Egg-shaped for anaerobic digestion
In south-east Asia a design dating from the 1950s has been found to be an effective and economic solution for anaerobic sludge digestion, as Gerhard Sutter of round formwork specialists, RSB Schalungstechnik GmbH explains.Since anaerobic digestion was developed at the beginning of the twentieth century, many approaches have been made to come up with the appropriate shape for sludge digesters. Designs were founded in the following process-related criteria: the digester vessel has to provide air-sealed containment for the sludge; and optimum conditions for the process: i.e. constant temperature and uniform distribution of the raw sludge for fast and thorough degradation of the organic matter.
The key issue was to achieve higher mixing efficiency and reduce grit and scum-handling problems within the vessel. When digesters with large capacity were required in the early 1950s the egg-shaped digester (ESD) was developed because of its structural benefits over the convention drum-shaped 'Anglo-American' design. The main benefits of the ESD design include:
- large volume on a small foot-print.
- uniform flow mixing pattern.
- reduced heat loss for comparable digester volume.
- due to the conical design there is a small sludge surface area so scum and grit build-up can be controlled by regular mixing.
- the conical shape at the base reduces sand and grit build up, and thus cleaning frequency.
Although construction costs for egg-shaped digesters are 20-40% higher, these are offset by annual operating cost savings of 40-50%, compared to the conventional design.
At present there are four major treatment plants in Singapore using ESDs for digestion of sewage sludge. The Kranji plant has three ESDs with a capacity of 7,800m3 each. In Seletar six ESDs each containing 6,500m3 began their start up phase mid-2000. At the Ulu Pandan plant eight ESDs are currently under construction.
The digesters at Seletar, Ulu Pandan and the Kim Chuan plant will have a reduced visual impact since a substantial proportion (55-65%) of the each structure remains below ground. In Japan, ESDs are nearing completion in Naha/Okinawa (8,000m3,), in Syunan (3,000m3), Otagawa (two plants of 3,050m3 each), Ichinoseki (1,700m3) and at Ashidagawa a plant of 5,000m3.
Phase I of Singapore's Kim Chuan treatment plant consists of three prestressed concrete ESDs, each 30 metres tall with a maximum mid-section diameter of 1860 metres.
Construction is divided into six sections. The foundation consists of a 45°-sloped bottom cone with an integrated circular ring foundation acting as a pile support. Remaining sections include a transition section, a double-curved geometry egg-shape section, the cone plus a final cylindrical section for the gas dome.
Construction began late December 1999, with an anticipated completion for the concrete shells at the end of this year. The digester will be prestressed both vertically and horizontally using the VSL bonded tendon system to create a watertight structure. The construction of each tank requires approximately 800m3 of concrete, 28 tons of prestressing tendons and 80 tons of mild reinforced steel.
The massive foundation plate approximately 10m below finished grade acts as a pile cap for 50 piles with a bearing capacity of 150 tons each. This arrangement provides a solid base for the digester structures.
No wall penetration
The RSB double-curved formwork consists of curved I-beams which are connected by specially fabricated tension members for the outer form and compression members for the inner skin. These members are designed to carry the formwork, rebar and concreting loads through a full ring of tension members.
Pre-cut trapezoidal form boards are inserted into the self-supporting steel skeleton and form the surface on which the concrete will be cast.
The inner formwork is designed and erected in the same manner as the outer formwork and the circumferential connection members between the vertical steel I-beams carry the compressive stresses.
Since the inner and outer forms are self-supporting, there is no need for through bolts or ties, resulting in a watertight concrete structure without any through-wall penetrations.
While the reinforcement and concrete are being placed in the conical section, a crew is pre-assembling the inner and outer formwork for the next section. Matching the curved shape of the digester, the I-beams for for the double-curved section making the egg-shape are polygonally curved. The design radius in these sections is not changed, thus providing a considerable reduction in material costs, since formwork for only about 50% of the actual contact area has to be provided.
After prefabrication on the ground, the easily manageable segments are lifted to their proper position by crane and attached to the previously erected sections.
The prefabrication ensures the formwork can be in place in the minimum time and helps to reduce the overall construction time. When the formwork is stripped, the prefabricated sections are taken down intact, then reused without further fabrication.
Using this repetitive method the RSB formwork system - including the working platform - moves up, section by section, until the final cone roof of the egg-shaped digester is completed.
Working closely with specialist contractors headed by Ng Huat Engineering Pte. Ltd, the project has brought together years of experience in the design and construction of axi-symmetric, double-curved structures. The result is watertight, low maintenance, long-life digesters.