A holistic approach to water supply network operation – using desalination to improve cost efficiency

Seqwater is the Queensland Government statutory authority responsible for providing a safe, secure and cost effective water supply in South East Queensland (SEQ). This is fundamental to the region's health and prosperity.

A particular water security challenge is the threat of drought. The Millennium Drought occurred between 2001 and 2009, and was one of the longest and most severe droughts in SEQ since European settlement. In response, $6 billion was spent on water security improvements, including construction of the Gold Coast Desalination Plant (GCDP) and Western Corridor Recycled Water Scheme (WCRWS), the connection of existing and new water supply sources, and increasing storage volumes across the region. Seqwater now manages $11.4 billion of water supply infrastructure including 26 dams, 51 weirs, 14 bores, 37 water treatment plants (WTPs), more than 600 kilometers of bulk water supply pipeline, and the GCDP and WCRWS. The total yield of the new bulk water supply system (the Water Grid) is 415,000 ML/annum.

Seqwater operates the Water Grid to maintain a reliable supply to 3.1 million customers – only 53,000 in SEQ remain on local supply. The effort is also made to deliver leading edge water supply planning to achieve the best value from the Water Grid assets and stay responsive to weather events, including drought and flood.

The Mudgeeraba WTP was commissioned in 1969, and treats water from Little Nerang River and Hinze Dam to a design capacity of 105 ML/d. Under current demands it operates continuously, averaging 52 ML/d. Together, Mudgeeraba, Molendinar and Mt Crosby WTPs to the north supply the Gold Coast region of the Water Grid. Mudgeeraba WTP also supplies approximately 173,000 customers directly.

The GCDP has been maintained in hot standby since mid-2011. It supplies a minimal volume of desalinated water to the Water Grid each week, and remains available to supplement demand if required. For example, in 2011 and 2013 it was used when surface WTPs were impacted by floods. GCDP can produce up to 133 ML/day, and is designed to be operated at 33% (three trains), 66% (six trains), or 100% capacity (nine trains).

The design of Mudgeeraba WTP did not allow the chlorine contact tank to be isolated for maintenance and/or upgrades, because there was no bypass pipework. The treated water reservoirs could not be taken offline without negatively impacting the chlorine contact time for disinfection. These assets required at least two weeks offline for maintenance/upgrade. An innovative approach to operating the Water Grid was required to maintain supply, including those supplied directly by Mudgeeraba WTP, while allowing that WTP to be taken offline.

The approach to planning the shutdown of Mudgeeraba WTP included:

• Reviewing existing site and ‘as-constructed’ documentation

• Identifying options for supply using the Water Grid

• Undertaking modelling to:

  • - Understand the extent of loss of supply (timing and impact duration)

  • - Determine the technical feasibility of the various options

• Nominating the shortlisted options

• Undertaking a ‘whole of supply system’ financial assessment of options (including net present value; NPV)

• Selecting the preferred option/solution

• Conducting risk assessment and mapping workshops to develop a ‘whole of system’ view of the needs, risks and opportunities, and promote early and active stakeholder engagement

• Developing an action plan for maintaining safe and reliable drinking water while the Mudgeeraba WTP was offline.

This process enabled each option to be developed with an understanding of Seqwater's long-term water supply and operating strategy, as well as short-term asset requirements and demands. It was also important to understand the relative cost of operating the Water Grid in each potential configuration. In particular, the risk and cost of operating the GCDP to supplement the bulk water supply was reviewed at this stage.

Members of the Seqwater treatment and network operations team were invited to a workshop concerning a range of options for supply maintenance. These included construction of a local treatment plant bypass (including sub-option alignments, and temporary or permanent installations), as well as options to operate the Water Grid in a different configuration, Table 1.

Early investigations quickly showed that local treatment plant bypass options could not be delivered cost effectively due to hydraulic, structural, and spatial limitations. As a result, the local bypass options were rejected. The options (including bypass and alternate network configurations) included:

With the above risks in mind, modelling of the network was undertaken to:

• Confirm the extent, timing, and duration of the expected supply interruption

• Determine the technical feasibility of the short-listed options.

Further modelling confirmed all short-listed supply options were technically feasible. However, maintaining supply under each option required a different blend of water from the various sources, and altered the pumping requirements. This in turn affected network operating costs. Most significantly, options which minimized the use of desalinated water had notably lower operating costs.

The modelling also identified the options with elevated risk of biofilm sloughing due to reverse flow through portions of the network. A system performance curve modelled the water age against chlorine decay curves, to indicate changes in free chlorine residuals across the Water Grid.

The modelling showed that the options had different risk profiles. Options 1 and 6 were rejected because of elevated risks to either Mudgeeraba WTP (neglecting maintenance – Option 1) or the network (increased pressure – Option 6). Option 3 was also rejected, because it had significantly higher Opex than the similar Option 5. Only two options were shortlisted, numbers 4 (supply from Molendinar WTP via a new pipeline) and 5 (supply from GCDP). These two were carried to detailed assessment.

The NPV assessment was completed using the total Water Grid operating costs calculated by Seqwater's ‘decision support system’ model, and capital costs developed to ±50% accuracy. The assessment also considered instances where the Water Grid may need to be used again in the same configuration (based on planned future maintenance activities, plus an allowance for unplanned outages or works occurring once every 10 years). Best value asset investment was central to the decision process, particularly the value of investing in a physical asset that has minimal chance of being reused (Option 4) versus an option that incurs only a short term operating cost (Option 5).

