Milliscreen milestone

Published: 5 April 2023

Last Updated: 8 May 2023

A major milestone has been reached today with Napier’s wastewater treatment plant now having its milliscreen back in action.

The milliscreen, along with the rest of the plant, was severely impacted by Cyclone Gabrielle including significant flooding.

Mayor Kirsten Wise sees the milliscreen as a sign Napier is on the road to recovery, but also as a huge achievement for the large, multi-discipline team working on the plant.

“For the community in Napier, having the milliscreen back up and running is a relief, for the team though it is a huge triumph that has taken many long days, many very clever people, teamwork and a can-do attitude,” says Mayor Wise. “I am very proud of the team here, and very grateful to have them working with us in Napier to get this complex project to its next vital step.”

Mayor Wise also acknowledged the role of community. “The issues at the WWTP have been major and have affected all of Napier but we have a great team of smart minds working long hours on solutions. We want to thank the community too for their patience and for doing their bit by managing household water use, it really does help.”

The milliscreen is one of two processes wastewater goes through before it is pumped out to sea. The second process relies on the biological trickling filter, which is still out of action. The BTF works on a system of bacteria that processes waste. The bacteria needs time to regrow. Also, requiring repair is the extensive electrical system that runs the plant. The WWTP team has put in place alternative systems to keep as much of the plant running as possible, while more long-term solutions are put in place.

Programme Director Wayne Newbury-Lee says the team is working to a target of July 2023 to have the first BTF operating.

“There are a number of things that may push out this timeline including weather and supply chains, but it is important for the team to have a goal in mind as they work hard to get every part of this complex system back online.”

Further on the WWTP:

There are two main parts of the WWTP. The milliscreen separates large solids, materials and waste products. The biological trickling filter (BTF) develops a biological environment – a culture of bacteria grown on plastic forms called petals – that breaks down bacteria. To enable these systems, there are complex electrical and control systems managed by the operators that run the plant. All of this was particularly badly impacted by flooding.

The first step was to make the site safe for workers. Then the team did a detailed condition assessment of buildings, systems, services, equipment and the plant in general. Over 60% of the plant was damaged to the point of disrepair and in some places the damage was even worse, up to 80%.

After the cyclone, neither the milliscreen nor the BTF or any other plant sub-systems were working. In the BTF the bacteria culture had died and it now needs to be regrown. Each BTF is different but this can take many weeks. In Napier’s system there are two relatively new BTFs. The WWTP can perform fairly well with just one but two gives a more resilient, future-proofed system.

The milliscreen has now been fixed to point where it can be adequately operated and this week it went into operation under detailed operational control. This will allow a basic level of filtering of wastewater before it enters the outfall pipe. The process of regrowing the bacteria in the one BTF is being advanced by working in parallel so the bacteria culture can get started before the WWTP is 100% ready. This will speed up the recovery process.

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