If you have ever heard of urban metabolism or urban ecosystems, this is one of the first studies/images you have probably seen:
This was done by P. Duvigneaud and S. Denayer De-Smet back in the 1970s and represented in the urban ecosystem of Brussels. It is largely recognised and cited as one of the seminal and most comprehensive studies in urban ecology and urban metabolism.
This study presented several subsystems of the urban ecosystem and highlighted its unsustainable functioning in a very synthetic and graphically appealing visualisation. However, at first glance this illustration is highly complex and not very accessible. If you have also struggled to figure out what all these numbers mean, well this post will disaggregate the different subsystems of this study in order to make it slightly more legible.
The first element presented in this study is the natural energy balance of Brussels. This represents natural energy received on the entire territory of Brussels (130.10^12 kcal), the export of energy reflected by Brussels’ surface (27.10^12 kcal), the import of energy due to atmospheric radiation (390.10^12 kcal) and the export of thermal energy due to soil radiation (435.10^12 kcal). The global balance of natural energy is therefore equal to 58.10^12 kcal
The second element mentioned in this study is the subsidiary energy or the imported energy that is coming directly from the sun. As no accurate data where available, Brussels imports of energy are estimated as 13% of Belgium’s consumption. This accounts for 26.10^12 kcal of coal, fuel, gasoline and natural gas, 4.10^12 kcal of electricity and 2.10^12 kcal of energy in the form of food.
The total energy balance is therefore the sum of the balance of natural energy (58.10^12 kcal) and of the imported subsidiary energy (32.10^12 kcal) which results to an export of 89.10^12 kcal (probably this is due to rounding figures) of energy. The researchers highlight that the subsidiary energy imported accounts for approximately half of the natural energy.
In order to estimate the air pollution of Brussels, a full combustion of the imported subsidiary energy was assumed. This lead to 5,931.10³ tonnes of CO2, 200.10³ t of CO, 30.10³ t of SO2, 20.10³ t of SO2, 45.10³ t of hydrocarbons, 2.10³ of PM and 200 tonnes of lead. An additional 450.10³ tonnes of CO2 are added to the atmosphere through human respiration and almost a quarter of this value (138.10³ tonnes of CO2) is absorbed by photosynthesis.
Similarly to the energy balance, the water balance of Brussels’ ecosystem consists as well of a number of elements divided in natural and man-made.
Annual precipitation accounted for 113.10^6 tonnes (or 113.10^9 litres) for an average precipitation height of 700 mm. The other input flow of water was imported water captured outside of Brussels’ ecosystem. This flow accounted for 61.10^6 tonnes (this is an exact figure coming from CIBE).
It was estimated that around 60% of precipitations were evacuated through runoff (60.10^6 tonnes, see figure above) and drainage (8.10^6 tonnes, see figure above). In addition, 40% of precipitations were released back to the atmosphere through evapotranspiration (45.10^6 tonnes). Finally, in addition to the 68.10^6 tonnes of water precipitations evacuated, another 57.10^6 tonnes coming from imported potable water are added to Brussels river, the Senne. Indeed, 5% of imported water (4.10^6 tonnes) is used for watering plants and therefore are going back to the atmosphere through evapotranspiration.
Material flows and waste
The last section presented in this post are material flows. In this study, material flows were not studied per se. The material flows presented here in green are a direct translation from the estimation of energy flows to matter. The rest of materials entering and exiting Brussels’ ecosystem were not accounted.
Similarly to some water values, waste figures (384.000 t in 1974 and 380.000 t in 1975) are precise data coming from the Conseil d’Administration. However, the figures mentioned in the text are different from the ones present in the following figure.
This seminal study allowed to underline the unsustainability of Brussels ecosystem 40 years ago. Brussels imported energy and water flows from outside of its territory while it received almost double the quantity of its inhabitant needs through natural and local flows. A recent study of Brussels urban metabolism shows that the situation has not changed (see figure below). This post wished to make the results of the original study more accessible and show that there are still relevant in the present discourse of urban ecology, urban metabolism and circular economy.
Duvigneaud, P. and S. Denayer-De Smet. 1977. L’écosystème URBS: L’écosystème urbain bruxellois. In Productivité biologique en Belgique. Gembloux: Scope.
BATir – ULB, EcoRes and Institut de Conseil et d’Etudes en Développement Durable. 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. Brussels: Institut Bruxellois de Gestion d’Environnement.