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The fluoride debate is set to resume in Dunedin when councillors consider a potential bill of up to $3.2 million to add the chemical to untreated water reservoirs across the city.
At present, 85% of the city's drinking water was treated with fluoride, but reservoirs supplying properties in Mosgiel, Port Chalmers, Waikouaiti, Outram and West Taieri were not.
A report requested by councillors last year, and to be considered at next week's long-term plan hearing, spells out the cost of adding fluoride to those areas not already treated.
It was estimated four of the five water treatment plants could be retrofitted with fluoridation equipment and a building to house it, at a cost of about $51,000 per site.
However, Mosgiel's water treatment process was different, with five pump stations and no central storage reservoir, so the same approach would not work, the report by council programme analyst Darrin Lane said.
Instead, the council would have to consider spending $2.5 million to $3 million on a new distribution network, or decommissioning Mosgiel infrastructure and distributing water to Mosgiel from the already treated Mt Grand plant.
That would cost between $500,000 and $1 million to do, Mr Lane said.
Previous proposals to supply water to Mosgiel via Mt Grand had been met with ''significant community opposition'', and extra costs could also be faced, he warned.
That included a new code for fluoridating drinking water, being developed by Water New Zealand, he said.
If the code were adopted and adhered to by the council, additional monitoring would be required at a cost of $25,000 per plant, he said.
In addition, the majority of water from the West Taieri plant went to rural properties and was not used for human consumption, while Port Chalmers' plant was already being assessed for decommissioning, he said.
The report recommended councillors consider the options.