The continuing issue of discoloured water in the Hauraki Plains has caused frustration to the community and Council alike.
All of system issue to solve
“We understand the frustrations the community are having, we are spending considerable time working through the issues and coming up with a programme of work that will cover all of the underlying problems,” says Adrian de Laborde, General Manager Service Delivery.
“It is a fairly complicated problem to resolve. It is an all of system issue, not just a treatment issue but also a reticulation and pipe maintenance issue.”
The underlying problem is that the water source that we draw our raw water from, the Waihou River, has variable and high levels of manganese. The treatment process can remove around 90-95% of the manganese in the water. The remaining manganese enters the treated water system.
The manganese that makes it into the treated water is mostly lower than the 0.04 ppm (parts per million) required for aesthetic value (i.e. colour) as per the Drinking Water Standards. The health level is 0.4ppm.
Changes to treatment approach
Previously the water was treated with Chlorine, but in more recent times we have moved to using potassium permanganate (potperm) which is a more effective treatment process but can turn the water pink if the dosage is wrong.
Council have purchased an online monitoring and dosing system, a first in NZ for treatment of fluctuating levels of manganese. Difficulty in getting components required for the system has meant it has taken a year to be delivered and installed and is only now being commissioned.
Manganese and water discolouration
“Through investigation and research, we have found that regardless of how low the percentage of manganese entering the treated water, it still finds its way into the reticulation network, comes out of solution and sticks to the sides of the pipes,” says Mr de Laborde.
“The manganese on the pipe walls builds up to the extent that any minor change in the network, such as small changes to the PH, flow, pressure or direction can cause the manganese to dislodge from the pipe wall and colour the water,” he added.
Removing manganese build-up
One of the most effective methods of removing the build-up on the pipe walls is ‘pigging’. This method involves forcing a foam bung through a water line. Although effective, this method can produce large volumes of black water that has to be disposed of in an environmentally sound manner. We have been trialling various ways to pig the lines over the last couple of months and have more appropriate ways of disposing of the manganese removed from the pipes. We are finalising the programme of work to start routine pigging of the lines on the Hauraki Plains in a rotation maintenance schedule. To start this work we will need to install an access point on each pipe, again supply chain issues are holding up this process.
“Unfortunately this will take time to start to get the brown water out of the system but we will target the worst areas and then move around," said de Laborde.
Pigging the lines
“We have been pigging key pipes as we work towards finalising our programme. The three main lines coming into Ngatea will be pigged this side of Christmas, should the materials be delivered on time. We will release the full programme once we have it,” added de Laborde.
Contact us to report issues
Please continue to let us know about any issues you have with your water. Although our physical offices will be closed between Monday 26 December and Tuesday 3 January, we are still contactable 24/7 on 0800 734 834 (from within the district) or 07 862 8609. Thank you for your ongoing patience while we resolve this complex issue. Ngā mihi.