Water fit to swim in but not to drink

An effluent irrigator. Photo / Mark Mitchell

Water agencies say it is too costly to recycle the millions of litres of water pouring daily into the sea from Auckland's effluent treatment plants.

Higher treatment standards at the two plants serving metropolitan Auckland make the discharge at their sea outfalls fit to swim in, though not to drink.

A claim by the North Shore City Council on Friday that its plant "discharges 99.9 per cent water" prompted a Herald reader to ask why this 68 million litres of water was not being reused.

The reader, Jenny Bickerstaff, questioned whether it could not be used for irrigating council gardens or in car washes, instead of being wasted by discharging it into the ocean.

The Auckland Regional Council member for the North Shore, Dr Joel Cayford, supports the idea.

"The tighter the standard, the more obvious it becomes to provide for reuse," he said.

"Auckland's need for washing and grassland irrigation water during dry periods could readily be met by limited reticulation of highly treated water."

Dr Cayford said reuse of water could delay the time when Auckland's peak demand consumption called for an extra source of water.

Such options included a second Waikato River pipeline and a dam at Riverhead, to the city's northwest.

These options would cost about $200 million each.

But so far, North Shore City Council has only a trial of using water from its Rosedale plant to irrigate nearby trees.

City Water operations manager Steve Singleton said the plant's discharge quality was "very high" but was not fit for human consumption or for, say, watering park grass "because it may raise health and cultural issues".

Steps could be added to the treatment process to make the water drinkable and reused in the water supply, as was done in dry parts of Australia.

But the costs and Auckland's high rainfall made this uneconomic.

A reticulation system from the plant to users would cost about $30 million.

The adjacent Albany and Wairau Rd business areas required a full drinking water standard and potentially would take only 1 per cent of the plant's output.

Watercare Services' plant at Mangere, on the Manukau Harbour, investigated land application of treated wastewater on several occasions, said Watercare chief engineer Jim Hodges.

"The volumes involved at more than 300 million litres a day mean there is insufficient land available within tens of kilometres of the Mangere plant with the required soil types to accept treated wastewater.

"Under winter conditions the land can accept little or no treated wastewater.

"When costs, conflict with other land uses and public perception are taken into account, the option is not practicable."

Mr Hodges said Watercare continued to try to find industries that could use treated wastewater.

In the long term, with improved technology, Watercare's Three Waters strategy expected treated wastewater to become a significant source of drinking water.

Recycling H2O

* Nearly the same amount of water supplied to Aucklanders is discharged from treatment plants to meet recreational water standards.

* At about 368 million litres, this amount would fill 157 Olympic-size pools.

* Auckland gets its water from 10 dams, the Waikato River and an Onehunga underground source.

* About 25 per cent of supply is used by businesses, schools and hospitals, a further 3 per cent by agriculture and irrigation and a further 22 per cent for reticulation maintenance, emergencies and firefighting.

* Auckland's water storage yesterday was at 86.33 per cent of capacity.