It is too early to tell how much sewer plant upgrades might cost the city of Honolulu — and the residents who will pay the bill, said Hawai'i Environmental Protection Agency spokesman Dean Higuchi yesterday.

The city has estimated upgrades to Sand Island and Honouliuli sewage treatment plants could cost $1.2 billion.

Higuchi says that without an analysis of each plant, "it's going to be difficult for any of us to guess what the fees may be and what the overall cost may be."

The federal Environmental Protection Agency oversees water quality standards across the country and each year takes action in various states, but Higuchi said each action is different so Honolulu's outcome can't be predicted by what may have happened in other cities.

"While other cities have similar issues, there's no exact parallel," Higuchi said.

"There's no easy, clear, exact example that's going to fit nicely," Higuchi said, when looking at past EPA actions to predict future costs in Honolulu.

Currently, the city's two largest wastewater treatment plants — Sand Island and Honouliuli — operate under a waiver that allows them to discharge sewage that has received advanced primary treatment but not the secondary treatment required in most other states. The waiver itself is fairly rare.

Fewer than 50 of the nation's 16,000 wastewater plants operate under such a waiver, the EPA reported earlier.

Honolulu has been granted the waiver in large part because the treated wastewater is discharged through deep-ocean outfalls, not into a river, stream or lake that transports one city's waste to another's backyard.

The cost of upgrading O'ahu treatment plants will also depend on each plant's current situation.

The Honouliuli plant, for example, already treats some of its wastewater to the point where it can be recycled, Higuchi said, which means that plant's cost to convert to secondary treatment may not be that expensive.

Reach Robbie Dingeman at rdingeman@honoluluadvertiser.com.