Palmerston North ratepayers could face a bill of up to $17 million to remove nitrogen from the city's wastewater discharge into the Manawatu River.
Horizons Regional Council has released its opinion that a $2.9m disk filtration system proposed by the city council to remove particulate phosphorus is not enough.
Horizons wants to impose new conditions on the discharge consent to also limit the amount of soluble inorganic nitrogen going into the river at low flow.
The city council is resisting that, and a decision will have to be made by independent commissioners after a hearing later this year.
The conflict over nitrogen has emerged after nearly three years of joint work between the two councils.
Both researched why nutrient-dependent algae grows so quickly downstream of the discharge when the river is in low flow, causing mayfly to move away, changing the diet available for fish.
"Obviously, we have a difference of opinion over whether nitrogen needs to be removed," said mayor Jono Naylor.
He said the city council wanted to play its part in cleaning up the Manawatu River.
"But there is also an economic issue.
"I wonder, if someone put $17m in Horizons' back pocket and said, go fix the river, how much of that would be spent on Palmerston North's wastewater treatment plant.
"My guess is, not very much."
Horizons' advice is that there are technologies available that would enable nitrogen levels to be lowered to the 7mg per litre target in the draft conditions.
Its estimates are that changes to the plant that would be needed would cost between $8m and $17m.
It said the improvements would prevent the adverse effects that both councils have agreed the discharge is having on the Manawatu River downstream of the plant. It would be up to the city council to decide how to achieve compliance.
Horizons chief executive Michael McCartney said the regional council appreciated the work the city council had done so far in what had already been a lengthy process.
"It is essential to get it right as the health of the Manawatu River is important to our region," he said.
City council chief executive Paddy Clifford said the two councils had done a great deal of work, guided by peer-reviewed scientific research, to find out why algae grew faster than expected downstream of the discharge.
Both agreed the effects were greater than anticipated when the discharge consent was first granted.
"It's the extent of the effect and the remedies proposed where we differ."
City council water and waste services manager Rob Green said studies were under way already to check that a proposed disk filtration system to remove particulate phosphorus would be effective in reducing algal growth.
Pending the results, and approval of the spending in the adoption of the 2014-15 Annual Plan, construction of that plant would begin next year.
- Manawatu Standard
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