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The supply and waste management company Hamburg Wasser is currently renovating a 100-year-old ring pipeline in the district of Rothenburgsort, the cradle of Hamburg’s central water supply. This ring line connects five drinking water reservoirs with a total storage volume of approx. 100,000 m³. Due to the importance of these reservoirs for the supply guarantee of Hamburg’s city center as well as the district in the southeast, these rehabilitation works during full operation can only be carried out during the season with the lowest consumption. With regard to engineering and logistics, this project is a big challenge which therefore had to be divided into five separate construction phases.

In 2014, the demand for drinking water from the supply system of Hamburg Wasser which not only provides water for Hamburg itself but also the neighboring communities amounted to approx. 115.5 million m³. The maximum daily discharge was 377,000 m³; the minimal discharge was 254,000 m³. The reservoir system in Rothenburgsort forms an important hub which supplies a big part of the city center as well as the eastern supply area with a volume of approx. 100.000 m³. The connections between these reservoirs were gradually expanded in the course of the 20th century and now need to be modernized. Due to their central location and importance for the water supply, these modernization works need to be carried out during full operation which requires a lot of planning for the construction process.

Hamburg’s drinking water supply system

The drinking water supply system of Hamburg Netz is divided into the supply areas center, northeast, south and west. 16 groundwater works feed into the network system with a net length of approx. 5,500 km. For reasons of energy efficiency, ten additional pressure zones have been installed inside these supply areas.

The water balance of the individual supply areas has to be met by the waterworks in this zone. In case this is not possible due to transport capacities or bottlenecks created by technical issues, the shortage has to be compensated by predefined points of supply from the neighboring supply areas and has to be taken into account in the water balances. The system outlet pressures are coordinated with the network operation and take into account the specifications of the network hydraulics for the compliance with the minimum pressures according to the technical regulations even in remote areas.

The waterworks in the respective supply areas are designed as base load plants or pressure regulation plants. The base load plants feed a constant daily quantity of drinking water into the transportation network with a fixed system outlet pressure. Within a supply area, the waterworks take over the function of pressure regulation plant. Only the largest area in the northeast has to be supplied by two plants in order to comply with the minimum pressure.

The importance of the central supply area

The central supply area includes Hamburg’s city center and the districts in the southeast. The water demand amounts to approx. 31 million m³ which equals 27 % of the total demand in 2014 and is mainly characterized by commercial influences. In favorable weather conditions, the maximum daily consumption is reached at the beginning of the week with up to 120,000 m³. The minimal consumption of about 60,000 m³ is reached on Sundays and public holidays. In the past years, the hourly demand peaks were always reached during special events such as football matches with up to 8,500 m³/h which exceeds the usual summer peaks by up to 20 %. The night consumption can sink as low as 700 m³/h during low-consumption times.

The base load plant in the district of Curslack and the peak load plant in the district of Billbrook are used to secure the drinking water supply in the supply area. The drinking water of both waterworks is mixed together in the container system of the main pumping station at Rothenburgsort to achieve a uniform water quality and is then stored and distributed in the central supply zone (fig. 1).

Due to the fact that the groundwater extraction capacities are not sufficient in times of high demand or for special supply situations, two points of supply for the supply areas in the northeast and in the south were installed. Via these points of supply, a predefined, schedule-controlled amount of water can be fed into the central supply area. In addition, it is also possible to pump a predefined amount of water from the central supply area into the western supply area via the pumping station Pferdemarkt in order to ensure the supply guarantee in that area.

Description of the main pumping station at Rothenburgsort

Since 1848, the year in which Hamburg’s central water supply was built, the premises of the main pumping station at Rothenburgsort have been used by different drinking water supply companies, first “Stadtwasserkunst“, then the “Hamburger Wasserwerke GmbH“ and now “Hamburg Wasser“. These days, systems essential to Hamburg’s water supply such as the central control room, the processing plant of the Billbrook waterworks, the container system and the two pumping stations of the main pumping station in Rothenburgsort are located on these premises next to the headquarters building.

