Summer cloud has silver lining for city

The soggy summer has a silver lining for Palmerston North, though it’s probably not enough to get ratepayers dancing in the puddles.

The weather could produce savings of up to $100,000 at Palmerston North’s wastewater treatment plant.

When the Manawatu River is running low, the city council has to remove phosphorus from treated wastewater before discharging to the river.

But so far this summer, it has only had to carry out that process on 15 days.

Water and waste services manager Robert van Bentum said in an average summer, alum treatment to remove the phosphorus was needed on 100 days.

The treatment was required as a condition of the city’s discharge consent, to reduce the nutrients available downstream of the plant to trigger algal growth on the river bed.

It cost between $2000 and $3000 a day for the chemical and sludge treatment and disposal, adding up in an average summer to between $200,000 and $300,000.

Van Bentum said it was still early in the season to be thinking about banking the savings, although weekend rain had already brought dosing to a temporary halt.

‘‘It is possible, if conditions are dry through into the autumn, that we may still approach 60 to 80 days.’’

Most summers, March and April had seen extended periods of dosing. In a couple of years, treatment continued as late as June.

The Totara Rd plant was upgraded to remove phosphorus by alum dosing when river flows dropped to 37 cubic metres a second at the gauge by the old Teacher’s College.

The process started at 41cumecs to ensure the resource consent condition was met in time.

That was about half of the median flow for the Manawatu River, and occurred about a quarter of the year.

In summer, dosing continued until the river rose above 100cumecs.

Van Bentum said the number of days that treatment was needed varied widely from year to year, with a low of 63 days in 2011/12 to a high of 139 days the next summer.

The other bonus from the wet summer was that the city’s main water supply, the Turitea Dam, had remained full to capacity for all but two spells.

It stopped overflowing for 17 days from December 14 to January 3, and for another 15 days from February 26 to March 13.

That has meant no hosing restrictions for the city.

But for those complaining about the rain, the latest figures from Niwa show Palmerston North received 230 millimetres of rain from December to February.

That was 104 per cent of normal summer rainfall – pretty much bang on average, despite January being the wettest January in a decade.

The rainfall that has fed the high river and dam levels had come from further up the catchments.