7.5km 'tank' could ease sewage woes

By WAYNE THOMPSON

A 7.5km-long tunnel for storing sewage beneath three North Shore suburbs is being mooted as a way of providing quick relief from the city's beach pollution woes.

The tunnel tank - and satellite waste treatment plants - are fresh ideas revealed yesterday in an engineering update on plans to improve the city's struggling wastewater system.

Overflows of raw sewage now happen monthly after heavy rain, resulting in bathing bans at city beaches as a health precaution.

System improvements costing from $100 million to $600 million are being assessed by engineers for consideration later in the year by the council and citizens in Project Care.

They are in addition to $60 million of work already under way to reduce the level of city sewer overflows in wet weather.

Moving and storing sewage in a tunnel would produce a quick and significant reduction in sewer overflows, the council's wastewater planning engineer, Jan Heijs, said yesterday.

It would also help operation of the city's ailing sewer network and take the pressure off the Rosedale treatment station when it could not cope with suddenly increased inflows.

Mr Heijs said Wairau, and Bush Rd in the central city, needed new pumping stations and transfer routes to the Rosedale plant.

Storage tanks would also need to be provided along these new routes.

These works would be disruptive to residents and take considerable time to prepare and build.

It was appropriate to assess the tunnel alternative in parallel with the conventional works.

A tunnel would run from the municipal golf course at Takapuna, to the west of the Northern Motorway, through Glenfield, Unsworth Heights and North Harbour to the Massey University campus at Albany.

Mr Heijs said the tunnel could cost $60 million - making it $10 million more expensive than conventional storage tanks.

But not all the benefits of such a tunnel had yet been calculated. It could take six months to work out whether a tunnel was preferable to conventional works.

Tunnels were being built in Hong Kong, Singapore and Sydney.

Mr Heijs said a study was also being made into the possibility of relieving the network by building local treatment stations - for example, at Long Bay.

But the chairman of the council's works and environment committee, Bruce Lilly, played down the possibility of Long Bay being a likely candidate for local treatment, despite its growth and the need to provide expensive sewer links to the Rosedale plant.