Broken Mt Albert sewer tunnel pipe: Costly effort to divert millions of litres of sludge

4:54 pm on 24 January 2020

Another broken wastewater pipe in Wellington means trucks are hauling millions litres of sludge every day to landfill in a fix that's expected to cost ratepayers $200,000 each week.

Sludge being pumped into trucks.

Sludge being pumped into trucks. Photo: RNZ / Jonathan Mitchell

The latest failure, which happened last week, occurred in the Mt Albert sewer tunnel - part of a 9km pipeline that carries sewage from Moa Point on the city's south coast to the Southern Landfill.

The costly move is an attempt to prevent wastewater having to be discharged into the harbour, but Wellington Water has warned a repair could be more than a month away.

"The complications are that the pipes that need repair are encased in concrete in the base of the sewer tunnel... so we have to arrange for that flow to be bypassed," Wellington Water chief wastewater advisor Steve Hutchison said.

The trucks are having to make up to 150 round trips each day, which took about an hour each. He said they may need to operate around the clock at times to keep up with the volume - about a million litres each day.

While there was some heightened risk of infection with sewage being moved above grounds, hygiene was being handled carefully among truck drivers, and all trucks were closed, he said.

Steve Hutchison from Wellington Water holding some sludge.

Steve Hutchison from Wellington Water holding some sludge. Photo: RNZ / Jonathan Mitchell

Hutchison emphasised this was a temporary fix, and work was underway to plan a repair with a temporary bypass, as well as a long-term fix.

He suspected an installation fault was to blame, as this particular system was only installed in 1998. They were meant to last at least 80 years, he said.

The network was built as a dual-pipe system that was meant to account for failures and maintenance but, in a "highly unusual" situation, both failed last week, Hutchison said.

In December, a wastewater pipe collapsed in the CBD, which caused more than five million litres of sewage to spill into the harbour.

The Mt Albert tunnel takes all the wastewater from Island Bay - about 100 litres per second of wastewater, Hutchison said.

Asked whether legal action could be taken if installation was to blame, he said "we'll look into that, but I suspect the length of time that's passed means limitations are past pursuing".

Several options were being explored including repairs, having the pipe out of the tunnel, or bypassing the pipe altogether.

"There's a number of issues to work through looking at those, which we'll be progressing with in the next couple of weeks," Hutchison said.

A similar repair, also due to a defect in the concrete in which the pipes are embedded, was carried out in 2013 and took about five weeks to complete.

Meanwhile, contractors continue to urgently try to fix the central city's wastewater system following the collapse of old pipes underneath Dixon and Willis streets.

Pipes being assembled at Willis St to be joined into a new sewer main.

Pipes being assembled at Willis St to be joined into a new sewer main. Photo: RNZ / Kent Atkinson

Currently an above-ground black plastic pipe was working in its place, but plans were well underway to put a "slightly larger, slightly deeper" pipe underground in its place.

But this meant pulling up an old 1890s pipe, which was wedged in among the extensive network of power and gas pipes beneath the city's narrow streets.

"You're putting quite a large pipe - 800mm - and the clearest alignment is where that 1890s pipe is."

Hutchison said, despite the unrest, Wellingtonians shouldn't notice anything awry when using their toilet.

Wellington Water chief executive Colin Crampton said that black pipe would be off Willis St in a couple of months.

"The reason that's significant is that the black pipe on the surface at the moment is not only disrupting traffic and public transport but also affecting the businesses in the area. So the council and ourselves are very focused on getting the black pipe off the road and underground."

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