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Wastewater treatment

The waste water currently generated by the Brussels Region (and a portion of the peripheral municipalities) and reaching the water treatment plants amounts to approximately 148 million m3 per year. Three quarters of this is processed at the North water treatment plant, and the remaining quarter at the South water treatment plant. The purification efficiency of the North plant has been good and stable over the last 4 years. The efficiency of the South plant is insufficient on the other hand, especially with regards to suspended matter and nutrients. In order to redress this situation, a huge building site began in 2014 to overhaul the installation. However, it is incorrect to assume that all waste water is processed by the treatment plants: recent measurements highlight the major role played by stormwater overflows in transferring pollutants to the Senne and the Canal.

Volumes of waste water entering the water treatment plants

In principle, depending on the nominal capacities of the water treatment plants, the North plant is designed for three quarters of the waste water of the population equivalents (PE) of the Brussels Region (and of a portion of the peripheral Flemish municipalities), and the South plant is designed for the remaining quarter (1,100,000 PE vs 360,000 PE). This estimate tallies with the reality since, depending on the years, the North plant receives between 70 and 75% of the total volume of waste water flowing into the plants. It should be specified that this volume includes a significant proportion of run-off rain water (given that the sewage system has historically been combined) but also water diverted from the hydrographic network (including complete waterways such as the Maelbeek). Monitoring the volumes flowing towards the plants should make it possible to account for connection works to the sewage system over the long term, but above all for the reduction of clear water or run-off which pass through it.

Volumes flowing into the South water treatment plant (2007-2014)
Source: Vivaqua, monthly and annual operation reports

Note: Profound methodological changes related to the calculation of volumes flowing in were introduced at the start of 2011, making the values less reliable since this date.

Over the period 2011-2014, the annual volume flowing into the South treatment plant fluctuated between 35 and 45 million m³ per year and the average daily volume between 90 and 120 thousand m³ per day.

As a reminder, these volumes are not totally directed towards the complete treatment line (biological treatment process, referred to as the dry weather facility): in the event that a certain flow is exceeded at the entry to the plant, the excess water is directed towards a treatment line with only a partially purification process (referred to as the wet weather facility). The distribution of the volumes of influents following the two treatment processes is not available for the South treatment plant. In the absence of measurements at the entry to these lines, the volumes can be estimated indirectly using the measurements of the volumes discharged by each of the two lines: the volume flowing out the biological treatment process represents in general between 90 and 95% of the total discharged volume.

Two new collectors have recently been commissioned in the South basin: the Vogelzangbeek collector in September 2012 and the Verrewinkelbeek collector - at the downstream section - in 2014. This last connection might lead to the difference in evolution observed in 2014 in relation to previous years between the level of rainfall and the volumes flowing into the plant.

Volumes flowing into the North water treatment plant (2007-2014)
Source: Aquiris, monthly and annual operation reports

The volume flowing into the North treatment plant over the period 2011-2014 fluctuated between 100 and 115 million m3 per year. Over this same period, the average volume flowing into the biological treatment process ranged from 270 to 285 thousand m3 per day. The proportion flowing into the biological treatment process has amounted to at least 90% of the total volume since 2011. It is interesting to highlight the fact that this percentage has tended to increase between 2011 and 2014 (going from 90% in 2011 to 95% in 2014) and even since 2007. In other words, the waste water passing through the North plant has undergone more advanced treatment over the last few years.

We can observe an overall upward trend in the volumes of influents over the period 2007-2014. The situation appears to have stabilised since 2010-2011. If rainfall is undeniably one of the explanatory factors, the connection of new sewed areas or even the works to reduce clear water or run-off rain water carried out since 2010 have also been contributing factors. In fact, for certain years, like for example in 2011 when the rainfall was clearly negative compared with 2012 and even 2010, the total inflowing volume was on the same scale of magnitude.