Option 5 required the Tarrant Drive Pumping Station controls to be reconfigured to push desalinated water backwards through the network towards Mudgeeraba WTP's direct customers. This would be the first time that desalinated water had been supplied to Mudgeeraba's direct customers.

The key benefits of selecting Option 5 were:

• Low technical risk (with implementation of risk management strategy)

• Minimal capital cost

• Public perception risks manageable through community engagement

• No carbon footprint from materials (mainly control updates)

• No net increase in asset base requiring on-going maintenance.

Planning this project required a thorough understanding of specific project risks as well as the GCDP's history, including public perceptions of both that asset and desalinated water. As a result, the planning process focused on early engagement with stakeholders including operations teams (Seqwater, City of Gold Coast, and Veolia – the GCDP operators), as well as water quality, program delivery, and the community relations teams. The latter were involved early to ensure the public were informed and a strategy was in place to manage their concerns.

Earlier modelling showed that an increase in free chlorine residual was expected in the Water Grid due to the reduced disinfectant decay from desalinated water. A significant proportion of customers were expected to experience an increase of 0.2 to 1.0 mg/L. Furthermore, approximately 50 m of DN600 pipeline was expected to suffer low velocity reverse flow, putting it at risk of biofilm sloughing.

The risk workshop identified that the GCDP normally only operates when the ocean water intake silt density index (SDI) is within an acceptable range. In other words, if the SDI is too high, GCDP does not operate and any deficit is provided by the Mudgeeraba, Molendinar and even the Mt Crosby WTPs. With Mudgeeraba WTP offline, the GCDP became a critical part of the Water Grid so that maintaining supply continuity was paramount. Interrupting GCDP production at high SDI was not an option.

An action plan was developed to address the outcomes of the risk mapping exercise. Key activities included:

• Flushing sections of pipeline subject to reverse flows to minimize the risk of biofilm sloughing, and taste and odor complaints

• Exercising the network valves

• Maintaining non-redundant assets at the Tarrant Drive Pumping Station

• Performing trial runs of the supply scheme prior to the offline window

• Arranging critical spares (e.g. membrane cartridges) for the GCDP to counter the risk of out of specification raw water SDI.

• Adopting twenty-four hour rolling shifts for the construction works at Mudgeeraba WTP.

The Mudgeeraba WTP was taken offline for 18 days from 1 to 18 September 2015. As planned, supply was maintained continuously throughout this period, involving the production of 930 ML of desalinated water from the GCDP. The solution allowed:

• No addition of physical assets to the asset base

• Faster delivery of urgent maintenance works at Mudgeeraba WTP by avoiding the time to build new assets

• Improved flexibility, efficiency and robustness of the Water Grid

• General acceptance of the ‘new’ water source (no water quality issues were experienced and only one complaint received during Mudgeeraba WTP's offline window, a net reduction in complaints over the period)

• Use of the GCDP for the first time, outside extreme weather events and hot standby (SDI instrumentation and some instrumentation accessibility at GCDP were reviewed in light of prolonged GCDP operation)

• Successful project delivery within schedule and budget, implementing strategic risk mapping mitigation measures

• Environmental benefits from avoiding the construction of new assets for a solution with an infrequent use case.

Prompted partially by this project, Seqwater is considering further operating alternatives for GCDP use and the delivery of projects. For example, the ability to facilitate longer WTP shutdowns, where it is cost-effective or it represents the only technically feasible option, is now considered normal. This represents an evolving water security strategy for SEQ, specifically targeted at providing a diverse range of water sources including dams and weirs, groundwater, and more climate-resilient sources including desalinated water from the GCDP and purified recycled water (drought measures).

This strategy has recently been documented in ‘Water for Life – South East Queensland's Water Security Program 2015–2045’ Version 1 (Seqwater 2015). Maintaining short-term capability of the GCDP (i.e. in hot standby mode) enables the asset to respond rapidly to operating issues, and maintain a reliable supply during floods and other emergencies, as well as during planned maintenance activities such as those for Mudgeeraba WTP. The next planned use for GCDP is the upgrade of backwash and electrical systems at Molendinar WTP. Molendinar will be offline for 8 to 16 weeks in early- to mid-2018, with supply supplemented by the GCDP. However, this will be simpler to implement because the Molendinar WTP reservoir complex is already designed to receive water from the GCDP.

As planned, supply during the Mudgeeraba construction work was maintained continuously. The project demonstrated that the GCDP can reliably deliver base load supply to enable planned works and for any unplanned outages at Mudgeeraba WTP.

The holistic planning approach resulted in the selection of a cost effective, sustainable, robust, whole-of-system solution that was delivered successfully by implementing mapped risk mitigation measures. The solution adds value to Seqwater's asset portfolio, without the addition of physical assets, by increasing the Water Grid's flexibility and robustness. This innovative method of using the Water Grid will be available as required to support planned projects and emergencies.

A project of this size and complexity cannot be successfully planned without assistance from a wide range of stakeholders. Particular thanks go to:

Phil Mogg and Romer Cantos (City of Gold Coast)

Scott Murphy, George Bellizia and Alf Cosgrove (Veolia Australia and New Zealand)

James Moffatt and Grant Gabriel (Engeny Water Management)

Paul Rogers, Duncan Middleton, Janine Tunny, Jeff Browne, John Gedge, Jim Fear, Mark Cullinan, Charles Kennedy and Paul Fozard (Seqwater)