Hydraulic engineering project ARA ProRheno BASEL
June 19, 2024
LARGE CONSTRUCTION PROJECTS ONLY SUCCEED AS A TEAM, EVEN IN BUILDING CONSTRUCTION
Around 270,000 inhabitants of the cantons of Basel-Stadt and Basel-Landschaft are connected to the Basel WWTP. With a large-scale expansion, the capacity will be designed for the population growth expected in the catchment area by 2040. The renovation and expansion of one of the largest wastewater treatment plants in Switzerland is only possible if everyone involved in the project pulls together and joins forces to develop the best possible solutions.
ProRheno AG's current Basel wastewater treatment plant, which was commissioned in 1982, has been reaching the limits of its capacity for several years. It no longer meets the required technical standards and only partially satisfies the requirements of federal water protection legislation. For example, it must be upgraded to purify the nitrogen compounds contained in the water and equipped with a stage to reduce micropollutants. The construction of a plant to produce biogas from the digestion of the sewage sludge produced will bring a further improvement.
KNOW-HOW AND EXPERIENCE REQUIRED
It was clear from the outset that this project was a perfect fit for ERNE, as it had the necessary experience and know-how. With the construction of several wastewater treatment plants throughout northern Switzerland, the ERNE teams have built up the best references for this huge structure in recent years.
The project was put out to public tender. Accordingly, the three-person calculation team led by Nicolas Scherer began the extensive tendering process in 2018. When studying the documents, it quickly became clear that the client's specifications were very demanding due to the complexity of the project.
A detailed bill of quantities had to be calculated for the tender and the staging requirements had to be adhered to. Nicolas Scherer's practical experience as a long-standing construction manager and head of the WWTP/hydraulic engineering division was a great advantage when preparing the tender documents. It was important to recognize the opportunities and risks of the project and weigh them up against each other.
The following criteria were decisive for the evaluation of the public tender:
The price
Key people involved
The contract analysis, which shows how the construction project will be approached
References in the field of WWTP construction
Work preparation: the be-all and end-all for successful implementation
In December 2018, ERNE was awarded the contract to implement the project. Now the detailed work preparation (AVOR) could be tackled. Once again, teamwork was required. Experienced ERNE project managers and site managers went through the individual services in detail and analyzed
component by component. The foremen who were to be deployed on the construction site also contributed their experience and ideas. Services, processes, products and staging were already optimized during this phase, always in consultation with the site management.
One important point was that the construction was to take place during ongoing operations. In addition, space for the construction site installation was very limited. In order to make the best possible use of the construction site infrastructure, the daily program was divided into hourly units. The required material had to be delivered "just in time" directly to the processing site.
BUILDING CONSTRUCTION WORK IN TWO PHASES
The construction work was carried out in two phases. Phase one started in June 2019 and was completed at the beginning of 2021. It comprised the following projects:
Conversion of the raw water lift and mixed water treatment: this is where the wastewater is pumped to the sewage treatment plant; solids (shredded paper, solid faecal matter, etc.) larger than approx. 7 centimetres are screened out.
Removal of the mechanical stage with screen, grit chamber and primary clarifier, where the remaining solids separate from the liquid and sink to the bottom. The wastewater then enters the primary clarifier.
SBR reactors with headworks for the biological treatment of wastewater using special bacteria (SBR = sequential biological treatment).
All new buildings were put into operation before phase two started in summer 2021:
Stage for the treatment of micropollutants. These organic trace substances are broken down by adding ozone.
Five digestion towers to recycle the sludge produced
New construction of the plant building
The plant was complete in December 2023.
SPECIAL REQUIREMENTS IN HYDRAULIC ENGINEERING
In hydraulic engineering and especially in WWTPs, the requirements for the tightness of the components are very high. The highest impermeability class 1 is required: the relevant component must be completely dry; no damp spots are permitted on the dry-side surfaces of the structure.
In order to meet the high requirements, a special film (Zemdrain) was applied to all components in contact with water, such as tanks, containers, channels and basins, in addition to sealing systems. ERNE has already gained important experience with this in the various wastewater treatment plants it has completed. However, the enormous scope - around 50,000 square meters of film were installed - was also new for ERNE.
