When Octa managed this ocean outfall project in 2009 it was the biggest construction project the Christchurch City Council had undertaken. The ocean outfall was designed to take the city’s treated wastewater from the oxidation ponds at the Wastewater Treatment Plant and transport it 3km out into Pegasus Bay instead of discharging it into the Avon-Heathcote Estuary.
A new pump station was built on the edge of one of the oxidation ponds at the Christchurch Wastewater Treatment Plant, and a 5km underground pipeline now takes treated wastewater from the pump station under the Estuary, South New Brighton Spit and sand dunes, and then 3km out to sea.
This project was a significant engineering coup for the Council – three separate connecting tunnels were built. The land sections involved micro-tunnelling, deemed to be a sound engineering solution with the lowest environmental impact and the lowest long-term operating costs.
The 3km marine section, made from polyethylene pipestring was assembled in Lyttelton, and towed into Pegasus Bay where it was installed and bolted together in the seabed.
Weather and sea conditions delayed the marine section installation by almost five months, requiring an abatement notice when the consent to discharge into the Estuary expired. Notwithstanding this delay, the project was completed on budget.