Toxic algae worries no barrier to filter plans

23:00, Apr 01 2014

Plans to install a disc filter to remove more phosphorus from Palmerston North's wastewater discharge are moving ahead.

The city council has resolved the $2.9 million project should remain in the draft Annual Plan, despite a warning that changing the balance of nutrients in the Manawatu River could favour growth of toxic algae.

The council this week received a report from consultant Keith Hamill that allayed concerns about the proposal to remove particulate phosphorus, while doing nothing about nitrogen levels.

The worry was that the algae, which produces toxins that can kill dogs, would flourish as a result of the change in nutrient ratios.

Hamill told councillors the plant would not increase the risk of benthic cyanobacteria flourishing in the river downstream of the treated wastewater discharge.

He said the algae, Phormidium, was better than others at extracting the phosphorus it needed from sediment and mats in the river.


By removing more particulate phosphorus, it would lose its competitive advantage.

But city councillor Chris Teo-Sherrell, who has a doctorate in science, points to a Cawthron Institute report that shows more toxic algae in the Manawatu River when there was a lot of nitrogen and very little phosphorus available.

He said the toxic algae depended on nitrogen in the water to flourish.

"I am not arguing that we should not install the disc filter, but that we should not do that alone. It must be accompanied by decreasing the nitrogen content of the discharge."

Water and waste services manager Rob Green said the disc filter was recommended as the first step.

Its installation did not rule out further refinements, nor would it be wasted if further treatment was introduced.

Teo-Sherrell said it would make sense to wait for a fuller report, due out this year, that would cover samples taken over two years.

However, Mayor Jono Naylor said Hamill's assessment made sense to him, and he was satisfied the council was set on the right course.

The council has removed a proviso about waiting for the results of further research, from its proposal.

Horizons Regional Council is pushing for changes to the city council's discharge consent conditions that would force it to develop further treatment options to remove nitrogen.

The city has so far resisted that call.

Manawatu Standard