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Focus: pollutant emissions into surface water

The 2010 "inventory of emissions" of the Brussels Region details the emissions of 86 pollutants into the Senne, the Canal and the Woluwe, at the regional territory level, according to their origin. Retracing step by step the route of each pollutant from its source until its arrival in the hydrographic network, this inventory represents a powerful management support tool. Although it has confirmed the major influence of the population, industry and businesses on water quality, it has also demonstrated the high contribution of diffuse inputs for some substances and the predominant role played by the stormwater overflows in the transfer of pollutants towards the Senne and the Canal.

The quantified emissions of 86 pollutants into the Senne, the Canal and the Woluwe

The Brussels Region possesses a powerful tool which allows it to precisely quantify throughout the whole territory the point sources and diffuse sources of pollution into waterways and the Canal: an "inventory of emissions". This tool complies with the European obligation to produce such an inventory for the 33+8 priority European substances and dangerous substances (cfr. article 5 of directive 2008/105/EC and see "chemical quality of surface water") and it goes even further, since 45 other relevant substances for the Brussels Region have been taken into account.

This tool quantifies the "gross" emissions of pollutants from both point and diffuse sources (for example nitrogen and phosphorus emissions by the population at the house level depending on the people living there). It then simulates the pathways, flows or transit of these pollutants (run-off, sewage, water treatment plants, etc.) from their source until the hydrographic network. And ultimately it provides the "net" emissions of pollutants into surface water, in other words the share of gross emissions which actually reach the waterway and which will influence the concentrations of pollutants in the water column, sludge or even biota.

In specific terms, the Brussels inventory focuses on the emissions of 86 pollutants into the three main waterways (the Senne, the Canal and the Woluwe) from 20 sources (including waste water from private individuals, industry and business, corrosion of buildings, pollution related to road, rail or waterway traffic, the use of pesticides and fertilisers, the leaching of pollutants stored in the sludge of waterways, atmospheric deposition).

The strength of this tool lies in its ability to geolocate each of these stages: the (gross) emissions are in fact calculated per grid of 50m x 50m. This makes it possible to identify the emissions of each pollutant at each point in the region's territory. This "geographic specification" (or spatialisation) of the estimates of emissions is quite unique in its kind, and has enormous potential as it enables these estimates to be validated by comparing them with the measurements taken on the ground.

A distinct methodology for point and diffuse sources of emissions

The inventory was drawn up for the reference year 2010 by the Vlaamse Instelling voor Technologische Onderzoek (VITO) (Flemish Institute for Technological Research) on behalf of Brussels Environment. The methodology is based on the WEISS system (Water Emissions Inventory Support System), which was developed in conjunction with the VITO and the Flemish Environment Agency during the European Life+ project.

The method of estimating gross emissions differs depending on their source (point or diffuse):

  • For point sources of emissions, such as direct discharges into surface water, the location data of the point of discharge and the annual loads discharged per pollutant (concentration x flow/volume) - measured or estimated - will be directly encoded in the tool.
  • For diffuse sources of emissions, which by definition cannot be characterised by a single discharge point, the WEISS tool estimates the emissions on the basis of an explanatory variable (for example: the number of homes, the number of kilometres of rail tracks, etc.) and an emissions factor taken from scientific literature (for example: x grams of nitrogen per inhabitant per year, x grams of mineral oil per switch on a rail track).

As such, all the sources and pollutants considered are either calculated based on measurements taken on the ground, or from land use information.

Validation of the results of the model

The output calculations and estimates of the model are compared with the concentrations measured in the surface water and the waste water at the entry to the water treatment plants, to quantify the margin of error, or unexplained loads. This enables a critical analysis of the results obtained.

Some methodological limits and areas for improvement

Although the tool is highly comprehensive, it has certain limitations:

  • The exchanges between polluted groundwater and surface water are not yet taken into account;
  • The sources of suspended matter and of salts/conductivity are not yet fully integrated into the tool;
  • For some parameters and/or sources, few emissions factors are currently available. So the tool cannot provide a complete picture of the main sources;
  • Doubts specific to the various hypotheses in the context of the estimates of certain sources, pathways and/or discharges.

On this last point, specific studies are planned in the coming years in order to refine and validate different estimates (including: the loads transported by run-off rain water, the emissions factors of roads and rail tracks, the incoming loads at the water treatment plants via specific measuring campaigns, etc.).

Illustration of a model output for the Biochemical Oxygen Demand (BOD)

One of the outputs of the WEISS tool is a diagram of the quantified loads of a pollutant at each stage of its course (at the Brussels Region level) until joining the surface water (see example below).

Diagram of the flows of pollutant loads of Biochemical Oxygen Demand (BOD) at the Brussels-Capital Region level (in tonnes for the year 2010)
Source: Brussels Environment, extract of the inventory of emissions into surface water (VITO)

The gross emissions appear in the blue inset at the top ("gross emission"), the net emissions in the blue inset at the bottom ("surface waters"), whilst pathways are in the white insets and seepage is in the red insets.

This diagram makes it possible to determine the proportion of net emissions in gross emissions and the annual load emitted into surface water. In the example of the BOD, the gross emissions across the whole territory of the Region amount to almost 17,000 tonnes and the net emissions into surface water represents slightly more than one quarter (28%), or almost 5,000 tonnes.

