Palmerston North Wastewater Treatment Plant

Palmerston North City Council

Palmerston North, New Zealand

Palmerston North City Council (PNCC) required a review and improvements to the performance of their Anaerobic Digestion Systems at the Palmerston North Wastewater Treatment Plant.

Our team’s anecdotal evidence suggested long-term under-performance had been occurring. Our team undertook hydraulic residence time testing on the digesters to reveal that nearly 50% of the reactor volume was poorly mixed and had been for many years despite a number of previous reviews.

Calibre acted as process designers, as well as mechanical and electrical commissioning engineers. First our team undertook hydraulic residence tests and from these recommended a short-term, transitional mixing system, followed by a longer term solution. The short-term solution improved performance sufficiently, allowing a single digester to accept the entire plant’s waste sludge. This allowed the second reactor to be removed and upgraded.


The modifications made to the system increased the system capacity more than 300%. On our recommendation the council developed the facility into a co-digestion facility for combined source segregated trade sludges and waste, as well as biosolids. As a result the gas production has increased three-fold, resulting in an upgrade payback period of less than three years.

The gas production performance of the co-digestion upgrade was so convincing that PNCC engaged Calibre in 2017 to design an additional capacity upgrade of the existing digesters for acceptance of solid food waste and pre-consumer food residuals. The installation of the upgrade works is expected to be completed in 2019.

Project highlights

  • Enabled new revenue streams
  • 300% digester capacity increase
  • Green electricity export to grid




  • 2015 ACENZ Innovate NZ Awards of Excellence – Silver Award


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