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New Viaduct Stormwater System Catches Contaminants

New Viaduct Stormwater System Catches Contaminants At Source

Relatively new stormwater technology will be used to reduce the amount of sediment reaching the Central Area Harbour Edge through stormwater outfalls.

The catchpit filters system (CFS) catches sediment at source, significantly reducing the amount of sediment and attached toxicants discharged into receiving waters.

The technology will be trialled on catchpits leading to a new stormwater outfall in the Port area, known as the “Captain Cook” outfall, which will largely replace four old outfalls at the foot of the CBD.

It is expected that installing the catchpit filters system throughout the Captain Cook catchment will reduce the amount of sediment entering the harbour by about 50 per cent, or between 100 and 150 tonnes a year.

The $300,000 system will also trap toxins attached to sediment, including copper, zinc and lead which are mostly attributable to vehicle use. It will be combined with dredging and trialled for a year in the 123ha Captain Cook catchment.

Nine outfalls covering a 708ha catchment currently discharge an estimated 1000 to 2000 cubic metres of sediment into the harbour each year. Much of the sediment settles near the outfalls, which are dredged as part of a Ports of Auckland dredging programme. However, City Works chair Cr Doug Astley says increasing environmental awareness has accelerated the need for more sustainable solutions like the CFS system.

“Interception at source has been trialled extensively with up to 80 per cent of stormwater-generated sediment being intercepted at source,” he said. “We’ll look at installing CFS on all of the other harbour edge catchments if this trial is successful.”

A CFS is a nylon-mesh bag which hangs within a metal frame from under the catchpit grate. Stormwater entering the catchment is “strained” through the bag, with debris and more coarse sediment retained within the bag.

The bags, holding up to 50kg of material, are emptied once every two to four months by a “sludge gulper” type of truck. The bag is replaced, with the old one being laundered before re-use.


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