Sewer tunnel desilting

By Manly Community Forum
September 16, 2023
2 min read

The Northern Suburbs Ocean Outfall Sewer, commonly known as the NSOOS, is an important part of our wastewater sewerage network. This vital piece of infrastructure runs along the north side of the Parramatta River, extending as far west as Blacktown and transports wastewater all the way to the North Head Water Resource Recovery Facility in Manly.

The NSOOS was built progressively between 1916 and 1933 and was mostly tunnelled through rock, deep underground. It was the third major wastewater system built to service a rapidly growing Sydney and was significant in the development of the north shore, inner and north western suburbs of Sydney. This sewer tunnel system is around 3.5 metres high by 2.6 metres wide – and delivers sewer and wastewater services to 1.7 million people across Sydney’s Western and Northern suburbs. The 28km-long NSOOS collects around 25% of Sydney’s total sewage each day.

When it was built, concrete was used to line the NSOOS and protect the rock from corrosion caused by wastewater. Over time, this concrete has deteriorated, resulting in large pieces falling from the roof of the sewer tunnel. These concrete pieces are now sitting on the tunnel floor, collecting further silt and debris from the hundreds of thousands of litres of wastewater that passes through it each day.

Work is ongoing inside the sewer to restore it to its optimum condition. Custom made machines have been developed to be driven through the wastewater tunnel by specialist workers to remove the debris. This includes a pontoon with a digger on the front which removes the silt and then relays it to the surface. The work is difficult and arduous taking months of planning and years to complete. A spray-on abrasion-resistant concrete is being applied to the walls and roof.

Work on the section from Clontarf to Manly is about to commence as the section under Mosman has been completed. Planning is also underway to access the tunnel from the North Head Wastewater Treatment Plant. This will involve the removal of a ‘room full’ of sandstone beneath the plant to facilitate the installation of a lift system to enable machinery and workers to access.

At the same time the North Head Wastewater Treatment Plant is increasing its capacity with the installation of new digesters and other solids collection and processing equipment. This work is almost completed and the huge crane that can be seen on the North head skyline is about to be dismantled and will be transported by huge semi-trailers down Darley Rd.

These once-in-a-generation restoration works taking place inside the NSOOS and up at the Treatment Plant at North Head are crucial for protecting Sydney Harbour and other waterways from sewage overflows and will extend the life of the structure by at least 50 years.


September 2023 Newsletter2023North Head
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