Long lost waters return to harbour

Coastal restoration takes a big step today as part of the Mangere sewage works upgrade. PHILIP ENGLISH on the restoration, and the pipeline that might have been.

For the first time in more than 40 years the sea will today flow back into an area taken decades ago for the Mangere sewage treatment works.

In a ceremony marking one of the final steps in New Zealand's biggest coastal restoration project, a seawall will be breached, allowing the sea to reoccupy the part of Manukau Harbour used since the late 1950s for oxidation ponds.

Two of the plant's four oxidation ponds will be history from today, returning an area of 200ha to the harbour.

By about this time next year, the last two ponds, covering about 300ha, will also have disappeared. In a few more years the Mangere coastal area surrounding most of Puketutu Island will be transformed into 13km of natural harbour foreshore.

By the end of the project, about 4 million cu m of polluted sludge will have been removed from the ponds to landfill.

The coastal restoration project is a big part of the $451 million upgrade of the Mangere plant by the region's publicly owned bulk-water supply and sewage treatment company, Watercare Services.

Started three years ago, the project involves not only the restoration of the coast but a change to a land-based sewage treatment process designed to substantially improve the quality of effluent discharged from the plant.

When it is finished, technology such as nine huge reactor-clarifiers to biologically strip organic pollutants from sewage and an ultraviolet disinfection chamber with 7776 UV lamps to kill bacteria and viruses will be used to clean up the waste.

The treated effluent will still discharge into Manukau Harbour.

The flow of treated sewage into the plant is about 300,000 tonnes a day, effectively making it Auckland's biggest river. But when it is discharged it will be cleaner and will be fed into the harbour on full tide twice a day at 25 tonnes a second through a new intertidal storage basin.

An improvement in water quality is expected to ultimately allow swimming and shellfish gathering nearby, except for a small mixing area close to the discharge.

Watercare Services believes the upgrade will significantly improve the quality of Manukau Harbour and eliminate odour and insect problems which made the ponds notorious for those living nearby and for others farther afield in sometimes windy Auckland.

The upgrade will also cope with the region's growing population.

Another part of the project will involve work with local iwi from the Makaurau Marae to rehabilitate the Oruarangi Creek, which will again flow into the sea.

For local Maori today will be a big day. Decades ago they gave up their fishing and shellfish grounds, their local beach and the creek estuary where they launched their boats so the ponds could be built. Their traditional places are now being restored.

Migratory bird roosts will also be rebuilt, and eventually a coastal walkway will link Mangere Mountain, Ambury Regional Park and the Otuataua stonefields.

Between now and June 7, Watercare Services advises people not to swim or collect shellfish in the water between Blockhouse Bay, Mangere Bridge and Ihumatao. Boaties are also asked to steer clear of the waters between the southern end of Puketutu Island and Ihumatao until at least June 7 because the area is still a construction zone.

People wanting information can telephone 0800 928-374. For more information on Project Manukau go to Watercare NZ