North plant: favourable evolution of the purification efficiency between 2007 and 2011, stable between 2011 and 2014

North water treatment plant - average annual concentrations at the exit of the biological treatment process and average annual pollution reduction rates between the entry and the exit of the biological treatment process (2007-2014)
Source: Aquiris, monthly and annual operation reports

In general, the purification efficiency (of the biological treatment process) of the North water treatment plant (both in terms of concentrations and pollution reduction rate) clearly progressed between 2007 and 2011 for all parameters. They were stable overall between 2011 and 2014, despite a slight dip in 2012 and 2013.
With regards to the pollution reduction rates, the progression was very pronounced for suspended matter and nutrients (around 10 points for suspended matter and phosphorus, and 15 points for nitrogen).

The standards set for annual averages in the Brussels decrees, which transpose verbatim the directive on urban waste water, have been respected (see methodological sheet). As a reminder, there are two types of standards set for the annual averages: those pertaining to concentrations, and those pertaining to pollution reduction rates. Although the purpose of this sheet is not to make an assessment of the conformity of the station (other criteria would have to be taken into account in that case - see methodological sheet and the focus "Treatment of waste water" from the State of the Environment 2007-2010), these standards serve as a reference for the results provided.

  • With regards to suspended matter, the standard pertaining to the average annual pollution reduction rate has been respected since 2010 (bearing in mind that the standard pertaining to the annual average has always been respected); it should be specified that these standards are optional under the decree;
  • With regards to nutrients, the standards pertaining to pollution reduction rates and concentrations have also been respected since 2010 (and even since 2008 for the pollution reduction rate of nitrogen).

As for the organic load (BOD and COD), the standard has been respected since the commissioning of the plant in 2007.

South plant: upgrading works in progress

Profound methodological changes relating to sampling were introduced at the start of 2011 which led to the observation of a hiatus between the purification efficiency levels before and after this date. As such, the results between these two periods are not comparable.

South water treatment plant - average annual concentrations at the exit of the biological treatment process and average annual pollution reduction rates between the entry and the exit of the biological treatment process (2007-2014)
Source: Vivaqua, monthly and annual operation reports

Note: The sampling method used changed considerably from 2011 onwards: the data is deemed to be much more reliable and representative of the water quality from this date (see methodological sheet). The exit of the biological treatment process here refers to the discharge point into the Senne.

The purification efficiency (of the biological treatment process) of the South plant shows rather mixed trends and results. Between 2011 and 2014, and especially since 2012, the annual average output concentrations of the plant decreased for all of the parameters considered. With regards to the pollution reduction rate, no clear trend can be observed between 2011 and 2014, with the exception of a gradual improvement for nitrogen. The average pollution reduction percentage for phosphorus also appears to have had an upturn between 2011 and 2013, although the 2014 result fell below the level of that observed in 2011.

The assessment in terms of organic matter is positive on the whole since the BOD and the COD respect (most of the time) the standards set by the Brussels decrees for annual averages. Values deviating from the standards have only been observed on three occasions since 2011: in 2011 and 2012 for the BOD concentration (higher than 25 mg/l) and in 2012 for the pollution reduction percentage of COD (lower than 75%). As a reminder, the comparison with these standards is for informative purposes only - as for the North plant - since other criteria come into play to assess the conformity of a station.

However, the situation demonstrates less satisfactory purification efficiency with regards to suspended matter and nutrients. On the one hand, the results for suspended matter systematically deviated from the standards between 2011 and 2014, both for concentrations and pollution reduction percentages. However there is a positive trend for concentrations. It should be specified that these standards are optional according to the Brussels decrees, but applicable since June 2012 according to the environmental permit. On the other hand, the South plant systematically deviates from the standards set for nutrients. This result is due to the absence of a more advancedtreatment for these parameters (tertiary treatment). In order to redress this situation, large-scale upgrading works were started in 2014, expected to last 3 years (at an estimated cost of €72 million ex VAT). It is important to underline the fact that water will continue to be treated during these works. Eventually, the South plant will be equipped with a membrane filtration process (which will replace the current conventional decantation system), which will enable its treatment yield to be improved and other pollutants than the five "conventional" ones to be retained.

