An underground world of its own
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 approximately 10,900 kilometres long. This is equivalent to the distance between Berlin and Lima, Peru.
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. There is a pumping station situated there, which then pumps the wastewater on to the sewage treatment plants. The Berlin wastewater sewers are a total of approximately 9,600 km long. This includes 4,330 km of wastewater sewers, 3,300 km of stormwater sewers and 1,920 km of combined water sewers. Around three-quarters 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
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 waste water must be drained away as compared to the amount of water 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 storage 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 2020, a total of 306,000 m3 of storage space is intended to be 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 the respective sewage treatment plant. If the level of stormwater exceeds the storage volume of the combined water sewer, 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.
The separate system
In the separate system, wastewater and stormwater are collected in two separate sewer systems. Wastewater from households and businesses 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 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 to take away any pollutants that may occur as a result. Around 62 % of the sewer network is made from vitreous clay pipes. Road sewers range from 20 cm in diameter up to 2.80 m for main collector sewer pipes and up to a size of 4.20 m wide by 3.20 m high for masonry sewers. The smallest building connection sewer is 15 cm in diameter.
The following materials are used for the sewer network:
vitreous clay pipe for wastewater and combined water sewers
concrete pipe for stormwater sewers
reinforced concrete pipe for all types of sewers
oncrete or masonry for all types of sewers, if a special section is required due to local circumstances