How much will it cost ratepayers?
Palmerston North businesses that flush large amounts of phosphorous to the city’s wastewater treatment plant are already paying a premium for the practice.
But the bill for city ratepayers to get the treatment working properly remains unclear.
Phosphorous has been identified as the culprit in pollution of the Manawatu River downstream of the city council’s Totara Rd plant.
The verdict was confirmed in a report released last week after a joint study undertaken for the Horizons Regional and Palmerston North City councils.
For about 120 days a year, the council adds aluminium sulphate to the treatment process to extract phosphorous before discharging to the river.
Water and waste services manager Rob Green said that treatment worked well.
The council met the conditions of its resource consent at the end of the pipe into the river at low flows when the plant was extracting phosphorous.
But the results of a summer of water quality testing showed there was still too much phosphorous in the river downstream, causing algae to grow too fast, destroying the habitat of desirable river dwellers such as mayflies.
The research suggests the river bed is acting like a sponge, soaking up phosphorous at times when it is allowed in the discharge, and releasing it when river levels drop.
This summer the council will have to treat for phosphorous for longer periods, at extra cost.
Last year it spent about $500,000 on alum supplies and the extra operational costs of firing up the phosphorous removal plant.
Mr Green said the recently-introduced trade wastes charges that include a cost to businesses that discharge a large amount of phosphorous will not go far towards the new bills.
In the first year since their introduction, to the end of June, 14 businesses had been billed $63,000.
Mr Green said the charges were also working to encourage businesses to change their practices, with some switching products or processes to avoid or minimise the charge.
Phosphorous, a common ingredient in fertilisers, is also used in many detergents and cleaning products.
The council runs tests at the big users’ points of discharge every three months, and if the concentrations drop, so do their charges.
The rest of the cost of removing the phosphorous falls on domestic users.
The cost is built in to the annual charge for wastewater services of $160 a year.
City Networks general manager Ray Swadel said extra costs this summer would have to be met by pruning operational budgets elsewhere.
The councils will be carrying out further studies on the suspected ‘‘sponge effect’’, and tracking the impact of increasing the number of days the phosphorous removal plant operates, to identify what else the city needs to do to reduce its discharge’s harmful effect on the river.