Mixing the numbers is creating confusion over exactly what the annual operating costs of Whanganui's new $1 million wastewater treatment plant will be.

Some letter writers to the Chronicle have claimed the yearly operating costs for the wastewater treatment plant (WWTP) have more than doubled, climbing from $5.6 million to $11.3 million.

But Mike Fermor, council's general manager finance, said those people were confusing the figures and as a result coming up with the wrong answers.

Mr Fermor said the council's wastewater activity had two parts to it - the wastewater treatment plant (WWTP) itself and the underground pipes connecting to houses and businesses which carry waste to the treatment plant.

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And by combining the costs of both activities some were claiming that total figure to be the annual operating costs of the new plant.

But he said the amended 2015-25 10-Year Plan has set annual running costs at $6.9 million.

"The total wastewater activity rate funding requirement - that includes the plant and that underground reticulation - is $11.3 million," he said.

Mr Fermor said based on 2014 estimates, the WWTP operating costs were $5.6 million but at that stage council was assuming it could dispose of sludge from the treatment plant at Bonny Glen landfill near Marton.

In late 2014 the council found Bonny Glen would not accept the sludge from the Airport Rd plant. The alternative was to transport it to the Hampton Downs landfill north of Hamilton which pushed out operating costs to $9.7 million.

"As shown in the original 10-Year Plan, council considered that unaffordable. Since then the preferred option is addition of the sludge drying plant at Airport Rd which has reduced WWTP operating costs from $9.7 million to $6.9 million," he said.

Mr Fermor said it should be noted that the WWTP operating costs also included costs such as interest on the capital costs of building the plant, costs related to the ocean outfall, trade waste monitoring costs, rates and insurance.

"These costs are legitimate costs but are often not taken into account when people start talking about alternatives at reduced operating costs," he said.