Underground World with a System

Drinking water turns into wastewater. It flows through the building connection sewers in freefall to the collector sewer pipes in the roads and through ever larger sewers to the pumping stations, which then pump it to the local sewage treatment plants. The Berlin wastewater sewers are 9,700 kilometres long. This is equivalent to the distance between Berlin and Cape Town, South Africa.

Berlin Underground

Berlin is divided into honeycomb-like drainage areas, whose boundaries are not identical with those of the Berlin residential districts, but rather follow the courses of rivers and canals. They also follow varying levels of terrain. Sewers always lead to the lowest point of a drainage area. A pumping station is situated there, which then pumps then wastewater on to the sewage treatment plants. Altogether, the Berlin wastewater sewers are more than 9,700 kilometres long. This includes 4,400 kilometres of wastewater sewers, 3,400 kilometres of stormwater sewers and 2,000 kilometres of combined water sewers. Around threequarters of the city area of Berlin are set up as one separate sewer network system and one quarter as a combined sewer network system.

The Combined System

combined system

In the combined system, wastewater and stormwater are transported together through one type of sewer. This system has definite advantages, especially in the inner city, where there is very little space under the roads, next to the subway and other lines. However, for the measurement of the sewers, the share of stormwater is very important, because, in the event of rain, many times the amount of wastewater must be drained away as compared to the amount drained during dry weather. In order to keep the size of the combined water sewers economical and relieve the network system and treatment plant of excess incoming stormwater, there are reservoir and stormwater overflow facilities situated at fixed points along the sewer network as well as in the proximity of local pumping stations. During a storm, nearly a quarter of a million cubic metres of water can be held back in the sewers of the combined system. By about 2022, the aim is to have a total of 300,000 cubic metres of storage space available. This prevents dirty water from the sewer network from overflowing into the bodies of water. During heavy rainfall, local wastewater treatment plants are not in a position to handle all the incoming water at once. In these rare cases, they would be overloaded and the delicate biological treatment process would be permanently disrupted. To prevent this, the maximum capacity of the pumping stations is limited to the maximum treatment capacity of then respective sewage treatment plant. If the level of stormwater exceeds the storage volume of the combined water sewer,n the stormwater and wastewater which is not held back by the pumping station then passes untreated through the stormwater overflows into the combined sewers and is subsequently discharged into bodies of water. However, this is the exception, not the rule, and only occurs during very heavy downpours.

The Separate System

Separate System

In the separate system, wastewater and stormwater are collected in two separate sewer systems. Wastewater from households, businesses and industry flows from wastewater sewers to pumping stations, which then pump it on to wastewater treatment plants. The advantage here is that the amount of wastewater going through the system is fairly uniform and the occasional large quantities of stormwater that accumulate due to bad weather conditions do not have to be transported to the treatment plant through the sewers and pressure mains. For this purpose, special stormwater sewers collect and discharge rainfall and other precipitation to nearby rivers, sewers, streams or lakes. In areas with permeable soil, stormwater percolates where it falls. For roads with heavy traffic, stormwater sewers have been installed to ensure run-off and divert any pollutants that may occur. Around 60 % of the sewer network is made from vitrified clay pipes. Road sewers range from 20 centimetres in diameter up to 2.80 metres for accessible main collector sewer pipes and up to a maximum size of 4.20 metres wide by 3.20 metres high for masonry sewers. The smallest building connection sewer is 15 centimetres in diameter.

Green Intensive Cleaning

A special form of these treatment plants are retention ground filters. These filters clean dirty stormwater from heavily travelled roads and industrial areas by filtering and retaining removable dirt from the water using a substrate and reed plants. So far, Berliner Wasserbetriebe has built four retention ground filters at the Baggersee Lake in Biesdorf, in Adlershof, in Blankenburg and at Halensee Lake.

The stormwater collected in the sewers first flows into a tank, where the dirt that has flowed in with it settles. In the actual filter itself, the loosened dirt and contaminants, such as phosphorous, nitrogen and heavy metals, are bonded to the substrate and in the plants. The reed plants ensure that the filter does not become blocked. After passing through the filter, the water is then freed of 80 % of the filterable substances – among other things, heavy metals adhere to them – and 70 % of the phosphorus compounds. Together with extensive renovation work in the sewers, these plants serve the purpose of achieving a good ecological balance in the Spree-Havel river system. The sewers transport the wastewater to one of thepumping stations within the metropolitan area.

Tanks Catch the Rain

Emergency outlets and stormwater overflows in Berlin are located on the rivers Spree, Havel, Panke, and on the Landwehr Canal, the Teltow Canal, and the Neukölln and Berlin-Spandau shipping canals. There are also several stormwater retention tanks at strategically important points. They catch the combined water during short and heavy storms in order to prevent it from overflowing into local bodies of water. With weirs, throttles and increased overflow thresholds, the sewage system itself is used as a reservoir. During long, sustained heavy rainstorms, it is possible to treat stormwater in the tanks directly.

With Pressure to the Sewage Treatment Plant

The pumping stations are connected by a 2,000-kilometre-long network with pressure pipes up to 1.60 metres thick. Main pumping stations transport the wastewater through these pressure mains to the sewage treatment plants. The system for transporting the wastewater from more than 300 plants, comprising pumping stations, weirs and stormwater tanks, is monitored and controlled from a control centre in Friedrichshain.

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© 2007, Berliner Wasserbetriebe.