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How Barwon Region Water Authority retired an 80-year-old historic sewer aqueduct, replacing it with a modern pump-boosted gravity system that's unique in Australia.
The historic sewage aqueduct reached the venerable age of 80 before it was replaced by a modern pump-boosted gravity system.
Australia's Geelong Region covers an area of 2,500 square kilometers. Located about 75 km southwest of Melbourne, Geelong was first declared a town in 1838 and gradually developed into a major trading port for wool exports to Britain. Textile industries flourished along the Barwon River, and following the establishment of an automotive plant in 1925, other international and national manufacturing industries also moved to the region. With a population of approximately 200,000, Geelong is now the largest provincial city in Victoria and the 11th largest urban center in Australia.
The city remains a significant trading port, especially for bulk grains and petrochemicals. Manufacturing remains the mainstay of the local economy and the region's bay and ocean coasts are the primary leisure and recreation destinations outside the state capital of Melbourne, attracting over a million tourist visits per year.
Urgent replacement for an aging aqueduct
In 1990, a local newspaper story with the headline "Historic Aqueduct in Danger" described the possible ill fate of an aging sewage aqueduct, a vintage structure from 1913 that was of major engineering heritage value and a familiar regional landmark.
The 750-meter-long aqueduct, a reinforced concrete framed bridge, crossed the Barwon River at Breakwater, Geelong, and comprised a key link in the Geelong sewerage system operated by Barwon Region Water Authority (previously the Geelong and District Water Board). However, its poor structural condition was beyond economic repair and a replacement was needed urgently. If the 80-year-old-structure failed, untreated sewage could drain directly into the important Barwon River. Barwon Water 's strategy for its wastewater system included an AUD 6.5 million bridge to replace the aqueduct. Before proceeding with this option, the authority commissioned the consulting engineering firm Gutteridge Haskins & Davey Pty. Ltd. (GHD) to evaluate other technical alternatives.
Options utilizing conventional sewage pump stations were quickly found to be uneconomic by GHD and the company instead focused on two new innovative concepts worthy of closer examination: an "air cushion" sewer and a pump-boosted gravity system based on submersible pump technology.
The pump-boosted gravity system proved to be the most economical solution. It represented a cost saving of AUD 2.3 million and was also more favorable in terms of general construction and operation. Environmental disturbance would also be minimized, particularly to the sensitive Goat Island reserve located beneath the existing aqueduct.
Another major environmental advantage of the pump-boosted concept was also recognized: the peak flow capacity of the existing downstream ovoid sewer would be increased up to 50 percent, thereby deferring the need for expanding this section of the sewer.
Concept
Since the development of separate sewerage systems in the 19th century, traditional industry approaches have aimed to transport all sewage flows by gravity, "downhill" if possible. Pumping is adopted as a last resort and then only for raising all flows "uphill."
In the case of downhill flows, a pump boosting concept differs in that peak flows are handled by booster pumping and dry weather flows (occurring around 95 percent of the time) flow by gravity.
In new systems, a pump boosting concept achieves significant economies of pipe size and reduces environmental disturbance and social disruption during construction. In existing systems, the operational life of structurally sound pipelines is extended considerably.
The new pump-boosted gravity system at Breakwater, Geelong, is an innovative and cost-effective solution consisting of:
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| an upstream inlet sewer section; | | | | | a major eight-meter-high pump station of unique design; | | | | | twin siphon pipes under the river and a temporary connection to the existing downstream ovoid outfall sewer.
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GHD developed the concept, designed the pump station and helped commission the new system.
Pump station with CP 3500s
The new booster pump station at Breakwater is the only one of its kind in Australia.
As the first step in the pump station detailed design, a computer hydraulic model was developed to predict the hydraulic performance of the overall system.
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During the design stage, GHD consulted with ITT Flygt Australia regarding the selection of the most suitable submersible pump type and model for the station," says GHD's Lindsay Mott. "The ITT Flygt bid was the lowest and the most suitable from a technical standpoint in internationally competitive tendering."
For the station ITT Flygt Australia has supplied three CP 3500s equipped with 60 kW motors and 1410 impellers. The contract was for supply, installation and commissioning of the pumps.
"Standard 'multirode' level control systems are used to operate the pumps in the normal way, except that pump starts are rotated," continues Lindsay Mott. "The controls are also arranged so that each pump operates to 'snore' per day, an arrangement that ensures regular pump exercise and regular transfer of sediment to avoid septic conditions from developing. The station operates only on 2 to 4 pump starts per day, instead of the more usual 8 – 10 starts per hour."
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The aging aqueduct, classified by the National Trust and Australian Heritage Commission, has been saved and stands as a monument to Australian engineering history today.
The CP 3500 was selected because it operates satisfactorily at low head (the normal operating head is only 1.5 meters) over the possible extreme operating range of 0.5 to 7.0 meters head.
"The CP 3500 is also the most suitable choice for unscreened sewage and wastewater, due to its large throughlet diameter of 170 mm. The pump also provides the lowest capital cost. Flygt pumps have proven reliability and good local service backup in Australia, as demonstrated by Barwon Water's experience," says Lindsay Mott.
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Barwon Water's Ian Sanderson, bSupervisor for Wastewater Pumping Stations, in the control room.
"Considerable attention was also given to achieve a distinctive yet functional and secure structure which does not visually conflict with the historic aqueduct, such as simple shapes, concrete surfaces and no visible external pipework and stairways."
The pumps deliver low-head pressure boosting only. Normally pump operating heads are only about 1.5 meters with an infrequent maximum of about 4.0 meters. These unusually low head conditions, combined with only peak flow pumping, result in low power requirements to reduce operating costs.
Higher than normal pump main velocities of up to 3.0 m/s are achieved by utilizing an open discharge tower between the pumps and the discharge mains to minimize water hammer effects. which further reduces main sizes and costs. The open tower also provides the operators with visual access to all functions and components of the station, thereby improving its operability and serviceability.
Benefits
Compared to the original idea of replacing the old aqueduct with a new bridge arrangement, the major benefits of the pump-boosted gravity system are:
| total capital cost saving of approximately AUD 2.9 million (2.3 million less than a new bridge and potential savings of 0.6 million to future downstream works). | | | | | increased flow capability of the pumping station – it can be upgraded to 2,0001/s, compared with 1,6001/s design flow, which greatly improves the flexibility of future system expansion strategies. | | | | | improved outfall flushing capabilities that reduce the sulfide generation potential of downstream sewer. | | | | | a 50 percent increase in the capacity of the existing downstream ovoid sewer to defer future augmentation works.
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The total project, completed in 1992, took four years from prestudy to start-up. In addition to providing an environmentally sound solution to wastewater pumping and transport, the new pump-boosted gravity system is achieving significant capital and running cost savings for Barwon Water. The venerable Breakwater aqueduct, now in its retired state, stands as a monument to Australian engineering history and may be able to be saved as a walkway across the river.
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