Cromwell Wastewater Treatment Plant
Big picture thinking delivers huge savings to Central Otago District Council.
Central Otago is the country’s most inland region, set amongst a stunning backdrop that is both vast and undulating. The Central Otago District Council services an area of 9969 km2 and has one of the lowest population densities per square kilometre. The community consists of a variety of small townships, such as Alexandra, Clyde and Cromwell, offering a wide range of services. The district is popular with holiday-makers, particularly over summer.
Off-peak growth has also increased across the district with a 25 percent population growth is expected over the next three decades.
This growth, increasing environmental expectations and regulation has dictated the need to improve the quality of urban wastewater discharges.
Infrastructure management is a core part of what Central Otago District Council provides to its communities. It accounts for the majority of council’s spending, across almost $1 billion worth of assets, including seven public wastewater schemes throughout the district.
As many councils know, accurately planning upgrades with the right budgets and timing is of utmost importance. One of the first and largest initiatives in the Central Otago District Council’s 10-year Plan is the Cromwell Wastewater Treatment Plant (WWTP) Upgrade. This project includes several phases and was devised to improve the quality of the district’s urban wastewater discharges into Lake Dunstan by January 2019.
The current project, which includes a new wastewater transfer from Bannockburn, sludge removal and the first phase of improvement at Cromwell WWTP, was estimated to cost $10.9 million. The final investment was $8 million, saving ratepayers $2.9 million dollars.
Simon Norton, major capital projects manager, Central Otago District Council, runs the major water projects. He credits the robust procurement process and innovative design, and construction methods by both Fulton Hogan and Downer for the early and efficient completion of the overall project.
The initial phase of the project was to connect the Bannockburn Wastewater System to Cromwell WWTP. This scheme was designed by Beca and constructed by Fulton Hogan under a traditional contract. The estimated $2 million project began in January 2016 and came in at $1.5 million upon construction completion. This was, in part, thanks to innovative construction by Fulton Hogan which removed the need for scaffolding on the Bannockburn Bridge to which the pipe was attached.
Simon comments; “The adapted crane and man-cage made a big difference to the price of the pipeline and was completed without incident.”
With over 30 years’ experience in water engineering, Simon believes that working with the right people and a thorough procurement process increased the success of the overall project right from the start.
The next phase of the project was the upgrade of the Cromwell Wastewater Treatment Plant. This started in December 2016.
Simon says; “When it comes to a design, build and operate procurement process, first you need a solid principal requirements specification document that is well considered by an experienced engineering and procurement team. It’s the absolute basis of everything else that follows.”
Under the council’s instruction, Beca created the principal requirements specification document and council commenced an expression-of-interest phase, with nine suppliers responding. Responders were marked against a series of technical parameters by Simon and a team of evaluators, and two respondents shortlisted.
The shortlisted consortia then had to complete a tender design under competitive tension.
Simon follows on, “The tender designs were received after an interactive process including two full-day face-to-face workshops to provide council confidence in their design and construction and the design.”
The tenderers then provided a final submission which was evaluated on a quality cost model – two-thirds on quality and one-third on cost (for both design and construction).
Simon says, “What we ultimately wanted was a lower whole-of-life cost. That requires good design, a thorough understanding of available technologies and a well-configured design and construction team.
“This wasn’t a typical procurement process – but it should be considered for a potentially $5 to 10 million project and with this level of complexity.”
Downer, with Harrison Grierson as the lead designer, was awarded the project in March 2017 with construction commencing in April 2017 (the operations contract completes in July 2020.)
Prior to the main contract, the treatment plant’s two ponds underwent sludge removal by specialists South Water. Owner Rob Grant had recently procured specific sludge removal machines which were made in Wales and sent to New Zealand. One of the machines was transported to the site from Nelson.
The main contract included for a new, enlarged inlet pipe to accommodate the future growth of Cromwell and a new inlet screen future-proofed with provision for a second screen when required.
Simon says, “The wastewater then goes into a new inlet meter so we can accurately measure the in-flow for consent purposes. The inlet works also include a flow splitter with actuated valves. These allow remote control of the split of flows to the ponds.”
The flow continues into the ponds which both have new Kiwi-designed Aquarators and new baffle walls to lengthen the flow path.
“The new Aquarators and baffles force both ponds into a more labyrinth-like flow,” says Simon. “Downer and Harrison Grierson engaged the Aquarators designer, and we had a peer review by Beca’s pond specialist to ensure the flow path was at optimal levels. The Aquarators were set to project their bubble streams in a race track formation for the best circulation.”
The council is the biggest user of the Aquarators this far south (Clutha also has a few), which is significant as air temperature is a major factor in water treatment.
Simon clarifies, “In the middle of winter we have weeks where it’s minus or below freezing for large parts of the day so microbiological activity slows down and aeration becomes more important. We wanted to include New Zealand technology where appropriate and have been impressed with the quality and post-installation back-up provided by the supplier.”
Continuing the wastewater treatment process, the flow goes into a new membrane pre-screen and is pumped through into a large new filtration system which is made up of more than one million hollow straws which further filter and clean the water. The filtering process is chemical free. The treated water is discharged into Lake Dunstan and is clean enough to swim in.
“We chose this plant because it provides cost-effective, reliable and proven technology. We liked that it was relatively low cost and easy to operate and results in a very high standard of filtrate. E.coli levels are at least 1000 times better. Actual pre-project screen levels were between 110,000 and 2,000 cfu/100ml, and we are now achieving 1-2 which is at least a 1000-fold improvement which is fantastic.”
Simon says there were no major issues in managing the project.
“There were the usual hundreds of items on watchlists and risk schedules which we were constantly monitoring. Through the course of the design and construction phase, we worked our way through these, and we’ve pretty much completed them all. Some projects this close to completion still have long lists, so now being down to single digits is pretty good.”
Simon ends, “The main outcomes of the project are that by coming in under time and budget, we saved our ratepayers money that can be invested into other projects and we’re ready to handle the demands of our growing population efficiently
For more information on the Cromwell Wastewater Treatment Plant Upgrade, visit the Central Otago District Council website at www.codc.govt.nz and view the video about this project.
This article was first published in the February 2019 issue of NZ Local Government Magazine.