Alexandra’s water supply is now free of lime scale with the new Lake Dunstan water supply coming down the pipeline and out of the town’s taps.

The $16.1M upgrade to the water supplies of both Alexandra and Clyde has been the biggest infrastructure project Central has seen in a generation. 

It involved the expansion of the Clyde bore field and the construction of a membrane treatment plant, water storage tanks and main pipeline build. 

Central Otago District Council (CODC) capital projects programme manager Patrick Keenan said he and the team were proud of the project.

“It’s been a major effort to get here.”

Marshall Projects managing director Tom Marshall showed the Central App the treatment plant.

“Essentially small filters take out the protozoa [germs] we add chlorine and send it to supply.”

Watch: Tom explaining the interior membrane racks of the treatment plant here.

A backwash occurs every 40 minutes to remove the solid particulate matter from the membranes and out to waste (some of this water is used to irrigate on-site plantings). 

The system does an integrity test every day, and holds its pressure to detect if any of the millions of fibres have failed. 

Tom said he would expect one or two possible failures every couple of years but it was straightforward to block a fibre off with a little stainless steel pin and the module would not need to be replaced.

An aerial view of the new Lake Dunstan water supply

In the event of a communication or power failure the treatment plant can continue operating as generators are on site, although the remote monitoring would not work.

“In an internet outage it all keeps working, it’s hardwired and we don’t need external comms,” Tom said.

In a statement CODC said residents may notice a temporary difference in the taste and odour of their water during the transition to the new supply. 

Over time, the last of the lime scale will pass through house pipes as the softer water dissolves any existing lime scale deposits. These deposits will be flushed out through household taps or water-fed appliances, and down the drain, without posing any risk to public health.

The new supply is compliant with government legislation and contains chlorine at a very low level of 1 milligrams per litre, it is not fluoridated. 

Chlorine gas is injected into a main pipe in a small room, nicknamed the “magic room”.

Two drums of chlorine gas will be on site, loaded on trolleys and in a monitored, sealed room

The magic room where chlorine gas is added to the treatment process

The plant has been designed with the future in mind, as there is room to add in another module of membrane filters in the building. 

“Extremely trying circumstances in terms of procurement” were managed well, Tom said, as there was a scarcity of the PVDF [polyvinylidene difluoride] product used to make the membranes.

He said they got on to it early and the membranes came from Japan and the racks from Germany.

Fulton Hogan plant operator Daryl MacDougall moved from Melbourne with his family to Alexandra a few weeks ago to take up his role looking after the plant.

He said his experience and skills gained working in water treatment in Melbourne were transferrable and he was excited about being in Central. 

A big day as water starts flowing (from left) newcomer Daryl MacDougall, Maddie Irving, Adele Eyers and Peter Hogan 

Work started in 2018 and has been completed amid a backdrop of covid challenges including building material supply shortages. The project also included the construction of 10.3km of 500mm diameter main pipeline between Clyde and Alexandra. This pipeline was completed in March 2020.

An official opening will be in June, and then a public open day in spring. 

Watch a time lapse of the work on the treatment plant site (from November 10, 2022 to April 21, 2023).

For more on the project see CODC’s infrastructure web page.

If you have any concerns about your water contact CODC by phone, or log a service request, under the Central App’s Council button.