Reduced treatment of waste water during severe bad weather

The waste water of the Brussels Region is almost completely collected at the current time (98%, according to the second water management plan). But as previously indicated, during heavy rainfall, a portion of the water flowing into the water treatment plants is diverted towards the "wet weather" treatment process, where the treatment applied is less advanced than in the biological treatment process. Despite this partial treatment, the discharges from the wet weather treatment facility represent a source of pollutant emissions - particularly of organic load - for the Senne (see focus "pollutant emissions into surface water").

During heavy rainfall, so as to avoid the sewage network being overloaded, a part of the water passing through is always offloaded towards the hydrographic network, at the level of the "stormwater overflows", without prior treatment (upstream of the stations then): acting as safety valves, these works prevent the sewage system coming under pressure. However these discharges are far from being negligible, both in terms of volume and quality. Data collected from some of these works attest to this, as detailed below.

Of the 81 overflow works of the sewage system which flow into the hydrographic network (and even in both directions for 36 of them), 14 situated along the Senne or the Canal are subject to telemetric monitoring by the Brussels Water Management Company (SBGE-BMWB) , or Aquiris. These measurements pertain to the volumes discharged and/or the frequency of overflowing. A one-off measuring campaign was also carried out by Brussels Environment in 2010. These observations highlight the regular and highly frequent operation of the monitored stormwater overflows, which is far in excess of the guideline of a maximum of 7 days with discharge per year, used as the reference in Flanders:

  • Based on measurements carried out between June 2008 and March 2010 at 5 of the largest stormwater overflows flowing into the Canal (all situated on the left bank), the overflows are still lower than the guidelines in general. The sole exception: the stormwater overflow of Molenbeek, for which around twenty annual occurrences are recorded. The volume discharged by all of these 5 works amounts to 0.7 million m3 per year. But this figure probably vastly underestimates the total volume discharged into the Canal, since it doesn't take into account the two other stormwater overflows which are considered important (Neerpedebeek and Broekbeek).
  • For this same period, 4 major stormwater overflows flowing into the Senne (Paruck, Molenbeek, Marly and Zwartebeek) have experienced around thirty occurrences of overflowing each year. With the other stormwater overflows connected to the left bank sewage outlet of the North treatment plant, the average flow discharged into the Senne would equate to almost 5 million m3 per year, the equivalent of 5% of the total volume flowing into the treatment plant.
  • Based on the one-off campaign in 2010, one of the other main stormwater overflows flowing into the Senne - the New Maelbeek - , which is connected to the right bank sewage outlet of the North plant, would be activated around fifty times a year and alone would discharge around 4.8 million m3 per year.

Although numerous stormwater overflows are present along the Woluwe, it appears that transfers are rare (see chapter 2 of the draft second water management plan).

In order to evaluate the pollution loads emitted, two measuring campaigns lasting 3 to 6 months were carried out in 2012 and 2013 at two large stormwater overflows: the New Maelbeek and the Paruck. The results of these campaigns have helped contribute to the inventory of emissions into surface water, which demonstrated that the stormwater overflows often represent the largest access routes of net pollutant emissions into the Senne and the Canal (see focus "pollutant emissions into surface water"). Given these results, adapting these stormwater overflows needs to be studied in order to reduce their emissions in the long run.

Date de mise à jour: 30/05/2020


Methodological sheet(s)

Indicator: Concentration of discharges from the waste water treatment plants (.pdf, in French and Dutch only)

Indicator: Pollution reduction rate of the waste water treatment plants (.pdf, in French and Dutch only)

Indicator: Volume of waste water flowing into the waste water treatment plants (.pdf, in French and Dutch only)

Study(ies) and report(s)

VIVAQUA or SBGE-BMWB, various years. « Monthly reports » and « Annual operation reports of the Brussels South water treatment plant ». Studies performed on behalf of Brussels Environment. Restricted.

AQUIRIS, various years. « Monthly technical reports » and « Annual technical reports of the Brussels North water treatment plant ». Studies performed on behalf of Brussels Environment. Restricted.