The Far North District Council in New Zealand realised it needed to upgrade its Paihia wastewater treatment plant (WWTP) when new environmental standards mandated that the effluent discharged from such facilities shouldn’t contain more than 2mg/l of ammonia. The existing plants, located in an isolated spot in the Waitangi Forest, could only achieve outflows of 40mg/l of ammonia.
The existing effluent from the WWTP discharges into the local stream and from there into the culturally significant Waitangi Wetland, home to the endangered Northland Mudfish (Waikaka), a Māori taonga.
A conventional upgrade to the pond was priced at $25 million, which was not affordable to the local community. But by innovating, the project was able to use US-developed Bioshell technology for only $6.5 million, achieving its effluent output goals at a fraction of the originally quoted cost.
Because the site is bounded by natural wetlands and two drains feeding into the local stream network, there was limited space for construction. Further limits were put on the site expansion by the wetlands, as well as forestry land and culturally significant archaeological sites. In addition, vehicle access is restricted by both working forestry land and narrow access tracks.
A further complication was the location of the nearest power lines being over three kilometres away, with power upgrades priced at $700,000. Again, this option was unaffordable for the community.
Finding the solutions
By choosing the Bioshell solution, the new treatment plant was able to fit within existing site boundaries. Around 553 Bioshells were installed at the existing pond, capable of removing up to 120kg of ammonia per day, while the Bioshells were arranged in rows, easing maintenance and installation. Each Bioshell is fed air from a common manifold by a duty air blower.
The challenge of getting power to the site was met by the installation of an on-site generator which also supports system controls and telemetry without the need for additional buildings.
As a result, the project was able to fit within the existing site footprint and comply with the stringent ammonia standards required.
There have been several innovations around the project, with the Paihia Bioshell being the largest installation of its kind outside the US. The Bioshells have also met the target goals for ammonia reduction, with output falling from an average of 40mg/l to less than 2mg/l.
The project is power efficient, needing 50 per cent less electricity than a conventional treatment plant. Minimal concrete was required for the installation and the plant falls within the existing site footprint, minimising its impact on the surrounding environment.
This was all achieved at a price affordable to the local community and provides a model for sewage treatment across New Zealand, particularly in environmentally and culturally sensitive areas.