An important decision on whether to require costly improvements to a major West O'ahu sewage treatment plant is in the hands of the federal government, and Honolulu officials are bracing for a similar ruling on a larger facility at Sand Island.

The plants handle the bulk of the island's wastewater, but the city contends that upgrading them to a level known as full secondary treatment would be unnecessary and could cost $1.2 billion. The money would be better spent on other projects, including repairs to the island's crumbling network of sewage pipes, officials argue.

But the U.S. Environmental Protection Agency recently found that bacteria levels around the West O'ahu plant's discharge outfall were higher than national standards adopted in 2004 to protect swimmers, surfers and others from gastrointestinal diseases.

The agency also found that the plant's effluent often proves toxic to sea urchins, and produces excess ammonia that can harm other aquatic organisms.

In a voluminous response to the EPA's tentative denial of a long-held treatment waiver for the plant, at Honouliuli, the city contends that the federal agency's "tentative conclusions are not supported by ... monitoring data, and are often arbitrary, conclusory, speculative, or not rationally related to the waiver criteria."

The quality of effluent from the plant has improved since the waiver was first granted in 1988, according to the document.

"We need to continue to concentrate our efforts on fixing our (wastewater) collection system," Mayor Mufi Hannemann said. "There are really no water quality or health benefits to be gained by spending hundreds of millions of dollars going to secondary treatment at Honouliuli."

The EPA will review the response, along with comments submitted by scientists and residents, before making a final decision. There is no specific time schedule for that decision.

Soon the agency also is expected to tentatively deny a waiver for the Sand Island plant, which treats most sewage from urban Honolulu. The city has indicated it would contest such a ruling.

Hannemann and the city's two previous mayors have maintained that upgrading the plants is not necessary because they discharge effluent into deep waters far offshore.

Most other treatment plants in the United States discharge into rivers, lakes or shallow coastal waters after secondary treatment. But all other plants in Hawai'i use secondary treatment, including six smaller ones run by the city.

The City Council has authorized Hannemann's administration to spend up to $1.2 million on attorneys to oppose upgrades at the Honouliuli and Sand Island plants, which treat about 85 percent of the wastewater handled by the city.

Reach Johnny Brannon at jbrannon@honoluluadvertiser.com.