| 61 CONTRACTING Central Otago District Council Fresh drop for Alexandra residents T T Russell Fredric The new plant will ensure that both Clyde and Alexandra townships are serviced by one supply with the ability to account for growth. After nearly a decade of planning, high quality tap water filtered through a new membrane treatment plant is on the near horizon for Alexandra residents. Central Otago District Council capital projects programme manager Patrick Keenan says initial planning for the Lake Dunstan Water Supply project which incorporates the membrane plant started in 2013. Part of the need for a new plant was due to the existing water supply, sourced from bores adjacent to the Clutha River, having high lime content, Patrick says. This caused problems with hot water cylinders, appliances and water filters and also affected the taste of the water. Investigations revealed Clyde’s existing bore to be a good water source in terms of both quantity and quality. The Lake Dunstan Water Supply Project was consulted on in the 2018 long-term plan. Installation of the pipeline to carry the water from the new treatment plant at Clyde to Northern Reservoir at Alexandra began in May 2019 and was completed in July 2020. Having one water treatment plant for the two towns will create efficiencies. The new plant will ensure that both Clyde and Alexandra townships are serviced by one supply with the ability to account for growth rather than trying to service the need from two separate plants, Patrick says. “We’ll be able to manage one source and to balance demand when there are interruptions to service in either township. This is done by utilising our reservoir storage capability at Clyde Hill and with a new 4,000m3 water reservoir at Alexandra.” It was originally thought a conventional chlorine and ultra violet water treatment plant could be installed, however testing in 2018 revealed that standard pre-treatment filters blocked very quickly. “There were two main issues creating the filter blockages, the first being Lindavia. This is a microscopic algae prevalent in Lake Wanaka. The algae produces a glue-like slime, commonly known as lake snow. The algae has travelled down the Clutha Mata-Au River to Lake Dunstan.” The second is rock flour, a very fine material from ground up rock created by glacial activity which originates from the Shotover River catchment. “Those two issues combined created a challenge for us, so in 2019 as a result of trials with different methodologies, the membrane filtration gave us the best outcome.” The new plant was originally scheduled to be completed by mid-2020. However, the need for further consideration for the project’s filtration system, plus covid-related setbacks including international supply and shipping delays, means the plant is now expected to be finished around April 2023. With the design changes necessary to meet New Zealand Drinking Water Standards plus cost increases, the original project cost estimate of $9.2 million has escalated to a total of $16.5 million. The contract to supply and install the membrane plant was awarded to Pall Marshall Water Consortium for $3.7 million. The contract for the remainder of the project has been awarded to Fulton Hogan with works including construction of the water treatment plant building, reservoirs, pipe works, and the bore field. Within their contract, Fulton Hogan will also project manage a large component of the work which was awarded as a direct appointment to Switchbuild which will undertake the telemetry and SCADA (supervisory control and data acquisition) interface works. The new plant will be commissioned to deliver a maximum of 14 megalitres a day. Its design allows for a capacity of 20 megalitres a day to accommodate future population growth. Patrick says that after nearly 10 years of planning and preparation, it is great to see the Lake Dunstan Water Supply project now progressing. “It’s taken a lot of work in the background, both from the council’s perspective, the investigation team and everyone who has put it together to deliver it; it’s been a huge task. “These sort of projects only happen once in a generation in a rural provincial district such as this.” “We’ll be able to manage one source and to balance demand when there are interruptions to service in either township.” Proud to be associated with the Central Otago District Council Switchbuild specialise in the integration of process & control systems and the presentation of plant data • Services: Design / ACAD / Commissioning • Dashboards: Customised Data Acquisition • Reports: Process / Operational / Compliance • Alarm Notification: Operator Plans • Programming: PLC / HMI / SCADA / Telemetry • Systems Engineering: Industrial networks / Communications • Switchboards: Motor Control Centres, Power Distribution • Electrical Technician: Switchboard and process upgrades
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