An innovative sewage solution to service Melbourne’s growing north

With the population of Melbourne’s Northern Growth Area expected to rapidly increase in the coming decades, Yarra Valley Water was faced with the task of needing to install major water, recycled water and sewerage infrastructure to meet the increasing demand. To deliver services in the coming years, a $400 million program of works is being completed, including the construction of the Craigieburn Sewage Transfer Hub (CSTH)—an innovative facility designed to expand with the community.

Melbourne’s growing north
The Northern Growth Area runs from Craigieburn up the Hume Highway to the Wallan area and covers roughly 20km. By 2040, the area is planned to accommodate an additional 128,000 properties and around 300,000 new residents.

At the time of the planning, there was no existing major sewerage infrastructure between Craigieburn and Wallan where most of the population growth will occur.

This presented a challenge, as the predicted growth could not occur without the construction of new infrastructure including sewage tunnels, water transfer mains, storage tanks, and new and upgraded treatment plants.

Yarra Valley Water is undertaking a $400 million program of works over the next five years to build this much needed infrastructure, which will be largely supported by the CSTH.

Once completed in late 2018, the CSTH will be the largest mid-system sewage storage facility in Australia, with the facility able to expand as development in the area continues.

It will send waste from the Amaroo Main Sewer downstream to the Western Treatment Plant, storing flows when required, allowing Yarra Valley Water to make the best use of the existing infrastructure that it has to service growth.

The facility will ensure large quantities of sewage flows associated with wet weather can be captured and managed without expensive upgrades to the downstream sewer network, reducing the risk of sewage spills.

By creating flexible infrastructure which can grow along with the community and keep up, it provides an innovative, cost-effective alternative to building a large sewer in the area at a much higher cost.

A different approach to sewage management
A sewage transfer facility of this scale is an unusual approach to sewage management, and traditionally Yarra Valley Water would have installed a large sewer tunnel — similar to the Amaroo Main Sewer—to transfer flows from the area at Craigieburn to the nearest downstream sewer with sufficient capacity — most likely sending it down to Melbourne Water’s main trunk sewer, the Northern Intercepting Sewer.

Following an innovative planning and design process, it was decided that the CSTH would be built to locally store and distribute flows, and designed to grow as needed to match development. This approach has resulted in a saving of over $100 million through this alternative solution.

Sewage generated in the growth area will flow into the site of the existing Craigieburn Sewage Treatment Plant via the Amaroo Main Sewer, which is currently reaching the final stage of construction, and the existing Aitken Creek Branch Sewer.

The existing sewerage network does not currently have the capacity to accept peak sewer flows from the area. As a result, the CSTH is required to store flows temporarily when there is insufficient capacity in the downstream network, reducing the risk of sewage spills.

The sewage will then be transferred to the Western Treatment Plant for treatment.

Innovative hub design
The CSTH is a unique sewerage flow storage and distribution facility consisting of two 16 Megalitre (ML) storage tanks making it the biggest in Australia, with plans for future on-site storage of over 200ML across 14 tanks. There will also be a built-in inlet drop structure to help regulate wet weather flows to the storages, and an emergency relief structure.

To manage the flows there will be three sewer pump stations with a total capacity of approximately 1000L/s, with provision to be upgraded to around 2000L/s if needed.

Andrew Cosham, Divisional Manager Major Projects, said the facility will receive daily dry weather flows which will then be pumped offsite into the existing network.

“During a wet weather event when the incoming flows from the new area are greater than the capacity of the downstream network, the flows in excess of the downstream capacity will be temporarily stored at the facility and fed back into the system when capacity becomes available.”

Picking the right pumps
Andrew said extensive research was undertaken in the design stage of the project on the pump and impeller options available.

“This helped to develop specific functional requirements and a list of preferred pumps. The contractor then undertook the procurement process as part of the construction phase of the project.”

Due to the high load of rags that are being encountered in newer developments, Yarra Valley Water said the key factors when selecting pumps for the CSTH were the pumps ability to handle solids—either by passing or chopping them. Other important factors that were a priority when considering pumps were energy efficiency, cost, replacement availability and lead times.

“Energy efficiency is always important in selecting pumps, however, it was not the primary selection criteria. Due to the highly varied operating points (300L/s at 2m head or 14m head) for the wet weather pumps, high efficiency was always going to be hard to achieve with a limited pump selection,” Andrew said.

“As the pumps have all been type tested, the on-site testing will be limited to checking that the design setpoints can be achieved. There will be a trial period to refine the pump cut in and out points, as well as the control programming.”

The project has pumps of varying sizes and functions:

  • The main dry weather transfer pumps are Sulzer XFP 205J-CB2 PE 900/4 pumps with contrablock impellers
  • The main wet weather pumps are Sulzer XFP 305M-CB2, PE 750/6 pumps with contrablock plus impeller
  • Mono progressive cavity pumps were chosen for sludge transfer
  • Three JWC Environmental Muffin Monster macerators were chosen to reduce ragging of the dry weather pumps

Sally Crook, Project Manager

Perfecting the process
The CSTH is an example of the type of innovation required to service growing cities where traditional infrastructure approaches are high cost and face significant technical challenges.

The process adopted has ultimately provided a significant direct saving to the community, maximised the use of existing infrastructure, and delivered a facility that is low risk and simple to operate and maintain, whilst still satisfying all involved stakeholders and providing an equivalent level of service to customers.

By implementing this unique strategy, Yarra Valley Water has been able to provide the community with an innovative lower cost sewerage network compared to what would have been implemented if traditional planning, servicing and design philosophies were adopted.

CPB contractors are the principal construction contractors, with Jacobs initially involved for three years from the creation of functional design through to detailed design. The project is in its final stages, with installation, testing and commissioning of equipment taking place before it is completed in mid-2018.

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