Mass death of fish in Berlin 2015, Landwehrkanal Blog
Combined sewer overflows
The sewer system was replaced down to the gutter in the late 19th century as the authorities became aware of the health hazard it posed to the public. As the future growth and degree of capacity utilisation was unforeseeable, the sewer was designed to the necessary specs of the time, resulting in a capacity too small to catch every spike in water flow these days. The modern combined sewer network is too small to cope with dirty water and large amounts of rainwater at the same time, which combined can overflow into the natural water bodies. The emergency outlets were first implemented in 1876 as „predetermined breaking points“ to relieve the sewerage system. The downside was already noticeable in 1886, when the first large scale death of fish occurred, one of many more to follow (Mohajeri 2005). This system exists everywhere within the area of the “S-Bahn” ring; the inner Berlin city. In a combined sewer system, the industrial, commercial and domestic wastewater as well as rainwater flow off into one sewer and are led to pumping stations and from there on to the purification plants outside of the city. Rain basins, underground storages, drainage channels and rain overflows have been implemented to store the water during precipitation and feed it intermittently into the waterworks to be distributed by volume according to the facilities’ capacity. However, in case of heavy rainfall, when the water reaches a certain level, the pumping stations can no longer cope with the amount and the combined sewage has to be dumped untreated into the rivers (SenStadtWoh 2009). The mixture of dirty water and rainwater is strongly diluted by the rain masses; mostly in a ratio of 1:8 to 1:12. Nevertheless, the direct discharges of faeces, rinsing residues, street dirt, trash, dog excrement and hygiene articles pollute the waters with nutrients that lead to oxygen depletion. The river is then so heavily polluted that direct contact is a serious health threat to humans and a mass death of fish may occur. Rivers can naturally clean themselves like a bio treatment plant but, as explained, at their source the rivers of Berlin flow very slow and thus are an especially prone and sensitive ecosystem. Depending on the weather and the frequency and intensity of the rain, combined sewer overflows occur roughly 30 to 40 times a year. Even if the Berliner Wasser Betriebe (short: BWB) are expanding the volume of the storage channels and rainwater basins continuously to tackle this exact problem. Despite the current plan of expanding rainwater storage capacity from 240.000m3 to 400.000m3 in 2024, it is still expected that up to 10-20 annual overflows will occur.
To put this in perspective; statistically there are rain events that only occur every few years or centuries. After the completion of the expansion planned by the BWB, the system will be capable of handling rain events which statistically only occur every 2 to 5 years. According to the BWB, there is simply not enough space under Berlin to construct big enough storage rooms for rain events that statistically only occur once in a century. These would be too large for the rest of the time and they could cause odours due to the slow-running of sewage. In addition they would be very costly, so consequently the occasional failure of the sewerage system is an accepted outcome by the BWB (BWB 2018).
What comes into questions is if the definition of a one in a century rain event is still valid as in the last 20 years, they took place at least three times in Berlin in 2006, 2016 and 2017. As for 2017, some of us might remember the videos of people swimming in the streets, flooded underground stations and tunnels and cars being washed away like toys. On June 29th 2017 within 18 hours as much water fell as would usual fall in a quarter of a year. On an average day without rain, 550,000 cubic meters of wastewater are discharged through the sewage system. On this day the amount doubled and 2.8 million m³ of mixed wastewater flowed untreated into Berlin‘s surface waters (BWB 2018). This cloudburst was not a record. In summer 2006 almost 130 litres per m2 of rain fell in Tegel within one hour, flooding the airfield, streets and nearby tunnels (Ehlerding 2016). In the context of increasing weather extremes, it is necessary to assess the situation in the long-term. Even taking into account the proposed extension of current rainwater storage, the system will gradually fall behind in preventing sewage overflows. Especially considering the subsurface space limitations Berlin progressively faces, it is important to work out a system which doesn´t rely on extending bulky and costly infrastructure.
Instead alternative approaches must be considered, which work within existing circumstances or even extend its purpose to serve larger needs on a multifunctional level. For example Ralf Steeg, an engineer from Berlin, envisioned in his project “SPREE2011” to create additional sewage storage by sinking several water tanks into the Spree. At the same time, these tanks were supposed to create artificial land surface to make the Spree more accessible to the public while helping to prevent overflows. Unfortunately the project appears to have stalled after a successful test phase (Jacobs 2015).