RENEWED INNOVATION IN BUILDING CONSTRUCTION
For a major project of this size, it was not possible to work with conventional timber formwork, as the area would be too large. However, little experience was available for the use of a Zemdrain membrane on framed formwork. An in-house ERNE solution was therefore required. With great commitment and labor-intensive special assignments, a team of experienced site managers, foremen and foremen worked on the optimum solution. They tested every idea and were only satisfied with the best solution. The self-developed system has established itself in the construction of the Basel wastewater treatment plant and delivers perfect components.
10 METER HIGH WALLS - A CHALLENGE FOR THE STRUCTURAL ENGINEERING TEAMS
Another challenge was the 10 meter high, 32 meter long and 70 centimetre thick walls of the nine SBR reactors for biological purification. The tender specified that these were to be concreted in two parts. In order to optimize the construction process, it was decided in collaboration with the site management and the engineer to build the reactor walls in one piece. In order to be able to produce these complex components to the highest quality, the foremen involved in the project later created a 1:1 sample before the start of construction. With increased demands on tightness, components of these dimensions must also be pre-stressed with cables to prevent the formation of cracks. With normal reinforcement, the reinforcement would be so tight that there would be practically no room for the concrete. This procedure was also discussed in detail within the team.
Because the components are repeated in the SBR reactors, the work processes were optimized through close observation and joint meetings. A learning process that paid off: in the beginning, one wall was built per week, but now the building construction crew only needs four days per wall with fewer people.
In this project, everything was one dimension larger than in conventional wastewater treatment plants. The grit chamber at ARA Basel consists of three streets where there is normally only one. As it is also particularly massive and high, shuttering the filling concrete wedges was a challenge.
INVESTMENT IN THE FUTURE
In order to save time and material, ERNE used single-sided wall formwork for the first time on the ARA at ProRheno AG. The advantage of this type of formwork is that it can be used on one side and the wall width is predetermined. Reworking at the tie points is reduced; fewer small formwork parts are required and the scaffolding only has to be erected on one side.
SAFETY IN THE TEAM
The structural engineering team is constantly aware of the increased dangers that arise when building the high walls of the SBR reactors, among other things. Kilometers of guardrails ensure safety; in addition, the workers always protected themselves with their personal safety equipment to prevent falls. Here too, daily work preparation was immensely important and had to be taken into account to ensure an optimal construction process. Such a large construction site, where between 50 and 60 ERNE employees are working at any one time, always presents those responsible with new, special challenges. Each morning, the foremen pointed out these critical points to their groups and made them aware of the dangers and the necessary safety measures.
26 METER HIGH DIGESTION TOWERS
During the second construction phase, the five cylindrical digestion towers were built as part of the master builder's work. The structural engineering team first concreted the floor slabs and then built the concrete walls. The digestion towers are 26 meters high and 16 meters in diameter.
As part of the micropollution measures, the west headworks, filtration, the PAH process, ozonation and the east headworks were built. A total of 770 supports were built for the 14 filter cells for subsequent filtration in the basins. A new office building was also erected for the construction.
ERNE stands out in this project as a specialist in the region. In recent years, the hydraulic engineering specialists at the sewage treatment plants in Kloten, Winterthur, Zuchwil, Schmerikon and Reinach have been able to acquire the know-how of circular structures and deepen their skills.
VIDEO Crane assembly at lofty heights
The crane fitters from WOLFFKRAN proved their endurance in wind and weather during the assembly of our new top-slewing crane. Strength but also a great deal of dexterity were required.
Over the course of a day, the experienced fitters assembled the various parts to create the 55-metre-high crane.
Incidentally, the crane was christened Alfred.
FACTS & FIGURES
Project title | Extension of ARA ProRheno Basel |
Project management | Nicolas Scherer |
Site manager | Martin Frei, Matthias Rohrer |
Foreman | Beat Senn, Andreas Waldmeier, Matthias Rohner, Till Wälchli, Roman Frei, Maik Sütterlin |
Services | Structural engineering, concrete construction |
Construction time | Phase 1 - June 2019 to early 2021 / Phase 2 - August 2021 to December 2023 |
IMPRESSIVE FIGURES
Concrete | 70'000 cubic meters | |
Mono concrete | 37'600 square meters | |
Fair-faced concrete | 4'500 square meters | |
steel | 8'000 tons | |
Cable prestressing | 56'000 meters of cable | |
Zemdrain foil | 50'000 square meters |
