After years of research and testing, an innovative passive water treatment system is becoming a reality at the Reefton Restoration Project on the West Coast of New Zealand’s South Island.  

Construction is underway on an innovative passive water treatment system at the Reefton Restoration Project (formerly the Globe Progress Mine) to effectively manage the site’s water seepage.

When OceanaGold commenced closure at Reefton, the company committed to developing a water management solution that would exceed compliance requirements and continue to perform long after the site has been handed back.

The system – known as a Vertical Flow Reactor – removes heavy metals from water seepages before the water is gravity fed into Devil’s Creek. Although these metals (including iron and arsenic) exist naturally inside the rock, they could threaten the natural ecosystems if released at elevated levels.

The Vertical Flow Reactor has been designed specifically to meet OceanaGold’s high sustainability expectations for best practice mine closure. It’s the result of more than four years of research and trials – and spades of dedication – into effective, chemical-free water treatment solutions, and it has been the major focus of the Reefton Restoration Project’s Environmental Coordinator, Steph Hayton.

Steph dedicated her Master of Science towards the project, and with the help of her supervisors, the commitment and funding of OceanaGold, and support of specialist consultants, the project team turned an ambitious idea into reality.

“Our first discussion about possible passive water treatment solutions occurred as far back as 2015, when we started to think about closure and what we could do to prepare the site,” Steph said.

“In 2018 I started my Masters with a literature review and studied all the water treatment systems that have been used and tested globally. We then set up trials at Reefton with two of the most favourable systems – Bioreactors and a Vertical Flow Reactor,” she said.

“The trials ran over a two-year period and it became evident that the Vertical Flow Reactor worked exceedingly well – there was a noticeable difference in the hydraulic residence times (the time it takes for the water to move through the system), when compared to the Bioreactors. It showed removal rates of metals were high at relatively low residence times, and the captured solids proved to be more stable.”

Steph says the Vertical Flow Reactor has real potential to be utilised at other sites.

“Previously, we treated water through a Water Treatment Plant before discharging it offsite. This process is expensive, uses chemicals to perform the treatment process, and it’s labour intensive requiring extensive maintenance,” she said.

“Our solution using the Vertical Flow Reactor removes suspended metals from the water with little running cost, using gravity flows, and no added water treatment chemicals. It has been trialled extensively, with the final design developed to exceed compliance requirements and run as passively as possible.

“At OceanaGold, we have mine closure at the forefront of our minds, not only towards the end of a mine’s life, but during mining.

“The trials at Reefton have shown how important it is to test systems and learn what works at each site early to effectively develop closure plans. It’s so much more practical and reliable to use a passive system long-term.”

 What is the Vertical Flow Reactor?

The concept originated at Cardiff University, and was adapted by OceanaGold with help from the Verum Group and Mine Waste Management.

By oxygenating the water before it enters the Vertical Flow Reactor, the iron within the water comes out of solution, turning it into a reddish-brown colour. The iron particulate then gently settles on a gravel filter bed at the bottom of collection ponds.

Iron naturally attracts other metals, so it captures free-floating arsenic from the water. The water then continues its gravity fed course through the gravel bed and exits the system into the nearby Devil’s Creek. The solids are left behind in the collection pond, then removed and stored safely in a controlled storage area.

Over time, the metals will eventually be exhausted from the leachable area of the surrounding rock, and the ponds will continue to naturally spill into Devil’s Creek.

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