Egg-Shaped Digesters at Woodman Point WWTP
An egg shaped digester (ESD), as the name implies, is an egg shaped vessel for the anaerobic digestion of sewage sludge. In the case of Woodman Point, two digesters were built by the Water Corporation and completed in 1997, each with a holding capacity of 8,000 m³. They are approximately 38 m high from the foundation to the top of the digester and 23 m in diameter. The lower 12.5 m of each digester is built below ground level. The digesters at the Woodman Point Waste Water Treatment Plant, located between Henderson Road and Lake Coogee, close to the Henderson Marine Precinct, were constructed using pre stressed concrete. Subsequently, a third digester has been constructed to supplement the two completed in 1997. The egg shape concept for digester vessels originated in Germany but many ESDs have been built in several locations including Japan, the USA and South East Asia.
One of the main advantages of ESDs is that, because of their shape, dead zones are avoided and this provides optimum conditions for sludge circulation and mixing.
The Water Corporation called for design and construct tenders and it was made clear that either steel or concrete could be used for their construction. Theiss Contractors were the successful bidders. They selected in their design team a blend of local and overseas experience. Oswald Schultz, a German based firm, who were very experienced in this type of work, were the chosen Process Engineering Consultants. Peter Jager in association with Bruechle Gilchrist & Evans, were chosen as the Structural and Civil Engineering consultants. RSB Roundtech of Austria were chosen to provide the complex shuttering system.[1] A contract was awarded in June 1995, based on the construction being in prestressed concrete.
The major design load on the digesters is the hydrostatic pressure generated by sludge which has a unit weight of 10.5kN/m³. Gravity loads are less significant and, in most places, add to the compression in the concrete. Other design loads to take into consideration are wind, internal gas pressure, earthquake and thermal effects, including differential thermal effects caused by inside and outside operating temperatures. Prestressing of the concrete in horizontal and vertical, or meridional directions resists the forces. The walls are designed to have a residual compression to prevent cracking and hence leaking of the contents.
Most ESDs built in recent years have been supported on a ring beam, at or near ground level, which in turn is supported on piles. At Woodman Point, however, the ground conditions consisted generally of 3 m of granular fill overlying limestone remaining within a former quarry. The limestone was variable in strength. Geotechnical studies were needed to assess this variability.
To avoid the use of piles and a heavy ring beam, a design proposal was adopted where the fill on site was replaced locally with a cement stabilised sand, and both this and the variable strength limestone below were sculpted to the approximate shape of the lower section of the ESDs. Computer modelling was then used to understand the structure’s response to ground conditions which then prevailed beneath the digesters. This approach had several advantages, principally that it eliminated both the wide ring beam and piles, and it also meant that only the internal face of the ESD required shuttering below ground level.
The egg shaped digesters were built in six vertical lifts. Formwork for inside and outside shutters consisted of “I” beams which were bolted together to form a doubly curved surface with the tension members being designed to carry formwork and concreting loads through a full ring of tension and compression members. The formwork was designed so that 0.5 m high trapezoidal form boards could be inserted in position immediately in advance of the concrete being poured and vibrated. In this way the structure was poured in a continuous operation between construction joints, with the pour being a helix proceeding continually upwards as the form boards were installed. Since the ESDs are symmetrical about their equator, the formwork could be inverted for the top half once concreting to this level was completed. For the top half the inner face was made continuous, as concreting took place from the outside.
In service the ESDs provide additional value to the Water Corporation. After treatment in the digesters for a period of 20 days, the sludge is dewatered in a mechanical system using centrifugal force and then dried prior to use as fertiliser or as a soil conditioner. Also, while the sludge is in the digesters, it produces methane gas that can be used to drive gas engines which generate electricity for use throughout the Woodman Point plant, with any excess sold to the power grid.
Engineering Significance
Design and construction of the first egg shaped digesters In Australia and the Southern Hemisphere comprised several innovations. These included support of the digesters directly by the ground instead of expensive pile footings thus also avoiding the need for a heavy ring beam at pile head level; and the use of post tensioned concrete design to form a complex geometrical shape, with the development of cost saving prefabricated formwork and shuttering.
Author:
Ernie Evans and Doug McInnes, May 2020.
- ↑ Depending on circumstances “shutters” and “formwork” may be used to describe the temporary element which retains concrete during a pour. In this case shutters provide the complex curved shape which was required on the internal surface, and exterior surface above ground level, of the concrete.
