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Once-in-a-lifetime diffuser job done

The once-in-a-generation job to replace Hastings’ wastewater outfall diffuser pipe nearly three kilometres out to sea has just been completed.

The diffuser is a 300 metre long pipe with holes along it which allows treated water from the Hastings District Council Wastewater Treatment Plant at Clive to be dispersed over an area large enough to ensure good mixing with the sea water. It sits at the end of a two-and-a-half kilometre concrete pipe.

Installing it involved teams of divers, boats, underwater mechanised cutting tools, and dragging 150 metres of pipe at a time over the sand to the sea; not to mention working around the weather.

Apart from delays due to storms the complex project had gone very well, said Hastings District Council’s asset group manager Craig Thew.

The 12 divers faced particular challenges, with “virtually zero visibility” near the sea floor. The area had naturally occurring very fine sediment which made for “cloudy” conditions – difficult when using the equivalent of underwater chainsaws and pneumatic wrenches. The divers were in constant communication with the boat-based teams, including by CCTV.

Just keeping the four dive boats still above the divers presented more challenges. Each boat had to have four anchors to secure it over its work site.

“It is very high-tech and health and safety has to be the number one consideration. The company we used, New Zealand Diving and Salvage Ltd, is extremely experienced in these areas and we are really happy with the way this project has gone,” Mr Thew said.

The final job of disconnecting the old diffuser and changing over to the new one has successfully been completed, said Council’s wastewater manager David James.

Planning for the nearly $3 million project had been extremely detailed which had been worth it with the project going without a hitch, Mr James said.

The job should not need to be done again for at least another 80 years, given the advances in composite plastics. It had a thicker plastic composite wall, would not corrode, and would bend and flex with the movement of water. It was held to the sea floor with anchor blocks.

The previous pipe was fibreglass and had held up very well, said Mr Thew. It was installed in 1981 and inspected every year. It had been expected to need replacing in 2006, however it had stood up so well to the conditions that the project could be delayed by another 11 years.

“That’s a significant saving to our ratepayers. We expect that this one will perform even better and that it could be nearing 100 years before the council of the time needs to consider replacing it.”

After discussions with environmental experts Golders, iwi, Hawke’s Bay Regional Council and the Department of Conservation, the decision had been made to leave the old pipe on the sea floor. It had become three quarters buried in sand and was inert so would not cause damage or have adverse environmental impacts. The cost of removing it had been quoted at anything up to $900,000 and would potentially cause more disruption than leaving it in place.

4 October 2017

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