports (National Bank of Belgium), the transport of goods loaded and unloaded in the BCR via road (FPS Mobility and Transportation), by rivers and sea (Port of Brussels), and by rail (SNCB);
  • for waste: municipal waste (Bruxelles Propreté, operators in the social economy, municipalities and the IGEAT-ULB study carried out on behalf of Brussels Environment for container parks), non-municipal waste (the Recydata study carried out on behalf of Brussels Environment, Bruxelles Energie, Recupel, the study on construction and demolition waste by CERAA and ROTOR  carried out on behalf of Brussels Environment, Recytyre, Febelauto, Valorfrit, Port of Brussels, SBGE, VIVAQUA, Brussels Environment);
  • for the stock of materials: buildings and structures (estimates by BATir-ULB based on cadastral data, Brussels Urban Development - Office observatory, of the FPS Economy - DG Statistics and Economic Information), small and large household electrical appliances (DGSEI, Récupel), vehicles (DGSEI, STIB and studies on the material composition of various types of vehicles), networks (HYDROBRU, Sibelga, STIB, SNCB, URBIS database, technical sheets and publications);
  • for water: waterways and canals (Flowbru, Brussels Environment, ULB - Water treatment and pollution unit), rainfall (RMI), waste water (Brussels Environment, SBGE), tap water and water catchment (VIVAQUA, HYDROBRU, Brussels Environment), run-off, seepage and evapotranspiration (ULB - Water treatment and pollution unit)

Main findings

The Brussels territory, which is highly urbanised and densely populated (by its inhabitants but also by its commuters, students, tourists, congress attendees, etc.), with an essentially tertiary economy and limited in size, is characterised by a strong dependence on the exterior for the resources which it consumes.

The balance above shows that more that 96% of the tap water consumed in Brussels is imported from the Walloon Region.  It is also interesting to observe that the inflow of rainwater equates to twice the volume of imported tap water. Yet, a significant proportion of this rainwater ends up in the sewage network without first being used (rainwater recovery has not been quantified within the scope of this study, due to a lack of adequate data). With regards to energy, only around 5% of the Brussels energy consumption originates from production in the Brussels Region. This is essentially electricity produced by the electric power station linked to the household waste incinerator, although there are other sources (biogas produced by the digestion of sewage sludge, solar energy and geothermal energy, etc.) (see the indicator "Production of renewable energy").

The balance has made it possible to estimate, roughly, the quantity of materials and fuels coming in to the Brussels Region to be 9,000 kt in 2011 (as a reminder, this category groups together e.g. minerals and metallurgy products, agricultural and food products, clothing, household appliances, furnishings, newspapers and books, etc.). These incoming flows primarily concern minerals and construction materials (25%), fuels (25%) and agricultural and food products (23%). In relation to the number of inhabitants and without taking fuels into account, this equates to an annual incoming materials flow of 7,981 kg/inhabitant (products from agriculture or food, construction materials, chemical and metallurgical products, machines and equipment, textiles, paper and publications, etc.). Only a small part of these flows is however intended for final consumption by Brussels households.  Most of these flows are used by Brussels economic activities, including intermediate consumption for manufacturing companies. However, it should be reiterated that the available data did not always make it possible to differentiate between the transport of goods which is actually destined for the Brussels economy, and the consumption of the passing traffic by inhabitants.  We can also observe that the balance does not take indirect flows (or hidden flows) into account, in other words all the resources which are not integrated into the final imported product, but whose utilisation or extraction are necessary for manufacturing and transporting the product.