Another interesting fact which emerges from this diagram is the location of the main "losses" (seepage). We can observe then that almost 70% of the pollutant load of BOD is retained at the biological facility of the water treatment plants ("treatment basin"): in other words, the purification carried out at the stations allows 70% of the emitted BOD to be reduced.

Another analysis which can be read from this diagram: the preferential transit routes of pollutants (or conversely, those which are hardly used, or not at all). Still referring to BOD, we can observe that almost 50% of the net emissions which reach waterways are brought there by stormwater overflows and 38% by the wet weather treatment processes of the water treatment plants ("stormwater basin"): this is therefore the place where we need to act as a priority to reduce emissions. In contrast, the areas not connected to the water treatment plants ultimately contribute very little to the emissions for this parameter (just 2%).

Exposure of waterways to pollution

Unsurprisingly, the Senne generally receives the vast majority of net emissions of pollutants - even if this varies depending on the parameter considered. It is in fact the receiving water of effluents from the two regional water treatment plants and also sustains the discharges from numerous stormwater overflows right throughout its Brussels course. The corollary of this observation is that the impacts of pollution are therefore the most significant on this waterway (see "general physico-chemical quality of surface water" and "chemical quality of surface water").

By contrast, the Woluwe is barely affected by pollution: it benefits from the protection provided by the Sonian forest on its upstream course and from stormwater overflows which although present, are rarely used in practice. And the Canal occupies the middle position: its quality is deteriorated by certain pollutants, but to a much lesser extent than the Senne.

Pollution in terms of organic load, suspended matter and nutrients

For the 5 substances which can be treated by the water treatment plants, the Senne receives on average almost 80% of net emissions, the Canal slightly less than 18% and the Woluwe 2% (see also "general physico-chemical quality of surface water").

Relative breakdown of annual net emissions of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Suspended Matter (SM), total nitrogen (Nt) and total phosphorus (Pt) depending on the waterway
Source: Brussels Environment, extract from the inventory of emissions into surface water (VITO), figures for the year 2010

Note: The sources of suspended matter are not yet fully integrated into the tool.

With regards to the relative breakdown per sector, it is evident that the population makes the largest contribution to pollution (between 71% and 88% depending on the parameter considered) with businesses accounting for the remaining percentage. The contribution of agriculture within the BCR is negligible.

As previously indicated, the dry weather treatment process of the two water treatment plants logically allows a considerable proportion of the gross emissions of these 5 substances to be reduced. The average pollution reduction rate of the organic load (expressed in BOD and COD) amounts thus to 92%. As for net emissions, they essentially reach the surface water via stormwater overflows, the wet weather treatment process of the water treatment plants, and to a lesser degree, the dry weather treatment process.

Other pollution

The Senne remains the waterway which receives the highest quantities of net emissions, compared with the Canal and the Woluwe. Among the main problematic substances identified in the Brussels Region (PAH, zinc, lead, nickel, cadmium and mineral oils), the Senne receives almost 70% of the net emissions of PAH and more than 80% of the emissions of other pollutants.

Relative breakdown of annual net emissions of Polycyclic Aromatic Hydrocarbons (PAH), certain metals and mineral oils depending on the waterway
Source: Brussels Environment, extract from the inventory of emissions into surface water (VITO), figures for the year 2010

For the Senne, the 5 main sources of pressure are waste water from households and businesses, but also the diffuse sources related to buildings and structures, traffic and atmospheric deposition. In the case of the Canal, navigation is a source of additional pressure. As for the Woluwe, it is primarily affected by diffuse sources of pollution of PAH.

The prevalence of sources of pressure vary depending on the parameter considered (see also "general physico-chemical quality of surface water"). Pollution from diffuse sources often represents a significantly larger proportion of the emissions of these pollutants compared to organic matter, suspended matter and nutrients (for example, 65% of the net emissions of zinc and 28% of lead emissions are due to the corrosion of construction materials). In the case of mineral oils, the pollution comes almost exclusively from diffuse sources (road and rail traffic).

For the substances which are properly treated or retained at the water treatment plants, for example the PAH, the most important access route of net emissions into surface water is at the stormwater overflow level (35% in the case of the PAH). For the Canal, the direct discharges from waterway navigation can represent a significant proportion of the contribution.

For more information on the inventory of emissions, the reader is invited to refer to chapter 2 of the draft of the second water management plan and the study by VITO.

Date de mise à jour: 30/05/2020


Study(ies) and report(s)

VITO, December 2013. «Inventarisatie van de emissies naar water in het Brussels Hoofdstedelijk Gewest» Post 1: voorstudie. Study performed on behalf of Brussels Environment. 78 pp. (.zip, in Dutch only)

VITO, June 2014. «Inventarisatie van de emissies naar water in het Brussels Hoofdstedelijk Gewest» Post 2: analyserapport en factsheets. Study performed on behalf of Brussels Environment. 346 pp. (.zip, in Dutch only)

VITO, January 2014. «Technische nota – Transport naar het oppervlaktewater binnen WEISS voor Brussels Hoofdstedelijk Gewest». Study performed on behalf of Brussels Environment. 32 pp. (.zip, in Dutch only)