There are five drinking water containers with a maximum storage capacity of up to 100,000 m³ available for the storage of the drinking water. This way, the enormous hourly fluctuations in the demand for drinking water can be buffered and the required output of the waterworks in Curslack and Billbrook can be kept at a constant level. The drinking water is pumped into the transportation network via two pumping stations. The pumping stations are hydraulically designed in such a way that each pumping station is able to cover the demand for drinking water.

The drinking water containers which are in use today as well as the associated connecting lines and sewers were built in 1928, 1957, 1976 and 1979. Gate and butterfly valves were used as shut-off and control valves for the connecting lines while bulkheads were used for the sewers. Due to the fact that the containers are connected through sewers and pipelines, shafts had to be built. The drinking water containers, sewers, pipelines and shafts were equipped with a mineral lining when necessary. The connecting lines have a nominal diameter of up to DN 2000; the sewers have a cross-section of up to 4.6 m². In order to ensure the consistent quality of the drinking water, the container inlet and the discharged water are slightly disinfected with chlorine gas in consultation with the public health authority.

Problem description and rehabilitation concept

For many years, the quality of the drinking water in the container systems has been monitored analytically and in great detail. The results of this analysis show that some sections of the sewer system and some connecting shafts are having negative effects on the bacterial quality of the drinking water. Of particular importance in this context is the influence of cracks in the structures, dead zones in sections with little flow-through as well as construction related offsets and transitions of sewer cross-sections and pipelines. As a result, the container casing has to be renewed.

In the course of this concept, several sewers that were in need of rehabilitation were replaced by pipelines, two shafts were restored and one shaft was replaced by a valve junction with special fittings. Securing the ongoing operation of the drinking water supply as well as the large nominal diameter of up to DN 2000 and the great installation depths posed as special challenge. In addition, the individual drinking water containers can only be decommissioned during times of low-consumption.

A particular focus is placed on the hydraulic inspection of the existing system and the dimensioning of the new pipelines. During the calculations, special demand peaks such as football matches, unscheduled water release situations like the bursting of main transport lines but also scheduled maintenance works on central energy supply systems, feed pumps or inspection and cleaning works on drinking water containers are taken into consideration.

The entire project is divided into 5 lots (fig. 2). The available time frame for the individual lots is from September until April. The complete construction period is scheduled for at least 5 years.

Lot 1 – Replacement of mixer shaft with valve junction
Lot 2 – Casing of sewer section south and shaft rehabilitation
Lot 3 – Casing of sewer section north
Lot 4 – Shaft rehabilitation
Lot 5 – Clearing of hydraulic bottleneck

Lot 1 - Replacement of mixer shaft with valve junction

The mixer shaft to be replaced which is located on the premises of the main pumping station in Rothenburgsort has a base area of 22 m² and a height of 5.4 m. Both the inlet line from the waterworks in Curslack with a nominal diameter of DN 2000 and the inlet line of the waterworks in Billbrook with a nominal diameter DN 1000 flow into this shaft.

A rectangular duct with a diameter of about 4.6 m² which is connected with a DN 2000 pipeline is used as an additional inlet line for the container system.

The mixer shaft has to be replaced by a valve junction due to structural defects. The works include the installation of a T-piece with an extension pipe DN 2000 and emergency access as well as the connection by means of a DN 1000 pipeline from the waterworks in Billbrook and another T-piece DN 2000 for the connection of a DN 2000/DN 1400 line from the waterworks in Curslack. Both fittings connect the inlet line with the container system DN 2000 and the rectangular duct as an emergency connection in DN 1200. The mixer shaft is preserved as a shaft construction with an above-ground access and contains the T-piece which is used as a mixer element with its associated valves and the emergency exit.

The engineering department of Hamburg Wasser with the help from CONSULAQUA Hamburg (CAH) is responsible for the planning, the tendering process for the various works as well as the award of contract and the project management.

The construction works for the first lot started in 2014. After a very short flushing period, the valve junction and its connections were commissioned in the middle of 2015. The civil engineering works in lot 1 are scheduled to be finished at the end of 2015.

Conclusion

The problem analysis and the developing of respective solutions for such a complex and historically grown structure as the container system of the main pumping station in Rothenburgsort require a substantial technical, economical and operational knowledge in order to produce economically feasible results. The implementation requires strong engineering capacities as well as efficient suppliers and companies.

(bbr, 10-2015)