With regards to waste, the balance estimated the flow incinerated by Brussels Energy in 2011 to be 448 kt of household and assimilated waste (in other words, household-type waste produced by inhabitants but also by shops, offices, business, schools, etc.) and the flow of outgoing waste from the Brussels Region to be 1,312 kt.  Around 46% (in weight) of this outgoing flow was composed of construction and demolition waste. The other flows which were quantitatively important were sludge from water treatment plants and swept up sludge (+/-11%), paper and cardboard (+/-8%) as well as metals (+/-7%) and incineration waste (+/-7%).  However, it should be specified that the outgoing flows are partially composed of waste which is sorted, regrouped or dismantled in the Brussels Region, and which is then sold on to be recycled or reused (paper/cardboard, plastics, glass, steel, metals, compost, textiles,  etc.). In this respect, the available data did not always make it possible to distinguish a possible re-circling in the outgoing flows, as a resource, of certain waste within the Brussels Region itself.

The quantitative and qualitative identification of the materials stock is a fundamental issue in the development of circular economy strategies, since it makes it possible to anticipate waste which will be produced in the future, and from which recoverable materials or components could be extracted (the concept of "urban mining").  A brief evaluation of this stock has been carried out in the context of the present study. It appears that, as a first approximation, the materials stock including building structures (a non-exhaustive inventory), the electric appliances in households, the vehicle stock (public and private) and infrastructure networks represent 185 million tonnes (or 165 tonnes per inhabitant). Although this appears quite significant in quantitative terms, this deposit of future "waste-resources" is nonetheless composed of materials whose environmental impact and economic interest, in terms of recovery, are highly variable.  Within the scope of this study, only the main flows have been estimated, but it is useful to bear in mind that there is important potential for the circular economy in terms of the re-circling of micro-flows (e.g.: specific metals such as rare earths).

Date de mise à jour: 30/05/2020


State of the Environment's sheet(s)

Supply and consumption of tap water (edition 2011-2014)

Focus: quantitative status of groundwater (edition 2011-2014)

Total energy consumption of the Region (edition 2011-2014)

Renewable energy generation (edition 2011-2014)

Emission of greenhouses gases (edition 2011-2014)

Emission of acidifying substances (NOx, SOx, NH3) (edition 2011-2014)

Emission of ozone precursors (NOx, COV, CO and CH4) (edition 2011-2014)

Emission of fine particles (primary PM10) (edition 2011-2014)

Focus : Tonnage of households and assimilated waste (edition 2011-2014)

Focus : Proportion and management of the waste prepared withe the aim of reuse and recycling (edition 2011-2014)

Focus : Monitoring of the main waste material flows from professionals (edition 2011-2014)

Focus : Sensible use of resources and prevention of waste via the social economy (edition 2011-2012)

Focus: production et gestion des boues et sédiments (edition 2007-2010, in French and Dutch only)

Pressions environnementales des activités, chapitre acteurs économiques (edition 2007-2008) (.pdf)

 L’empreinte écologique, chapitre politique et gouvernance environnementale (edition 2003-2006) (.pdf, in French and Dutch only)

Study(ies) and reports

ECOLIFE 2003. « L'empreinte écologique des habitants de la Région de Bruxelles-Capitale: Rapport synthétique », study performed on behalf of Brussels Environment, 30 pp. (.pdf, in French and Dutch only)

ECOLIFE 2004. « De ecologische voetafdruk van de bewoners van het Brussels Hoofdstedelijk Gewest : Technisch rapport », study performed on behalf of Brussels Environment, 77 pp. + appendices. (.pdf, in Dutch only )

ECORES, ICEDD, BATir (ULB) 2015. « Métabolisme de la Région de Bruxelles-Capitale : identification des flux, acteurs et activités économiques sur le territoire et pistes de réflexion pour l’optimisation des ressources », study performed on behalf of Brussels Environment, 289 pp. + appendices. (.pdf, in French only)

RDC ENVIRONMENT 2008. « Etude préparatoire pour l'évaluation de l'empreinte écologique des activités localisées en Région de Bruxelles-Capitale  », study performed on behalf of Brussels Environment, 232 pp. + appendices. (pdf, in French only)

KAMPELMAN R. 2016. « Mesurer l'économie circulaire à l'échelle territoriale : Une analyse systémique de la gestion des matières organiques à Bruxelles »,  In « Observatoire français des conjonctures économiques », 24 pp. (pdf, in French only)