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General physical-chemical quality of surface water

The good physical-chemical quality of water is a necessary and vital condition for the survival and development of aquatic life. The Woluwe and, to a lesser extent, the Kanaal, are of good physical-chemical quality. The quality of the Zenne seems to be improving, thanks partly to the increasing purity of the waste water from the Brussels Region.

Target: "good condition"

In accordance with the Water framework directive (WFD), every Member State must set up networks of quality checks for its water and take the necessary measures to realise "good condition" by 2015, both on a chemical and ecological level, within the bodies of surface water. Three bodies of surface water have been selected in the Brussels Region: the Zenne, the Kanaal and the Woluwe.
The parameters that determine the general physical-chemical quality of the water (water temperature, turbidity, purity, salt levels, oxygen capacity, nutrient concentration,...) contribute towards the ecological improvement of the waterways. Even though there are specific quality targets for these parameters (the Environmental Quality Standards), the WFD does not really define the physical-chemical "condition" of surface water. Given that the physical-chemical quality supports aquatic life, it is indirectly reflected in the ecological condition or the ecological potential of the surface water (see "Ecological quality of the waterways and ponds").

The physical-chemical quality of the surface water in the Brussels Region and explanatory factors

The water of the Woluwe is of a very good quality and that of the Kanaal contains relatively low levels of pollution. The same, however, cannot be said for the Zenne. Analyses show that the general physical-chemical quality of the water of the Zenne when it leaves the regional territory has generally, significantly improved. During the last few years, the most noteworthy positive evolution can be put down to the realisation, in March 2007, of the second regional treatment plant for wastewater (in the north of Brussels) (the South water-purifying plant, with a lower processing capacity and which is not fitted with a powerful installation for the removal of nitrogen and phosphor, was put into operation in August 2000). The physical-chemical quality of the Zenne water as it enters the Region also seems to have improved since 2003-2005; this favourable evolution may also be linked to the improved water purification upstream of the Region itself.
Alongside the purification of the polluted water, this evolution could also be explained by other factors, such as the gradual limitation of the use of phosphates in washing products, the reduction of the atmospheric supply of nitrogen or even the reduced supply of nitrogen via agriculture and dairy farming.

Recent evolution in the Zenne

Evolution of the physical-chemical quality of the Zenne (2001-2012)
Source: Bruxelles Environnement - Leefmilieu Brussel, Department State of the Environment, 2013
Evolution of the physical-chemical quality of the Zenne (2001-2012)

Comment: For the calculation of the 2010 average a deviant value of orthophosphates concentration was excluded.

This positive trend is reflected in the evolution of various parameters, more specifically:

  • Since 2004, the limitation of organic oxygen demand (BZV), very notable at the Regional exit point, between 2003 and 2007 (-85%). As a result, the standard is getting close or even being achieved (as in 2008, 2010 and 2011) with BZV levels that are comparable at the entry and exit points of the Region (the BZV is an indicator of pollution by biodegradable organic substances whereby the decomposition process uses dissolved oxygen);
  • Since 2006, an increase in the average level of dissolved oxygen both in the inflows and outflows (2.5 times higher in 2012 than in 2006). This has meant compliance with the standard for the inflow into the Region since 2007, and for the outflow from the Region since 2011 (dissolved oxygen is vital for aquatic life and for the decomposition of biodegradable polluting substances and this is essential for self-cleansing);
  • A trend towards a breakdown of concentrations of ammonium nitrogen (NH4+), very pronounced in the outflows of the Brussels Region from 2007: the average concentration upon leaving the Region thus evolved from 19.5 mg N/l in the period 2001-2006 to 4.2 mg N/l in the period 2007-2012 and compliance with the standard in 2011 and 2012. (NH4+ is the result of the aerobic breakdown of organic nitrogen that generally comes from the discharge of non or insufficiently purified waste water; the breakdown of NH4+ into nitrites and then nitrates consumes dissolved oxygen and thus contributes towards the phenomenon of eutrophication of the North Sea);
  • Since 2007 the concentrations of orthophosphates upon leaving the Region have reduced slightly; they are gradually levelling up with concentrations at entry points (with the exception of the year 2012); nevertheless, they remain slightly higher than the standard (these orthophosphates come from the breakdown of organic phosphates that are partially the result of the discharge of waste water and the use of fertiliser; they form the basis of the eutrophication of waterways and ponds).

Compliance with quality standards of Zenne water

The recent improvement of the water quality of the Zenne is reflected in increased observance of the water quality standards. This has already affected positively the aquatic life in this waterway both up and downstream of the Region. Within the Brussels Region there seems to be a slightly positive trend; this must be further confirmed in the future (see "Ecological quality of the waterways and ponds").
Continuing efforts are, however, required both within the Brussels Region and upstream of the Region, in order to observe the environmental quality standards that have been in force since 2011. In 2012 for example, the standards were exceeded at the entry and/or exit of the Region for conductivity, BOD, orthophosphates, dissolved zinc and so on.
These targets are exceptionally hard for the Zenne. This waterway, with a very limited flow rate, receives all the effluents - purified to 80 to 90% in accordance with the prevailing legislation - from the North and South purification plants (1,460,000 IE in total) and from many other upstream plants. Depending on the circumstances the flow of purified water that is discharged by the North purification plant into the Zenne can double or even triple the daily average flow rate of the Zenne as it leaves Brussels. The fact that the river is covered in almost its entire Brussels section and the often artificial nature of its riverbanks considerably limit options in terms of the development of aquatic life and oxygenation. Under these conditions, the Zenne water will not achieve the "good ecological potential" by 2015 as required by the WFD (as a reminder, the physical-chemical quality is indirectly integrated in the ecological potential). As a consequence, a delay (postponement) until 2027 has been requested from the European Commission (see "Ecological quality of the waterways and ponds").

Compliance with water quality standards for the Kanaal and the Woluwe

Few breaches of the basic quality standard have been observed for the Kanaal. Nevertheless, this waterway has to put up with specific pollution on the regional territory: this includes the direct inflow of low quality water from the Neerpedebeek, the Broekbeek and the Zenne (via pumping) and the overflow during heavy rainfall form collectors or from the Zenne. The pollution is also the result of a few specific discharges of waste water, contamination by traffic on the water or the process whereby polluting substances that are present in the sediment are brought into suspension (as a result of dredging and turbulence). Like the river Zenne the Kanaal cannot realise "good ecological potential" by 2015; a postponement until 2021 has thus been requested from the European Commission.
The Woluwe, which receives very little polluted discharges in its course through Brussels, flows out of the Region with a good quality level; the quality standards are almost always achieved. This waterway should be able to realise the good chemical and ecological potential standards by 2015.


  • Leefmilieu Brussel 2011. “Milieueffectenrapport van het ontwerp van het maatregelenprogramma dat het Waterbeheersplan van het Brussels Hoofdstedelijk Gewest begeleidt”, 390 pages.
  • Leefmilieu Brussel, various years. Technical report of results from annual analyses of the physical-chemical quality of surface water in the Brussels Region.


Date de mise à jour: 29/05/2020