Management of Rain and Waste Water 2022

Introduction

Precipitation and waste water from private households, public institutions, industry and trade, as well as run-off from public road areas, generate large quantities of rain and waste water in Berlin. These need to be drained, and if necessary, treated. In 2022, some 680,000 m³ of waste water from private households, trade, industry and public institutions, as well as rainwater from Berlin and its surrounding areas was treated in the sewage plants per day. This volume corresponds to a rate of almost 8 m³/sec. It therefore represents about 15 % of the run-off of the Unterhavel river below Berlin at average water flow. The amount of waste water produced in Berlin could fill the Großer Wannsee lake in three weeks.

A sewerage network with a total length of 9,768 km, operated by the _Berliner Wasserbetriebe_ (BWB, Berlin Waterworks), is used for the disposal of waste water. It was constructed based on two different systems, the combined and the separate sewerage system, and consists of a total of 4,421 km of waste water drains, 1,927 km of combined water drains and 3,349 km of rainwater drains, as well as numerous other drains and structures, serving special purposes, such as rainwater overflows, retention basins and culverts. With the help of 166 pumping stations and a network of 1,194 km of pressurised sewage pipes, the collected waste water is pumped to the sewage plants.

The combined sewerage system was built from 1873 onwards to drain the entire urban area of Berlin as it was then. It was based on a design by James Hobrecht. However, towns and communities outside the historic city centre of Berlin and Spandau, which were independent until 1920, mainly built systems based on separate drainage. After their incorporation into Berlin, their facilities were combined into today’s two systems. Drainage areas are aligned with river courses and canals, and follow elevations. The boundaries of the drainage areas differ from those of the boroughs. About four fifths of Berlin’s areas connected to a sewerage system are drained by the separate system, and the remaining fifth by the combined system (SenStadt 2001).

The Separate System

In the separate system, waste water and rainwater are discharged into two separate networks. Waste water drains carry household, commercial and industrial waste water to the pumping stations. It is then sent to the Ruhleben, Münchehofe, Schönerlinde, Waßmannsdorf, Wansdorf, and Stahnsdorf sewage treatment plants via pressurised pipes. The sewage plants discharge treated water into Berlin’s bodies of water.

Pumping stations are usually equipped with emergency outlets, via which waste water can flow into the receiving bodies of water in case of a technical defect. Of the 77 emergency outlets, 29 lead to the Spree, 3 to the Dahme, 18 to the Havel, and 20 to the Teltowkanal; 6 connect to standing surface waters, and 1 emergency outlet is connected to other pumping stations via sewage drains. Emergency outlet activity varies greatly from year to year.

Rainwater drains collect precipitation from impervious surfaces, cooling water from factories, and water from drainage ditches. This is carried directly to smaller or larger surface waters. In some cases, very large areas of the city drain into very small receiving waters. In total, the separate sewerage system discharges some 50 million m³ of rainwater into Berlin’s water bodies per year (SenStadtWohn 2017). Rainwater from the separate sewerage systems is heavily polluted by dust, air pollutants, rubbed-off road surface and tire particles, leaked oil, leaves, animal faeces, road grit in winter etc. Especially in small standing waters and canals with a relatively small water volume, heavy rainfalls have been causing fish die-off. This is due to depletion processes caused by organic matter starting to decompose as soon as it is discharged into the water and the resulting oxygen consumption. To reduce the pollution of water bodies, retention basins and retention soil filters have been installed at the main discharge points to treat the rainwater. By 2022, 32 rainwater treatment facilities were put into operation by the Waterworks. More than ten additional facilities have already been set up along the city motorways.

Tab. 1: Treated wastewater volume and receiving waters of the large sewage plants in Berlin and surroundings in 2022

Furthermore, there are some areas along the inner city boundaries which were originally equipped with a combined sewerage system, but later received a rainwater drainage system (a modified combined sewerage system). Rainwater continues to be discharged, however, into the rainwater overflows of the combined sewerage system.

The Combined System

This system almost completely drains the old city centres of Berlin and Spandau also including the area surrounded by the inner S-Bahn Circle Line (city rail). In this combined sewerage system, household, commercial and industrial waste water as well as rainwater is collected in one drain, and then sent to the next pumping station. From there, the combined sewage usually takes the same route as the sewage from the separate sewerage system. The existing combined sewerage system network is comprised of 16 storage sewers and rainwater overflow basins as well as 9 other facilities, such as weirs (as of 2022), in which the combined sewage is retained when it is raining and fed into the sewage treatment plant with a time delay. During short periods of heavy rainfall, the network is therefore able to collect all of the combined sewage. Long-lasting, heavy rain, however, forms an exception to this rule. As soon as the water has reached a certain level in the drains, or the pumping stations can no longer cope with the volume of water they receive, the combined sewage – which mainly consists of rainwater (at a ratio of about 1:9 of waste water to rainwater) during such downpours – flows untreated into the water bodies via rainwater overflows.

The combined sewage overflow depends on the weather. Annual analyses reveal how strongly it fluctuates. Between 2012 and 2022, the number of days on which an overflow of combined sewage was registered ranged between 31 and 69 days per year. The overflow volume ranged between 0.78 to 7.22 million m³ per year.

When analysing days with a recorded overflow of combined sewage, it should be noted that heavy rain often occurs locally and the overflow of combined sewage is confined to a limited area and period of time.
In order to achieve the environmental objectives of the Water Framework Directive and to meet the requirements of the water authorities’ permission to discharge combined sewage into Berlin’s bodies of water, the BWB and the Senate launched a construction programme to create a total of 300,000 m³ of retention capacity (current storage capacity is about 264,000 m³) in the inner-city combined sewerage system by 2024. This will considerably reduce the overflow frequency and volume of combined sewage flowing into the Berlin water network.

Areas without Rainwater Drainage

On the outskirts of the city, there are areas which are connected to a sewerage system that does, however, not include rainwater drains. In these areas, rainwater seeps into the ground. Local rainwater management therefore means that rainwater is not discharged into a water body. This not only reduces peak runoff during rainfall events in the sewerage system and thus in the water body, it also increases the evaporation capacity in the vicinity of the site, which may improve the microclimate. In addition, it may also increase groundwater recharge (SenUVK, 2018). Decoupling measures help to tap into this positive potential. In recent years, decoupling measures and other rainwater management measures have often already been applied in the planning of new construction areas and are now mandatory for new buildings. As part of these measures, more rainwater is retained in the area (SenUVK, 2018, see BReWa-BE, SenUMVK 2021). Areas with local rainwater management still need to be defined accurately. Existing green roofs have already been mapped in the Environmental Atlas Map 06.11.

In rare cases, rainwater from roads is discharged into the waste water drainage system. These roads are marked as such on the map.

Areas without Waste Water Drainage

Despite considerable efforts by the Berlin Waterworks (BWB), not all inhabited areas have been connected to the sewerage network as yet. In those built-up areas of Berlin with no house connections, sewage is collected in septic tanks with no discharge, from where it is taken to sewage treatment plants by authorised transport companies.

There are also many areas that are connected to the rainwater drainage system, but do not have waste water drainage. Most of these areas are roads and paths. In some cases, however, this is also true for other areas. Roofs of railway facilities or car parks near green spaces are often connected to the rainwater drainage system. Waste water does not usually accumulate on those surfaces.

Separate maps were developed to document the runoff situation exploring the discharge of rainwater into bodies of water. They show the catchment areas of the rainwater drainage (Map 02.09.2 and 02.09.3). Each area connected to the rainwater drainage system is matched to the water body into which the rainwater is discharged.