Risky job ridding Napier of its wastewater

A tour of the BTF Wastewater Treatment Plant on Waitangi Road, Awatoto. The damaged outfall extends to 1.5km off the coast. Photo / File

An increasing number of leaks in Napier's wastewater outfall have forced Napier City Council into urgent repairs and a replacement which could include the end of disposal at sea.

At least seven leaks have been discovered and mainly-repaired in the 1.5km outfall which stretches into Hawke Bay from the council's Biological Trickling Filter plant.

The BTF had been commissioned in 2014, the previous milliscreening plants had been operating since 1991, and the outfall has been in place since 1967.

None of the leaks was discovered to have significant environmental impact and the pipeline was considered to be still in good condition, but steps need to be taken to try to avoid what council environmental solutions manager Cameron Burton said at a council meeting on Wednesday could be "a bit of a catastrophe environmentally."

All options are considered to have some risk, including possible complete failure during repairs, which themselves might not fix the latest leak in a fibreglass joint about 700 metres offshore.

Discussing immediate repairs, council manager asset strategy Catherine Bayly described the outfall as a "live asset" where a shutdown for repairs was only possible for three hours at a time.

"We want to the lowest-risk, least-invasive repairs in the next six months," she said. "We don't know how successful it's going to be."

The council decided a specialist dive team should be engaged for a job, which could take place in near-zero visibility; that an emergency response plan should be developed for implementation in the event of failure; to apply for a resource consent to cover the situation, and to start planning for early replacement, the cost of which could be up to $40 million.

Repairs would be constrained by the visibility and short window of three hours at a time available for a shutdown, and the plan would likely need the construction of a mechanism to pump the wastewater from a leak point through tube further out to sea.

The planning for the long term is expected to include all possible futuristic options, including replacing the outfall with a drying system and disposal of remaining dry matter on land, as higher standards are put in place.

With the possibility the outfall has been damaged by impacts, such as fishing vessel anchors and nets, the council, working with the Hawke's Bay Regional Council, expects to have a "no-go" zone in the area of the repairs.