Sewage water discharged in the Senne and the Canal

In the future, the Senne (Zenne) will be uncovered in the Maximilian park. Together with the extention of the park, it will contribute to the creation of a place where people can be close to nature. It is also a symbolic act to undo the mistakes of the past when the Senne was used as an open sewage and had to be covered. But is the Senne really ready to be exposed in the center of the city? Is it free from sewage water discharges? The answer is no, and far from it. Actually, the Senne is polluted whole year round by sewage water discharges through the sewage system overflows.

Number of overflows in the canal at Sainctelette in 2020: 21 days

Number of overflows in the Senne at Sainctelette in 2020: 79 days

Sainctelette: Overflow events in February

To the Canal
To the Senne

New Maelbeek: Overflow events in February

To the Senne

Video of sewage water overflow to the canal

Every sewage system has emergency overflows for heavy rains. To avoid floodings in the city, the excess water is discharged through the overflows into a canal, a river or a storm water bassin. According to the actual Flemish standards, overflows should function 7 days a year at most, in case of exceptionally heavy rains.

In Brussels, there are 81 overflows but 3 of them account for 75% of all sewage water discharged into the Senne and the canal: Sainctelette (Paruck), Molenbeek and Maalbeek. Let us take a closer look to the one at Sainctelette. When it rains, the water level rises in the sewage collector. When it reaches a certain point, the excess water is discharged to the Senne through the overflow and a sort of siphon that passes underneath the canal. This siphon is the initial sewage pipe that used to connect the sewage system directly to the Senne prior to the construction of the two sewage water treatment facilities in Brussels in 2000 and 2007. If the water in the collector continues to rise despite the first discharge, the second-level overflow is activated and the water flows directly into the canal. And since there is no grid or retention of trash, everything, rats included, ends up in the canal together with the polluted water.

How often are these security mechanisms activated in Brussels? If we look at the Sainctelette overflow, there were 9 overflows to the Senne and 3 to the canal in October alone (see graph above). These numbers are obtained by counting the number of days that the overflow is activated and discharching a significant amount of water. It is possible that the overflow is activated a couple of times in one day but this is counted as one overflow event. In October alone, the standard of a maximum of 7 days a year, used for the construction of new sewers, has been exceeded. Since the beginning of this year until the end of November, there have been 68 overflow days to the Senne and 20 to the canal, unimaginable numbers. And what happens at the Sainctelette overflow is representative for the other overflows in Brussels. Throughout the whole year, polluted sewage water flows directly into the Senne and the canal during rain events. Overall, the Senne receives a volume of sewage water that is 10 times superior to that of the canal. Sometimes, an overflow event can continue for hours. For example, on March 10 and 13 this year, sewage water flowed twice for 12 straight hours directly into the Senne (see graph below). What we can see as well on the same graph is that the flow of the overflow becomes negative twice. This means that water flowed from the Senne into the sewage, generating a kind of short circuit. A non-return valve is needed to prevent this. The pollution in the water is bad for the ecosystem as it causes the oxygen level to drop, among other things, preventing many types of fish and plants to settle in the Senne. Moreover, plastic trash finds its way into the river and finally into the sea. Not to mention that it can also be a source of health issues for people as well. 

In the last years, we have seen more news coverage about the improvement of the water quality of the Senne. The fish population has increased, seals swim much further into the Scheldt etc. The reason for this is the construction of the two sewage water treatment facilities in 2000 and 2007. Before that, all the sewage water was directly dumped into the Senne! Can we really be proud about the improvement of the water quality of the Senne? The construction of the two facilities was a very big step for sure but it was just a necessary and logical step for a city, and it was taken very late. Today, 98% of the city is connected to the treatment facilities, but that does not mean that 98% of the water is treated, far from it. Together both facilities treat 125 million m3 of water each year but at least 10 million m3 never reach them and are directly discharged into the Senne and the canal through the overflows. What is more, 5% of the water that reaches the facilities only goes through the emergency treatment process when it rains where it is only lightly filtered and the trash is removed before it is discharged into the Senne. 

It is time to act. If Brussels really wants to introduce more nature in the heart of the city, it is time to respect the nature we already have. The European Water Framework Directive, published in 2000, commits all European Union members to achieve good qualitative and quantitative status of all water bodies by 2027. For Brussels, this applies to the canal, the Senne and the Woluwe. Today, the canal and the Senne suffer from bad water quality and are far from reaching the good chemical and ecological status. How will they ever reach this status if they continuously receive sewage water discharges? Will Brussels just pay the fines to Europe for not committing to the demanded standards?

The sewage system has not changed much since it was first constructed and it is not adapted to a city with a population of 1,2 million people. The perfect solution would be to prevent rain water from flowing into the sewage system. This is done by building and renovating in such a way that water is held before it can flow away. This transformation is necessary and has to take place but it will take decades to really show results while the population keeps growing and rain becomes heavier. Underground storm water bassins are a necessary and complementary solution to reduce the amount of sewage overflows in a much shorter period of time. When it rains, these bassins store the excess water to release it back later into the sewage system when the water levels are normal again. Currently, the city plans to optimize the use of the existing Belliard storm water bassin to reduce the amount of overflows of the Maalbeek. Nowadays, this water bassin is only used to prevent floodings. But, since the volume of the Belliard storm water bassin is quite limited (17.000 m3), this will probably not have a really big impact. For the Molenbeek, plans were made to construct a storm water bassin because there is a real risk of flooding but the realisation of these plans seems to be blocked. For Sainctelette, only small adjustments will be done. Right next to the Sainctelette overflow, the construction of the new Beco park will soon begin. Why not use this opportunity to build a storm water bassin underneath the park before it is constructed? This is a golden opportunity. However, the current regional government has decided not to invest in these solutions which would help solve the problem of sewage overflow within the foreseeable future. This is not understandable or unacceptable.

If the discharges of these three overflows could be eliminated, we could reduce the amount of sewage water discharges in Brussels by 75% and the efficiency of the treatmant facilities would rise significantly because it would receive less rainwater. This too is a necessary and logical step for a city to take. So what are we waiting for?